Population dynamics of Diadema antillarum (Echinodermata: Echinoidea) following mass mortality in Panam�

Abstract

In 1983, Diadema antillarum suffered mass mortality throughout the Caribbean Sea and the western Atlantic Ocean. I followed the dynamics of populations at the San Blas Islands, Panama from April 1983 to November 1987. Density measurements indicate that populations of D. antillarum have not recovered from the die-offs that killed nearly 97% of the individuals. There was recruitment to the 1 to 1.5 cm class immediately after the mass mortality, but there has been little additional influx of juveniles since then. The low number of observable juveniles could not be attributed to elevated rates of predation on very small individuals. Rates of recruitment did not differ between reefs with artificially increased densities of D. antillarum and reefs kept free of sea urchins; thus, the lack of recruitment did not arise from absence of adults that could provide settlement cues to the larvae or protection to newly settled juveniles. Other species of sea urchins did not show a clear pattern of increase after the demise of D. antillarum. Therefore, interspecific competition directed towards D. antillarum juveniles did not increase after the mass mortality. Two reefs where Echinometra viridis, Eucidaris tribuloides and Lytechinus williamsi, were removed showed no significant differences in recruitment of D. antillarum relative to two reefs where these species were allowed to remain at their natural densities. Resident D. antillarum after the mass mortality produced gametes with the same per capita intensity and lunar synchrony as before the mass mortality. However, it is possible that the probability of fertilization of their gametes decreased because of low population density. The most likely explanation for lack of recruitment is that the reduced numbers of reproducing adults at Panama and upstream locations resulted in levels of larval supply that were inadequate to sustain recruitment on Panamanian reefs.