Some aspects of the biology of Velacumantus australis (Quoy and Gaimard) (Gastropoda: Potamididae)

Abstract

Velacurnantus australis (Quoy and Gaimard) (Gastropoda: Potamididae) is common in estuaries, coastal lakes and sheltered bays along the eastern coast of Australia, south of the Tropic of Capricorn, along the eastern part of the Victorian coast and the southwest part of the coast of Western Australia. It also occurs in Tasmania. Recent fossils of the species are present in Western Victoria and South Australia.The life cycle can be divided into a number of stages that may be distinguished by characteristics of the shell. There is probably only one breeding season each year, and most snails probably reproduce in their second year. The life span is probably at least five years and possibly six.The males are aphallic and the sexes can be recognised from the appearance of the gonads. Males tend to be longer than females, but in most age groups the females tend to be heavier than males of the same length and age.The habitat of the species is described and it is suggested that the distribution is limited by the availability of suitable habitat. The distribution of V. australis is similar to that of a number of other molluscs which live in similar habitats and it is suggested that the distribution of these species is more likely to be governed by intolerance to high temperatures rather than low ones. Adjacent populations of V. australis sometimes vary considerably in mean length. However, length increases clinally down the eastern coast of Australia.V, australis harbours several larval trematode parasites which infect the digestive glands and gonads, The incidence of these infections varies considerably in different populations. Within a single population incidence varies with age, The oldest snails usually have the highest incidence of infection, and infections do not occur in the smallest Juveniles, The incidence of infection also varies with length, in snails of the same age, and the relationship between the incidence of infection and length is different in snails of different age groups. Snails with oysters attached to their shells have a higher incidence of infection than comparable snails with clean shells. In most snails harbouring heavy infections of larval trematodes no traces of gonad remain, and in snails with light infections the gonad tissue is reduced and usually atrophied. Infected snails tend to be heavier than comparable uninfected snails and the increase appears to be due to an increase in the weight of the digestive gland-gonad. Boring gastropods and fish are important predators of V. australis. Examination of one sample of shell material suggested that about 60% of the snails had died of predation. V. australis is polymorphic in respect to shell banding. A white banded form occurs in all age groups of all the populations examined from the Australian mainland. It has a frequency exceeding 1% in all populations examined, except one from Tasmania and those from Pt. Phillip Bay, Victoria. The frequency of this form tends to be highest in small Juveniles and lowest in Old Adults. It is usually about the same in living and fossil samples from the same or nearby localities which suggests that the polymorphism is balanced and very";stable. Adjacent populations usually have similar frequencies which suggests that the selective forces are much the same in adjacent • I C'C localities.Banded snails are less often infected with larval trematodes than unbanded snails, These infections cause either complete castration or considerable degeneration of the gonads. It is suggested that parasitism plays an important part in the maintenance of the polymorphism. If this is so, there should be a relationship between the incidence of larval trematode infection and the frequency of banded snails, and data from a number of populations suggest that there is such a relationship. Certain boring gastropods and fish may be responsible for differential predation on banded and unbanded snails.Selection pressures other than those due to parasitism and predation probably play a part in,the maintenance of the polymorphism. The frequency of the white banded form decreases clinally with latitude Os. Living populations from Queensland and northern New South Wales have frequencies between 7 - 18%, whereas all populations from southern New South Wales and eastern Victoria, except those from Lake Illawarra, have frequencies between 2-5%. The frequency in Port Phillip Bay is less than 0.1%. The frequency in a sample from Swan River, Western Australia, was 9.1% about the same as in samples from localities of the same latitude on the eastern coast of Australia. Show more