Cohabitation on Algal Habitat Islands by Two Hermaphroditic Tanaidacea (Crustacea: Peracarida)

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Abstract
Two very similar tanaidaceans, Leptochelia forresti (Stebbing) and L. dubia (Kroyer), were the most abundant macrofauna associated with the capitula of the calcareous green macroalga Penicillus capitatus Lamarck in Puerto Rico. Both tanaids are protogynous hermaphrodites with highly skewed sex ratios. Abundances of tanaids on P. capitatus were related to habitat size. Densities (numbers per macrophyte dry weight and surface area) were much higher on the alga than on adjacent sea grasses. Abundances of male L. forresti on the habitat islands was correlated with the abundance of females of the species. Sex ratios (female to male) between five and than to one may be related to sex reversal patterns. In light of the fact that L. forresti and L. dubia were found in equal abundance on the tops of O. capitatus, several factors suggest that males of the species are not distributed independently and that distributional patterns and sex reversals and ratios may be influenced by interspecific interactions: (1) Males of L. dubia were rare on the alga, making up less than 1.0% of the population. (2) Males of L. forresti and L. dubia never co-occured, despite co-occurrences of females. (3) Only very large males of L. dubia were found on P. capitatus. (4) The percentage of females L. dubia in reproductive condition was very low on P. capitatus, compared to that in adjacent habitats. Hypothesis are presented to explain differences in sex ratios; these include interspecific competition among males and interspecific inhibition of sex reversal.