SHORT-TERM COMPETENCE IN LARVAE OF QUEEN CONCH STROMBUS-GIGAS - SHIFTS IN BEHAVIOR, MORPHOLOGY AND METAMORPHIC RESPONSE

Competent larvae of the prosobranch gastropod Strombus gigas Linne (queen conch) require an environmental cue to induce metamorphosis. In this laboratory investigation the planktotrophic veligers of S. gigas were exposed to 2 cues to observe changes in metamorphic response as a function of age. The cues were extract of Laurencia poitei, an artificial cue, and detrital blades of Thalassia testudinum, a natural cue. Onset of competence began 18 d post-hatch, and veligers were competent for only 6 d at 28 to 30 degrees C, Both cues induced the highest percentage metamorphosis on Day 23 (88 % for extract and 78 % for detritus), and response dropped dramatically by Day 25 (3 % for extract and 15 % for detritus). Shell length for successful metamorphosis ranged between 952 and 1258 mu m. Pigmentation change from orange to dark green on the propodium and larval operculum indicated competence, and competent veligers exhibited a swim-crawl behavior associated with settlement. This study revealed that veligers of S. gigas have a unique developmental history. The competence period was shorter than the precompetence period. Most other planktotrophic larvae are competent for a time equal to or longer than the precompetence period. Short-term competence is ordinarily associated with metamorphosis to a broad spectrum of cues. This may explain why veligers of S. gigas were observed to metamorphose to a variety of benthic cues found in juvenile conch seagrass habitats.