Host specialization of an obligate sponge-dwelling brittlestar

Abstract

On coral reefs off the Florida Keys (USA), the obligate sponge-dwelling brittlestar Ophiothrix lineata lives almost exclusively in the tube sponge Callyspongia vaginalis. We examined chemotactic recognition by O. lineata to assess sponge host preferences using a Y-tube assay cham- ber. O. lineata preferentially selected seawater conditioned by C. vaginalis relative to seawater con- trols and showed no preference for seawater conditioned by the infrequent host sponge Niphates dig- italis or the non-host sponge Aplysina archeri. When offered seawater conditioned by C. vaginalis and N. digitalis, O. lineata preferentially chose C. vaginalis. Field manipulations examined growth of O. lineata confined to live in all 3 sponge species and the impact of habitat size on growth of O. lin- eata living in C. vaginalis. Growth of O. lineata was significantly greater in longer (12 cm) tubes of C. vaginalis than in shorter (6 cm) tubes, and while O. lineata was able to survive in A. archeri, growth was greater for brittlestars living in C. vaginalis and N. digitalis. Surveys of O. lineata in C. vaginalis revealed that 74.3% of sponges had at least 1 male and female brittlestar, with 37.1% of sponges having a greater proportion of males, which may increase brittlestar fertilization success. Abundance of O. lineata increased with sponge size and did not differ based on the presence of brooded larvae in sponge tissue. The obligate association of O. lineata with C. vaginalis likely evolved as a consequence of some combination of host abundance, enhanced food availability, and greater probability of mating success in the multi-tubed sponge.