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Hydrothermal vents along the mid-ocean ridges host ephemeral ecosystems of diverse endemic fauna including several crustacean species, some of which undergo planktonic development as larvae up to 1,000 m above and 100 km away from the vents. Little is known about the role of vision in the life history of vent fauna. Here we report that planktonic zoea larvae of the vent crab Bythograea thermydron possess image-forming compound eyes with a visual pigment sensitive to the blue light of mesopelagic waters. As they metamorphose and begin to descend to and settle at the vents, they lose their image-forming optics and develop high-sensitivity naked-retina eyes. The spectral absorbance of the visual pigment in these eyes shifts towards longer wavelengths from larva to postlarva to adult. This progressive visual metamorphosis trades imaging for increased sensitivity, and changes spectral sensitivity from the blue wavelengths of the larval environment towards the dim, longer wavelengths produced in the deeper bathypelagic vent environment of the adults. As hydrothermal vents produce light, vision may supplement thermal and chemical senses to orient postlarval settlement at vent sites.

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.

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