Development and behavior of megalopa larvae and juveniles of the hydrothermal vent crab Bythograea thermydron

Abstract

We collected megalopa larvae and early juveniles of the crab Bythograea thermydron from a depth of 2500 to 2600 m at a hydrothermal vent field along the East Pacific Rise (ca 9" to 10" N, 104' W). Taxononlic identification of the megalopa larvae was accomplished through the use of morphological characteristics corroborated by molecular genetic analysis of an amplified portion of DNA from the mitochondnal 16s rRNA gene. We successfully reared megalopa larvae through metamorphosis and through subsequent juvenile molts at atmospheric pressure in the laboratory. This is the first time that this has been reported for any vent species. Laboratory data were combined with measurements of field-caught juveniles to ailow estimation of carapace xvidth, dry weight, and Stage duration of the first 5 juvenile Stages. Results of behavioral experiments indicated that B. thermydron megalopae swim actively over the range of temperature expected near the vents (2 to 25'C). Swimming speed varied with temperature (4 to 10 cm s-'), but generaily exceeded the speed of bottom currents at the vent fields. Moreover, the propensity to swirn was inversely related to temperature. These results suggest that swimming behavior may be an important component of locating warm vent Settlement sites in the otherwise cold waters surrounding a vent field.