Neritopsina

(Nerites and relatives)

Phylum Mollusca

Class Gastropoda

Order Neritopsina

Number of families 6

Thumbnail description
Widely diverse gastropods, generally small to medium-sized, that coil their shells differently than other gastropods and lack a central shell axis

Evolution and systematics

The earliest unequivocal record of the order Neritopsina is Late Sularian-Devonian (428–374 million years ago). Earlier Ordovician records are based on protoconch and adult shell similarities. The Neritopsina were noted as being very distinct from other "archaeogastropods" in the early twentieth century, but only since the 1970s has that placement been broadly accepted.

Neritopsina coil their shells differently than other coiled gastropods and therefore lack a central shell axis, the columella, and most species absorb the internal partitions of the shell as they grow, permitting the snail's body to be more limpet-like rather than coiled, irrespective of its shell.

The Neritopsina are the first clade in the gastropod lineage that has undergone the extensive evolutionary radiations observed across the Gastropoda. The group has shell morphologies that range from coiled conical snails (Hydrocenidae) to limpets (Phenacolepadidae), and even slugs (Titiscaniidae). While conical shells are seen in Hydrocenidae, none appear to have developed very high-spired shells. Multiple terrestrial (Helicinidae, Hydrocenidae) and freshwater invasions have occurred (Neritidae), with some freshwater taxa still having an estuarine or marine larval phase.

The order Neritopsina contains the families Neritidae, Phenacolepadidae, Neritopsidae, Helicinidae, Hydrocenidae, and Titiscaniidae.

Physical characteristics

Neritopsina shell morphology covers most forms seen in the Gastropoda: coiled to limpet, and even shell-less slugs. The Neritopsina have a distinctive juvenile shell, or protoconch, and most species absorb the internal partitions of the shell as they grow, permitting the snail's body to remain more limpet-like rather than coiled. Neritopsis radula does not absorb the internal partitions of its shell and, based on this and other anatomical characters, is thought to represent the most basal taxon. The most typical coiled morphologies are seen in the terrestrial groups Helicinidae and Hydrocenidae, and are similar to those seen in coiled vetigastropod groups such as the Turbinidae. All limpet morphologies are marine groups and include both conical and coiled forms, again similar to that seen

in vetigastropods groups (e.g., Fissurelloidea and Haliotoidea). The most common marine form is a coiled limpet-like morphology with a flat shelf posterior to the aperture. This porcelanous shelf is often ridged, forming tooth-like structures. Shell sculpture varies widely from simple concentric growth lines to taxa with heavy radial ribbing. Several freshwater species have spines that extend from the penultimate whorl, and are often covered by a dark, thick periostracum. Most species have a calcium carbonate operculum that is used to cover the aperture, and the operculum has an apophysis, or spur, on the internal surface. The animals are supple and they have a single pair of cephalic tentacles (hydrocenid species lack tentacles, but the eyes are stalked). Some deepwater and limpet-like taxa bear sensory epipodial tentacles.

The neritopsine female reproductive system is complex, with multiple openings, fertilization is internal, and a muscular copulatory structure is typically found on the right side of the head; a penis is lacking in the terrestrial groups. Spermatophores to facilitate sperm transfer are present and sperm morphology is also distinct in the group. Gelatinous egg capsules are produced and these may be further packaged into calcium carbonate-impregnated egg masses. Free-living veliger larvae are planktotrophic, but direct development is present in some freshwater species and in all terrestrial taxa. The neritopsine radula is rhipdoglossate, as in the Cocculiniformia and Vetigastropoda.

Neritopsines are generally small- to medium-sized gastropods, and range between 0.07–1.5 in (2–40 mm), a relatively small size range within the Gastropoda. External color patterns are variable from solid dark and light colors to bright greens. Shell markings are often geometrical in zigzag shapes. Terrestrial neritopsines often have bright color markings and glossy shells, while others are more subdued, mottled, and blend into their habitats well.

Distribution

Neritopsines are distributed worldwide and have radiated and diversified throughout all tropical and subtropical oceans of the world. Terrestrial species are also distributed worldwide, primarily in the tropics. More temperate taxa occur in the freshwater systems of Europe and Asia, and terrestrially in both the Old and New Worlds.

Habitat

In marine and freshwater realms, neritopsines are typically associated with hard substrates such as coral, beach rock, basalt outcrops and cobbles, and others. Several taxa (Smaragdiinae) are also associated with marine angiosperms in shallow near-shore marine habitats. Two terrestrial invasions, the Helicinidae and the Hydrocenidae, are both associated with moist forest floors and the trunks of trees. The Helicinidae are also found in xeric habitats, and may be partially or completely aboreal. Both terrestrial groups are often associated with limestone geologies. Multiple freshwater invasions have also occurred within the Neritidae, including the Septaria of freshwater streams on tropical Pacific islands and the radiation of Theodoxus species in the river systems of Europe and central Asia. In the tropical Pacific, some freshwater taxa still have an estuarine or marine period in their larval phase before returning to freshwater streams and rivers. Neritopsines also occur in the deep sea, and are associated with dysoxyic habitats and vent and seep communities.

Behavior

Mating behavior of neritopsines is poorly known. Because of the presence of separate sexes and copulatory structures, some mating behavior is likely present in this group. Clustering is one of the most common behaviors seen in intertidal neritopsines and the clusters are thought to reduce risks of both predation and desiccation. In these multilayered clusters, water loss is reduced and evaporative cooling helps regulate body temperature. Nerites are also known to have trail-following behavior; they can follow both their own and conspecifics' mucus trails on the substrate. These trails may be important in relocating microhabitats for both feeding and resting, and may play a role in building aggregations.

Display behaviors are unknown in the neritopsines, but the anti-predator responses to other predatory gastropods include shell elevation, rotations, flailing of tentacles, and rapid movement. Territorial behavior, or defense of a home range, is also unknown. Most activity patterns in intertidal species are mediated by light and tidal patterns. Up-shore migration is common in intertidal species, with the largest individuals often found at the highest intertidal levels. Freshwater taxa with an estuarine or marine larval phase have behaviors that cue the larvae to settle at the mouths of rivers and streams, followed by movement against the flow, literally crawling upstream into the freshwater habitat.

Feeding ecology and diet

Neritopsines are grazers on algal spores, diatoms, and detritus. Some freshwater species such as Theodoxus are also carnivorous, feeding on aquatic insect larvae. Terrestrial representatives feed on detritus, algal spores, moss, and lichens. Deep sea and vent taxa are likely detritivores as well.

Foraging behavior for marine intertidal forms is closely tied to both light and tidal cycles. Many intertidal species move only when awash, and not when full exposed to aerial conditions or when completely submerged. Movement at low tide at night is often common. Many intertidal species also rest at a higher level in the intertidal zone and feed at a lower one.

Predators of neritopsines include crabs, fishes, and other predatory gastropods. Several tropical crab species have large, heavy claws that can effortlessly crush the shells of intertidal nerite species. Tropical reef fishes are also well equipped to both remove and crush nerites. Predatory gastropods are also common predators on neritopsine species, and several species have escape responses to the approach of a carnivorious species.

Reproductive biology

Little is known of the courtship and mating behaviors of neritopsine species. What is known is that the sexes are separate; the female reproductive system is quite complex with as many as three distinct openings into the mantle cavity; and a penis is present in all but the terrestrial species. Eggs are laid in gelatinous capsules, either singly or as multiples in an egg mass. In most marine species, the embryos pass through a trochophore stage within the capsule before hatching out as feeding veliger larvae. Some freshwater species and all terrestrial species have direct development, and hatch as juvenile snails. There is no record of parental care or brooding in this group, and seasonality in broadcast spawning individuals appears correlated with food availability both for the adults and the larvae. In the freshwater Theodoxus species, most of the eggs function as food for the single surviving juvenile.

Conservation status

Most nerite marine species are quite common on open shores. However, freshwater and terrestrial species are often more restricted in their distribution and more susceptible to disturbance; this is certainly the case in the Neritopsina. Ten freshwater members of the Neritidae and 23 terrestrial snails (17 Helicinidae and six Hydrocenidae) are listed on the IUCN Red List. Habitat destruction is the largest threat to these species, with deforestation directly affecting terrestrial species and increased sedimentation load impacting freshwater species. Freshwater species are also affected by water diversion and reservoir projects, as well as pollution.

Significance to humans

The Neritopsina, especially the smaller species, have been used by humans as decorative "beads." Larger marine species have been used as food items by subsistence gatherers since prehistoric time. Freshwater, and some marine, species often serve as intermediate hosts for trematode parasites, and terrestrial species in North America are sometimes considered as agriculture pests.

Species accounts

List of Species

Plicate nerite

Nerita plicata

family

Neritidae

taxonomy

Nerita plicata Linne, 1758.

other common names

None known.

physical characteristics

Globose, but relatively high-spired shell for the group; strong, spiral ribs, with two large denticles inside aperture. Colors range from almost solid dark to cream colored, with sparse dark markings. Length 0.39–0.59 in (10–15 mm).

distribution

Indo-Pacific.

habitat

Intertidal zone on beach rock and coral.

behavior

Nocturnal feeding excursions during low tide periods.

feeding ecology and diet

Grazer on algal spores, diatoms, and crusts.

reproductive biology

Internal fertilization (spermatophore); intertidal egg mass impregnated with calcium carbonate and attached to substrata.

conservation status

Not listed by the IUCN.

significance to humans

Common in shell craft, and used as experimental organism in tropical zonation studies.

River nerite

Theodoxus fluviatilis

family

Neritidae

taxonomy

Theodoxus fluviatilis Linne, 1758.

other common names

German: Gemeine Kahnschnecke.

physical characteristics

Globose, with large penultimate whorl; aperture is oval with raised edge. Exterior shell surface primarily smooth with weak spiral grooves, black to dark brown in color with white blotches and markings. Length 0.23–0.47 in (6–12 mm).

distribution

Europe.

habitat

Freshwater river systems and estuaries of Western Europe and Great Britain, and drainages into Baltic and Black Seas.

behavior

Uses escape behavior from predatory leeches.

feeding ecology and diet

Grazer on algae, diatoms, insect larvae, and detritus.

reproductive biology

Internal fertilization; benthic egg masses attach to hard substrata.

conservation status

Historical distribution substantially reduced by pollution, but currently returning to rivers and tributaries. Not listed by the IUCN.

significance to humans

Host for trematode larvae, and a freshwater pollution indicator species.

Resources

Books

Ponder, W. F. "Superorder Neritopsina." In Mollusca: The Southern Synthesis. Part B, Fauna of Australia, vol. 5, edited by P. L. Beesley, G. J. B. Ross, and A. Wells. Melbourne, Australia: CSIRO Publishing, 1998.

Periodicals

Bourne, G. C. "Contributions to the Morphology of the Group Neritiacea of Aspidobranch Gastropods. Part I. The Neritidae." Proceedings of the Zoological Society of London (1908): 810–887.

——. "Contributions to the Morphology of the Group Neritiacea of Aspidobranch Gastropods. Part II. The Helcinidae." Proceedings of the Zoological Society of London (1911): 759–809.

David Lindberg, PhD