Abstract
Mechanisms of larval settlement in the marine hydroid Tubularia mesembryanthemum have been investigated. Actinula larvae demonstrated two forms of attachment behavior: temporary attachment by atrichous isorhiza (AI) nematocyst discharge from the tentacle tips, and permanent settlement by cement secretion from the basal protrusion. During settlement, the larvae discharged numerous AIs by rubbing the tentacle tips on the substrata surfaces, and the other types of nematocytes synchronously migrated to the tentacle tips. Thus, the nematocyst composition of the tentacles radically changed after larval settlement in parallel with the functional change (from attachment to feeding and defense) of the tentacles. AI discharge was induced by tactile stimuli as well as high K+; a simultaneous transitory surge of [Ca2+]i was observed in the tentacle tips. This was inhibited by≤0.9mM[Ca2+] and by 10-50μM[Gd3+]. Actinula settlement was induced by direct contact of larval tentacles to microbial films, particularly films of the benthic diatom Cocconeis sp. The inducing activity of biofilms was inhibited by treatment with HIO4 and lectins, especially LCA, thus indicating the involvement of polysaccharides in the activity. GLWamide peptides, which are peptidic neurotransmitters, were found to induce actinula settlement, and the occurrence of GLWamidepositive neurons was confirmed in ontogenetic stages varying from competent larvae to juvenile polyps. These results suggest that an influx of extemal Ca2+via mechano-sensitive (stretch-activated) cationic channel activation and LCA-binding sugar-chains may play important roles in larval mechano-/chemoreception, and that GLWamide peptides may control larval morphogenesis as the internal signals. AI nematocytes are likely to act as sensors and effectors of actinular settlement
