EFFECTS OF SEA URCHINS (PARECHINUS ANGULOSUS) ON RECRUITS AND JUVENILES OF ABALONE (HALIOTIS MIDAE)

Abstract

Much current research on community ecology concerns indirect effects, cas- cading trophic interactions, and the role of top-down vs. bottom-up processes, as well as subsidization of matter and energy between different systems. We addressed all these topics by exploring interactions between the urchin Parechinus angulosus and both recruits and juveniles of the abalone Haliotis midae, in the light of the fact that juvenile abalone shelter beneath the urchins. In field experiments at six sites, urchins were eliminated from removal plots to determine the degree to which juvenile abalone (individuals 3-35 mm in length) depend upon urchins. Comparisons were made with undisturbed and disturbance-control plots. We hypothesized that after removal of urchins: (1) there would be an immediate decline of juvenile abalone, since they are intimately associated with the urchins; (2) abalone recruits (recent settlers <3 mm in length) would be unlikely to suffer immediately or directly because, unlike juveniles, they are only weakly associated with the urchins; (3) foliar algae would proliferate due to decreased grazing by urchins; (4) there would be a consequent reduction in crustose coralline algae due to overgrowth by foliar algae; and (5) the decline of crustose corallines would reduce recruitment of abalone due to a loss of settlement and nursery habitat. In urchin-removal plots, juvenile abalone declined dramatically, becoming virtually extinct locally. Their disappearance was slowest in crevices compared with exposed habitats, but even there they failed to survive longer than five months. Contrary to expec- tations, abalone recruits also disappeared from urchin-removal plots, possibly because the cover of sediment almost doubled there. Also contrary to our hypotheses, neither foliar macroalgae nor encrusting corallines responded to urchin removal. The explanation of these results is that P. angulosus was able to gather drift kelp when this food source was available, rather than acting as an active grazer. This feeding pattern has profound consequences and explains why urchins failed to influence algal community composition. By sheltering be- neath urchins, juvenile abalone gain protection from predators and may also benefit from an enhanced food supply. Bottom-up subsidization of urchins by drift kelp is thus respon- sible for the feeding behavior of the urchins, and indirectly facilitates their role in harboring juvenile abalone. This role is of immense importance for the lucrative abalone fishery, and depletion of urchins could lead to a collapse of abalone populations. Human exploitation of top predators that consume urchins, such as rock lobsters and some fishes, has conceivably exaggerated the role of urchins in sustaining juvenile abalone.