THE CELLULAR BASIS OF THE AGGRESSIVE ACRORHAGIAL RESPONSE OF SEA-ANEMONES

Some sea anemones possess structures called acrorhagi at the base of the tentacles. The acrorhagi are utilized solely for aggression. Acrorhagial aggression involves exquisite intra- and inter specific recognition. This study examined acrorhagi and putative acrorhagial analogs or homologs in 4 spp. [Anthopleura krebsi, Bunodosoma cavernata, Anemonia sargassensis, Bunodactis texaensis] of sea anemone. The morphology and ultrastructure of tentacles, pseudoacrorhagi, column vesicles and verrucae (adhesive column vesicles) differed form that of acrorhagi. Coral capitate tentacles and acrorhagi have different surface morphology, nematocysts and functions. Based 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, require prior acrorhagial expansion in Anthopleura krebsi and Bunodosoma cavernata. A mechanical mechanism is suggested whereby 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.