The demography and ecology of the pulmonate limpet Siphonaria zelandica at Kaikoura, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Siphonaria zelandica is a common intertidal pulmonate limpet in New Zealand. On the Kaikoura Peninsula, east coast of the South Island, S.zelandica is abundant on rocky platforms in damp crevices. This study examines the demography and ecology of S.zelandica at two sites on the peninsula. The distribution and abundance was examined both spatially and temporally. S.zelandica are found primarily in the mid tidal area with an average of 90limpets/m² over all sites although abundance does vary between sites. S.zelandica are, however, not strongly associated with any other gastropod species, although they are correlated with algae. Two size measurement methods were used to ascertain growth and mortality of S.zelandica, and the reproductive cycle was also determined. Growth is variable over time, being quicker in winter and spring than in summer/autumn. Growth is also variable with initial size of the animal with smaller size classes (5-10mm) growing rapidly and the larger size classes having slower growth. Mortality is greater in the larger S.zelandica than in small size classes and is affected by physical disturbances such as storms. Reproduction in S.zelandica is continuous over the year as shown by histological slides which showed that there were mature oocytes present throughout the year. There are peaks in spawning however that occur in February/March (late summer) and September/October (early spring) which indicate main spawning phases. Hatching after the eggs had been laid took between six and ten days and settlement was fairly rapid after this. The grazing effect of S.zelandica on algal abundance and the effects of differing densities of limpets on intraspecific survival and growth were examined through experimental enclosures. 0.25m² enclosures were constructed and randomly assigned treatments at two sites. The different densities used were 0,15, 30 and 50 limpets which represent half ambient, ambient and double ambient densities and the experiment was run for twelve months. It was found that although S.zelandica had some effect on algal abundance at differing densities, overall they cannot reduce micro or macroalgae by significant levels. An increase in density of the limpets had no effect on S.zelandica survival. Growth was significantly different between sites although there was no significant treatment effect. Experimental enclosures were also used to examine whelk predation on S.zelandica. Treatments used were a series of total and half cages with 30 S.zelandica and either with or without whelks. The treatments were duplicated at both mid and low shore. The experiment had a two-fold nature, it indicated the level of whelk predation at different shore heights and the survival of S.zelandica at the different shore heights. Whelks consume reasonable numbers of limpets at both mid and low shore heights but predation is increased at the low shore. S.zelandica does not survive well at the low shore regardless of being protected from whelks. Oystercatcher predation was examined by observations of foraging flocks. These found that oystercatchers have a major effect on limpet populations leaving, on average, only 10% of S.zelandica untouched in a patch after the birds foraging session. The behaviour of the oystercatchers was unusual as they turned the limpets over but never actually consumed them. The main conclusions of this study are that S.zelandica contribute to the patch dynamics of the Kaikoura shore by having both direct and indirect effects. The direct effects of S.zelandica are the localised population effects on algal abundance. Indirectly, S.zelandica are prey for both birds and whelks, which is a factor in their patchy distribution that in turn creates space for other organisms to settle.