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.