MUTUALISMS AMONG SPECIES OF CORAL REEF SPONGES

Intimate associations among individuals of three common species of Caribbean coral reef sponges can be mutually beneficial. Both growth rate and survival are enhanced when heterospecific sponges adhere to each other, as demonstrated by experiments in which sponges of the same size and genotype were grown (1) in intimate association with conspecific vs. heterospecific sponges, (2) alone vs. in intimate association with conspecific sponges, and (3) alone on the primary substratum vs. attached to an intact branch of a conspecific or heterospecific sponge. Natural development and long‐term dynamics of these associations were observed in unmanipulated individuals and also modeled by an experiment in which branches were exchanged between neighboring sponges. The three species studied ( Iotrochota birotulata, Amphimedon rubens, and Aplysina fulva ) share an erect branching growth form but differ in tissue and skeletal characteristics sufficiently that they are in different orders of the Class Demospongiae. Reflecting these differences, the species are demonstrated to differ from each other in their susceptibility to a variety of environmental hazards, including predation by angelfishes and trunkfishes, predation by starfish, smothering by sediment, breakage by storm waves, pulverization by storm waves, toppling by storm waves, fragment mortality, and pathogens. Although the mechanisms by which growth rate is enhanced by adhering to a heterospecific sponge are unknown, these sponges appear to be able to decrease their loss rate by adhering tightly to sponges of species that differ from them in chemistry, tissue density, and skeletal construction, thereby increasing survival of hazards to which they would succumb when growing alone.