Inhibition of settlement and metamorphosis of the ascidian Herdmania curvata by non-geniculate coralline algae

The surfaces of non-geniculate coralline algae (NCA) are known to induce th e settlement and metamorphosis of disparate marine taxa. In this study we i nvestigate the responsiveness of larvae of Herdmania curvata (Ascidiacea: S tolidobranchia) to three species of NCA (Neo-goniolithon brassica-florida, Hydrolithon onkodes, and Lithothamnium prolifer) that cohabit the slope and crest of Heron Reef, Great Barrier Reef. H. curvata larvae were first expo sed to these NCA at or within 2 h of hatching, which is 1 to 2 h prior to a ttaining competence, and then cultured continuously with the NCA for 12 to 14 h. Rates of settlement and metamorphosis of H, curvata cultured in labor atory chambers in the presence of the different NCA were significantly lowe r than spontaneous rates in seawater. The limited settlement in treatments containing NCA were confined entirely to the chamber periphery, and settlem ent never occurred on the surface of the NCA. The inhibitory effect was dos e-dependent and was stronger in H. brassica-florida and H. onkodes than in L. prolifer. Larvae that did not settle in treatments with NCA had rounded anterior trunks and, in extreme cases, kinked tails with rounded and dissoc iated tail muscle cells. In some individuals, we observed the anterior chem osensory papillae being sloughed off the larval body. Morphological analysi s of trunk ectodermal and mesenchymal nuclei of larvae cultured in the pres ence of the NCA revealed that general necrotic cell death was occurring. Im portantly, H. curvata larvae that were exposed to NCA could not subsequentl y be induced to metamorphose in KCl-elevated seawater, whereas larvae not e xposed to NCA metamorphosed at high rates in KCl-elevated seawater.