HABITAT ASSOCIATED DIFFERENCES IN TEMPERATE SPONGE ASSEMBLAGES - THE IMPORTANCE OF CHEMICAL DEFENSE

Sponges are important members of marine benthic communities, but, in g eneral, little is known about the factors that influence their distrib ution and abundance. Here we investigated the importance of chemical d efence in sponge assemblages from two adjacent shallow subtidal habita ts in temperate south-eastern Australia; kelp (Ecklonia radiata) fores ts and urchin dominated barrens (barrens). Surveys at two sites reveal ed a disjunct pattern of distribution for sponges between habitats, wi th ten species of sponge recorded in each habitat, but none occurring in both habitats. The cover of sponge beneath the Ecklonia forest (12. 7%) was significantly higher than the cover of sponge in the barrens ( 2%). Furthermore, sponge assemblages beneath Ecklonia forests consiste d of high densities of small colonies, whereas in the barrens, relativ ely large colonies in low densities predominated. Crude extract of the most abundant encrusting sponges from each habitat was incorporated i nto an artificial diet and fed to the sea urchin Centrostephanus rodge rsii at 50% and/or natural concentrations. Extract of three species of encrusting sponge from the barrens (Chondrilla australiensis, Clathri a sp.1 and Darwinella australiensis) significantly deterred feeding by C. rodgersii at 50% of their natural concentration. Extract of a four th barrens species, Pronax sp.1, did not significantly deter feeding a t 50% or natural concentration. Feeding by C. rodgersii was similarly not deterred by extracts of four sponge species from the Ecklonia fore sts at 50% or natural concentrations. Levels of inorganic material in sponges from the two habitats were also investigated. In general, encr usting species of sponge from the barrens had very low percentages (by weight) of inorganic material in their tissue, whereas encrusting spo nge species from the Ecklonia forests had very high percentages of ino rganic material, ranging from 83% to 93%. This study represents the fi rst experimental test of the effect of sponge extracts on feeding by s ea urchins. It reveals differences in sponges from the two habitats an d demonstrates an important anti-feedant role for extracts of encrusti ng sponges from the barrens habitat. (C) 1997 Elsevier Science B.V.