Impact of invertebrate predation on the New Zealand scallop (Pecten novaezelandiae)

Abstract

This study investigated the potential predatory effects of four invertebrate species (11 armed starfish Coscinasterias calamaria, 7 armed starfish Astrostole scabra, hermit crab Pagurus novaezelandiae, and the paddle crab Ovalipes catharus) on medium sized (50-60 mm shell length), and large sized (80-90 mm shell length) scallops Pecten novaezelandiae (Reeve 1853). Experiments using homogenised predators revealed that P. novaezelandiae is able to distinguish between predator species and detect predators without contact. It is concluded that detection of predators without contact is likely to: decrease the risk of scallop predation; help to save energy; maintain feeding and; maintain recessed position in the substratum. This is consistent with evolutionary adaptation of predator recognition as a result of these benefits. Tests with live 11 armed starfish showed that scallop size does not affect their acceptance of a scallop as prey. Both medium and large sized P. novaezelandiae were found to be significantly more likely to swim upon contact with the starfish species than with the crab species. Thus, there is a behavioural difference in escape responses towards the two different types of predators. This difference in responses is likely to be based on energy expenditure versus the predation risk. Therefore, the observed scallop responses are consistent with evolutionary adaptation of the most beneficial response for each predator. Live tests were found to evoke a stronger response by the scallops than homogenate tests, which reflects the increase in predation risk. Hermit crabs were found to be unable to consume P. novaezelandiae of the sizes used, though they have been found to consume scallops of 10 m m size (Bartrom 1989). This shows that P. novaezelandiae attains a refuge in size from predation by hermit crabs. Eleven armed starfish were able to consume both P. novaezelandiae sizes, showing that they remain effective predators of scallops through to commercial fishery size. No significant association was found between starfish and scallop numbers in the wild, in the Southern Scallop Fishery. It is possible that as both starfish and scallops are mobile any pattern of starfish association with scallops is countered by scallop dispersal away from the starfish. In the predator extract tests damaged scallops did not show a significant difference in the number of responses they exhibited amoung the predator extracts used, whereas undamaged scallops did. This indicates that damaged scallops repond to chemical traces of a predator more uniformly than undamaged scallops whose number of responses differed amoung the predators. Using live predators damaged scallops exhibited no significant difference in the swim response between 11 armed starfish and hermit crabs, whereas undamaged scallops did exhibit a significant difference. Thus, damaged scallops are not selective in their use of the swim response. Both the predator extract and live test results show that damaged scallops not only have the physiological costs of repair but also expend more energy, stop feeding more often, and more readily lose their position recessed in the substratum, all of which decrease their fitness and survival.