Sponge-rhodolith interactions in a subtropical estuarine system

The interactions between sponges and red macroalgae have been widely documented in tropical and subtropical environments worldwide, and many of them have been documented as mutualistic associations. Sponges, however, have also been frequently described as part of the associated fauna of rhodolith habitats (aggregations of free-living non-geniculated coralline macroalgae). Nonetheless, the types of interaction they establish as well as the role of sponges in these habitats remain unknown. In this study, the associations between sponges and rhodoliths were investigated in an estuarine ecosystem of the Mexican Pacific based on qualitative and quantitative data. A total of 13 sponge species were identified in five newly discovered rhodolith beds dominated by the non-geniculate coralline macroalga Lithophyllum margaritae. The sponge assemblages were strongly restricted to rhodolith habitats. The best predictor of sponge abundance (from 5.1 to 51.7 ind m^-2) and species richness (from 2.6 to 6.1 sponge species m^-2) was the rhodolith density rather than other population descriptors assessed (e.g., average size, branch density and sphericity). The identified sponges included a variety of forms: massive (46 %), encrusting (23 %), excavating (15 %), cushion-shape (8 %) and digitate (8 %). Moreover, more than 50 % of sponge species recorded (mainly massive and encrusting forms) were frequently found overgrowing and binding rhodoliths. Halichondria cf. semitubulosa and Mycale cecilia were the most common binding agents; these species bind an average of 3.1 and 6.6 rhodoliths per sponge individual, respectively. These findings reveal the importance of rhodoliths as habitat forming species, since these seaweed beds notably increased the substrate complexity in soft bottom environments. In addition, the relatively high abundance of sponges and their capability to bind rhodoliths suggest that these associated organisms could have an important contribution to rhodolith bed stability.