The systematics of the New Zealand species of Potamopyrgus (Mollusca: Hydrobiidae) and studies on the biology of Potamopyrgus antipodum : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University
This investigation has shown that only three species of Potamopyrgus Stimpson can be recognized from New Zealand, compared with the six species and three subspecies recognized by Suter (1913). The species are, P. antipodum Gray 1843, P. pupoides Hutton 1882, and P. estuarinus n. sp. P. dawbini Powell 1955 from the Auckland Islands is probably referable to P. antipodum, but the position of (?) P. melvilli (Hedley 1916) from the Kermadec Islands has not been determined. The European species P. jenkinsi (Smith 1889) cannot be separated from P. antipodum on morphological or anatomical grounds and may also be referable to that species. All species now placed in Fluviopupa Pilsbry 1911 should probably be referred to Potamopyrgus. P. estuarinus and P. pupoides are both smooth-shelled, bisexual, non-ovoviviparous and confined to brackish water. P. antipodum is highly variable in shell size, shape and ornamentation, inhabits fresh and brackish water, is ovoviviparous, and populations may consist entirely of parthenogenetic females, or contain variable numbers of sexually functional males. Rearing of snails in the laboratory has shown that snails do not necessarily breed true with respect to shell ornamentation, and that shell shape and ornamentation is not determined primarily by environmental factors. The shell of P. estuarinus cannot be distinguished from that of some P. antipodum but P. pupoides may be readily identified using shell characters alone. No significant interspecific differences in operculum, external morphology, body pigmentation or structure of the male reproductive system are found but P. pupoides possesses minor radular differences, and P. antipodum differs in the condition of the female reproductive system. The diploid (2n) chromosome number of all three species is 24. Qualitative paper chromatography of crude foot muscle and mantle edge extracts, and quantitative ion-exchange chromatography of shell periostracal protein have disclosed no important biochemical differences between species. P. antipodum is widely distributed in fresh waters and no clear relationship between shell shape and ornamentation, and different kinds of habitat have been found. P. estuarinus has a fairly restricted brackish water habitat and is frequently found near river mouths in harbours where snails may regularly be exposed to the air for part of each tide cycle. P. pupoides is also restricted to brackish water but normally remains fully aquatic at all times. Experimental studies on salinity relationships, habitat selection and the effects of desiccation have demonstrated important differences in the environmental relationships of the three species which can be correlated with their distributions. Life history and population studies made in three populations of P. antipodum (two ponds and a stream) over a 13-14 month period have shown that reproduction occurs throughout the year with peak activity in spring and summer. Generation time as indicated by laboratory rearing of snails is 9-12 months. Population age structures differed markedly between ponds and stream and reflected differences in the physical environments of the two habitats. Distribution, occurrence in drift and effects of floods were examined in the stream. Snails were generally most abundant in places sheltered from the main current or among vegetation, and large numbers were present within mats of willow roots. P. antipodum is a regular member of the drift fauna and floods have an important role in regulating population age structure. The distribution of P. antipodum in thermal waters was also investigated, and experimental work indicates that high water temperature is probably the most important factor limiting distribution. The maximum temperature at which snails were found in the field, 28°C, is also the temperature at which activity ceases and the snails enter a comatose state. Finally, a study has been made of the parasites of Potamopyrgus. An unidentified protozoan (Sporozoa : Porosporidae), occurring in an encysted state, is the most important internal parasite with infection rates as high as 86% having been recorded. The larvae of 13 species of Trematoda were identified and briefly described and their rates of infection determined. The monostome cercariae are the most important group of parasitic trematodes. The commensal oligochaete Chaetogaster limnaei limnaei was found in association with P. antipodum in Lake Pupuke, Auckland, and was observed to be predacious on embryonic snails.