New Zealand's fingernail clams... New Zealand has three members of the relatively primitive family Sphaeriidae (also known as 'Pisidiidae'), or finger-nail clam/ pea mussel family in our freshwaters. This family is also widely documented in Australia, Europe, and throughout North America; indeed the only continent that it is absent from is Antarctica. Together with the species of the freshwater mussel Hyridella species (family Hyriidae) these constitute the main elements of our bivalve freshwater fauna. However, other species less frequent and less easily collected may occur. All three pea mussels may be found in a variety of habitats and depths in many parts of New Zealand and may be found living together. The native pea mussel Sphaerium novaezelandiae is found only in New Zealand and is widespread. It is the largest of the pea mussels in New Zealand, and inhabits creeks, lakes, lagoons, silty streams and ponds. Along with the other two less common pea mussels Pisidium casertanum and P. hodgkini they are the most southern members of their family. The species of the Sphaerium genus are distinguished from Pisidium by the presence of two siphons and paired gills in the former, and either a reduced or absent exhalant siphon, and a single or reduced set of second gills in the latter. Sphaerium has a very thin and brittle shell to a width of only 9 mm maximum length, with Pisidium shells typically only reaching a maximum of around 5 mm. Shell shape is sometimes used as a distinguishing feature between species, but this is often very variable and thus unreliable as a diagnostic tool.

A native pea mussel Pisidium sp.

Sphaerium is usually found in the shallow margins (littoral habitat) of lakes and streams, even when these waterbodies are subjected to seasonal drying. Sphaerium is tolerant to desiccation during prolonged emergence via a strategy known as aestivation in which the animal enters a state analogous to a hibernation until water returns. Our other species the Pisidium are usually found in deeper more permanent waters (e.g. the deep, cool 'profundal' habitat of lakes) and therefore is less tolerant to drying. Movement is largely through continual burrowing through sediments with the shell hinge angled downward. Their extensile, muscular foot allows them to be unexpectedly agile and active at night. They sometimes also settle on the stems of plants. Whilst most bivalves e.g. our larger native mussels Hyridella feed largely on small suspended plant plankton (phytoplankton), living in the waterbody’s sediment filter-feeding is largely by filtration of bacteria from water contained in small spaces between sediment particles. A more substantial fraction of absorbed bacteria comes from direct sediment ingestion, and feeding with the foot ('pedal') on organic detritus on the surface of the sediment. With very high densities being not unusual, pea mussel community harvest a significant proportion of the primary productivity of their habitats on a daily basis. The major reproductive type of the pea mussel family is through hermaphroditism, although brief phases of being either initially male and then maturing to a female sex and vice versa also may occur. Reproduction occurs throughout the year, for as long as the pea mussels are feeding and growing, with the adult activities of these molluscs being devoted to reproduction; eventually leading to the death of the animal. Young appear not as a planktonic larvae as with marine bivalves which would then be distributed by current to new habitats, or as a parasitic larvae as with freshwater mussels (Hyridella species), but as exact tiny shelled miniatures of the adults. This is a reproductive strategy known as 'ovoviviparity' in which, although less young are produced, these fewer young have a much greater chance of survival having matured significantly within marsupial pouches of the gills. Life-spans are typically of no more than 12-15 months, with life-cycles being usually completed in around 1 year. The members of this family are thought to have great potential as 'bioindicators' of pollution events in freshwater and to the toxicities of these pollutions. Back to MOLLUSCS FURTHER READING AND REFERENCES Roa, S. A. 1997: Studies on the pea mussel Sphaerium novaezelandiae (Deshayes, 1853) in Lake Rotorua. MSc thesis, University of Waikato. Hamilton, New Zealand. 91p. Winterbourn, M. J. 1973:A guide to the freshwater mollusca of New Zealand,/I>. Tuatara 20: 141-159. Winterbourn, M. J.; Mason, K. 1983: Freshwater life: streams, ponds, swamps, lakes and rivers. Mobil New Zealand Nature Series. Reed. Wellington, New Zealand. 76p.