Tricolia: Fluorescent Seashells

Tricolia pullus, copyright Ar rouz.

Search among patches of seaweed along the shores of Africa, Australia or warmer parts of Eurasia and you may be able to find represents of the marine gastropod genus Tricolia. Tricolia are small shells, less than a centimetre in height, with shiny shells that may be smooth or spirally ribbed. Most species have a moderately high spire and an ovate shape but some are lower and more globose (Knight et al. 1960). The shell may or may not have an umbilicus, and there is a calcareous, externally convex operculum. Tricolia belongs to the Phasianellidae, commonly known as pheasant shells, presumably in reference to the bold, intricate colour patterns of many species. Species of Tricolia and the closely related genus Eulithidium, which replaces it in the Americas, have shell pigments containing porphyrin that fluoresce under ultraviolet light (Vafiadis & Burn 2020). Over forty species of Tricolia are currently recognised with the highest diversity in southern Africa (Nangammbi et al. 2016). However, the taxonomy of the genus has historically been confused due to polymorphic species being named multiple times; it is possible that at least some of the apparent African diversity is an artefact of the genus being largely unrevised in that region. An analysis of some of the southern African taxa by Nangammbi et al. (2016) found that some 'species' could not be distinguished genetically. They were, nevertheless, distinct geographically and the authors suggested that they may be variants of a single species responding to different environments.

Variants of Tricolia kochii, copyright Brian du Preez.

Like other members of the Vetigastropoda (the clade containing most of what used to be called the 'archaeogastropods'), Tricolia species have a simple life cycle without an actively feeding planktonic larva. The basic mode of reproduction is by broadcast spawning with separate males and females releasing gametes into the water column. After fertilisation, a brief non-feeding planktonic phase is nourished by yolk from the egg before the larva settles. The brevity of this phase is reflected by the resultant form of the protoconch which accounts for less than an entire whorl. In the Indo-West Pacific species T. variabilis, the male is smaller than the female and sits directly on her, waiting to fertilise her eggs as they are laid as gelatinous capsules rather than freely broadcasted. A temperate Australian species, T. rosea, takes things a step further as the female broods embryos (up to nearly fifty at a time) within the cavity of the last shell whorl (Vafiadis & Burn 2020). How the eggs are actually fertilised remains unknown but all embryos within a brood are about the samesize and stage of development, indicating a single fertilisation event; perhaps males associate with females as in T. variabilis. After the young pheasant shells hatch or settle, they initially feed on diatoms and other microalgae until they eventually grow enough to move onto the seaweed fronds that will comprise their adult diet.

REFERENCES

Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: R. C. Moore (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1. Mollusca—general features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—general features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I171–I351. Geological Society of America: Boulder (Colorado), and University of Kansas Press: Lawrence (Kansas).

Nangammbi, T. C., D. G. Herbert & P. R. Teske. 2016. Molecular insights into species recognition within southern Africa's endemic Tricolia radiation (Vetigastropoda: Phasianellidae). Journal of Molluscan Studies 82: 97–103.

Vafiadis, P., & R. Burn. 2020. Internal embryonic brooding and development in the southern Australian micro-snail Tricolia rosea (Angas, 1867) (Vetigastropoda: Phasianellidae: Tricoliinae). Molluscan Research 40 (1): 60–76.

Platyschismatinae

Platyschisma helicoides, from Knight et al. (1960).

In an earlier post on this site, I commented on some of the various ways that gastropods deal with the fact that their development tends to put their anus uncomfortably close to their mouth. A common solution is the development of a sinus or slit in the shell that provides spaces for the anus to be moved backwards.

One of the major gastropod groups exhibiting such a feature is known as the Pleurotomarioidea. In the modern fauna, pleurotomarioids are not hugely abundant, with living species restricted to deep waters. However, they were one of the dominant gastropod groups back in the Palaeozoic when they were represented by a number of families. One Palaeozoic pleurotomarioid group is the Platyschismatinae, known from the Lower Ordovician to the Middle Permian (Knight et al. 1960). Platyschismatines went with the sinus option, with a sinus present at or above the midpoint on the outer edge of the shell opening. Knight et al. (1960) included five genera in the Platyschismatinae. The type genus, Platyschisma, has a slightly flattened spiral and a relatively thin shell. Some of the other platyschismatines were also relatively flat.

REFERENCE

Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169-I331. Geological Society of America, and University of Kansas Press.

What We Have Here is a Failure to Communicate

To those without much of a background in taxonomy, the various rules governing the naming of organisms can seen frustratingly byzantine and laborious. "Surely," they think to themselves as they despairingly attempt to come to grips with concepts of holotypes and lectotypes, synonyms and homonyms, "there must be an easier way of doing this". Nevertheless, the easiest way to develop an appreciation for just how valuable it is to have a set of rules governing nomenclature is to attempt to deal with anything dating back to the days before such rules were established. Settle back, readers, while I tell you a tale. Pour yourself a drink. You're going to hate this.

Collonista glareola, a species that just might be related to the subject of this post, copyright Huang, Fu & Poppe.

These days, it is generally accepted that before a new name can enter general use, it should be clearly established in some form of formal, widely-accessible publication just to what it is that the name is supposed to refer. Back in the day, however, this was not always the case. A century or two ago, the communities of researchers working on a particular group of organisms were often small, and it was not uncommon for names to effectively spread through personal correspondence or word of mouth alone. One naturalist might refer to a new genus he had come to recognise in a letter to another, and the latter naturalist may then assign his own species to that genus without the first naturalist ever publishing a formal description. At the time, this might not be seen as much of an issue: after all, if there was ever any question as to the first naturalist's original intent, surely it could be clarified by simply writing to him personally?

The name Leptothyra seems to have been established in this kind of way in the mid-1800s by the American naturalist James Graham Cooper for a genus of small marine gastropods (belonging to the vetigastropods, related to the top shells and cat's-eyes) found on the coast of California. In 1871, W. H. Dall attributed the name to an unpublished manuscript of Cooper's and cited the type species as Linnaeus' Turbo sanguineus, a Mediterranean species to which Cooper had also attributed specimens from the Pacific. As it happens, Turbo sanguineus was already the type species for an earlier genus name, Homalopoma, so Dall's 'Leptothyra' would be considered invalid and give precedence to Homalopoma. However, in 1869 the name Leptothyra had been used by W. H. Pease for L. costata, a species from Hawaii, without direct reference to any other species (thus making L. costata the effective type species of Leptothyra). Subsequent authors often considered 'Leptothyra Pease' to be a separate genus from Homalopoma/'Leptothyra Dall'. At least one author who did not, Henry A. Pilsbry (1888), nevertheless used the name Leptothyra under the mistaken belief that the name Homalopoma was preoccupied. Over time, numerous species both living and fossil from around the Pacific were assigned to Leptothyra in one way or another.

It was not until over a century later that Coan (1986) pointed out that the name Leptothyra had appeared in print even earlier than Pease's usage. Cooper himself had used the name in a list of Californian molluscs in 1867. Even though Cooper's list lacked any descriptive details, this counts as enough to validate the name because he included species for which descriptions had already been published under other genera. One of these was Turbo sanguineus, which Coan officially designated as type species and fixed Leptothyra's status as an invalid later name for Homalopoma.

Which leaves open the question of what one should call the genus formerly known as 'Leptothyra Pease'. It doesn't help matters that Pease's 'Leptothyra costata' has apparently never been illustrated and its identity has been open to question. Iredale (1918) proposed the name Collonista for use with species previously included in Leptothyra, stating that the latter "proves to have been first published by Pease in connexion with a juvenile shell of a different genus", but gave no further elaboration or explanation how he reached that conclusion. The online resource WoRMS lists L. costata as a junior synonym of the widespread Pacific species Collonista verruca, but I have been unable to find where that synonymy was published. Nevertheless, any sort of replacement name for 'Leptothyra Pease' seems like it would be misguided at best. It is unlikely that L. costata represents any genus otherwise unknown and, even without any explicit statement to the effect, it is quite possible that Pease only intended to assign his species to Cooper's manuscript genus rather than establish a new genus of his own. Any concept of a genus Leptothyra is best left to sink into the annals of history.

REFERENCES

Coan, E. 1986. Some additional taxonomic unites that first appear in publications by J. G. Cooper. Nautilus 100 (1): 30–32.

Cooper, J. G. 1867. Geographical catalogue of the Mollusca found west of the Rocky Mountains, between latitudes 33° and 49° north. Geological Survey of California: San Francisco.

Dall, W. H. 1871. Descriptions of sixty new forms of mollusks from the west coast of North America and the North Pacific Ocean, with notes on others already described. American Journal of Conchology 7 (2): 93–160, pls 13–16.

Iredale, T. 1918. Molluscan nomenclatural problems and solutions.—No. 1. Proceedings of the Malacological Society of London 13 (1–2): 28–40.

Pease, W. H. 1869. Descriptions of new species of marine Gasteropodae inhabiting Polynesia. American Journal of Conchology 5 (2): 64–79.

Peering through a Limpet's Keyhole

Keyhole limpet Fissurella latimarginata, copyright Jan Maximiliano.

In an earlier post, I introduced you to the slit limpets, conical- or flat-shelled gastropods in the family Fissurellidae that possess a longitudinal slit at the front of their shells in order to help achieve the imposrtant condition of having one's anus as far away from one's mouth as possible. The image above shows another member of the same family, but this time known as a keyhole limpet. In the keyhole limpets of the genus Fissurella, the slit has been closed off and modified into a rounded opening bound by a callus at the shell's apex. The apex is located sub-centrally on the shell which is also radially ornamented (Simone 2008). Other interesting features of the genus include a tendency for the radula to be asymmetrical with the three- or four-cusped lateral teeth larger on one side than the other. Two related genera, Amblychilepas and Macroschisma, differ primarily in having larger soft bodies that cannot be retracted under the shell whereas Fissurella species are able to seal themselves in (Aktipis et al. 2011).

Fissurella volcano, copyright Jerry Kirkhart.

Various Fissurella species are found around the world. They have been divided between several subgenera, but Fissurella taxonomy is complicated by the fact that the overall shape of the shell is strongly affected by the nature of the substrate each individual makes its home. Truly reliable identification of distinct taxa requires detailed knowledge of the soft anatomy which is apparently still little-known for many species. According to Simone (2008), there is a correlation between shell height and energy level of each species' preferred habitat: species found in higher-energy environments (such as shorelines subject to heavy surf) tend to have higher shells (which surprises me because, if you'd asked me to guess, I might have expected the opposite).

As far as humans are concerned, though, most keyhole limpets have fairly little economic impact. Larger species, which can get up to about ten centimetres in size (many are much smaller), are harvested for food around the coast of South America. I also came across a reference to a Fissurella species being regarded as a pest in abalone aquaculture, as both species are algae-grazers and compete for food. Other than that, one imagines that their pre-perforated shells could be very useful for children wanting to make a (possibly somewhat malodorous) necklace as a souvenir of a trip to the beach.

REFERENCES

Aktipis, S. W., E. Boehm & G. Giribet. 2010. Another step towards understanding the slit-limpets (Fissurellidae, Fissurelloidea, Vetigastropoda, Gastropoda): a combined five-gene molecular phylogeny. Zoologica Scripta 40: 238–259.

Simone, L. R. L. 2008. A new species of Fissurella from São Pedro e São Paulo Archipelago, Brazil (Vetigastropoda, Fissurellidae). Veliger 50 (4): 292–304.

Marginal Limpets

Shells of Emarginula solidula, photographed by Jan Delsing.

For today's subject taxon, I've drawn the Emarginulini. This is a tribe within the gastropod family Fissurellidae, members of which are commonly known as keyhole or slit limpets. They get these names because of openings in their shells: keyhole limpets have a distinct hole at the apex of their shell, while slit limpets have a longitudinal slit running back from the front of their shell. In both groups, the slit or 'keyhole' functions in excretion. Gastropods undergo a process early in development known as torsion: the viscera of the embryo twists around so that it reverses its original direction (you can see a basic diagram of the process here). The reasons why this happens remain somewhat uncertain (one early suggestion was that it is what allowed the larval gastropod to retract into its shell and close the shell opening with an operculum) but a potentially negative side-effect of the process is that the anus comes to open directly above the mouth. Unless you want to go through your life with a bit of a funny taste in your mouth, this is not ideal. Therefore, many torted gastropods develop some sort of sinus or recess in their shell so the anal opening can be moved rearwards, away from the mouth.

Live individual of Tugali parmophoroidea, from here. Tugali has an expanded mantle, but is still able to retract it body underneath the shell for protection.

The members of the Emarginulini as recognised by Bouchet et al. (2005) are slit limpets rather than keyhole limpets, though in some species the slit has become much reduced and may only be visible on the underside of the shell (i.e there is a ventral groove rather than a full slit). Bouchet et al. (2005) divided the Fissurellidae between the Fissurellinae and Emarginulinae, a classification based on the structure of the radula and shell muscles (Aktipis et al. 2011). The Fissurellinae are all keyhole limpets, but Bouchet et al.'s Emarginulinae included (in addition to the Emarginulini) the tribe Diodorini, whose members are keyhole limpets like the Fissurellinae but have emarginuline internal anatomy. Other authors have recognised this group as a third intermediate subfamily. Two further tribes of Emarginulinae, the Scutini and Fissurellideini, include species with expanded mantles and reduced shells that may be entirely concealed within the soft body of the slug-like animal.

An elephant snail or shield limpet Scutus sp., from here. This genus has a greatly enlarged mantle, which usually folds over to conceal the reduced shell.

However, this classification of the fissurellids was challenged by the molecular analysis of Aktipis et al. (2011). The results of these authors indicated that the Emarginulini of Bouchet et al. (2005) is para- or polyphyletic. A clade of Fissurellinae with Diodorini indicates a single origin of keyhole limpets, with the emarginuline radula and muscle structure being ancestral for fissurellids as a whole. This result is also consistent with the fossil record: 'emarginulines' are known as early as the Triassic, but fissurellines and diodorines have not been found earlier than the Caenozoic. Aktipis et al. (2011) therefore recognised a more restricted monophyletic Emarginulinae containing the genera Emarginula, Montfortula, Tugali and Scutus, while more basal forms (sister to all other fissurellids) were separated as the Hemitominae (Aktipis et al. did not analyse the position of the Fissurellideini). Of Aktipis et al.'s Emarginulinae proper, only Emarginula has a well-developed slit (the others have ventral shell grooves; Scutus has a quite reduced shell), but this genus was also not monophyletic. Instead of aligning by morphology, the species analysed formed clusters corresponding more to their biogeography: a Mediterranean Emarginula clade, a Pacific clade of Emarginula and Montfortula species, and an Australian clade of Scutus and Tugali. The relationships between these three clades varied by analysis method. The Australian Scutus clade was not necessarily sister to the remaining Emarginulinae, so it may not be worthwhile at this point in time distinguishing the tribes Emarginulini and Scutini.

REFERENCES

Aktipis, S. W., E. Boehm & G. Giribet. 2010. Another step towards understanding the slit-limpets (Fissurellidae, Fissurelloidea, Vetigastropoda, Gastropoda): a combined five-gene molecular phylogeny. Zoologica Scripta 40 (3): 238-259.

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.

Name the Bug: Alaskiella medfraensis

Alaskiella medfraensis (from Frýda & Blodgett, 1998)

No-one successfully identified this one. I guess Palaeozoic gastropods just don't have the same following as other animals.

Alaskiella medfraensis is a member of the Porcellioidea, a superfamily of gastropods containing the Palaeozoic Porcelliidae and the Mesozoic Cirroidea. Porcellioids are distinguishable from other gastropods by virtue of being the only heterostrophic vetigastropods. I'll explain what that means, and I apologise in advance if it's a little hard to follow. I know it confuses the hell out me.

I've previously explained the difference between dextral and sinistral gastropods and how to distinguish the two. The method that I described therein is the correct one for orthostrophic shells. Orthostrophic growth is the standard gastropod growth pattern with the shell growing in a downwards spiral. However, some types of gastropod are hyperstrophic which essentially means that from a developmental perspective they grow upwards rather than downwards. Because the shell's "correct" orientation is therefore rotated 180° from that of an orthostrophic shell, if you hold a hyperstrophic shell with the aperture downwards a dextral shell is going to appear sinistral and vice versa. In life, of course, the aperture will still be held downwards in a hyperstrophic gastropod, but they can still be distinguished because the positions of the organs (gastropods are bilaterally asymmetrical) will be reversed - the stuff that you'd expect to see on the left side will instead be on the right. In the case of fossil gastropods, where you can't look at the organs, distinguishing hyperstrophic shells becomes a lot more difficult and I have to confess that I really don't see how they do it (apparently the shape and orientation of the aperture may offer some indications).

Heterostrophic shells like Alaskiella start off life growing upwards like a hyperstrophic shell but then change the direction of growth to downwards. As a result of this, they also change the direction of spiralling - so Alaskiella starts off spiralling upwards dextrally before it spirals downwards sinistrally. Among living gastropods, heterostrophic coiling is characteristic of the Heterobranchia, the clade that includes opisthobranchs (sea slugs and related animals) and pulmonates (lung-breathing snails). However, porcellioids developed heterostrophy independently of heterobranchs; instead, they belong to the Vetigastropoda, the clade including trochids (top shells) and turbinids (cat's-eye or turban shells). The main characters showing porcellioids to be vetigastropods are the presence of a vetigastropod-type protoconch (the embryonic shell) and nacre (the shiny inner layer of, for instance, a paua shell [Haliotis iris]; Frýda et al., 2008). Neither of these features is really visible in the figure above (nacre has only been identified as preserved in one cirrid genus so far) but you can readily see the large selenizone, the groove running around the outer edge of the whorls, which tends to be a characteristic of vetigastropods*. Porcellioids can also be distinguished from heterobranchs in that while heterobranchs change the direction of coiling at the transition point between the protoconch and the teleoconch (the post-embryonic or post-larval shell), porcellioids change directly during the early part of the teleoconch (Frýda & Blodgett, 2004).

*To be scrupulously correct, not all vetigastropods have a selenizone and not every gastropod with a selenizone (which may be a groove or may be a row of openings) is a vetigastropod. However, in general, non-vetigastropods with selenizones have only small ones.

Alaskiella medfraensis differs from other porcellioids in that other porcellioids have the axis of coiling of the protoconch parallel to that of the teleoconch, but Alaskiella has the axes offset at an angle. The change in the axis of coiling is what gives Alaskiella its characteristic looped look where the shell changes direction, which always puts me in mind of the looped peak that tends to form at the top of a meringue when you spoon it out.

REFERENCES

Frýda, J., & R. B. Blodgett. 1998. Two new cirroidean genera (Vetigastropoda, Archaeogastropoda) from the Emsian (late Early Devonian) of Alaska with notes on the early phylogeny of Cirroidea. Journal of Paleontology 72 (2): 265-273.

Frýda, J., & R. B. Blodgett. 2004. New Emsian (Late Early Devonian) gastropods from Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska (Farewell Terrane), and their paleobiogeographic affinities and evolutionary significance. Journal of Paleontology 78 (1): 111-132.

Frýda, J., R. B. Blodgett, A. C. Lenz & Š. Manda. 2008. New porcellioidean gastropods from Early Devonian of Royal Creek area, Yukon Territory, Canada, with notes on their early phylogeny. Journal of Paleontology 82 (3): 595-603.

A Different Kind of Shell

Stomatella varia, photographed by Scott Collins.

Both of the readers of this page will doubtless be glad to know that the Taxon of the Week post is bringing a respite from my temporary obsession with Palaeozoic cephalopods. Instead, I'm going to shift my focus wildly through the world's biodiversity and introduce you to... an entirely different type of mollusc. Prepare to meet the marine gastropod genus Stomatella.

Stomatella is a genus of the Trochidae, one of the largest families of marine snails. I'll refrain from giving any actual numbers - there are a number of 'trochid'-type subfamilies, and every single treatment I've seen seems to have a different idea about which subfamilies should be included in the Trochidae proper and which should be placed in separate families (Bouchet et al., 2005; Williams et al., 2008). So I really should say that Stomatella is usually a genus of Trochidae - the two references just cited both include Stomatellinae in Trochidae, but Hedegaard (1997), for instance, indicated that it should be a separate family based on shell microstructure.

Most trochids are conical shells commonly referred to as "top shells". Stomatella, in contrast, is an example of what might be called the "shield-slug" morphology - the shell is flattened and ear-shaped (the technical term is "auriform") and relatively very small compared to the rest of the animal. Not surprisingly in light of this form, Stomatella also differs from other trochids in lacking the otherwise usually well-developed operculum. Being unable to retreat into its shell like other trochids, Stomatella has developed a different defense - it is able to autotomise the back part of the foot, which then wriggles in the manner of a lizard's tail while the animal makes good its escape. Stomatella are broadcast spawners, and are apparently regularly known to spawn and breed in captivity.

Stomatella are well-known animals in the marine aquarium hobby, though a scan of various noticeboards and such suggests that they are not so much something deliberately trafficked in as something that tends to turn up of its own accord, brought in as hitch-hikers on rocks and such, and is then tolerated because of their usefulness in controlling algae. Despite this, there seems to be a spectacularly frustrating lack of research in these animals. A Google Scholar search on "Stomatella" brings up a little over seventy results, including only a single 117-year-old two-page paper specifically focusing on Stomatella (Pilsbry, 1891), and a bit of searching reveals that the species described in that paper isn't even included in Stomatella any more. There isn't even a Wikipedia page for Stomatella. So let this be a lesson to you all - just because something is familiar, doesn't necessarily mean that it's well-known.

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.

Hedegaard, C. 1997. Shell structures of the Recent Vetigastropoda. Journal of Molluscan Studies 63 (3): 369-377.

Pilsbry, H. A. 1891. Note on the soft parts and dentition of Stomatella. Proceedings of the Academy of Natural Sciences of Philadelphia 43: 71-72.

Williams, S. T., S. Karube & T. Ozawa. 2008. Molecular systematics of Vetigastropoda: Trochidae, Turbinidae and Trochoidea redefined. Zoologica Scripta 37: 483-506.