Larvae of the sea hare Aplysia californica settle and metamorphose on an assortment of macroalgal species

Larvae of the sea hare Aplysia californica (Molluscs: Opisthobranchia) spend several weeks feeding in the plankton pnor to settlement and metamorphosis Previous work indicated that metamorphosis was tnggered by only one (or a t most a few) algal species. However, in the present laboratory study, a mean of 30 % or more of the larvae of this sea hare metamorphosed in response to 10 of 18 species of intertidal macroalgae (9 red. 7 brown, 2 green). Metamorphosis was greatest in response to the red algae Rhodymenja californica, Corallina officinalis, Plocamjurn cartilagineurn and Laurencia pacifjca. Juveniles of A. californica that had metamorphosed on the last 2 species grazed on them and began to grow, whereas juveniles on the other species tended to crawl off the alga and around the assay dish. Of the 8 algae least preferred, only 1 was red, the remainder brown or green. For larvae of A. californica, metamorphosis on a relatively wide spectrum of algal species may be more efficacious than metamorphosis on any one alga, because juvenile sea hares can readily crawl to nearby algal species that they prefer to eat after they have metamorphosed on an alga that is not t h e ~ r preferred food. Most marine invertebrates pass through a planktonic larval stage prior to settlement and metamorphosis, and the recruitment of their larvae is important in structuring benthic marine communities (Keough & Downes 1982, Keough 1983, Gaines & Roughgarden 1985). Larval settlement is largely regarded as a response to complex, and often highly specific, environmental stimuli (Burke 1983, Crisp 1984), especially chemical cues (reviewed in Hadfield 1986, Pawlik & Faulkner 1986). However, few naturally-occurring chemical inducers of larval settlement and metamorphosis have been isolated and identified (Kato et al. 1975, Cuomo 1985, Pawlik 1986). ' Present address: Friday Harbor Laboratories, University of Washington, 620 University Road, Friday Harbor, Washington 98250, USA O Inter-Research/Printed in F R. Germany Marine molluscs of the genus Aplysia are among the most intensively studied animals on earth and have been the subject of research on development, growth and energetics, circadian rhythms, and the neural basis for learning, memory and behavior (reviewed in Kandel 1979, Carefoot 1987). Research interest in one of the best studied sea hares, A. califol-nica, from the coast of California, prompted the laboratory cultivation of the planktotrophic larvae of these animals through metamorphosis (Kriegstein et al. 1974). In the field, young recruits of these herbivorous snails were most frequently found eating red algae, primarily of the genus Laurenc ia (Kupfermann & Carew 1974). Moreover, several putatively defensive halogenated natural products isolated from adult A. californica were found to be derivatives of metabolites of L. paci f jca (Stallard & Faulkner 1974), suggesting some dependence of the molluscs on this alga. Larval settlement of Aplysia californica was reported to be highly substrate-specific, and chemical inducers were believed to be responsible for substrate choice (reviewed in Carefoot 1987). Kriegstein et al. (1974) found that laboratory-reared larvae would settle and metamorphose on Laurenc ia paci f ica , but not on species of Plocarnium, Polysiphonia, Daysia , C h o n d r u s or Ulva. Subsequently, Capo et al. (1979) reported that 2 red algae from the New England coast, N e o a g a r dhei l la baileyl and Graci lar ia sp. , would also induce settlement and metamorphosis of A . californica. The study described herein was initiated in a n effort to isolate and identify compounds from L. paci f ica that induced metamorphosis of A. californica. It was discovered, however, that metamorphosis of A. californica was much less specific than had been previously reported. 196 Mar. Ecol. Prog. Se Materials and methods. All larvae used in experiments were the progeny of 2 specimens of Aplysia californica that had been cultured as larvae in the lab, had metamorphosed upon exposure to branchlets of Laurencia pacifica, and had been reared from juveniles to reproductive adults on a diet of Plocamium cartilagineum and L. pacifica. Small pieces of egg mass were placed in aerated beakers containing l pm-filtered, natural seawater (hereafter referred to as 'seawater') until hatching occurred. Larvae were then transferred to 2 1 Fernbach flasks filled to the neck with seawater containing 40mg 1-' each of the antibiotics streptomycin sulfate and sodium penicillinG (Sigma Chemical Co., St. Louis, Missouri) and 104 cells ml-' of the green flagellate Pavlova lutheri. Larval entrapment a t the air/water interface was prevented by spreading flakes of cetyl alcohol (l-hexadecanol, Sigma) on the water surface. Flasks were kept a t 20°C in a chamber 35 cm beneath 2 continuously-illuminated 60 W fluorescent lights. Every 4 d , culture vessels were cleaned.