Growth dynamics and mortality of the encrusting sponge Crambe crambe (Poecilosclerida) in contrasting habitats: correlation with population structure and investment in defence

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 defences. Previous studies have demonstrated that this sponge is better defended (chemically and physically) in shaded animal-dominated habitats than in well-illuminated habitats, dominated by algae. It was determined whether these habitat-associated differences in investment correlate with differences in growth, regeneration and mortality rates of small specimens (initial average area less than 100 mm(2)) of this sponge. 2. In the 2 years of the study the sponge grew slowly (size increased 2.5 times on average), and showed high interindividual variation in growth rates. A seasonal pattern was evident, with marked size increases from May to October. Significantly higher growth rates (on a monthly 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 differences were found in sponge regeneration rates between communities. 3. Mortality was significantly higher in individuals from the well-illuminated habitat, and mainly affected the smallest sponges (< 150 mm(2)). 4. This sponge was significantly more abundant in the shaded habitat, but the mean size of the sponges was greater in the well-illuminated habitat. The size distributions in the two communities were also significantly different. Sponges of the smallest size classes were more abundant in the shaded community. 5. It is concluded that sponges in the shaded habitat, in which investment in defence was greatest, featured higher survival but grew more slowly than those in the well-illuminated habitat. Differential mortality and growth observed in the study were sufficient to explain the patterns of abundance and size distributions of the established sponge populations from these two habitats.