Experimental evidence of minimal effects on octocoral hosts caused by the introduced ophiuroid Ophiothela mirabilis

Highly abundant and widespread populations of the introduced eastern Pacific ophiuroid brittle star Opthiothela mirabilis occur in southeast Florida, extending the range of this new non-native species from southern Brazil northward to the Caribbean Sea and Florida, now spanning about 6000 km of latitude along western Atlantic tropical/subtropical shores. This brittle star species, representing two lineages, is an epizoite on shallow-occurring (3–15 m depth) plexaurid octocorals (e.g., Eunicea spp., Muricea elongata, and Plexaurella dichotoma). Overall body color (disk and arms) is commonly beige or orange; a few individuals display combinations of both colors: orange disk/beige arms or beige disk/orange arms. Population densities in situ commonly ranged between 5 and 12 individuals of O. mirabilis per 10-cm-long octocoral branch with up to six arms generally curled around their octocoral host’s branches. In a 33-day experiment, M. elongata colony surface areas with reduced (contracted) polyps and reduced polyp extension lengths were not statistically different between octocorals with brittle stars versus octocorals without brittle stars. At the end of 33 days, brittle star densities demonstrated an overall decline. In a 66-day experiment, increasing brittle star densities resulted in reduced extended polyp areas in M. elongata, but no changes in Eunicea flexuosa. Polyp extension lengths decreased in both octocoral hosts with increasing densities of O. mirabilis. Overall, experimental brittle star densities did not change, with several octocoral hosts revealing increases in O. mirabilis by means of fissiparity. The presence and increasing densities of O. mirabilis causing a reduction in octocoral polyp extension were greatest in M. elongata but did not result in visible adverse effects to host condition. Black spotting on octocoral stems, a sign of tissue deterioration, was not observed during or after the two experiments. Nearly all experimental octocorals demonstrated tissue growth in the 66-day experiment, expanding over relatively large areas of the supporting pedestals.