The Litiopids: Small Sea-Snails among the Weeds

I may have commented before that biodiversity tends to increase as one moves to a smaller scale. If I haven't, I certainly should have. The number of small representatives of a group will almost always be greater than the number of large ones. And if one considers the molluscs, for instance, the diversity of large, eye-catching species is considerably smaller is considerably smaller than the diversity of the micro-mollusks that usually go unnoticed.

Litiopids Alaba virgata crawling about on seagrass, copyright Ria Tan.

The Litiopidae are a group of marine gastropods that are found living among and feeding on seaweeds and seagrasses; though little recognised, they can be very abundant. They have high-spired, conical, translucent shells that reach about an inch in length in the largest species, but seem to be more commonly less than a centimetre. They belong among the larger gastropod clade known as the Cerithioidea and can be difficult to distinguish from other members of this clade by their shells alone. Most members of the Litiopidae are placed within the genera Litiopa and Alaba.

The soft anatomy of the family is more distinctive (Houbrick 1987). Litiopids have a long, narrow foot with a median slit in the rear part of the underside marking the opening of a large mesopodial mucous gland. The sides of the foot carry several epipodial tentacles; in other gastropods with such tentacles, they provide a sensory function. A pair of long tentacles is also present on the head, which is produced into an extensible bilobed snout. A pair of small eyes is present at the base of the tentacles.

Soft anatomy of Alaba incerta, from Houbrick (1987).

Litiopids glide about on the underwater vegetation at some speed; they are also able to glide upside-down on the water surface, hanging from the surface tension. The trail of mucus laid down by the mesopodial mucous gland functions like the drag-line laid down by a spider. If the animal finds itself torn away from its substrate, the mucous strand tethers it in place, and it can then haul itself back into place.

REFERENCES

Houbrick, R. S. 1987. Anatomy of Alaba and Litiopa (Prosobranchia: Litiopidae): systematic implications. Nautilus 101 (1): 9–18.

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.