AB MacDiarmid

Ontogenetic changes in the behavior, spatial distribution, or habitat use of a species are presumably adaptations to ecological forces that dlffer in their effect on various life stages. The New Zealand rock lobster Jasus edwardsii is one of several species of spiny lobster that exhibits dramatic ontogenetic shifts in sociality and spatia.1 distribution, and we tested whether such changes are adaptive. We first surveyed several natural populations of J. edwardsii to document size-speclfic differences in aggregation. To determine i f chemical cues discharged by conspecifics promote aggregation of certain ontogenetic stages, we tested the responsiveness of lobsters of 3 ontogenetic stages (early benth~c juvenile, juvenile, and subadult) to the chemical cues produced by conspecifics of different sizes. Finally, we tethered lobsters of different ontogenetic stages alone and in groups to test the effect of lobster size and aggregation on mortality. Our results offer compelling evidence that pre-reproductive J. edwardsii undergo an ontogenetic change in sociality that alters their spatial distribution and sunrival. Our field surveys show that J. edwardsii are solitary as early benthic juveniles and become social and aggregate as they grow larger. We then demonstrate, using laboratory experiments, that there is a sizespecific increase in the response of pre-reproductive J. edwardsii to the chemical cues of larger conspecifics which facilitates these ontogenet~c changes in aggregation. Finally, our tethering results confirm that this change in social condition is selectively advantageous: aggregation does not increase the survival of small lobsters, but larger lobsters survive better in groups. Thus, in this study we demonstrate the linkage between ontogenetic changes in the spatial distribution of a species, the behavioral process that creates the pattern, and the selective advantage conferred by these developmental changes.

Patterns of abundance and movement of the spiny lobster Jasus edwardsii (Hutton) (F. Palinuridae) were examined within a coastal reef system. Overall population size remained relatively stable over a 3 yr study period. Marked differences in density, sex ratio and size frequency of spiny lobsters occurred between depths on the reef over a horizontal scale of 300 m. These differences varied seasonally and were related to moulting, reproductive and feeding cycles. They were not directly Linked to fluctuations in sea surface temperature, sea surge or photoperiod. Similar annual trends occurred at other sites. Highest densities of females in shallow (< 10 m) water coincided with moulting in May. Males moved inshore only briefly to moult in October-November often with no concomitant increase in male density in shallow (< 10 m) water. Increases in density of females at the deep (25 m) seaward edge of the coastal reef occurred during the egg-bearing period. In summer (December, January) the density of large males at a deep patch reef (25 to 30 m) increased. These males foraged nocturnally for bivalves on the surroundmg sand flat. Resighting of tagged individuals confirmed that the observed fluctuations in density, sex ratio and size frequency of spiny lobsters at different depths were caused by males and females moving inshore-offshore at different times of the year.

The interrelation between the sea urchin Evechinus chloroticus and the spiny lobster Jasus edwardsii was investigated in the shallow subtidal zone of rocky reefs in northern New Zealand. Both species were found in large numbers in the Shallow Broken Rock hab~tat. During the day spiny lobsters and sea urchins were spatially segregated on a small scale. Movement patterns of spiny lobsters indicate that sea urchins are accessible to nocturnally foraging lobsters. Laboratory experiments demonstrated that large lobsters ate all sizes of sea urchins. All sizes of lobster ate small sea urchins (< 50 mm TD) in preference to larger sea urchins. The provision of herbivorous gastropods (also eaten by lobsters) and shelter for sea urchins did not mean that more larger sea urchins were eaten. The removal of large brown algae or sea urchins and gastropods from areas of reef did not cause significant reductions in the daytime density of J. edwardsii. We argue that differing micro-habitat requirements of the 2 species mean that large abundances of E. chloroticus are unlikely to depress 3. edwardsii densities. The neccessary experiments to test this hypothesis are discussed.