Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Mollusca
Class Gastropoda
Clade Neritimorpha
Family Neritidae

Common name: nerites

Overview

Neritidae, or nerites, are a family of mostly marine snails that includes some brackish and freshwater species. They are characterised by a thick, coiled shell of a generally rounded or drop-like shape into which they can completely withdraw. The calcium carbonate operculum is typically thick and semicircular or �D�-shaped. Eyes are in short stalks near the base of the typically short, invaginable tentacles and they possess a single ctenidium (gill). Nerites are generally small to medium-sized snails (2�40 mm) often with highly patterned and brightly coloured shells ranging from solid dark and light colours to bright greens with shell markings often geometrical in zigzag shapes. Shell sculpture varies widely from simple concentric growth lines to species with heavy radial ribbing. Shell morphology can also vary according to microhabitat (river flow, substrate, etc.).

Distribution and diversity

Neritidae occur across all tropical and subtropical marine intertidal zones and oceans throughout the world. Freshwater species also mostly occur in the tropics with some temperate taxa. Neritidae are found in freshwater and brackish streams or rivers throughout coastal northern Australia. There are five freshwater species from two genera, Clithon and Neritina, which are found in northern Queensland and northern Northern Territory, which are also widely distributed in the islands north of Australia and south-eastern Asia. Several other species occur on Pacific islands.

Life cycle

The mating behaviour of Neritidae is poorly known but the sexes are separate and fertilisation is internal. The male bears a penis located on the right side of the head that is used to transfer sperm to the female who has a complex reproductive system with as many as three distinct openings into the mantle cavity. Females lay pale, gelatinous eggs either singly or in groups in an egg capsules on rocks or shells, from which larvae hatch into the water. After a period in the plankton, larvae settle out and begin growth as juveniles. However, some freshwater species have direct development and hatch as juvenile snails while other species have an estuarine or marine period in their larval phase before returning to freshwater streams and rivers.

Feeding

Neritidae graze on algae, diatoms, and detritus by scraping their radula (teeth) over the substrate. Some freshwater species such as Theodoxus (found in Europe and central Asia) are also carnivorous, feeding on aquatic insect larvae.

Ecology

Neritidae that occur in brackish and freshwater habitats are found on stream margins attached to rocks and wood. They are often restricted in their distribution and populations may be susceptible to disturbances such as water pollution and diversion and reservoir projects. Larger freshwater and marine species are harvested by humans for food, particularly by subsistence gatherer cultures. Some freshwater, and a few marine, species often serve as intermediate hosts for trematode parasites. Several species are sold in the aquarium trade for their attractive shells and algae control in freshwater fish tanks.