Nerita plicata Linnaeus, 1758
Nerita plicata is a marine 'nerite' snail, characterized by a solid, globose shell with a high spire and heavy sculpture. Color of shell ranges from cream to rose, sometimes with blackish markings that can yield an overall grey appearance.
Nerita plicata represents a common and broadly distributed species of marine intertidal snail, ranging throughout the rocky shores of the Western Pacific and Indian Oceans. The species is characterized by having planktonic veliger larvae, resulting in broad dispersal capabilities. Upon settlement in the upper intertidal, juveniles and adults feed by scraping microalgae and cyanobacteria from hard, rocky substrate using a radula. Sexes are separate, with males and females engaging in copulation; fertilization is internal. Following mating, females deposit egg capsules containing multiple eggs that develop and hatch into veliger larvae.
Nerita plicata is characterized by a globose shell, elevated spire, and strong spiral ribs. Body whorl convex, spire pointed at apex, and rounded spiral ribs thickened with deep interstitial grooves. Shell aperture bordered by 5-7 prominent denticles on outer lip and 3-4 squared denticles on columellar lip. Columella convex and marked with strong wrinkles and pustules. Operculum concave in shape, smooth with narrow strip of small granules, and tan to fawn in color. Color of shell varies from cream to rose, sometimes with blackish markings that become so dense in some specimens to yield an overall grey appearance. Outer lip often bordered by yellow line. Shell length up to 35 mm.
Original description: "N. testa sulcata, labiis dentatis: interiore rotundato; exteriore utrinque dentibus acutis conicis" (Linnaeus 1758).
Type specimens: http://www.linnean-online.org/view/shells/nerita_plicata.html
Similar to other nerites, Nerita plicata remodels the interior of its shell by resorption of the inner shell walls, yielding a single vaulted chamber to house its body and maintain a water reservoir. In addition to an external shell, Nerita plicata possesses a distinct head, a visceral mass, and a foot. The head is comprised of a pair of cephalic tentacles (with eyes), a snout, and a buccal cavity (mouth). For feeding, N. plicata uses a rhipidoglossan radula that is comprised of a single, quadrate central tooth, flanked on each side by 5 lateral teeth and numerous (~60-80) marginal teeth (Komatsu 1986). Nerita possess a single left gill through which respiration primarily takes place; however, when the snail is out of water, the mantle cavity serves as a lung. Males are distinguished from females by the presence of a cephalic penis, which is used to transfer spermatophores to the receptaculum of the female. For a more detailed description of Nerita morphology and anatomy, see Bourne (1908) and Fretter (1965).
Individuals can reach a maximum of 35 mm in shell length, but average approximately 25 mm.
To help regulate temperature and avoid desiccation, individuals of Nerita plicata often cluster or form dense aggregates.
Nerita plicata possess a diploid chromosome number of 2n = 24, including 11 pairs of autosomes and one pair of sex chromosomes (n = 11 + X). Males are characterized by the sex-determining chromosomes XO (Komatsu, 1985).
Despite inhabiting the marine upper intertidal zone, Nerita plicata is able to regulate temperature and avoid desiccation by maintaining a large water reservoir within its shell, which in turn facilitates evaporative cooling.
Nerita filosa, Nerita versicolor
Ecology and Distribution
Indo-West Pacific: northeastern South Africa to Hawaiian Islands; occasionally on Easter Island and reported from Clipperton Island.
Marine intertidal: rocky shores; supra littoral; specializes primarily on limestone substrate, but occasionally found on volcanic rock (Vermeij 1971).
Similar to other species of Nerita, N. plicata is an herbivore, grazing on a mixture of epilithic microalgae and cyanobacteria.
Sexes are separate (= dioecious), with males and females engaging in copulation. Males insert a cephalic penis into the mantle cavity of a female, leading to the transfer a spermatophore; fertilization is internal. Following mating, females deposit flattened egg capsules on nearby hard, rocky substrate. Each capsule contains multiple eggs, which develop and hatch out into planktonic veliger larvae.
In Nerita plicata, dispersal occurs during the larval stage, as veliger larvae feed on plankton (= planktotrophic). To date, there are no precise estimates of larval duration for N. plicata; however, Nerita species may spend up to 6 months in the plankton (Underwood 1975), resulting in broad dispersal potential.
Common and usually abundant throughout geographic range, although somewhat patchy in distribution due to habitat specialization.
Evolution and Systematics
Nerita plicata serves as the type species for the subgenus Ritena Gray, 1858. Recent molecular phylogenetic findings suggest that N. plicata is most closely related to Nerita picea; however, apart from a high spire, these species show relatively few morphological similarities (Frey 2008). Future research is necessary to determine sister species relationships and exact systematic positions.
A phylogeographic study revealed two divergent clades within N. plicata, separated by a large genetic break (2.3% COI sequence divergence). Both clades span the Pacific and Indian oceans, showing evidence of gene flow between basins; however the smaller clade is more commonly distributed within the Central Pacific, suggesting a sharp cline in clade frequency in the region (Crandall et al. 2008).