Presence and absence of monthly reproductive rhythms among eight Caribbean echinoids off the coast of Panama

Abstract

Monthly reproductive rhythms of five species of regular and two species of irregular echinoids were studied in the San Blas Archipelago during 1984. The annual reproductive periodicity of these species had been studied at the same locality in 1982–83. The data were subjected to Rayleigh's tests, autocorrelation analysis, and cross correlation analysis with the phases of the moon. Previously published data for an additional species, Diadema antillarum, that had been studied with the same sampling regime during the same time period in the same area, were re-analysed with the statistical procedures used in this paper. The eight species could be classified in four categories with regards to their mode of spawning: (1) Tripneustes ventricosus (Lamarck), Echinometra viridis A. Agassiz and Leodia sexiesperforata (Leske) spawn at random during the lunar cycle. (2) Clypeaster rosaceus (Linnaeus) and Lytechinus williamsi Chesher show nonrandom distributions of their spawning activity around the lunar cycle, but do not appear to follow any lunar periodicity. (3) Eucidaris tribuloides (Lamarck) and Diadema antillarum Philippi have a lunar rhythm in their reproduction, each spawning during a different lunar phase. (4) Lytechinus variegatus (Lamarck) follows a semilunar cycle, spawning every new and full moon, a pattern that had also been found in Bermuda in 1938. The phylogenetic position of the eight species indicates that lunar periodicity is not a lineage-specific trait, inherited from a common ancestor. It must, instead, have evolved independently in different echinoid taxa, possibly as an adaptation to serve the reproductive requirements of each species. However, attempts to identify its adaptive significance through comparisons between the species failed to support any of the hypotheses examined. Because water level on the Atlantic coast of Panama is determined by meteorological conditions as well as base tides, lunar spawning is unlikely to be cued by the tide, or to be an adaptation for flushing of fertilized zygotes away from reefs. Species in which lunar rhythms are present are not distinguished from species that spawn at random by their special photosensitivity, they are unlikely to be doing so because of food limitations during part of the month, and they do not appear to be under stronger selection for reproductive synchrony than species that have no lunar rhythms.