The Lithoglyphidae: Let's Get Fresh

Live individuals of Lithoglyphus naticoides, copyright Jan Steger.

In previous posts on this site (see here and here), I've introduced you to members of the Hydrobiidae, a diverse family of mostly freshwater gastropods. Hydrobiids have long been recognised as a tricky group to work with, because of their small size and general shortage of distinctive shell features. In recent years, an understanding has developed that the 'hydrobiids' may include a number of lineages that became independently adapted to fresh water, and a number of previously recognised subfamilies of the Hydrobiidae have come to be recognised as their own distinct families. One of these ascended subgroups is the Lithoglyphidae.

Flat pebblesnails Lepyrium showalteri with eggs, copyright Friends of the Cahaba River National Wildlife Refuge.

The Lithoglyphidae are a family of about 100 known species, mostly found in the Holarctic region (Strong et al. 2008), though they have also been recorded from South America. Most lithoglyphids have distinctively squat, relatively thick shells, and for a long time this was treated as one of the main defining features of the group. However, Thompson (1984) pointed out that the sturdy lithoglyphid shell was probably an adaptation to living in fast-flowing streams and rivers, and could also be found in other 'hydrobiid' groups. As well as reducing the shell profile, the lithoglyphid shell possesses a broad aperture that allows for a proportionately large foot, increasing the snail's clinging power. Thompson (1984) identified a number of other features characteristic of lithoglyphids, including a spirally sculptured protoconch and a simple, blade-like penis that lacks accessory lobes or glandular structures. As the soft anatomy of many 'hydrobiids' has not yet been described, it is still possible that some taxa currently identified as lithoglyphids are in fact impostors. Conversely, confirmed lithoglyphids now include some taxa more divergent in shell shape, such as the limpet-like Lepyrium showalteri from Alabama. This species is distinctive enough that when first described it was identified as a neritid, a member of a group of gastropods not even closely related to lithoglyphids (imagine a new species of rodent being identified as a ratfish). Sadly, Lepyrium is also now endangered, being extinct in one of the two river catchments it was historically known from (see here). Thompson (1984) notes that another North American lithoglyphid genus, Clappia, may be entirely extinct. For at least one species, the cause of extinction was pollution from coal mining; no cause was specified for the other species, but according to Wikipedia its native habitat in the Coosa River has been modified by the construction of hydroelectric dams.

Shells of Benedictia baicalensis, from Baikal.ru.

Also closely related to the lithoglyphids are the Benedictiinae, a group of 'hydrobiid' gastropods endemic to Lake Baikal in Russia. A single species of benedictiine has been described from Lake Hövsgöl in Mongolia, but has not been collected there since; it seems likely that its original location was an error (Sitnikova et al. 2006). Baikal is a remarkable place: one of the world's largest freshwater lakes (and easily the largest in terms of the volume of water it contains), it is basically a freshwater sea. While other large lakes such as the Rift Lakes of Africa are poorly oxygenated at deeper levels, effectively restricting most animal life to the surface layer, Baikal has oxygen-rich deeper waters allowing a rich deep-water animal community (this may also be related to the numerous hydrothermal vents in the depths of Baikal). Some of you may have heard of the endemic Baikal seal Phoca sibirica, but Baikal is also home to a wide diversity of endemic fish (including a dramatic radiation of sculpins), a remarkable array of endemic amphipods, and even its own endemic family of sponges. The Benedictiinae are currently classified as a separate subfamily of Lithoglyphidae, with the remainder of species in the Lithoglyphinae (Bouchet et al. 2005), but as the relationship between the two subfamilies has not yet been examined in detail it is possible that the lithoglyphines are paraphyletic to the benedictiines. The benedictiines generally have thinner shells the lithoglyphines, possibly related to the differences in their usual habitats.

REFERENCES

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Sitnikova, T., C. Goulden & D. Robinson. 2006. On gastropod mollusks from Lake Hövsgöl. In: Goulden, C. E., T. Sitnikova, J. Gelhaus & B. Boldgiv (eds) The Geology, Biodiversity and Ecology of Lake Hövsgöl (Mongolia), pp. 233-252. Backhuys Publishers: Leiden.

Strong, E. E., O. Gargominy, W. F. Ponder & P. Bouchet. 2008. Global diversity of gastropods (Gastropoda; Mollusca) in freshwater. Hydrobiologia 595: 149-166.

Thompson, F. G. 1984. North American freshwater snail genera of the hydrobiid subfamily Lithoglyphinae. Malacologia 25 (1): 109-141.

The Velutinidae: Sea Not-Quite-Slugs

Velutinid, probably Lamellaria. Copyright Bill Rudman.

In previous posts on this site, I have introduced you to some of the incredible animals that shelter under the misleadingly unprepossessing name of 'sea slugs'. However, marine mollusc diversity being what it is, it should probably come as no surprise that not all that squashes is slug-y.

The Velutinidae (sometimes referred to in older references as Lamellariidae) are a group of small gastropods that often look rather like sea slugs on the outside, but are misleading it that they actually do have shells. In one subfamily, the Lamellariinae, the mantle has expansive lobes that wrap up around the shell, covering it over. Velutinids are far from being the only molluscs that do this: the glossiness of cowrie shells, for instance, is because live cowries have the shell protected from the elements in this way. Lamellariines differ from cowries, however, in that the mantle lobes are more or less fused to each other and cannot be retracted back. In the other subfamily of Velutinidae, the Velutininae, the shell is not entirely covered by the mantle, as can be seen in the photo below. In both subfamilies, the shell makes it through one or two small loops before broadening out into an abalone-like shape. Lamellariines also differ from velutinines in lacking the marginal teeth of the radula (Beesley et al. 1998).

Velutina prolongata, copyright Dave Cowles.

The velutinids are all specialised predators of ascidians (sea squirts), which they tear into with hard chitinous jaws contained in the buccal mass. The mantle of lamellariines is often coloured to look like the sea squirt they are feeding on, which can make them very difficult to see: to a casual observer, they're just one more squashy blob amongst a whole bunch of squashy blobs. The unfortunate sea squirts are used as nurseries as well as dinner: the female velutinid inserts each egg capsule into a hole that she bites into the sea squirt, with only a narrow neck protruding from its skin through which the velutinid larva hatches.

Velutinid identified as Coriocella nigra, but looking a bit different from other photos online supposed to be this species, from here.

The velutinids are not close relatives of the classic sea slugs, but closer to gastropods such as cowries and periwinkles. They are regarded by most authors as closest to the Triviidae, a group of cowrie-like gastropods that resemble velutinids in their expanded mantle lobes and preferred diet of ascidians. Velutinids and triviids also resemble each other in having an unusual type of larva called an echinospira. Echinospira larvae appear to have two shells, with a mineralised inner shell that is quite separate from a transparent, glassy outer shell. In the Velutinidae, these two shells even coil differently: the outer shell is planispiral, but the inner shell is helical. When the larva metamorphoses, the outer shell is lost. As a result, earlier authors believed that the inner shell corresponded to the true adult shell, while the outer shell was a novelty unique to echinospirae. In recent times, however, a more popular interpretation is that the two shells each correspond to the inner calcareous layer and the outer periostracum of more usual shells, with their growth having become decoupled. The only other gastropods known to possess an echinospira larva are members of the family Capulidae; whether this larval type indicates that all three families form a single clade remains uncertain.

Echinospira larva of Lamellaria perspicua, from Lebour (1935).

REFERENCES

Beesley, P. L., G. J. B. Ross & A. Wells (eds) 1998. Fauna of Australia vol. 5. Mollusca: The Southern Synthesis, pt B. CSIRO Publishing: Melbourne.

Lebour, M. V. 1935. The echinospira larvae (Mollusca) of Plymouth. Proceedings of the Zoological Society of London 105 (1): 163-174, pls 1-6.

Raphitoma histrix

The shell in the photo above (from G. & Ph. Poppe) belongs to Raphitoma histrix (Bellardi 1847)*, a small marine gastropod (the shell shown is just over 6 mm long). Raphitoma histrix was first described as a fossil from the Pliocene of north-western Italy, but the name has also been used for Recent shells from the Mediterranean and western Africa. Rolán et al. (1998) implied (but did not state) that the identity of the modern specimens could do with a double-check. Raphitoma histrix is found in soft-bottomed, reasonably calm waters.

*There has been a bit of confusion over the exact name for this species. Many authors have referred to it as 'Raphitoma hystrix (Cristofori & Jan 1832)'. However, as noted by Pusateri et al. (2012), Cristofori & Jan's name was a nomen nudum (meaning that it lacked an accompanying description), making it not validly published. The name was not validated until it was used by Bellardi (1847), who used the spelling 'histrix'.

Raphitoma histrix is the type species of the genus Raphitoma, which is in turn the type genus of the subfamily Raphitominae. Past authors have classified this genus within the family Turridae, and the Raphitominae broadly corresponds with the group that has often been called the 'Daphnellinae' (other 'turrids' have been featured on this site before: Comitas, Antiplanes, Kuroshioturris, Asperdaphne and Paradrillia). However, with the recognition that the old 'Turridae' was largely just a dumping ground for less differentiated members of the superfamily Conoidea, the Raphitominae is now usually treated as part of the Conidae, and so a member of the same family as the cone shells. Like other members of this family, including other species of Raphitoma, R. histrix is probably a predator, possibly feeding on other marine invertebrates such as worms.

REFERENCES

Pusateri, F., R. Giannuzzi-Savelli & M. Oliverio. 2012. Revisione delle Raphitomidae mediterranee 1: su Raphitoma contigua (Monterosato, 1884) e Raphitoma spadiana n. sp., specie sorelle (Gastropoda, Conoidea). Sociedad Espanola de Malacologia—Iberus 30 (1): 41-52.

Rolán, E., J. Otero-Schmitt & F. Fernandes. 1998. The family Turridae s.l. (Mollusca, Gastropoda) in Angola (West Africa), 1. Subfamily Daphnellinae. Sociedad Espanola de Malacologia—Iberus 16 (1): 95-118.

The Hydrobiinae: North Atlantic Mud-snails

The fine-looking animal above (photographed by Roy Anderson) is Hydrobia acuta neglecta, a member of the subfamily Hydrobiinae of the family Hydrobiidae. Most members of the Hydrobiidae are freshwater snails (another hydrobiid has been covered on this site here), but Hydrobia and genera closely related to it are found in brackish or marine environments. They are grazers on algae and detritus, and can be very abundant. The group is found in coastal, muddy regions on either side of the North Atlantic, extending on the European side through the Mediterranean and into the Black Sea. Hydrobiids as a whole are not well-studied animals, mostly for one ultimate reason: they're really tiny. Most hydrobiids are only a few millimetres in length. And coupled with that small size is a strong conservatism in external appearance. Compare Hydrobia acuta neglecta above with another hydrobiine, Ecrobia ventrosa, also photographed by Roy Anderson:

The external shell of a hydrobiid supplies few details to distinguish and classify taxa, and dissecting out a snail that small to examine its soft parts is no cake-walk.

Monophyly of the saltwater hydrobiids is supported by molecular data (Wilke et al. 2013), and they form the core of the Hydrobiinae. Bouchet et al. (2005) also listed the family-group taxa 'Pyrgorientaliinae' and 'Pseudocaspiidae' as synonyms of Hydrobiinae. These were both established for freshwater species: the Pyrgorientaliinae for two genera from Turkey, and Pseudocaspiidae for two genera from central Asia (Kabat & Hershler 1993). Whether these taxa are truly associated with the hydrobiines, I suspect, requires further investigation (it should also be noted that many other authors have used 'Hydrobiinae' to refer to more extensive groupings included taxa listed by Bouchet et al. as separate subfamilies).

REFERENCES

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Kabat, A. R., & R. Hershler. 1993. The prosobranch snail family Hydrobiidae (Gastropoda: Rissooidea): review of classification and supraspecific taxa. Smithsonian Contributions to Zoology 547: 1-94.

Wilke, T., M. Haase, R. Hershler, H.-P. Liu, B. Misof & W. Ponder. 2013. Pushing short DNA fragments to the limit: phylogenetic relationships of ‘hydrobioid’ gastropods (Caenogastropoda: Rissooidea). Molecular Phylogenetics and Evolution 66 (3): 715-736.

Paradrillia

Paradrillia patruelis, from Joop Trausel and Frans Slieker.

Paradrillia is a genus of conoid gastropods found in the Indo-Pacific region, with a fossil record going back to the Miocene (Powell 1966; previous CoO posts on conoids can be found here, here, here and here). Species of Paradrillia are small shells, about one to three centimetres in length, with a relatively tall spire and short siphonal canal. The sculpture on the outside of the shell usually consists of nodular spirals; the operculum is leaf-like, with a terminal nucleus. The long, awl-shaped radular teeth are trough-shaped in cross-section (Kilburn 1988).

Paradrillia regia, from G. & Ph. Poppe.

The classification of Paradrillia has shifted around over the years, like most of the less differentiated conoids that were lumped by Powell (1966) under the heading of 'Turridae'. Powell (1966) classed them as Turriculinae, Kilburn (1988) transferred them to the Stictispirinae. Kilburn also synonymised Powell's separate genera Paradrillia and Vexitomina. Powell had distinguished them on the basis that Paradrillia supposedly had an operculum with a mediolateral nucleus instead of the terminal nucleus of Vexitomina. However, this was based on a single specimen that Kilburn regarded as teratological after he found terminal nuclei in Paradrillia melvilli. Most recently, Paradrillia was placed by by Bouchet et al. (2011) on the basis of molecular data in their new family Horaiclavidae.

REFERENCES

Bouchet, P., Yu. I. Kantor, A. Sysoev & N. Puillandre. 2011. A new operational classification of the Conoidea (Gastropoda). Journal of Molluscan Studies 77: 273-308.

Kilburn, R. N. 1988. Turridae (Mollusca: Gastropoda) of southern Africa and Mozambique. Part 4. Subfamilies Drilliinae, Crassispirinae and Strictispirinae. Annals of the Natal Museum 29 (1): 167-320.

Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: An evaluation of the vaid taxa, both recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1-184.

Asperdaphne, I Don't Know Who You Are Any More

A true Asperdaphne: the type species A. versivestita, photographed by Des Beechey.

The subject of today's post has been going through something of an identity crisis recently. Asperdaphne was listed by Powell (1966) as a genus of small conoid gastropods found in Australia, New Zealand and the Pacific coast of Asia, with a fusiform shell and coarse clathrate (lattice-like) ornamentation. This remains the sense in which it has been most commonly recognised. However, in a paper published just last year, Beu (2011) revealed that this picture of Asperdaphne was a fraud. The majority of species assigned to Asperdaphne by Powell (1966) were not members of the same genus as the type, A. versivestita. Instead, they belonged to another genus, Pleurotomella, the type species of which Powell had not been familiar with. Meanwhile, A. versivestita was more appropriately placed with what Powell had called Tritonoturris, an Indo-Pacific genus of larger conoids with a more ovate shell shape. As Asperdaphne was an older genus name than Tritonoturris, this meant that what had been Tritonoturris was now Asperdaphne, while what had been Asperdaphne was now Pleurotomella. The identity of the two east Asian species assigned to Asperdaphne by Powell (1966) was not discussed by Beu (2011).

Not an Asperdaphne: Pleurotomella hayesiana, also photographed by Des Beechey.

We have encountered this paper of Beu's before, when I cited it in the post on another conoid genus, Kuroshioturris. As with that genus, the recognition of Asperdaphne had been confused by differences in protoconch morphology related to larval nutrition. Species assigned to 'Tritonoturris' had a tall conical protoconch, indicating a planktotrophic (feeding on plankton) lifestyle as a larva, while Asperdaphne versivestita has a blunt-tipped paucispiral protoconch, indicating that its larvae are lecithotrophic ('fed' by energy reserves in the yolk).

Diagram of the foregut of 'Tritonoturris' subrissoides, from Fedosov (2008).

Slightly more mysterious are Asperdaphne's feeding habits as adults. Foregut structure has been investigated for one presumed Asperdaphne species, under the name Tritonoturris subrissoides (Fedosov 2008). T. subrissoides is one of a number of members of the family Raphitomidae to show a reduction in foregut structures, and has lost the radula and venom gland of most conoids. Instead, it has a large introvert (extendable proboscis) that probably functions in prey capture. However, the roof of the introvert has a large and elongate outgrowth, unlike any found to date in any other conoid, with a well differentiated muscle system indicating that it is capable of complex movement. Presumably, this outgrowth functions somehow in prey capture (perhaps as a grasping 'finger'?) but its exact purpose remains unknown.

REFERENCES

Beu, A. G. 2011. Marine Mollusca of isotope stages of the last 2 million years in New Zealand. Part 4. Gastropoda (Ptenoglossa, Neogastropoda, Heterobranchia). Journal of the Royal Society of New Zealand 41 (1): 1-153.

Fedosov, A. E. 2008. Reduction of the alimentary system structures in predatory gastropods of the superfamily Conoidea (Gastropoda: Neogastropoda). Doklady Biological Sciences 419: 136-138.

Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: an evaluation of the valid taxa, both Recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1-184, pls 1-23.

The Coral-lovers

The free-living (but, from the look of it, not particularly mobile) coralliophiline Latiaxis mawae, photographed by Merlin Charon.

While the subjects of today's posts, the gastropods of the Coralliophilinae, might have a name that translates as 'lovers of coral', it seems likely that the corals do not love them. Members of the Coralliophilinae are a group of about 250 species of specialised predators/parasites of corals and other cnidarians. They vary in habits from free-living grazers to species that live embedded within their coral host, and as a result they vary in external appearance. However, their internal anatomy is fairly consistent, with all species lacking a radula and having a similar gut anatomy. As far as is known, they are all protandrous hermaphrodites, maturing from males to females as they grow larger. The females brood their eggs within the mantle cavity. Phylogenetic analysis has indicated a relationship with the subfamilies Rapaninae (the oyster drills) and Ergalataxinae in the Muricidae, and the recent tendency has been to treat the coralliophilines as a subfamily of the latter (Barco et al. 2010).

The endobiotic Magilus striatus, with a close-up of the coiled juvenile shell, photographed by Femorale.

Like other muricids, many of the free-living coralliophilines (such as species in the genera Latiaxis and Babelomurex) are strikingly ornamented with spiny shells. Those that live in closer association with their coral hosts, however, show a corresponding reduction in ornamentation. Members of the genus Rapa, feeding on soft corals, have inflated, relatively flat-spired shells. Most derived of all, perhaps, is the boring genus Magilus, in which the shell, after an initial coiled section, becomes uncoiled and tubular to form a channel to the outer wall of the host coral. The simplified shell morphology of the endoparasitic forms means that species can be exceedingly difficult to distinguish. A study of the genus Leptoconchus, internal parasites of mushroom corals, by Gittenberger & Gittenberger (2011) identified fourteen molecularly distinct species, each associated with a different species of coral. However, morphological variation between specimens did not reliably correlate with molecular and host-association data and could not be used to distinguish species. In comparison, a study of the host-nonspecific free-living species Coralliophila meyendorffii found that, despite the clear distinction of a larger form feeding on sea anemones vs a smaller form feeding on corals, the two forms were not resolved as molecularly distinct and there was inadequate support for the recognition of the forms as separate species (Oliverio & Mariottini 2001).

The bubble turnip Rapa rapa, photographed by Eddie Hardy.

REFERENCES

Barco, A., M. Claremont, D. G. Reid, R. Houart, P. Bouchet, S. T. Williams, C. Cruaud, A. Couloux & M. Oliverio. 2010. A molecular phylogenetic framework for the Muricidae, a diverse family of carnivorous gastropods. Molecular Phylogenetics and Evolution 56: 1025-1039.

Gittenberger, A., & E. Gittenberger. 2011. Cryptic, adaptive radiation of endoparasitic snails: sibling species of Leptoconchus (Gastropoda: Coralliophilidae) in corals. Organisms Diversity & Evolution 11: 21-41.

Oliverio, M., & P. Mariottini. 2001. Contrasting morphological and molecular variation in Coralliophila meyendorffii (Muricidae, Coralliophilinae). Journal of Molluscan Studies 67: 243-246.

Fifteen Seconds of Mediocrity

Specimen assigned to 'Gemmula aff. G. asukana' by MacNeil (1960).

This is going to be another one of those posts where I try to actually build something on the back of some obscure taxon about which I've only managed to find enough to fill about two sentences. You have been warned.

Kuroshioturris asukana was described as Drillia asukana by Yokoyama in 1926 in the Journal of the Faculty of Science, Imperial University of Tokyo. That might in itself explain why I haven't been able to access the original description (for my American readers, the appropriate issue is on Google Books here, in case this is another one that Google is only making available within the United States). The species is a Pliocene member from Japan of the gastropod superfamily Conoidea, the 'poison-tongued' gastropods, in the family Turridae. The exact position of asukana in the turrids has shifted around a bit: it has appeared in Clavatula, in Gemmula, in (the most horrifying turrid genus of all) Pleurotoma, before Powell (1966) placed it into Kuroshioturris (treated by Powell as a subgenus of Ptychosyrinx, but as a separate genus by later authors such as Beu 2011). A second specimen other than the holotype assigned by Yokoyama to Pleurotoma asukana in 1928 may not in fact be the same species, and MacNeil (1960) noted that the holotype appeared to be a juvenile, further complicating identification.

The type species of Kuroshioturris, K. hyugaensis, from the East China Sea. Photo from here.

Powell (1966) stated that Kuroshioturris was restricted to the Miocene and Pliocene of Japan, but this was an error. Two of the species listed by Powell (including the type species of the genus) are in fact members of the Recent fauna. Beu (2011) also assigned two species from the Pliocene to Recent of New Zealand to Kuroshioturris. Powell distinguished Kuroshioturris from Ptychosyrinx sensu stricto on the basis of the protoconch morphology, noting that the adult shell was otherwise nearly identical. However, other cases of conoid genera being separated solely on the basis of protoconch form have since been regarded as invalid (Beu 2011). Protoconch morphology often reflects to mode of larval lifestyle for the animal: a tall, narrow protoconch (such as in Ptychosyrinx) indicates an actively feeding, planktotrophic larva, while a blunt, short protoconch indicates a lecithotrophic larva nourished by yolk reserves. Changes from one nutritional mode to another seem to have happened repeatedly, leading to cases such as the lecithotrophic 'genus' Maoritomella turning out to be a polyphyletic assemblage of Tomopleura species that had independently abandoned planktotrophy. Many such genera have now been synonymised, but many (such as Kuroshioturris?) still require examination.

REFERENCES

Beu, A. G. 2011. Marine Mollusca of isotope stages of the last 2 million years in New Zealand. Part 4. Gastropoda (Ptenoglossa, Neogastropoda, Heterobranchia). Journal of the Royal Society of New Zealand 41 (1): 1-153.

MacNeil, F. S. 1960. Tertiary and Quaternary Gastropoda of Okinawa. Geological Survey Professional Paper 339.

Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: an evaluation of the valid taxa, both Recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1-184, pls 1-23.

Snails that Never See the Light of Day

Diagrams of Hauffenia tellinii from Bodon et al. (2001). Figure 67 is the shell, figures 68-71 are opercula, 72-75 are male anatomy, 76-80 are female anatomy.

Hauffenia is a genus of freshwater snails of the family Hydrobiidae found in south-eastern Europe (Slovenia, Croatia, adjoining parts of Italy and Austria, etc.) Hydrobiids are a very diverse but very minute group of gastropods: Hauffenia species, for instance, are less than three millimetres in diameter and less than 1.5 millimetres in height. Hauffenia species differ from many other hydrobiid genera in having much flatter shells with only the slightest of turrets. This shape, which commenter 'tf' described as "almost but not quite planiform", is known as 'valvatiform', after Valvata, another freshwater snail with a similar shell. Though the shells of Hauffenia and Valvata are similar enough that the two genera have been confused in the past, the internal anatomy of Valvata shows that it is not a hydrobiid or even closely related. Hydrobiids belong to the major gastropod clade known as caenogastropods but Valvata is a heterobranch, more closely related to garden snails or sea slugs than to hydrobiids (Dayrat & Tillier, 2002). Valvata species are also hermaphroditic while hydrobiids such as Hauffenia have separate males and females.

Distinguishing Hauffenia from other valvatiform hydrobiids is difficult and requires examination of the internal anatomy. Bodon et al. (2001) characterised Hauffenia as possessing a penis with a stylet in the male, while females possessed proximal seminal receptacles only (no distal receptacles) and a reduced bursa copulatrix. Identifying these characters can be difficult because Hauffenia species are subterranean, mostly living in caves and springs in limestone karsts though the Hungarian Hauffenia kissdalmae was recently described from a spring in andesite (Erőss & Petró, 2008), making collecting fresh material difficult. Most early studies on Hauffenia were based on shell morphology only, and species previously assigned to the genus from western Europe or North America were regarded by Bodon et al. as belonging to other genera. Even more doubtful is the assignation of Miocene marine fossils to this genus, refuted by Iljina (2010) on the basis that the Hauffenia-like opercula attributed to the fossils were probably not validly associated, and possibly not even gastropod opercula. Some Hauffenia species, such as H. tellinii in the figure at the top of this post, possess a distinctive knob on the inside of the operculum that distinguishes them from other valvatiform hydrobiids. Earlier authors distinguished separate subgenera in Hauffenia based on whether or not a species possessed such a knob, but Bodon et al. (2010) did not use such a formal distinction.

And if you've just been reading this post to see who won the ID challenge, three points go to 'tf' who recognised the animal as a hydrobiid and provided the diagnostic features; two points go to 'intercostal' who mistook it for a valvatid but pointed out that the presence of an operculum meant that it couldn't be a pulmonate.

REFERENCES

Bodon, M., G. Manganelli & F. Giusti. 2001. A survey of the European valvatiform hydrobiid genera, with special reference to Hauffenia Pollonera, 1898 (Gastropoda: Hydrobiidae). Malacologia 43 (1-2): 103-215.

Dayrat, B., & S. Tillier. 2002. Evolutionary relationships of euthyneuran gastropods (Mollusca): a cladistic re-evaluation of morphological characters. Zoological Journal of the Linnean Society 135 (4): 403-470.

Erőss, Z. P., & E. Petró. 2008. A new species of the valvatiform hydrobiid genus Hauffenia from Hungary (Mollusca: Caenogastropoda: Hydrobiidae). Acta Zoologica Academiae Scientiarum Hugaricae 54 (2): 159-167.

Iljina, L. B. 2010. On the taxonomic position of Miocene valvatiform gastropods and their ecological features. Paleontological Journal 44 (4): 391-394.

Careful with that Spelling (Taxon of the Week: Barleeiidae)

The two millimetre long Barleeia angustata from Japan (assuming that this is not one of the "gastropods previously confused with Barleeia angustata" referred to in this abstract). Photo from here.

Seriously, watch it. There are some taxon names out there that seem to have been deliberately designed to provoke misspellings, and the double-e, double-i combination in Barleeiidae definitely puts it up there*. If Google Scholar search results are any indication, then publications using misspellings of Barleeiidae outnumber those using the correct spelling by a factor of ten. Barleeiidae and the type genus Barleeia derive their name from a George Barlee, a retired solicitor who regularly accompanied malacologist J. G. Jeffreys on collecting trips in the early 1800s (Fretter & Graham, 1978). The common name of "barley snails" is sometimes given to barleeiids; it looks more likely to be a mangling of the generic name rather than indicating any specific connection between barleeiids and barley.

*As well as providing a good example of the principle that taxonomic names are primarily designed to be written, not spoken. You can try saying that one aloud, but don't be surprised if passers-by think you've sprung a leak.

Barleeiidae are a family of marine gastropods found around the world. They belong to the Rissooidea (another tricky name to spell correctly), the same superfamily that includes the Caecidae. Unlike caecids, barleeiids have a gastropod-ordinary spired shell, usually about one and a half times as tall as wide. One noticeable feature that barleeiids do have in common with caecids, on the other hand, is their size - like caecids, barleeiids are minute, only about one or two millimetres tall. Species such as the north-west Atlantic Barleeia unifasciata live on macroalgae (i.e. seaweed), though their diet seems to be diatoms sitting on the weed more than the weed itself (Fernández et al., 1988). Barleeiid shells are smooth and mostly unornamented except for the protoconch (the very tip of the shell) with numerous fine spiral ridges (Fretter & Graham, 1978). The shell has an inner chitinous layer and the osphradium (the organ a marine gastropod smells with) is relatively large (Kabat & Hershler, 1993). Rissooids as a rule have separate males and females with the internally-fertilised females laying their eggs in lens-shaped capsules; in barleeiids and many other marine rissooids, the glands that secrete the capsule have a fairly basic structure, but in other rissooid families, particularly those including terrestrial and freshwater species, the oviduct glands become more complex.


Barleeia unifasciata. Photo from here.

The number of described species of barleeiids seems to be very small, possibly even less than twenty (see the Atlas of Living Australia listings, for instance), all but a few of which are included in Barleeia. The ALA listing (and other sources such as Wikipedia) includes the genus Amphithalamus in Barleeiidae, but Bouchet et al. (2005) placed that genus in the related but separate family Anabathridae. In view of their wide distribution, it seems certain that the small number of described barleiid species indicates a low level of study, and anyone willing to take on the study of such small animals would be bound to be rewarded with a wealth of new taxa.

REFERENCES

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Fernández, E., R. Anadón & C. Fernández. 1988. Life histories and growth of the gastropods Bittium reticulatum and Barleeia unifasciata inhabiting the seaweed Gelidium latifolium. Journal of Molluscan Studies 54: 119-129.

Fretter, V., & A. Graham. 1978. The prosobranch molluscs of Britain and Denmark. Part 4 - marine Rissoacea. Journal of Molluscan Studies Supplement 6.

Kabat, A. R., & R. Hershler. 1993. The prosobranch snail family Hydrobiidae (Gastropoda: Rissooidea): review of classification and supraspecific taxa. Smithsonian Contributions to Zoology 547.

(Belated) Taxon of the Week: The Bishop's Mitra

Mitra cardinalis. Photo from Sydney Shell Club.

The marine gastropods of the genus Mitra get their genus name (as well as their common name of 'mitre shells') from the resemblance of many species, at certain angles, to the pointy hat of a bishop (and indeed, the species names Mitra episcopalis, M. pontificalis and M. papalis all appear to be floating around out there). They are fairly middling-sized shells - three or four centimetres long would seem to be a respectable Mitra size - and most of them are slender and pointed at one end (the technical term is 'fusiform', and the discription 'cigar-shaped' gets bandied about regularly). Members of the subgenus Strigatella, however, are shorter, more globular animals.

Mitra are members of the family Mitridae, which is in term a family of the Neogastropoda. Neogastropods have been featured at this site before (here and here), albeit without having been identified as such, and there is a fair probability that if you go looking for gastropods on a trip to the beach that the first one you find will be a neogastropod. This is not so much because neogastropods are that much more abundant than other marine gastropods (although they are a fairly speciose bunch) as because neogastropods tend to be a lot more active than other gastropods, and are much more likely to be visibly on the move while other gastropods are sitting clamped to rocks. And the reason for the greater mobility of neogastropods is a matter of diet.


Mitra idae. Note the siphon protruding in front of the shell. Photo from Wikipedia.

The ancestral diet for gastropods was a reliable, if somewhat unexciting, scraped meal - algae rasped off rocks, or the fruits of scavenging. As a result, mobility is not at much of a premium for most gastropods - it doesn't take much speed to chase down a patch of algae - and the only reason to move is to get to the next patch of algae. Neogastropods, however, tired of this diet and went for something a little more exiting - they became active predators. Mobile neogastropods at the beach are on the hunt for prey (or, alternatively, pre-deceased animals to scavenge off). One of the most distinctive features of neogastropods to the casual observer is their elongate siphon, which in live animals can usually be seen extended from the front of the shell (which has a distinct notch or anterior extension for it to extend through), waving back and forth as the animal moves, sniffing for any appetising scents. The radula (the tongue-like structure covered with teeth in the mouth of a gastropod) has become adapted to the predatory life-style, with the number of teeth reduced but each individual tooth much larger and sharper. The fusiform body-shape as seen in most Mitra also appears well-suited to mobility, and is shared by a significant proportion of neogastropods.


Mitra mitra everting its proboscis. Photo from here, where they've made the mistake of thinking this individual is swallowing a worm.

Members of the family Mitridae possess a particularly elongate proboscis, often longer than the rest of the animal. Running along the inside of the proboscis is the radula and a muscular rod called the epiproboscis, which can be even further extended. Mitrids are specialist feeders on sipunculan worms, which live buried in sediment or burrowed into corals (Taylor, 1989), and the epiproboscis is used to capture their prey. Suggestions that it is used to inject digestive enzymes into the prey for external digestion are incorrect, as the prey is usually swallowed directly without allowing time for digestion (Taylor, 1989) The method used by Mitra idae to capture a sipunculan was described by West (1990), and as the morphology of the epiproboscis is fairly constant within the Mitridae other species probably use the same or a very similar method. After locating a sipunculan with its siphon, the gastropod would extend its proboscis until it contacted the worm, then the epiproboscis to grab onto the worm. The first move of the Mitra would then be to try and suck the worm directly out of its burrow. If this failed (which I suspect would be the norm), it would then use its radula to rasp a hole through the worm's skin before inserting the epiproboscis through the hole. The epiproboscis would entwine itself around the worm's viscera and grab directly onto its intestines. The viscera would then be hauled out through the hole in the sipunculan's skin and slurped down the waiting proboscis. Once the Mitra had pulled as much of the worm's guts out as it could, it would close its proboscis over the remaining husk and finish drawing the worm out from its hole. Insertion and retraction of the epiproboscis took a little under ten seconds. The whole process, from initial insertion to final withdrawal, could take up to twenty minutes.

REFERENCES

Taylor, J. D. 1989. The diet of coral-reef Mitridae (Gastropoda) from Guam; with a review of other species of the family. Journal of Natural History 23: 261-278.

West, T. L. 1990. Feeding behavior and functional morphology of the epiproboscis of Mitra idae (Mollusca: Gastropoda; Mitridae). Bulletin of Marine Science 46 93): 761-779.

Stop Giggling (Taxon of the Week: Fartulum)

The minute marine gastropod Caecum (Fartulum) occidentale, all of 2.5 millimetres long. Photo by Maurio Pizzini.

It has to be admitted that some organisms have rather unfairly copped it when it comes to the names that biologists have chosen to bestow upon them. There are birds called Turdus and Arses, a beetle called Dermestes haemorrhoidalis, even the fungus Rectipilus doesn't sound entirely comfortable. Compared to those unfortunates, today's subject perhaps got off lightly. Still, I don't think I would want to be known as Fartulum.

Fartulum is a taxon in the gastropod family Caecidae. Depending on where you look, it's treated as either its own genus or a subgenus of the genus Caecum (ranking issues again, not really important). Species of Fartulum are distinguished from other species of Caecum or closely related genera by their combination of a cap-shaped apical plug (more on that in a moment) and perfectly smooth mature shell without the rings or ridges of other caecids.


Caecum (Fartulum) magatama, even smaller at 1.8 millimetres. Photo from here.

Caecids are one of the more distinctive groups of gastropods. They belong to the superfamily Rissooidea, so are closely related to families with periwinkle-type shells such as Rissoidae and Hydrobiidae, but quite honestly you wouldn't know it to look at them. Mind you, first you'd have to be looking at them, and not many people do that. Not because they're uncommon, but because they're tiny. Many would be pushing it to get past two millimetres. Even if you were sharp-sighted enough to spot a caecid, you might dismiss it as a fragment of something else. Caecids start out life as a flat-spiralling shell, but after a couple of turns the whorls open up and the caecid leaves its tight spiral (Carpenter, 1861). In Caecum and its subgenera or related genera, the growing gastropod then produces an apical plug with which it seals off the upper part of the shell, so the living animal is restricted to the anterior section. With no internal tissue holding it in place, the forsaken spire breaks off, so the mature caecid is a short, slightly curved tube, open at one end and plugged at the other (Carpenter, 1861, described Fartulum specimens as looking like "tiny sausages"). As the caecid continues to secrete new shell at the front, it draws forward the plug at the back and continues to shed old shell.

Caecids are detritivores, and live buried in marine sediment, or among sponges or algae. Despite their obscurity, they are far from uncommon. For instance, an ecological survey of the intertidal zone at Mazatlán Bay on the Pacific coast of Mexico by Olabarria et al. (2001) found Fartulum to be the most abundant deposit-feeder there by a fairly significant margin.

REFERENCES

Carpenter, P. P. 1861. Lectures on Mollusca, or "Shell-fish" and their Allies. Prepared for the Smithsonian Institution. Congressional Globe Office: Washington.

Olabarria, C., J. L. Carballo & C. Vega. 2001. Spatio-temporal changes in the trophic structure of rocky intertidal mollusc assemblages on a tropical shore. Ciencias Marinas 27 (2): 235-254.

In Which I am Defeated by Shells

The tiny Mediterranean marine snail Fossarus ambiguus. Photo by A. Piras.

I knew this day was coming. Every Monday I assign myself a random taxon to write a post on, and so far I've generally been pretty successful. But I always knew that some day I'd assign myself a target that would prove hopeless. That day has come.

The Fossarinae are a group of marine gastropods belonging to the superfamily Cerithioidea that also includes the famous gastropod radiation of Lake Tanganyika in Africa (which, now I think about it, would have probably been a much more promising post subject). As a rule, fossarines are pretty tiny - the best-known species, the Mediterranean Fossarus ambiguus, is only three to five millimetres in size - and they are usually short, squat shells with fairly prominent ribbing. Older references place the fossarines in their own family, Fossaridae, but they were placed in the Planaxidae as a subfamily by Kowalke (1998) (which I haven't read). Fossarines and other Planaxidae (Planaxinae) are fairly distinct in appearance (planaxines are longer and more turret-shaped) but they have similar protoconch morphologies and are both larval brooders. After fertilisation, the female does not lay eggs but incubates her embryos in a pouch behind the head, eventually releasing them as free veliger larvae. One individual of the possible (see later) fossarine Larinopsis turbinata held as many as 400 embryos in its brood pouch.

And that is pretty much it, as far as I've been able to find. And to be honest, most of that I lifted off one webpage. Research-wise, Fossarinae seem to have been almost criminally ignored. They don't seem to be particularly rare, and seem to be found pretty much worldwide, so I can only ascribe this to their small size and unassuming natures. Also, as a result of this lack of study, there seems to be an underlying implication that the subfamily is poorly defined and many of the taxa currently regarded as "fossarines" may not be so.


One of these doubtful fossarines is this rather neat little shell, Larinopsis ostensus (photo from here). The shell in the photo is the holotype of this species, collected off Jervis Bay in New South Wales, and still the only known specimen. Larinopsis is big for a fossarine (more than three times the size of Fossarus ambiguus) and lacks the prominent ribbing of most fossarines. It also has that very neat loosely coiled shell, so thin that you can see right through it.

Completely unrelated postscript: While looking stuff up for this, I made the mistake of looking into Finlay (1927). Why are old taxonomic works on molluscs always so horrendously painful to read? There seems to have been this great conspiracy among malacologist prior to, say, 1950 to only ever present the most turbid of prose, to never be explicit when they could possibly be obscure, and to never present explanatory details. For instance, they might state that "Species A is so obviously distinct from species B that I am creating a new genus C" without (a) giving any actual detailed description of these "obvious" differences, or even (b) stating whether it is species A or B that is to belong to this new genus. Take this all-too-typical example from Finlay (1927):

Phasianella huttoni Pilsbry, 1888

As already noted, Thiele includes Prisogaster in his subfamily Phasianellinae (which is better regarded as a family), with two other genera, Phasianella and Tricolia. For the latter, Humphrey's name Eutropia must be used, and the family name would become Eutropiidae, but the Neozelanic species does not belong to the genus Eutropia. Pilsbry has pointed out that the small Australian species have a radula of the Phasianellid style, not of the Tricolia (=Eutropia) form. Consequently one may propose for the Neozelanic species the new generic name, Pellax, associating with it the Australian rosea Angas, virgo, Angas, etc.

And that is all Finlay has to say about that particular genus. He continues in this manner for 159 pages (excluding references), establishing no less than 177 new taxa in the process. In some places, even a single page is enough to inspire thoughts of suicide in the suffering reader.

REFERENCES

Finlay, H. J. (1927). A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320-485.

Kowalke, T. 1998. Bewertung protoconchmorphologischer Daten basaler Caenogastropoda (Cerithiimorpha und Littorinimorpha) hinsichtlich ihrer Systematik und Evolution von der Kreide bis rezent. Berliner geowiss. Abh. E, 27: 1–121, 11 Taf., 13 Abb.

A Whole New Twist on Things, or Just Shifting Back and Forth?

Once again, the Taxon of the Week here at Catalogue of Organisms is probably not worthy of the title. Rectiplanes Bartsch, 1944 was recognised by Powell (1966) as a separate subgenus of Antiplanes Dall, 1902, a genus of turrid shells found in cold waters of the North Pacific. Rectiplanes species, at 18-36 mm in length, were slightly smaller than Antiplanes (Antiplanes) species that reached up to 55 mm. However, a search on Google Scholar suggests that Rectiplanes disappears off the radar in about the mid-1970s. Why should this be?


Good pictures of 'Rectiplanes' seem to be unavailable on line. This photo of Antiplanes vinosa from the Jacksonville Shell Club is labelled Rectiplanes vinosa at the site I found it on, but is obviously sinistral rather than dextral.

The distinguishing feature of the two subgenera was that Antiplanes had sinistral coiling, while Rectiplanes had dextral coiling. The two terms refer to the direction in which the shell coils (its chirality). If you hold a gastropod shell with the tip pointed upwards and the opening at the bottom facing towards you, then in a dextral shell the opening will be on the right-hand side while a sinistral shell will have the opening on the left. The two forms of coiling are mirror images of each other. This has more than merely aesthetic consequences for the individual snails - because snail anatomy is asymmetrical, these differences in orientation are also reflected in the position of the genital openings. Snails with different orientations have the genital openings on different sides of the head, and in species that mate face-to-face the genital openings cannot be brought into contact between snails of different orientations. Usually the orientation of the body matches that of the shell, but there are some gastropods in which the two are opposite - as pointed out by Gould et al. (1985), this condition (called hyperstrophy) is actually a kind of developmental illusion caused by a reversal in direction of growth between the protoconch (the larval shell) and the teleoconch (the adult shell).


A rare mating between dextral and sinistral individuals of the edible snail (Helix pomatia). The awkwardness of such an attempt is readily apparent. Photo by Peter Leonhardt.

Dextral coiling is far more common in gastropods than sinistral coiling. Gould et al. (1985) noted that from hundreds of thousands of examined specimens of the West Indian snail Cerion, only six sinistral examples had been recorded*. When Powell (1966) compiled his catalogue of turrids, Antiplanes contained eleven dextral species compared to four sinistral species. The reasons for this imbalance remain entirely unknown. In many cases that have been investigated, a single gene appears to be involved in determining gastropod chirality. Differences between dextral and sinistral forms other than mere chirality are generally minor - Gould et al. (1985) did note morphometric differences in e.g. size of the aperture between sinistral and dextral Cerion, but Dietl & Hendricks (2006) found that sinistral individuals of Planaxis were more resistant to predation by crabs than dextral individuals. Some of the few clades of predominantly sinistral taxa have been perfectly successful, thank you very much, while a few gastropod taxa even maintain populations that are stably polymorphic for shell chirality.

*In the typical Stephen Jay Gould style that people either love or loathe, the paper opens with, "Vishnu, the great Hindu god of preservation, holds in one of his several hands a shell of the genus Turbinella".

To get back to Rectiplanes, the obvious question is whether chirality can be regarded as a significant factor in taxonomy. Does it indicate a close exclusive relationship between members of one or the other subgenus, or has chirality changed back and forth multiple times in the history of the genus? Unfortunately, no-one seems to have looked at the interspecific phylogeny of Antiplanes itself, but the general indication from other taxa seems to be that chirality alone is not a reliable indicator of phylogeny. Because only a single gene is generally involved in determining chirality*, mutations might be expected to happen fairly recently, and even if selected against somehow, recessive mutations might still persist in the population via heterozygote carriers. What is more, chirality seems to be determined not by the genotype of the individual, but the genotype of its mother (Gittenberger, 1988; Anonymous, 2005). In other words, a genetically sinistral snail born of a dextral mother will be phenetically dextral, but all of its offspring (whether genetically sinistral or heterozygotic) will be born sinistral. This means that even though sinistral and dextral individuals may be unable to mate directly, gene flow can still occur between sinistral and dextral morphs in the population. It also means that rather than appearing as isolated individuals doomed to pass away without finding a suitable mate, reversed-chirality individuals are more likely to appear in numbers, allowing for a greater chance of establishment.

*A big caveat needs to be slapped across what I'm saying here. Probably because of matters of pragmatism, genetic determination of chirality has been almost exclusively studied in members of the Pulmonata, the clade of gastropods that includes the vast majority of terrestrial snails and a fair proportion of freshwater ones. Therefore, caution should probably be exercised in extrapolating what we know about chirality in Pulmonata to gastropods that lie outside Pulmonata - such as Antiplanes.

Such factors mean that we might reasonably expect reversals in chirality to occur multiple times within a clade that includes examples of both chiralities, and that monophyly of one or the other chirality cannot be assumed. For instance, Ueshima & Asami (2003) found that while sinistral species of the land snail genus Euhadra had derived from a single ancestor, reversals to dextrality had occurred multiple times within the sinistral clade. Indeed, the dextral E. aomoriensis had potentially arisen polyphyletically from its sinistral ancestor E. quaesita. Therefore, there is little justification in maintaining Antiplanes and Rectiplanes as separate taxa without anything to separate them other than chirality.

REFERENCES

Anonymous. 2005. Speciation begins, but doesn't end, with the twist of a shell. PLoS Biology 3 (9): e330.

Dietl, G. P., & J. R. Hendricks. 2006. Crab scars reveal survival advantage of left-handed snails. Biology Letters 2 (3): 439-442.

Gittenberger, E. 1988. Sympatric speciation in snails; a largely neglected model. Evolution 42 (4): 826-828.

Gould, S. J., N. D. Young & B. Kasson. 1985. The consequences of being different: sinistral coiling in Cerion. Evolution 39 (6): 1364-1379.

Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: An evaluation of the vaid taxa, both recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1-184.

Ueshima, R., & T. Asami. 2003. Single-gene speciation by left-right reversal. Nature 425 (6959): 679.

The Life of an Ostrich Foot

Before I start, just a reminder that Linnaeus' Legacy is happening in two days' time, so get your posts in to me quick!

This week's Taxon of the Week is the Struthiolariidae, a family of marine gastropods (specifically caenogastropods, a large clade characterised by the possession of a feeding proboscis) commonly known as "ostrich foot shells". I have to confess complete ignorance as to why they are supposed to resemble an ostrich's foot. If you look at the photos above of Struthiolaria papulosa (from gastropods.com), I suppose you might be able to see a resemblance if you exercise a significant amount of imagination, but I doubt it would be the first thing I would think of.

Ostrich foot shells are an entirely Southern Hemisphere family, found in cooler waters. Despite a fossil record extending back to the Cretaceous, the Struthiolariidae is currently not a very speciose family - gastropods.com lists only seven species. New Zealand is the current centre of diversity (four species), and also the source of the oldest known fossil struthiolariids of the genus Conchothyra. A single species is known from Australia, one from South Georgia, and one from South Georgia and Kerguelen. Struthiolariids can reach a respectable size - Struthiolaria papulosa can be nearly 9 cm long. The thickened lip that can be seen around the shell opening in the photo above is considerably stronger than the rest of the shell, and often remains intact long after the remainder has worn away - they are a common find on sandy New Zealand beaches.



Struthiolariids show two major modes of movement. They may crawl about on the foot like most gastropods, or they may use a more dramatic mode of movement in which fluid is drawn from the foot and the small operculum (visible in the photo at the top) is directed downwards. The operculum ends in a sharp hook, and this is used to push against the sediment and lunge the animal forward in a clumsy leap (in the related Strombidae, the conches, the sole of the foot has become permanently reduced and this second method is their only way of moving). This opercular movement can also be used to right the animal when it becomes tipped upside-down. Crump (1968) recorded that opercular movement was used when the animal came in touch with a sea star to throw itself into a series of rapid somersaults to carry itself away from the potential predator (figure above from Crump, 1968). Morton (1951) also suggested a defensive function for the operculum - when a live specimen was held away from the substrate, the operculum would be extended out in an attempt to gain a purchase. Even if this was primarily the usual righting action, the sharp, hooked operculum being waved about could quite possibly deter a potential predator.



Despite being quite capable of moving about, struthiolariids are actually sessile filter-feeders by habit. The above figure from Morton (1951) shows a specimen of Struthiolaria papulosa buried beneath the sand in feeding position. The animal digs itself into the sand using its foot, and the long proboscis is used to construct two mucus-lined tubes that house the inhalent and exhalent siphons. A continuous stream of water and particulate matter is taken in through the inhalent siphon, filtered through the mucus-producing endostyle and the gills, and the filtered water is expelled through the exhalent siphon while the particulate matter is trapped in a string of mucus and carried along a specialised food grove to the front of the animal, where it can be ingested through the proboscis.

The gastropod family most closely related to the Struthiolariidae, the pelican's foot shells of the Aporrhaidae, share the characters of the paired siphons and infaunal living habit. However, the aporrhaids do not have the gills specialised into feeding structures as in the struthiolariids. Instead, the aporrhaids use the strong current produced the movement of water over the gills to draw particulate matter towards the front of the animal, where it is picked up by the proboscis in the normal caenogastropod fashion. In 1937, before the lifestyle of the Struthiolariidae had ever been investigated, Yonge suggested that the mode of feeding found in Aporrhaidae was a possible precursor to ciliary feeding as found in many mollusc groups. It therefore came as significant support to Yonge's speculations when the exact development in feeding mode he had suggested was discovered by Morton (1951) in the aporrhaids' closest relatives.

REFERENCES

Crump, R. G. 1968. The flight response in Struthiolaria papulosa gigas Sowerby. New Zealand Journal of Marine and Freshwater Research 2: 390-397.

Morton, J. E. 1951. The ecology and digestive system of the Struthiolariidae (Gastropoda). Quarterly Journal of Microscopical Science s3-92: 1-25.

Taxon of the Week: Toxic Sea Snails

Taxon of the Week has been delayed a day, as I wasn't actually at uni yesterday. But never fear - I know how much you've all been hanging out to see what the supreme organism will be this week, and this week's chosen organism is the turrid gastropod genus Comitas (image from here).

Turridae is a family of predatory gastropods, mostly fairly small though Comitas is one of the larger members, reaching up to 95 mm in length. The turrids belong to a clade called Toxoglossa that also includes the better-known Conidae (cone shells). The name Toxoglossa means "poison tongue", and a well-developed poison gland is associated with the radula of toxoglossans for the capture of prey. This has been taken to the greatest extent in cone shells, at least some of which are toxic enough to be dangerous to humans. Compared to other gastropods, toxoglossans show reduction in the numbers of radular teeth while individual teeth become larger and more elaborate (figure below from Kantor & Taylor, 2000). In cone shells, the individual teeth are long and spiral in cross-section to form a hypodermic toxin injector.



Though there are far more species of Turridae in the world than Conidae, the latter receives a lot more attention than the former, probably because turrids are generally smaller, more retiring and more likely to be found in deeper waters. Comitas is found in deeper and cooler waters of the Indo-Pacific (Powell, 1966).

REFERENCES

Kantor, Y. I., & J. D. Taylor. 2000. Formation of marginal radular teeth in Conoidea (Neogastropoda) and the evolution of the hypodermic envenomation mechanism. Journal of Zoology 252: 251-262.

Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: An evaluation of the vaid taxa, both recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1-184.