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

Abstract

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.

Bet-hedging dispersal strategy of a specialist marine herbivore: a settlement dimorphism among sibling larvae of Alderia modesta

“…Larvae of aplysiid opisthobranchs generally exhibit a graded settlement response, metamorphosing in varying percentages on different algae. When competent larvae of the sea hare Aplysia californica were exposed to 18 species of macroalgae, 30% or more metamorphosed on 10 of the algae, and 15% or more settled on 15 of the algal species (Pawlik 1989). Larvae of A. occulifera gave a mean 25% or more metamorphosis in response to 4 of 12 macroalgae, but no metamorphosis was induced by 6 of 12 algae (Plaut et al 1995).…”

“…Many species with an identified settlement requirement exhibit a low level of spontaneous metamorphosis during the first few days after larvae attain competence. A survey of the literature indicates that levels of spontaneous metamorphosis range from 0 to 10% for diverse taxa including aplysiids (Switzer-Dunlap & Hadfield 1977, Pawlik 1989, Plaut et al 1995, other opisthobranchs (Chia & Koss 1978, 1988, Hubbard 1988, Lambert et al 1997, Avila 1998, abalone (Morse & Morse 1984), echinoderms (Burke 1984, Pearce & Scheibling 1990, Kitamura et al 1993, Johnson & Sutton 1994, polychaetes (Williams 1964, Pawlik 1986, Toonen & Pawlik 1996, hydroids (Leitz & Wagner 1993), and barnacles (Matsumura et al 1998). The mean 34% spontaneous metamorphosis reported here for lecithotrophic larvae of Alderia modesta is at least 3 times greater than the percentage of metamorphosis in sea water controls reported for any other species with a specific settlement-cue requirement.…”

“…Larvae of A. occulifera gave a mean 25% or more metamorphosis in response to 4 of 12 macroalgae, but no metamorphosis was induced by 6 of 12 algae (Plaut et al 1995). Aplysiid larvae may respond to a variety of algae due to the less restricted diet of juveniles (Switzer-Dunlap & Hadfield 1977, Pawlik 1989. In contrast, more than 50% of cultured larvae of the ascoglossan Elysia chlorotica metamorphosed on the adult food, Vaucheria spp., but only 0 to 2% settled in response to the algae Bryopsis plumosa, Enteromorpha sp., and Cladophora sp.…”

Larval metamorphosis of Phestilla spp. in response to waterborne cues from corals

“…P. minor has the most restricted diet range and the shortest competency phase, all of which are strategies for recruiting into its natal habitat. Larvae of the sea hare Aplysia californica metamorphosed in response to a variety of macroalgal species but as juveniles would crawl onto preferred macroalgae (Pawlik, 1989). A. californica has planktotrophic larvae (that reach metamorphic competence 35 days after hatching), and larvae of A. californica can metamorphose onto a variety of algal species after dispersal between habitats.…”

“…This is a similar strategy as P. melanobrachia which eats corals in multiple genera and has planktotrophic larvae. Pawlik (1989) describes the trade off between settlement specificity and larval dispersal "…larvae appear to be selective enough to increase the likelihood of juvenile survival, but sufficiently broad in their response so as to minimize larval mortality." Some opisthobranchs have solved this trade off with a bet hedging strategy and have both lecithotrophic and planktotrophic larvae (Krug 2001;Smolensky et al, 2009).…”