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. The New Zealand P. corinna seems to have haemoglobin in only the buccax mass, as is common in gastropods.

HABITAT

Most types of freshwater are frequented by molluscs. Potamopyrgus species are the most ubiquitous. Pulmonates generally prefer quiet waters except for the freshwater limpets which cling to stones in streams Lymnaeids and planorbids live in lakes and ponds or quiet streams, crawling on weeds and algal-covered rocks and often floating upside-down on the surface film.

In the mud and sand on the bottom of lakes and streams are found the freshwater mussels and the minute sphaeriids. The latter are actively moving and are also located in the masses of roots along the banks The larger mussels live buried and motionless in the substrate.

Some prefer brackish water, especially Melanopsis and Potamopyragus pupoides. P. antipodum is often very abundant in brackish water but is by no means restricted to it

COLLECTING AND STORING

Freshwater molluscs are easily collected and stored. By shaking weeds into a bucket of water many of the gastropods can be collected. On and under stones are found the freshwater limpets and some snails - these can be scraped off with a sharp knife if necessary and then picked up with forceps. Sphaeriids can be collected by seiving mud or washing out root masses. Care must be taken in handling these tiny fragile bivalves The freshwater mussels can be located in the substrate by feeling with feet or hands, or, if in deeper water, by using a naturalist's dredge.

Specimens can be stored in stoppered bottles and tubes containing alcohol or neutral formalin, or dried and kept in glass tubes with cottonwool plugs. Labels should include the species' name, the locality, and other relevant data as to the sort of micro- 16.

habitat. The date of collection should always be included.

TAXONOMIC METHODS

As we have seen variation is to be expected in the freshwater mollusca. Early systematists did not realise this and many of their "species" have been shown to be within the range of variation of one variable interbreeding group. Modern systematists study shells of whole populations from as many localities as possible to try to gain an idea of the variability seen within one species. Other methods have been applied using the animal as a guide to relationships. Anatomy is useful especially at the generic level and micro-anatomy may prove to be valuable. Paper chromatography of the mucus secretion, (e. g. Wright 1959), and chromosome morphology, (e. g. Burch I960) have both been applied to freshwater pulmonate snails recently with some success. Breeding experiments, however, are the best test, (e. g. Boray and McMichael, 1961). By breeding different forms under different conditions the range of variability of the species can become deductive rather than inductive.

SYSTEMATICS

The families Hydrobiidae, Sphaeriidae and Ancylidae are in need of revision. At present this is being attempted in part by various overseas workers.

Descriptions of species are as short as possible and the diagrams should be used to supplement the description. References to Suter (1913) are given for most species or to subsequent papers and these should be referred to for a fuller descrip• tion. Radulae and operculae are not described and here again Suter (1913) is the best source for descriptions and references.

The species recognised here follow Powell's (1962) list with a few necessary changes, some already in the literature. Other changes are made where forms called subspecies do not conform to the recognised biological definition of that category. One recognised species is reduced to a subspecies and three others are tentatively discarded. Higher classification is essentially that of Thide (1931).

Class GASTROPODA Sub-class Prosobranchia Super-family Rissoacea

Animal always small, shell usually with a fairly tall spire and rounded apeture. The eyes are on the outside of the tentacles. Radula with seven teeth per row. A penis is usually present in the male.

Family HYDROBIIDAE

Fresh water rissoids with long tentacles and with a crystalline style in the stomach. 17.

Sub-family Hydrobiinae

Shell moderately tall or turbinate, operculum thin, horny, spiral

Genus Potamopyrgus Stimpson 1865

Animal with long pointed tentacles, the eyes on prominent tubercules. Shell with or without spines, highly polymorphic within each species - smooth, shouldered and spined specimens occurring within one species. The adult size is also quite variable.

Distribution - New Zealand, Australia, Tasmania, West Africa, Tropical America, and Europe (introduced)

Since most species are normally parthenogenic the taxonomic problems are especially difficult because the great variety of forms must belong to true breeding lines since no interbreeding occurs Warrick (1952) has shown the British P. jenkensi (which is possibly P. badia introduced from New Zealand) is represented in Britain by a number of strains and in any one population there may be 1-3 strains living side by side. Moreover many of the strains may show substrains. Pigmentation of the mantle, depth of sutures, keel- ling and/or bristling all occur to different degrees in different strains. Genetic and environmental factors appear to be interlinked in producing the different types of shell forms Extensive searching has found only one male specimen of P. jenkensi. Thus Potamopyrgus is the only undoubted parthenogenic prosobranch known Most species are viviparous, the pallial oviduct being used as a brood pouch.

Fairly clearly there have been too many species named in New Zealand. Some suggestions are made as to likely synonyms and some species are reshuffled.

P. antipodum (Gray 1843) Fig. la (Suter p. 231)

This species is fairly large, growing to about 6 mm. in height and is often covered with a black coating It is distinguished by its large size combined with flat whorls. Some specimens of Potamopyrgus have the shape of P antipodum but have spines

Animal - see P. zelandiae It is viviparous

Distribution - North and South Islands, often in brackish water but also found in many other habitats, especially streams

P. corolla (Gould 1847) Fig. 2a. (Suter p. 232)

The spines of this species make it fairly distinctive but the length of the spines is highly variable as is the shell-shape and the amount of keeling, number of whorls with spines etc. A so-called sub-species of P. corolla has been recognised to date as a narrower shell with short spines on the last four whorls, differing therefore from the typical form with a wider shell and long spines on the last three whorls. Fig 2 and 2a show how these characters overlap. Since, also, the distribution of the P. salleana form has always been recognised as coinciding with P. corolla it can no longer be regarded a sub-species but merely as a variation of P. corolla. Some small specimens resemble P. zelandiae. (Fig 26) 18.

Animal - see P. zelandiae. It is viviparous.

Distribution - North Island.

P. corolla badia (Gould 1848) Fig. 3, a. (Suter p. 231).

Previously recognised as a distinct species, P. badia has so many points in common with P. corolla but with a distinct geographical distribution it is best regarded a sub• species of P. corolla It usually has shorter and finer spines than typical P. corolla but some have only a keel on the last whorl with no spines while others have all whorls evenly convex. The size is variable but reaches about 8 mm. in height. It is vivi• parous.

Distribution - South Island and Chatham Island.

P. zelandiae (Gray 1843) Figs. 4, a, 8. (Suter p. 231).

This species is fairly distinct in its smaller size (height about 5mm. ), convex, smooth whorls and tall spire I have not seen any spined specimens. It is clearly not a sub-species of P. antipodum as its distribution coincides with that species as well as having several important differences.

Animal (Fig. 8) - The tentacles are long and pointed with a black strip along the outer edge but otherwise colourless. The eyes are in prominent bulges at the outer bases of the tentacles. There is a large black rostrum but the small mouth lobes are white. The foot is greyish and ciliated with black opercular lobes. The operculum is horny and thin. There is almost no difference in the external form of P zelandiae, P. corolla, and P. pupoides. The other species have not been examined.

Distribution - In lakes and streams in North and South Islands. Suter's P. subterraneus is P. zelandiae (R. K Dell Pers. Comm )

P. egenus (Gould 1848) Fig. 5 (Suter, p 233).

A tall spined shell with convex whorls is fairly distinct in its typical form but many intermediates between P. egenus and P. zelandiae are found It is probably only a form of P. zelandiae and grows to about the same size (5 mm high) in some popu• lations but others are somewhat larger

Distribution - Typical forms are found in Nelson and Wellington

P. spelaeus (Franenfeld 1862) Fig. 6. (Suter p. 234).

A small shell (height about 3 mm. ) rather similar to P. zelandiae and probably referable to that species.

Distribution - North and South Islands.

P. pupoides (Hutton 1882) Fig. 7, a, b. (Suter p. 234).

A very distinctive shell of very small size (usually less than 2 mm high) and

20.

not closely related to any other species, certainly not a sub-species of P spelaeus as it has been listed previously. The spire is cylindrical with flat whorls and the suture is usually margined

Animal - The small size of this species give a few characteristic features to the animal P. pupoides differs from all other species in the genus in having normal sexual repro• duction as males make up about half of the population. The eggs are large but incubation of the young was not observed in the one population studied.

Distribution - North and South Islands in brackish water.

Super-family Cerithiacea

Shell tall, with an anterior canal and the males have no penis

Family MELANIIDAE

The animal has a large snout with its eyes on bulges at the outsides of the tentacles A rudimentary siphonal fold is present anteriorly. Reproduction is often viviparous. The stomach has a crystalline style.

Sub-family Melanopsinae

The animal has a smooth mantle edge.

Genus Melanopsis (Ferussac pere 1807)

Animal with the tentacles long and pointed. The shell has a short spire and a long body whorl with the aperture notched below

Distribution - New Zealand, New Caledonia, Spain, Northern Africa and Asia Minor.

Sub-genus - Zemelanopsis (Finlay 1927).

Certain radula differences seem to be the only feature separating off this sub-genus

PLATE 1. Potamopyrgus species.

Fig. 1 P. antipodum. Auckland. (a) small variety Auckland. 2 P. corolla Lake Pupuke (a) short spined form Auckland. Auckland. (b) small specimen resembling zelandiae Auckland. P. corolla badia Dunedin (a) smooth form, Dunedin P. zelandiae Auckland. (a) tall variety resembling egenus P. egenus Nelson. P. spelaeus Hawke's Bay. P. pupoides. Melson. (a) Hawke's Bay : b) Stewart Island P. zelandiae The animal extended. Auckland PLATE 2.

9 Melanopsis (zeamelanopsis) trifasciata.

M. (Z, ) trifasciata (Gray 1843) Fig. 9 (Suter p. 236)

The shell is smooth and thin with the spire often badly eroded. Longitudinal weak folds may be developed. It is dark olive in colour with 1-3 spiral brown bands best seen inside the light bluish aperture. It grows to a height of 30 mm. but is extremely variable in adult size between populations. Morton (1952) describes the morphology of the stomach

Distribution - North and South Islands in brackish streams but of rather spasmodic occur• rence.

Sub-class Pulmonata

Order Basommatophora

Super-family Hygrophila (The freshwater pulmonates).

Family LYMNAEIDAE

The tentacles are flattened with the eyes at their inner bases The pulmonary aperture is on the right side but the kidney opening is on the left side of the neck. There is no pseudobranch developed. The shell is thin, dextrally coiled, with a prominent spire and an oval aperture. There is no operculum. The eyes are laid in gelatinous masses on plants and stones. 22.

Genus Lymnaea (= Simlimnaea) Lamarck 1799

Characters as for family.

Distribution - World wide.

L. tomentosa Pfeiffer. (=L. alfredi) Figs. 1 0, a, b, c, d, e, 11, a (Dell 1 956 p. 80)

This is the only species of Lymnaea in New Zealand and Australia. It is very variable and many species and even genera have been named. Dell (1956) and Boray and McMichael (1961) have shown they belong to one variable form, even the radula differing between populations. Spiral sculpture is developed in some populations.

L. tomentosa is the intermediate host for the liver fluke, Fasciola hepatica.

Animal - (Figs. 11, 11a. )

The cephalic tentacles are flat and leaf-like with the eyes at their inner bases. The rather wide foot has a strongly ciliated sole, is broadly rounded in front and tapers behind. The colour varies from dark brown to yellowish brown, and white mucus glands are scattered over the head and tentacles. The mantle has a straight edge and is hardly reflected over the shell. Pale blotches can be seen through the shell on the body whorl, while the digestive gland is visible through the spire whorls. A siphon-like extension to the pulmonary chamber reaches to the surface film and takes in bubbles of air. The egg masses are rather long and oval in shape with several eggs embedded in the jelly matrix.

Distribution - North and South Islands from Ninety Mile Beach to Otago but not known from East Cape, Fiordland, Marlborough nor from the off-shore Islands such as Great Barrier, Stewart Island and the Chathams.

Usually in standing water in small ponds. It is one of the animals appearing in temporary pools of water.

The English L. stagnalis (Fig. 18) is introduced to New Zealand. It occurs abundantly in Lake Taupo.

Family PLANORBIDAE

The animal has the visceral mass sinistrally coiled and a pseudobranch is present. The tentacles are long and pointed. The shell is sinistral.

Genus Planorbis Mueller 1774.

Animal with a short foot, long tentacles and smooth mantle margins. The shell is discoidal with a flat or depressed spire.

Distribution - World wide, usually in still waters.

P. corinna Gray 1850. Figs. 15, a 17, a (Suter p. 610)

The shell with four or more whorls when over 3. 5 mm. wide. The colour is

24.

light brownish or yellowish and the spire is slightly depressed. A small species not over about 4. 5 mm. in width.

Distribution - North and South Islands.

Animal - (Figs. 17, 17a. )

The cephalic tentacles are long and slender, the foot short and oval in shape. The animal is yellowish-brown in colour but with dark pigmentation behind the head. The orange-red buccal mass can be seen through the integument.

P. kahuica Finlay and Laws 1931. Fig. 16, a. (Finlay and Laws 1931 p. 23)

The shell is very like that of P. corinna and probably conspecific with that species The typical P. kahuica shell has fewer whorls (only three of 3. 5 mm. in width), with a more sunken spire and deeper sutures. It grows up to 4 mm. wide

Distribution - Hawkes Bay (P. corinna is apparently absent from this area).

P. kahuica is probably at best only a sub-species of P. corinna. The likelihood of P. kahuica emerging unscathed after a systematic study of the genus in New Zealand is remote indeed.

The large red P. corneus (Fig 20) is introduced to New Zealand. Found mainly in aquaria, it does not seem to have established itself in this country.

Genus Physastra Tapparone-Canefri 1883.

Animal with a smooth mantle edge. See Hubendich 1948 for details of anatomy. The shell is sinistral with a fairly tall spire.

Distribution - Dutch East Indies, New Guinea, Australia and New Zealand.

PLATE 3.

10 Lymnaea tomentosa. a-e» showing variation (from Dell 1956). 11 Lymnaea tomentosa. The animal extended in ventral view, (a) dorsal view of the same. 12 Physastra variabilis. a-f showing variation (from Dell 1956). 13 Physastra variabilis. The animal extended in ventral view, (a) dorsal view of the same. 14 Physastra oconnori. Subfossil (from Dell 1956). 15 (a) Planorbis corinna (from Finlay and Laws 1931). 16 (a) Planorbis kahuica (from Finlay and Laws 1931). 1 7 Planorbis corinna. The animal extended in ventral view (a) dorsal view of the same. PLATE 4. Introduced Species.

18 Lymnaea stagnalis (from Adams 1853 The General of Recent Mollusca). 19 Physa fontinalis (original) 20 Planorbis corneus (from Adams, 1853 The General of Recent Mollusca) (modified).

P. variabilis (Gray 1843) Figs. 1 2, a, b, c, d, e, f, 13, a. (Dell 1 956 p. 88)

All of the New Zealand "species" of "Isidora" are now covered by this one species. It has a very variable shell, the body whorl can be rounded or keeled, the spire short or tall, sculptured or smooth.

Distribution - North and South Islands but not Stewart Island or the Chathams. Prefers still water.

Animal (Fig 1 3, 1 3, a. )

Colour dark reddish brown, probably due to haemoglobin, with long cephalic tentacles. There is a prominent pseudobranch on the left side. The foot is wide in front, tapering gradually behind.

P. oconnori (Cumber 1941) Fig. 14. (Cumber 1941 p. 359. Dell 1 956 p. 89).

A subfossil, larger and with a longer spire and narrower shell than P. variabilis. Occasional spiral folds are usually developed.

Distribution - Waikari Moa Swamp, Glendhu Road, Lake Wanaka.

Family PHYSIDAE

The animal is sinistral with the mantle margin digitate. 26.

Physa fontinalis L Fig 19

An introduced species occurring commonly around Auckland in ponds, lakes and streams and it appears to be fairly well established in other parts of the country. It is easily distinguished from Physatra by its digitate mantle margin and greyish coloured animal.

Family ANCYLIDAE

Animal entirely aquatic, not spirally coiled, the tentacles short with the eyes on the inner sides of the bases There is usually a pseudobranch and the pallial cavity is rudimentary. There is a jaw present in the mouth The shell is limpet-like, the apex posterior (unlike the marine limpets in which it is anterior).

Genus "Gundlachia"

The typical forms of Gundlachia have a large flat lamella much like that seen in the marine Maoricrypta The New Zealand shells do not show this feature and for this and other reasons the genus Gundlachia is clearly not represented in New Zealand. The New Zealand shells have the embryonic part (apex) distinctly radially ribbed, a feature unnoticed by Suter and other workers. This feature is seen in the North American gen• us Ferrissia. Moreover as freshwater limpet taxonomy is highly confused at present a genus cannot be assigned with any certainty (see Robertson 1959)

"G". lucasi Suter 1905 Figs 21, a, 24. (Suter p 617)

The shell is oval. Dimensions L. 3 W. 2 Ht 1 mm.

"G". neozelanica Suter 1905. Figs 23, a. (Sute r p. 617)

The shell is oval-oblong with subparallel sides. Dimensions L. 3 W 2 Ht. 0. 75 mm. Both "species" occur scattered throughout the North and South Islands "G". lucasi typically lives under stones and "G" neozelanica lives on weeds. Inter• mediate specimens (Fig 22a) are found and probably G. neozelanica is merely an ecotype similar to the marine Notoacmaea helmsi scapha, a form of helmsi living on the seagrass Zostera

Animal (Fig. 24) - Gundlachia is transparent white in colour, with many mucus glands and strong, posterially directed cilia. The edge of the mantle is transparent but below this there is a fair amount of black pigment. The tentacles are rather flattened with the eyes at their inner bases. The respiratory aperture is on the left side. Immediate• ly inside this on the outer wall is a thickened area of epithelium, probably a rudimen• tary pseudobranch

Family LATTIIDAE

The eyes are at the outer bases of the tentacles and there is no jaw present. The respiratory opening is on the left side.

The shell has the apex at the posterior margin and is slightly coiled. A thin ledge is developed on the underside and a sharp lamella protrudes from this.

28.

Distribution- the North Island of New Zealand.

Genus Latia Gray 1850.

L. neritoides Gray 1850 Figs. 25, a, b. 26 (Suter p. 616).

The only species of this family, growing up to 6 mm. long. These animals are highly phosphorescent as can be seen by agitating the water or adding an irritating substance.

Animal (Fig. 26) - There is black pigmentation on the sides of the foot and head but the mantle is lightly pigmented and, like the rest of the animal, is well supplied with mucus glands. The sole is white. The tentacles are long and slender, not flattened, the eyes at their outer bases The respitory opening is on the right side where a pseudobranch is present.

Class: BIVALVIA

Sub-class Eulamellibranchia

Order Schizodonta

Super-family Unionacea (The freshwater mussels )

Family MUTELIDAE

The freshwater mussels with a glochidium or a lasidium larva They are variable in form, sculptured or smooth. The hinge is primitively a lamella but becomes reduced or more complex. The siphons are generally well developed, the supra-anal and anal siphons fused and the mantle bridge between the anal and branchia opening is well devel• oped, the branchial opening is rarely closed below. A marsupium is found in the inner gills and is variously developed.

PLATE 5.

21 (a) "Gundlachia" lucasi. Coromandel. 22 (a) "Gundlachia" intermediate between lucasi and neozelanica Coromandel. 23 (a) "Gundlachia" neozelanica. Hutt Valley. 24 "Gundlachia" lucasi Ventral view of the animal The arrows show ciliary currents 25(a, b) Latia neritoides Dorsal, lateral and ventral views of the shell 26 Latia neritoides Ventral view of the animal The arrows show ciliary currents Sub-family Hyridellinae Iredale 1934.

This is made up of a series of solid, elongate shells, the beaks sculptured with V- shaped ridges, the shell smooth or variously sculptured. Hinge with two strong cardin• als and two long laterals posteriorly in the left valves and one cardinal and one lateral in the right valve. The cardinals are usually grooved. The palps are small and sub- triangular. A marsupium is well developed in the middle third of the inner gill. The glochidium is subtriangular and hooked.

Genus Hyridella Swainson 1840.

The only genus in the sub-family but there are several sub-genera recognised

Distribution - Eastern Australia, Tamar River System of Tasmania, North and South Islands of New Zealand, Solomon Islands and New Guinea.

Sub-genus Hyridella Swainson 1840

Shell generally elongate, the hinge strong and the anterior muscle scars impressed, the anterior retractor and adductor separate. Siphons prominent but not heavily develop• ed.

Distribution - Eastern Australia, Tamar River System of Tasmania, North and South Is• lands of New Zealand (rare), South West New Guinea.

H. aucklandica (Gray 1843) Fig. 27. (McMichael and Hiscock 1958 p. 454).

Shell elongate, typically sinuate ventrally margin and low dorsal margin. Anter• ior retractor and adductor almost fused. Length 76 mm. height 38 mm. (type).

Anatomy - Gould 1852.

Distribution - Scattered localities in the North and South Islands (see map).

Boray and McMichael have placed this species in the sub-genus Hyridella on Gould's anatomical data and on shell characters. It seems possible that it is but another ecotype of the very variable H. menziensi. Dr. R. K. Dell has pointed out (personal communicat• ion), that in Hyridella populations where the shells are packed closely together the sinuate ventral margin is produced together with a long narrow shell. Anatomical characters separating the two sub-genera are presumably capable of variation, and it may be found that when a good series of specimens of both H. menziesi and H. aucklandica are examined these differences and the shell features may be found to be apparent only.

Sub-genus Echyridella McMichael and Hiscock 1958.

Shell variable, hinge frequently reduced and the anterior adductor and retractor muscles fused. The anatomical features are as in Hyridella but with thick fleshy siphons and the labial palps differ in certain details.

Distribution - The North and South Islands of New Zealand except the South West part of the South Island. 30.

H. (Echyridella) menziesi (Gray 1843) Figs. 28 a. b. c. (Dell 1953 p. 228. McMichael and Hiscock 1958 p. 477).

The dorsal margin is typically elevated posterior to the beaks to make a distinctly winged shell but often only slightly so. The ventral margin is straight or convex.

Distribution - as for sub-genus (see map).

The extreme variability of this species has caused systematists to name many forms as full species and sub-genes. Possibly regional subspecies exist but these have not yet been demonstrated adequately.

The life cycle of this species has been investigated by Percival (1931). The glochid- ium larva is shown in Fig. 29.

Sub-family Cucumerunioninae

Beaks and shell sculptured, shell elongate with a strong unionid hinge having very erect cardinal teeth, grooved, serrated and frequently denticulate. Shell sculpture typically of nodules or ridges on the posterior ridge and spreading dorsally and ventrally from it. Anterior retractor muscle scar tending to separate from adductor. Supra-anal opening absent, siphons large but not prominent and of equal size. The mantle bridge between the siphons thick, branchial siphon papulate and open below. Marsupium occu• pies the middle four-fifths of the inner gill of females. Glochidium unknown.

Distribution - Eastern Australia, North Island of New Zealand, Southern New Guinea.

Genus Cucumerunio Iredale 1934.

Shells elongate, medium to large, beaks sculptured with V-shaped ridges, the sculp• ture continuing onto the posterior part of the shell as lachrymose nodules or ridges. Hinge well developed. Anterior muscle scars fused. Anatomy as for the sub-family.

Distribution - Central part of East Coast of Australia and the North Island of New Zealand.

C. websteri websteri (Simpson 1902). Fig. 30. (Dell 1953 p. 225, McMichael and His• cock 1958 p. 476).

Shell characters as for genus, growing to about 70 mm. long The colour is brownish or greenish.

Anatomy unknown.

Distribution - Scattered localities South of Auckland as far as Foxton (see map).

C. websteri delli McMichael and Hiscock 1958 Fig. 31 (McMichael and Hiscock pp. 477-479).

Differs from C. websteri websteri in its lack of sculpture and more elongate shape (length up to 90 mm. ). Beaks sculptured with V-shaped ridges but the rest of the shell bears only traces of nodulation. Colour olive-green to dark brown.

32.

This genus is believed to have arrived in New Zealand in fairly recent times (McMichael 1958) probably by the young mussels being carried on the feet of birds.

Order Heterodonta

Super-family Sphaeriacea (The small sphaeriid freshwater bivalves).

Family SPHAERIIDAE

Animal with one or two siphons, hermaphrodite, the young incubated in the gills of which there are one or two pairs. The shell is small and fragile with an epidermis. There is a feeble short ligament and a simple pallial line. There are two cardinals and the laterals are double on the right valve and single on the left.

Genus Sphaerium Scopoli 1777.

The animal has two moderately long separate siphons and two pairs of gills, the outer smaller. The shell is fairly rounded with the anterior end nearly the same length as the posterior but may be slightly longer or shorter.

PLATE 6.

27 Hyridella aucklandica. 28 Hyridella menziesi (a) Lake Takapuna (from McMichael 8t Hiscock 1958). (b) Lake Rotorua (from McMichael & Hiscock 1958). (c) Lake Manapouri (from Dell 1953). 29 Glochidium larva of Hyridella menziesi (from Percival 1931). 30 Cucumerunio websteri websteri (from Dell 1953). 31 Cucumerunio websteri delli (from Dell 1953). 32 Map of New Zealand showing the distribution of the freshwater mussels (based on several sources. ) H Hyridella (Echyridella) menziesi. Hyridella (Hyridella) aucklandica. X Cucumerunio websteri websteri + Cucumerunio websteri delli. Y5.

PLATE 7

33 Sphaerium novaezelandiae. The extended foot and siphons seen from the side. 34 Sphaerium novae zelandiae. Left and right valves. 35 Pisidium aucklandica. The extended foot and siphons seen from the side.

S. novae zelandiae Deshayes 1853. Figs. 33, 34. (Suter p. 934).

This rather variable shell is easily distinguished by its rounded shape and rather large size for the family (up to 5. 5 mm. high and 7 mm. long).

Distribution - North and South Islands, Auckland Islands, Chatham Islands. This is our common Sphaeriid and it appears to include "Pisidium" novazelandiae Prime 'P'. hodgkini. Suter is also possibly a synonym, the material available being very close to S. novaezelandiae.

Genus Pisidium Pfeiffer 1821.

The upper siphon only is developed and there is only one pair of gills. The anterior end is longer than the posterior.

P. aucklandica Suter 1907 Fig. 35. (Suter p. 936).

This small bivalve is easily distinguished by its very long anterior end. 34.

Distribution - Scattered localities in the North and South Islands.

Mr. J. G. Kuiper is examining New Zealand members of the family at present but his results are not yet available. Possibly additional species will be found.

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