Nerites Old and New

Four-toothed nerites Nerita versicolor, from here. Some tropical nerite species can be vary variable in their patterning.

Like many New Zealand kids, I spent a large number of my early days at the beach (my great-grandparents and great-uncle lived beside a bay near Pataua north of Whangarei, and we used to camp there most summers). Most of my time at the beach tended to be occupied with the search for animals under rocks: mud crabs, snapping shrimp, whelks, even the occasional worm. The tops of the rocks would be home to oysters and nerites, and if you pulled a nerite off the rock you could see it close itself up, hiding behind its green and white operculum.

At the time, I wasn't aware of much difference between the nerites and any other marine snail, but there is one. Nerites and their allies, the Neritimorpha (sometimes called Neritopsina) are one of the major basal lineages among gastropods. They have a distinctive protoconch (larval shell), with closely convolute whorls (Frýda & Heidelberger 2003). Most living neritimorphs also dissolve out the columella, the central whorl of the shell, as they grow, so the interior of the shell is a single open cavity. Their shells lack nacre, and are closed with an operculum.

The fossil record of neritimorphs stretches back to the Palaeozoic, though the crown group probably originated close to the Permian-Triassic boundary (Nützel et al. 2007). Among taxa identified as stem neritimorphs in the Palaeozoic are the Naticopsidae (named for their superficial resemblance to the living moon snails, Naticidae) and the Platyceratidae, open-whorled forms that include species found in apparent symbiotic associations with other invertebrates such as crinoids. However, the discovery of preserved protoconches in some 'platyceratids' have demonstrated that, while some species had protoconches comparable to those of modern neritimorphs, others had distinctive open hook-like protoconches unlike those of any other gastropod (Frýda et al. 2009). Those species with the hook-shaped protoconches have been separated out as the Cyrtoneritimorpha, while the modern neritimorphs and those with comparable protoconches are called the Cycloneritimorpha. Despite the similarities in adult shell form between cyrtoneritimorphs and cycloneritimorphs, the distinct protoconch form suggests that the two lineages may not be closely related. However, no features have been identified as yet aligning cyrtoneritimorphs with any other gastropod group, and their true affinities are a mystery.

Neritopsis radula, from here.

Among the crown group neritimorphs, the two living species of the genus Neritopsis are distinctive in being the only species to retain the columella, and molecular analysis corroborates this morphological distinction in identifying Neritopsis as basally divided from most other neritimorphs (Kano et al. 2002). Neritopsis does form a clade with the genus Titiscania, but questions of columella retention are irrelevant for that genus, as its two species lack a shell entirely (instead, they protect themselves from predators by discharging white threads from glands on their back). Neritopsids were abundant during the Mesozoic, but became progressively rarer from about the mid-Cenozoic. The living species of Neritopsis and Titiscania are found in secluded habitats such as submarine caves and crevices under rocks.

The terrestrial neritimorph Helicina clappi, photographed by Robert Pilla.

The clade formed by the remaining neritimorphs was more successful, containing about 450 living species. As well as marine species, they include a number of brackish- or fresh-water taxa, and two families (the Hydrocenidae and Helicinidae) of terrestrial snails. Members of the family Phenacolepadidae are mostly limpet-shaped inhabitants of low-oxygen, sulphide-rich environments underneath rocks or sunken logs. Other families such as the Neritidae have remained mostly more conservative (though the Neritidae also include a limpet-like genus, Septaria), but are widespread throughout the world. The species I encountered as a child, offhand, was Nerita melanotragus—and if anyone out there can tell me why a small snail should have been given a name that appears to mean 'black goat', I'd be interested to know.

REFERENCES

Frýda, J., & D. Heidelberger. 2003. Systematic position of Cyrtoneritimorpha within the class Gastropoda with description of two new genera from Siluro-Devonian strata of Central Europe. Bulletin of the Czech Geological Survey 78 (1): 35-39.

Frýda, J., P. R. Racheboeuf, B. Frýdová, L. Ferrová, M. Mergl & S. Berkyová. 2009. Platyceratid gastropods—stem group of patellogastropods, neritimorphs or something else? Bulletin of Geosciences 84 (1): 107-120.

Kano, Y., S. Chiba & T. Kase. 2002. Major adaptive radiation in neritopsine gastropods estimated from 28S rRNA sequences and fossil records. Proc. R. Soc. Lond. B 269: 2457-2465.

Nützel, A., J. Fŕyda, T. E. Yancey & J. R. Anderson. 2007. Larval shells of Late Palaeozoic naticopsid gastropods (Neritopsoidea: Neritimorpha) with a discussion of the early neritimorph evolution. Paläontologische Zeitschrift 81 (3): 213-228.