Growth dynamics in four Mediterranean demosponges

Growth dynamics of the demosponges Crambe crambe, Hemimycale columella, Oscarella lobularis and Chondrosia reniformis were studied in the north-western Mediterranean over a 2 year period. Independent growth and shrinkage rates were quantified for first time by a newly developed photographic and computer-assisted image analysis. Overall, our results indicated relatively slow growth dynamics with low growth and shrinkage rates. Interspecific comparison showed that C. crambe, H. columella and O. lobularis grew and shrank at significantly higher rates than C. reniformis (differences of over one order magnitude). Measured rates were, in general, similar to those previously reported for other species thriving in the Mediterranean, temperate and tropical habitats. Seasonal growth and shrinkage patterns were examined in C. crambe. This species showed significant differences in growth rates over time but not in shrinkage rates. Division and fusion events were observed in all species except in specimens of C. reniformis. Approximately 30% and between 14-30% of the specimens experienced division and fusion respectively. Data on growth, shrinkage, division and fusion rates were interpreted as evidence for distinct biological strategies used by these four species to persist and occupy substratum. Chondrosia reniformis had a conservative strategy, with slow growth but with greater resistance to damage. Crambe crambe seemed to enhance its rate of space occupation by a high division rate. Hemimycale columella grew quickly and shrank at low rates, thus spreading rapidly over the substratum. Finally, O. lobularis grew and shrank rapidly, showing great growth plasticity. (C) 2001 Academic Press.