The cellular basis of the aggressive acrorhagial response of sea anemones

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Abstract
Some sea anemones possess structures called acrorhagi at the base of the tentacles. The acrorhagi are utilized solely for aggression. Acrorhagial aggression involves very exquisite intra‐ and interspecific recognition. This study examined acrorhagi and putative acrorhagial analogues or homologues in four species of sea anemone. The morphology and ultrastructure of tentacles, pseudoacrorhagi, column vesicles, and verrucae (adhesive column vesicles) differed from that of acrorhagi. Coral capitate tentacles and acrorhagi have different surface morphology, nematocysts, and functions. Besed on morphology, acrorhagi seem more likely to be homologous to tentacles than to verrucae. Acrorhagial nematocyst discharge and ectodermal peeling, the culmination of the response, were shown to require prior acrorhagial expansion in Anthopleura krebsi and Bunodosoma cavernata. A mechanical mechanism is suggested where‐ by distention of the acrorhagus opens a ciliary pit on the nematocyte surface and exposes the pit wall and microvilli, which may contain the chemoreceptors for the peeling process, including nematocyst discharge. A similar system may also be responsible for changing the threshold of nematocyst discharge in sea anemone tentacles. A case of possible neurosecretion in an anthozoan was also shown in this study.