Comparison of reproductive patterns among 7 Caribbean sponge species does not reveal a resource trade-off with chemical defenses

Abstract

Among sponge species on Caribbean coral reefs, some have defensive metabolites in their tissues that deter fish predators, while others tolerate predation. Assuming that resources are limited and chemical defenses are costly, sponge species that produce defensive metabolites should have less energy to allocate toward growth or reproduction. In a previous study, we documented a resource trade-off between chemical defenses and growth among 7 branching sponge species from shallow coral reefs off Key Largo, Florida. In the present companion study, we investigated the relationship between chemical defenses and reproduction among 7 sponge species (6 branching species from the previous study and 1 vase-shaped species) from the same location. From November 2007 to October 2008, monthly tissue samples were collected from the undefended species Iotrochota birotulata, Niphates erecta, Callyspongia armigera and Callyspongia vaginalis, and the defended species Aplysina cauliformis, Aplysina fulva and Amphimedon compressa. Using a standardized procedure, tissue samples were fixed, processed for histology, sectioned, stained, and photographed for the presence of reproductive propagules (oocytes, embryos or larvae). A reproductive output index (ROI: % area of propagules/total area of tissue scanned) was calculated for each sponge species. The ROI was highly variable across species and there were no significant differences in ROI between undefended and chemically defended species. Unlike the relationship for growth and chemical defenses, the absence of a clear trade-off between reproduction and chemical defenses is probably due to the additional confounding trade-offs between propagule formation and asexual reproduction by growth and fragmentation, which was previously demonstrated for C. vaginalis and C. armigera. In combination with our past studies, we provide a 3-dimensional graph of the relative investment in growth, reproduction and defense by the 7 sponge species and discuss how resource allocation has influenced the evolution of sponge communities on Caribbean coral reefs.