On the Caribbean coast of Panama Branchiosyllis exilis (Gravier) is associated with Ophiocoma echinata (Lamarck). They have not been found in symbiosis in other localities although both species are widespread. B. exilis was not collected on the ophiuroid species
sympatric with O. echinata, but we speculate that it may associate with other hosts elsewhere in its range. Larger ophiuroids are more often infested than smaller specimens. In the majority of infestations a single polychaete is found near the mouth or on the arm on the ventral surface
of the ophiuroid. The consistent occurrence of one polychaete per host may be an indication of aggression between individuals of B. exilis. The polychaete freely enters and leaves the mouth of O. echinata and may steal food captured by the ophiuroid. The incidence of infestation
varies at different collecting sites and seasonally at a single locality. It is possible that fluctuations in the incidence of infestation are related to seasonal stress and mortality of B. exilis, or are effected through changes in the density of the host population.
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