12. THE FRESHWATER MOLLUSCA OF NEW ZEALAND by W. F. Ponder The small group of drab-looking freshwater mollusca found in New Zealand, though devoid of striking colour and form are of great interest to the biologist because of their inherent variability and adaptability. These same characteristics give the taxonomist many a headache and a number of their problems will remain unsolved until the geneticists, physiologists and biochemists have investigated them. Only two of the five main classes of mollusca have entered freshwater, the Gastro• poda and Bivalvia. THE GASTROPODA The six families native to New Zealand belong to the Prosobranchia and Pulmonata The Prosobranchia: A very large and diverse group usually having a spiral shell but also including the marine limpets and tube molluscs. The animal usually has an operculum and the sexes are separate. The mantle cavity faces anteriorly instead of posteriorly as typical of other molluscs because of a process known as torsion during development. Except in the most primitive families there is only one gill with a single row of filaments in this cavity. Other structures in the mantle cavity include a mucus secreting hypo-branchial gland, a sensory osphradium, kidney aperture, anus and genital duct. Suppression of the free laval stage by enclosing the eggs in capsules was necessary for the gastropods to invade freshwater and thus copulatory organs had to be developed. A high proportion of freshwater molluscs are viviparous, that is they give birth to their young, a feature rarely seen in marine mollusca, and the prosobranchs are no exception. There are two freshwater prosobranch familes, the Hydrobiidae belonging to the Rissoacea, a large superfamily of minute conical snails which are mainly marine. The second family is the Thiaridae in the superfamily Cerithiacea which also contains the marine tube molluscs (Vermetidae) and turret shells (Turritellidae). Both families are algal-detritus feeders and they have a crystalline style in the stomach (see Morton 1952), a stiff rotating rod of muco-protein which contains enzymes. The Pulmonata: Contains the other four families of freshwater gastropods, all similar in not having a true gill. The mantle cavity is transformed into a lung with a small opening on the right side. The kidney and rectum open outside and other pallial structures are lost. In some of the aquatic families a new gill has evolved to facilitate respiration in their secondarily acquired habitat. The operculum is lost in all but one marine family. All pulmonates are hermaphrodite with complex genital tracts. The freshwater members range in form from snails to limpets but all are closely related, belonging to the Basommatophora, having their eyes at the bases of the tentacles. The 13. Stylommatophora have the eyes at the tips of their tentacles and include the pulmonate land mollusca. These families all browse with a broad, many toothed radula and have a crushing gizzard in the stomach but no crystalline style. The snail-like Lymnaeidae, Planorbiidae and Physidae crop green plants but the Ancylidae and Latiidae scrape the surface for finer particles. The Physidae is present in New Zealand as one introduced species. THE BIVALVIA The bivalve mantle cavity is even more important than the corresponding structure in gastropods and it envelops the entire visceral mass. It contains two gills on each side and though these are used in respiration their main importance is in the collection of food. They are ciliary feeders, suspended material in the inhalent water current being trapped in mucus on the gills and wafted to a food groove running along the bottom edge of the gill. Here it is rolled into a string of mucus-bound food and passed to the two pairs of labial palps which reject heavy material on to the mantle and pass fine particles to the mouth from where it is transported to the complex stomach for further sorting. There is a long cryst• alline style in most bivalves. A large laterally compressed foot is used in locomotion. Anterior and posterior adductor muscles close the valves while the elastic ligament tends to force them open. The chief characters used in classification are the hinge and gill structure. On this basis the freshwater bivalves can be placed into two orders. The Heterdonta contains many of the familiar marine bivalves including also the minute, fragile, freshwater family, the Sphaeriidae. The Schizodonta is a small order which includes the freshwater mussels (Mutelidae. ) Both families are hermaphrodite, sperm and ova being formed in different regions of the same gonad, and they both incubate their young, the Sphaeriidae liberating small replicas of the adult from the gill pouches. The freshwater mussels are released at a much earlier stage and are extremely numerous from several hundred thousand to a million at one time. The larvae are called glochidia and are equipped with a small triangular shell with hooks for attaching themselves to small fish. They soon become encysted by the host's tissues where they liquify the food and assimilate it by the margin of their mantle. When the host tissues are exhausted the larva histolyses, the adult organs and tissues are recon• stituted, and the metamorphosed young mussels drop off to resume a free existance. THE BIOLOGY OF FRESHWATER MOLLUSCA The very nature of the habitat demands three adaptations of all successful freshwater animals. Firstly mechanisms to overcome the osmotic differences between body tissues and the very dilute external medium. Secondly an efficient means of dispersal to escape changes in the unstable habitat, and, thirdly, a genetic plasticity to cope with the extreme variability of their environment. EXCRETION There are two main problems confronting freshwater animals. Since inorganic 14. waste ions are at a very low concentration in the external medium they must be conserved and most salts are therefore reabsorbed into the kidney so that the urine becomes hypoton• ic to the blood. Since there is a considerable osmotic difference between the animal tissues and the external medium there is a large unavoidable influx of water. This must be continuously baled out so that these molluscs can maintain their blood concentration. Picken (1937) has demonstrated a high water output from the kidney of Anodonta (a fresh• water mussel) and Lymnaea (a pulmonate snail). Even so Anodonta has a much lower blood concentration than seen in marine mollusca. Land snails excrete large amounts of uric acid (uricotely) instead of ammonia which comprises much of the nitrogen waste of most other molluscs. The freshwater molluscs however show varying degrees of uricotely, especially some prosobranchs, thus demonstrating their fairly recent origin from a terrestrial stock. The marine-derived rissoid Potamopyrgus excretes no uric acid. DISPERSAL Most of the freshwater molluscs in New Zealand are widespread throughout both islands, though some surprising gaps do occur. Also neighbouring bodies of water, though apparently similar in physical properties, may show differences in the species present. There is also the phenomenon of local distribution seen in most species, that is, small centres of distribution within the total range. However, dispersal on the whole is fairly good. Mechanisms for dispersal vary from the accidental transport in weed or mud on the feet of aquatic birds to behavioural adaptions. These may be simple, such as the habit of laying eggs on floating vegetation or as complex as the release of parasitic larvae by freshwater mussels as a response to the presence of a suitable fish host. VARIATION The lack of stability of the habitat combined with an annual life span does not allow natural selection to act on a large number of generations. However a high degree of short-term, small scale isolations can occur because freshwater habitats are geographically discontinuous and mostly of small extent. A result of this is that there is little full speciation but much intra-specific variation (Hunter 1961). It seems as though the adaptive plasticity, both phenotypic and genotypic, seen in the physiological make-up of these animals is of great selective value. Unfortunately for the systemat-ist this is often expressed in the shell (Dell 1953, 1956; Boray and McMichael 1961). It is of interest that though over most of New Zealand the pattern of freshwater bodies is very recent, the Northern Peninsular is rather older and it is here that most differentiation has occurred. LIFE CYCLES Little is known of the life histories of our freshwater molluscs, though overseas several papers have been written on the subject. Most pulmonates live for one year thus not competing for food with the next generation. Some freshwater mussels may live for several decades. What is known of the life history of the New Zealand fresh• water mussel, Hyridella menzesi, is covered by Percival 1931. 15. RESPIRATION Various methods are adopted, the prosobranchs and bivalves employ a gill whereas some pulmonates, showing their terrestrial origin, must come regularly to the surface to breathe. Hunter (1953) has shown that Physa fontinalis and Lymanaea peregra, two European species, the former introduced into New Zealand, vary in their method of respir• ation. Some individuals surface regularly, others remain submerged Some of the latter have a water-filled lung and must therefore breathe cutaneously, the rest have a bubble of air which probably acts like a physical gill as seen in some aquatic insects. Other pulmonates are entirely aqi atic and have developed a secondary gill (pseudo branch) derived from the mantle wall, the pallial cavity not being used Many Planorbids have haemoglobin to facilitate respiration at low oxygen pressure. The colour of Phy-sastra suggests it has haemoglobin throughout its body though it does not survive as well in stagnant conditions as the European species Pianorbis corneus, which has been long known to have a very high haemoglobin content.