Interactions among Florida sponges. I. Reef habitats

Spatial interference competition is well described for many sessile marine invertebrates, but few studies have addressed interactions between neighboring sponges. We conducted transect surveys to assess interspecific interactions among sponges in coral reef habitats in Key Largo, Florida, USA. In total, 15 transect sites were surveyed at 5 reef locations with mean densities ranging from 13.0 +/- 3.6 to 33.3 +/- 5.9 sponges m(-2). The most abundant of over 43 sponge species observed were Amphimedon compressa, Aplysina cauliformis, Aplysina insularis, and Niphates erecta, while the least abundant were Hippospongia lachne, Ircinia campana, and Pseudaxinella lunaecharta, A nearest-neighbor technique was used to determine the degree and frequency of interactions between randomly selected sponge pairs. Overall, 40.4% of sponges occurred alone, 31.0% in proximity to, and 28.6 % in contact with other sponge species. Among sponges in contact interactions, 18.7% were epibionts, 19.4% were basibionts, and a relationship could not be resolved for the remainder. Statistical analyses of the frequencies of each interaction category revealed that the ability to overgrow or resist overgrowth varied among species. For example, Aka coralliphagum, Ectyoplasia ferox, and Phorbas amaranthus occurred alone in > 60 % of their interactions and were never overgrown by other species with which they came into contact. In contrast, Agelas schmidti, Ircinia felix, Dysidea janiae, and Scopalina ruetzleri occurred alone in < 30 % of their interactions, with the first 2 species overgrown by, and the latter 2 overgrowing, most other species with which they came in contact. All contact interactions were further statistically examined, and 41 interspecific sponge pairs revealed significant overgrowth ability by 1 of the interacting species. While the overgrowth ability of a species is dependent on sponge morphology, the ability to resist overgrowth is associated with the production of allelochemicals.