METAMORPHIC RESPONSES TO NATURAL SUBSTRATA IN A GASTROPOD LARVA - DECISIONS RELATED TO POSTLARVAL GROWTH AND HABITAT PREFERENCE

Laboratory experiments were conducted to test the adaptive significanc e of settlement and metamorphosis responses in competent veligers of S trombus gigas Linnaeus (queen conch). When competent veligers were tes ted for metamorphic response to 15 substrata collected from nursery gr ounds in seagrass beds of the Florida Keys, 0-38% underwent metamorpho sis. Substrata with complex physical and biotic structures such as the calcareous red alga Neogoniolithon strictum (Foslie) Setchell and Mas on, the green alga Dasycladus vermicularis (Scopoli) Krasser, and the matrix of algae and sediment attached to rock substrata elicited highe st responses. No larvae responded to live blades of the seagrasses Tha lassia testudinum Koenig and Syringodium filiforme Kuetzing, indicatin g that these plants are not the primary inducement for recruitment of conch to seagrass meadows. When newly-settled conch were grown for 16 days on the same substrata used to test for metamorphic responses, gro wth rates ranged from 7-62 mu m/day and were weakly correlated (r = 0. 71) with frequency of metamorphosis. High growth rates were associated with substrata that elicited high, low, or no metamorphic responses ( e.g., on Thalassia testudinum), but low growth was always associated w ith low metamorphosis. High metamorphosis occurred with substrata that were preferred as habitat by postlarval conch and yielded high growth rates. Settlement decisions by queen conch larvae appear to have impo rtant adaptive significance for newly metamorphosed recruits.