Bivalves and Gastropods
Alison MacDiarmid and Taoho Patuawa
Bivalves and gastropods are the two most diverse and abundant classes in the phylum Mollusca that also includes chitons, tusk shells, octopus and squid. Estimates of the number of living mollusca worldwide range from 50,000 to more than 200,000, the vast majority of which are gastropods. There are 3667 marine molluscs in New Zealand waters, including 680 bivalves and 2738 gastropods. In both groups more than a third of known species remain to be described. The level of endemism is extremely high; with 85.5% (589) of bivalves and 86.6% (3183) of gastropods not known from outside New Zealand. Some groups are particularly speciose. For instance, the majority of the world's bivalves in the family Spheniopsidae and the glass-sponge eating gastropods in the family Trochaclididae occur in New Zealand. The uniqueness of New Zealand's bivalve and molluscan fauna is typified by certain taxa. Several species are the largest or among the largest of their group worldwide. The toheroa is one of the largest in its family, the Mesodesmatidae; the purple nudibranch Jason mirabilis is huge for an aeolid; several of New Zealand's trochid gastropods are larger than any others worldwide; and the giant Kermadec Island limpet, the sole representative of its family in New Zealand, reaches an impressive 150 mm in length.
Species from the northern part of New Zealand often occur or have close relatives in southeastern Australia, Norfolk Island, Lord Howe Island, or the tropical Pacific. Bivalves and gastropods from New Zealand's more southern marine environments have affinities to molluscs from Macquarie Island and South America.
Bivalves are characterised by a dorsally hinged shell comprising two parts or valves and usually a wedged-shaped muscular foot. They include the familiar cockle, pipi, oyster and scallop. Gastropods are usually characterised by a single shell, often spiralled, a large muscular foot used for locomotion, and a well-defined head carrying prominent stalked eyes and mouth bearing the radula, a toothed belt used for scraping food into the mouth. The many familiar sea snails such as the cat's eye, the limpets, the paua, and the whelks fit this pattern. In some gastropods, such as sea slugs and nudibranchs, the shell is reduced or absent.
Bivalves are normally suspension or deposit feeders and use their large gills as a specialised apparatus to filter particles that are then passed to the mouth. In suspension feeders water is pumped through the mantle cavity, normally by the action of tiny beating cilia, but in one group, the Poromyoida, the gills are reduced to a muscular perforated base that pumps by regular contractions. While some bivalves are permanently cemented in place, many can slowly dig their way through sediments or, in shallow-water species, move to the sediment surface where they rely on wave action to transport them up or down the beach. Some groups, such as scallops, are much more mobile and can swim using jets of water created by rapid repeated opening and closing of the valves. Even pipi, Paphies australis, normally seen as rather sedentary, can exude a large buoyant mucous thread that suspends an individual while it is carried to a new position by tidal currents.
Gastropods have evolved a wide range of feeding modes. Many shallow water species are herbivorous, grazing upon microscopic benthic algae or larger seaweeds and kelp. Cat's-eye and Cook's turban shell are good examples of this group. Others species, such as the oyster borer, Lepsiella scobina, and the large cask shells (genus Tonna), are predatory and prey on a variety of other species, using the radula to rasp a hole in bivalves, other gastropods, and echinoderms. Some species are highly specialised and prey upon just a single species or closely related group of species. For instance the nudibranch, Jason mirabilis, is usually only found on and predates the hydroid Solanderia ericopsis, while most of the feathery-gilled nudibranchs in the family Doridae consume sponges and bryozoans. Similarly, the long, narrow-bodied snail Volva longirostrata predates gorgonians. Other gastropods, such as species in the genus Cominella, are specialised scavengers, quickly moving up-current to feed upon dead or moribund fish and invertebrates.
Bivalves and gastropods have an exceptionally good fossil record, and are among the oldest groups of hard-shelled organisms, dating back more than 500 million years to the Early Cambrian. The New Zealand Triassic and Cenozoic molluscan fossil records are of particular importance because they occur almost nowhere else on land in the southern hemisphere. Hence, they are of particular importance for dating and characterising sedimentary rocks.
No living marine bivalve or gastropod appears threatened by extinction in New Zealand, although some species are known from just a single or a few localities. This, however, may reflect very patchy collection rather than actual rarity.
Around New Zealand, bivalves and gastropods are widely distributed from the Kermadec and the Chatham Islands to the subantarctic islands. Some species, such as paua, are widespread on rocky reefs around mainland New Zealand and the offshore islands, while some species have a restricted northern or southern distribution. Specialised mussels, in the genera Bathymodiolus and Gigantidas that harbour chemosynthetic bacteria in their tissues, live around submarine sulphur-rich hot-water vents on the flanks of seamounts, particularly in the Bay of Plenty and along the Kermadec Ridge. Some predatory species of nudibranch normally occur on only a single prey species.
Summary of Threats
Direct threats include over-harvesting of coastal species such as toheroa, paua, and greenshell mussels, trawl bycatch of continental shelf and slope species in bottom-trawl fisheries, and sedimentation of coastal and shelf habitats and pollution, especially the effect of tributyltin on coastal and harbour whelk species. Rare species known from just a few localities may be at risk from collectors.
Indirect threats include introduction of alien species and the effects of global warming, which will mostly affect those species with restricted geographical distributions or a limited range of temperature tolerance. These tolerances are unknown for the vast majority of New Zealand bivalve and gastropod species.
Burrowing bivalves such as cockles and pipi live in sediments but are normally adapted to a rather narrow range of sediment type (gravel, sand, mud etc) or a restricted depth range. Boring bivalves such as shipworms, Teredo, are found in wood or soft rock, while bivalves such as mussels and rock oysters may attach themselves to rock or wood or other species. Scallops live freely on the sediment surface. Some recently discovered mussel and clam species are specialised to live around submarine hot-water vents or cold-water seeps.
New Zealand marine gastropods are found in a great range of benthic habitats, from high on the intertidal to the depths of the Kermadec Trench. Some species, such as the surface-dwelling oceanic nudibranch Glaucus atlanticus, are entirely pelagic and live their complete life cycle in the water column. Some species occur only on a single host species on which they feed.
State of Information
A third to half of the known species of New Zealand's bivalves and gastropods remain to be formally described. The larger bivalves and gastropods are reasonably well known, though new species from deeper water and specialised habitats, such as vents and seeps, continue to regularly occur with the increasing exploitation and exploration of these areas. In contrast, the micro-gastropods are mostly undescribed and new species are found on a very regular basis.
The state of life-history information on bivalves and gastropods varies enormously. Commercially important species such as paua, rock oyster, dredge oyster, rock oysters, scallops, cockles, and pipi have been extensively studied and many important details of their biology and ecology are well established. A few of the ecologically significant species, such as the dog whelk, Lepsiella scobina, horse mussel, Atrina zelandica, and cat's-eye, Turbo smaragdus, have also received some attention, mainly in the form of student theses. The vast majority of bivalves and gastropods, however, have received almost no detailed research attention.
Significance for Maori
Bivalves and gastropods were a plentiful and important food source for Maori in early times, and several species could be gathered with relative ease. Important bivalve species include toheroa and tuatua (surf clams), pipi, hinangi (cockles), kutai (green-lipped mussels), tio (oysters), and kuakua (scallops). The main gastropods that were consumed were black-foot and yellow-foot paua. These paua were also referred to by many other names depending on where you were from. Other marine snails found on the rocky shore were used as bait for catching various species of rocky reef fish, however the snails themselves were rarely eaten. Many of these species are still gathered today by Maori, however the distribution and catch limitations have been severely affected by overfishing. The meat of the paua was also the most highly regarded bait used in crayfish traps.
Shellfish were prepared in a number of traditional ways, and this was most apparent when the shellfish were "fat', that is, had high roe content, as this was the opportune time to gather large volumes of this kai. Shellfish were eaten raw, cooked simply in boiling water or on the coals of a fire. Maori also used to put all forms of shellfish into harakeke (New Zealand flax) baskets, place them in streams of running fresh water, and leave them for periods of weeks to a month. This process was a form of fermentation and was a preferred method of preserving shellfish. This method is still conducted today in certain parts of New Zealand.
The shells of some of bivalve and gastropod species were used as tools, while some were also highly valued. The shells of mussels were, and still are by some Maori, used to open other mussels, thus acting like a knife. Paua was highly valued for the brilliant colours of the inside of the shell, and pieces of the shell were used as a decoration on all types of garments, ornaments, and carvings.
Powell, A W B. 1979. New Zealand Mollusca: Marine, land and freshwater shells. Collins, Auckland. 500pp.
Spencer, H G, Marshall, B A, Maxwell, P A, Grant-Mackie, J A, Stilwell, J D, Willan, R C, Campbell, H J, Crampton, J S, Henderson, R A, Bradshore, M A, Waterhouse, J B and J Jr Pojeta. Phylum mollusca: chitons, clams, tusk shells snails, squids and kin. In Gordon, D P (ed). In press: The New Zealand Inventory of Biodiversity Volume 1. Kingdom Animalia: Radiata, Lophotrochozoa, and Deuterostomia. Canterbury University Press, Christchurch.
Willan, R C. 2003. Nudibranchs. In Andrew, N and Francis, M (eds). The Living Reef: The ecology of New Zealand's rocky reefs. Craig Potton Publishing, Nelson, New Zealand.
Table 24: Number of endemic and total number of species in Bivalve and Gastropod orders (Phylum Mollusca) in New Zealand's marine environment
|Subclass Order||Endemic species||Total species|
|Subclass Order||Endemic species||Total species|
Figure 65: Turbo Turbo smaragdus annual distribution.
Figure 66: Paua Haliotis iris annual distribution.
Figure 67: Scallop Pecten novaezelandiae annual distribution.
Figure 68: Toheroa Paphies ventricosa annual distribution.
Figure 69: Bathymodiolus Bathymodiolus tangaroa annual distribution.