Life histories and ecological interactions of Potamopyrgus antipodarum and Physa acuta in relation to temperature.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The New Zealand freshwater snail Potamopyrgus antipodarurn has recently invaded freshwaters of North America and as a result its occurrence and spread have led to growing concern regarding its potentially negative impacts on members of the native fauna. These include several species of pulmonate gastropods in the family Physidae. Physa acuta, an endemic of Mediterranean Europe has successfully established itself in New Zealand, Australia and Africa. and occurs alongside P. antipodarum in a variety of Canterbury freshwaters. Its occurrence provided the opportunity to compare and examine the life histories and interactions of two highly successful invading species. Growth, reproduction and competitive interactions between P. antipodarum and P. acuta were investigated with particular reference to water temperature at four sites (including one influenced by a geothermal source) at Hanmer Springs, North Canterbury. Field and laboratory experiments were carried out over a 12 month period from January to December 1998. Water temperature in the range 4 to 15°C had a strong effect on the growth rate of juvenile P. antipodarum, which grew slowly at 4°C and fastest at 15°C. In contrast, P. acuta was not affected by water temperature and grew at similar rates when kept at 4, 8 and 15°C. Recruitment of young occurred year round in P. antipodarurn, whereas P. acuta appeared to require a higher water temperature to instigate oviposition. P. antipodarum from all four study sites had similar seasonal patterns in the number of embryos carried per individual female, but reproductive output differed between sites. Although large populations were sustained at the thermally influenced Site 4, reproductive output was low at all times. Crowding by conspecifics had strong effects on growth and fecundity of both P. antipodarum and P. acuta, but similar levels of crowding by the other species stimulated growth and reproductive output of both species, most notably P. acuta. Furthermore, growth of juvenile P. acuta was stimulated in the presence of chemical substances released by adult P. antipodarum, indicating that it can detect a potentially competing species without physical interference. My findings indicate that P. antipodarum can exist at a variety of temperature regimes, but prolonged exposure to moderately high temperatures are likely to be at a cost to reproductive output and population growth. The ability of P. acuta (and possible other physids) to increase its growth rate and reproductive output in the presence of another snail species suggested that the presence of P. antipodarum in North America may not necessarily be at the expense of endemic physids at least.