Tentacle autotomy in the hydromedusa Aglantha digitale (Cnidaria): an ultrastructural and neurophysiological analysis

Tentacles of the hydromedusa Aglantha digitate readily detach (autotomize) at a predictable site (autotomy plane) when a tentacle is pinched and tugged. The response is reversibly inhibited in seawater containing excess Mg2+. Behavioural, electrophysiological and ultrastructural analyses indicate that passive and active processes are involved in tentacle autotomy. Inherent structural weakness at the tentacle autotomy plane is suggested by thinning of mesoglea and reduction of muscle tails at this site. The autotomy plane is also characterized by a ring of distinctive ectodermal cells with vacuoles concentrated along the basal cell membrane bordering the mesoglea. These vacuolated autotomy plane (VAP) cells are extensively innervated and, except for a few slender muscle tails, they interrupt the sheath of myoepithelium encasing each tentacle. Tentacles fixed after an autotomizing stimulus but before tentacle breakage show loss of basal vacuoles in VAP cells and holes in the underlying mesoglea. We suggest that the active mechanisms for tentacle autotomy include strong contraction of muscle tails on either side of the autotomy plane and activation of VAP cells, which somehow lower the tensile strength of autotomy plane mesoglea. Autotomizing stimuli do not appear to evoke a special conduction system within the tentacle. Highfrequency nerve-impulse activity in the tentacle conduction system that otherwise directs graded tentacle contractions is initiated by autotomizing stimuli and precedes tentacle disjunction. Tentacle autotomy in Aglantha, a delicate holopelagic hydromedusa that inhabits densely populated pelagic zones, may allow escape when the tentacles inadvertently entangle and overly large and potentially damaging zooplankters.