Mercado-Molina, Alex E.

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    Population ecology of the demosponge Amphimedon compressa
    (2008) Mercado-Molina, Alex E.; Yoshioka, Paul M.; College of Arts and Sciences - Sciences; Weil, Ernesto; Schizas, Nikolaos V.; Aponte, Nilda E.; Department of Marine Sciences; Maldonado Ramírez, Sandra L.
    The population ecology of Amphimedon compressa was investigated at La Parguera, Southwest coast of Puerto Rico. A preliminary survey on the distribution, abundance and population size-structure of this sponge was initially done on three inner shelf and two mid-shelf coral reefs at various depths (3m, 5m, 8m, and 12m). In addition, the population dynamics of A. compressa was studied for one year at Las Pelotas (LP), an inner shelf reef experiencing low water motion, and at Media Luna (ML), a mid-shelf reef exposed to high water movement. Results showed that abundance of A. compressa varied significantly among sites and depth zones. Colonies were absent in the shallower zone (3m) in 4 of the 5 reefs, however, there was not a clear pattern in the abundance with relation to sites or depths. Size-frequency distributions in shallow depths (5m-8m) were generally dominated by colonies less than 20cm tall, meanwhile the size-structure in the deepest zone (12m) was characterized by a combination of small (<20cm) and large (>20cm) colonies. Median colony heights increased with depth, in 4 of the 5 reefs. When comparing median colony size among reefs within depths, larger colonies were found in inner shelf sites. There was little variation in the abundances during the studied period at both sites. However, higher numbers of colonies were found in February and April as a result of higher number of sexual and asexual recruits. Sexual and asexual recruits contributed similarly to the total recruitment of new colonies. Overall, survivorships did not differ statistically between sites (X21=1.759, p=0.185). However, survivorship at ML was size dependent with the lowest survivorships (<40%) for large colonies (>20cm) and the highest (85.7%) for small colonies (<10cm). In contrast, survivorships did not differ at LP averaging about 67% among size classes. The high mortality of large sponges at ML may be due to fragmentation and/or detachment from drag forces caused by high water motion. Growth rates were significantly higher at LP where growth rates increase with increasing size. This size-dependent growth pattern was not observed at ML. A stage-based population matrix model predicted an increase in abundances in both populations, with the population at ML increasing more rapidly. Projected abundances at LP showed a decline unless both sexual and asexual recruits are included in the model. Conversely, abundances at ML decline only when sexual recruits are not considered. No evidence of biological interactions (predation, grazing, inter/intra specific competition, disease) affecting the growth, survivorship and recruitment, were observed in this study indicating that physical factors may play a more important role in structuring the populations of A. compressa.