Morphological plasticity in the tropical sponge Anthosigmella varians: responses to predators and wave energy

The goal of the research presented here was to examine phenotypic plasticity exhibited by three morphotypes of the common Caribbean sponge Anthosigmella varians (Duchassaing & Michelotti). We were interested in examining the biotic (and, to a lesser extent, abiotic) factors responsible for branch production in this species. We also tested the hypothesis that the skeleton may serve an antipredator function in this sponge, focusing on vertebrate fish predators (i.e., angelfish) in this work. In transplant and caging experiments, unprotected forma varians replicates were immediately consumed by angelfish, while caged replicates persisted on the reef for several months. These findings support the hypothesis that predators (and not wave energy) restrict forma varians to lagoonal habitats. Branch production was not observed in A. varians forma incrustans when sponges were protected from predators or placed in predator-free, low-wave-energy environments. It is not clear from our work whether forma incrustans is capable of producing branches (i.e., whether branch production is a plastic trait in this morph). Additional field experiments demonstrated that A. varians forma varians increased spicule concentrations, compared to uninjured sponges, in response to artificial predation events, and A. varians forma rigida reduced spicule concentrations, compared to uncaged controls, when protected from predators. These findings indicate that spicule concentration is a plastic morphological trait that can be induced by damage, and that A. varians may be able to reduce spicule concentrations when environmental conditions change (e.g., in the absence of predators). The potential significance of inducible defenses and structural anti-predator defenses in sponges is discussed in relation to recent work on sponge chemical defenses.