Reference: Biol. Bull. 165: 286-304. (August, 1983) SETTLEMENT AND METAMORPHOSIS OF A TEMPERATE SOFT-CORAL LARVA (ALCYONIUM SIDERIUM VERRILL): INDUCTION BY CRUSTOSE ALGAE KENNETH P. SEBENS Biological Laboratories and Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138 ABSTRACT The temperate soft-coral Alcyonium siderium Verrill has a demersal planula larva which usually settles and metamorphoses on vertical rock surfaces near the parent colony. Such surfaces are covered by a variety of encrusting invertebrate species and by three common crustose algae (Lithothamnium glaciale, Phymatoli-thon rugolosum, and Waernia mirabilis). Larvae settle and metamorphose most frequently on these three algal species in the field (Sebens, 1983). Contact with each of the three crustose algae induced settlement and meta-morphosis within 1 -5 days in laboratory experiments. Rock or shell fragments, even with naturally filmed surfaces, did not induce metamorphosis in the same time period. A few larvae did metamorphose on the rock, shell, and glass or plastic surfaces of the containers, taking up to 30 days to do so. Larvae were kept alive up to 194 days but their competence to metamorphose declined significantly after ten days. The half-life of larvae that did not metamorphose was approximately 25 days. Larvae presented with coralline algae in darkness delayed metamorphosis by ap-proximately 10-20 days, but most of them did metamorphose by 30 days. Neither sea water incubated with coralline algae, nor coralline algae in close proximity (4-5 mm) to the larvae, but without contact, induced metamorphosis. Induction of settlement and metamorphosis is thus mediated by surface contact with the algae and probably not by a dissolved chemical. Presence of the colonial ascidian, Apli-dium pallidum, inhibited metamorphosis even when larvae were able to contact coralline algae, and also caused early larval death. INTRODUCTION The planulae of octocorals are usually brooded by the adult colony to a swim-ming stage (Matthews, 1917; Gohar, 1940; Hartnoll, 1975, 1977; Weinberg, 1979; Weinberg and Weinberg, 1979) which settles and crawls on the substratum. They may also be released as demersal crawling larvae (Hartnoll, 1977). The swimming larvae are similar in morphology and behavior to those of certain scleractinian corals (Abe, 1937; Atoda, 1947a, b, 195 la, b, c, 1953; Kawaguti, 1941, 1944; Harrigan, 1972a, b; Lewis 1974), hydroids (Nishihara, 1967a, b, 1968a, b; Donaldson 1974), scyphozoans (Brewer, 1976a, b; Neumann, 1979), and sea anemones (Chia and Spaulding, 1972; Siebert, 1973). Behavior of the demersal planulae is similar to that described for scleractinian corals (Gerrodette, 1981; Fadlallah and Pearse, 1982; Fadlallah, 1983), certain hydroids (Williams, 1965, 1976), and hydrocorals (Ostar-ello, 1973, 1976). Settlement and substratum choice has been studied for few an-thozoans [reviewed by Chia and Bickell (1978)], and for even fewer octocorals (Theodor, 1967; Chia and Crawford, 1973; Weinberg, 1979; Weinberg and Wein-berg, 1979). Received 23 November 1982; accepted 1 April 1983. 286
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