GROWTH DYNAMICS AND MORTALITY OF THE ENCRUSTING SPONGE CRAMBE-CRAMBE (POECILOSCLERIDA) IN CONTRASTING HABITATS - CORRELATION WITH POPULATION-STRUCTURE AND INVESTMENT IN DEFENSE

1. A trade-off was predicted between investment in defence and growth rate in the encrusting sponge Crambe crambe (Schmidt), while survival rates were expected to correlate positively with the production of def ences. Previous studies have demonstrated that this sponge is better d efended (chemically and physically) in shaded animal-dominated habitat s than in well-illuminated habitats, dominated by algae. It was determ ined whether these habitat-associated differences in investment correl ate with differences in growth, regeneration and mortality rates of sm all specimens (initial average area less than 100 mm(2)) of this spong e. 2. In the 2 years of the study the sponge grew slowly (size increas ed 2.5 times on average), and showed high interindividual variation in growth rates. A seasonal pattern was evident, with marked size increa ses from May to October. Significantly higher growth rates (on a month ly basis) were found in the well-illuminated habitat during the second year of study. There was also a negative relationship between monthly growth rate and toxicity (measured in previous studies). No differenc es were found in sponge regeneration rates between communities. 3. Mor tality was significantly higher in individuals from the well-illuminat ed habitat, and mainly affected the smallest sponges (< 150 mm(2)). 4. This sponge was significantly more abundant in the shaded habitat, bu t the mean size of the sponges was greater in the well-illuminated hab itat. The size distributions in the two communities were also signific antly different. Sponges of the smallest size classes were more abunda nt in the shaded community. 5. It is concluded that sponges in the sha ded habitat, in which investment in defence was greatest, featured hig her survival but grew more slowly than those in the well-illuminated h abitat. Differential mortality and growth observed in the study were s ufficient to explain the patterns of abundance and size distributions of the established sponge populations from these two habitats.