Symbioses in Amphipholis squamata (Echinodermata, Ophiuroidea, Amphiuridae): geographical variation of infestation and effect of symbionts on the host's light production.

Populations of the polychromatic and bioluminescent species Amphipholis squamata from eight locations were examined for internal and external symbionts. At three locations (two in the United Kingdom and one in Papua New Guinea), no symbionts were present, while four species were recovered from the remaining locations: Cancerilla tubulata and Parachordeumium amphiurae (copepods), Rhopalura ophiocomae (orthonectid) and an undescribed species of rhabdocoel turbellarian. No ophiuroid individual hosted more than one symbiont species, despite the presence of two or more within a population. Symbiont presence and prevalence varied with location, and with colour variety, but with no apparent pattern or trends. Light-production characteristics of the host were affected by the presence of all symbionts except C. tubulata. These effects, however, did not vary between colour varieties or between geographical locations, but were specific to the symbiont species: the presence of P. amphiurae resulted in enhanced intensity of light production, while that of R. ophiocomae and the turbellarian species resulted in reduced intensity. The kinetics of light production (time until maximum output) were altered only by the presence of the turbellarian. Changes in the light-production characteristics are discussed in relation to morphological, energetical and physiological effects of the symbioses.