As the nudibranch Berghia verrucicornis (A. Costa, 1867) feeds on the symbiotic sea anemone Aiptasia pallida (Verrill), zooxanthellae present in the anemone's tissues are transferred into extensions of the digestive diverticulum that are present in each of the nudibranch's numerous cerata. There the algal cells are phagocytosed by nutrient processing cells (NPCs) that line the diverticulum walls and are retained intracellularly in peri-algal vacuoles. Field collected specimens of B. verrucicornis always have high concentrations of zooxanthellae in their ceratal tissues. If, however, B. verrucicornis is starved, most of the algae are lost within 6 d of starvation. In addition, there is no evidence of active digestion of the algae in the digestive diverticula. Thus, it seems likely that the zooxanthellae are transient within the NPCs, any given algal cell being eventually exocytosed and passed out of the animal in its faeces. Regular feeding in the field ensures replacement of defaecated zooxanthellae. The results of this study suggest that the association between zooxanthellae and B. verrucicornis is a primitive mutualistic symbiosis. The observations that this particular species can be cultured and experimentally manipulated in the laboratory, and that a number of nudibranch molluscs are known to exhibit differing degrees of host-symbiont integration, suggest that nudibranch-zooxanthellae symbioses will be convenient models for investigations concerned with the evolution of mutualistic symbiosis.
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