JINKS, R.N.*; MARKLEY, T.L.; PEROVICH, G.M.; EPIFANIO, C.; CRONIN, T.W.: Ontogeny of vision in marine crustaceans.

Eyes of most adult crustaceans are specialized for image formation and frequently for color vision or polarization vision as well; but those of larvae probably have more basic functions. In contrast, eyes of adult crustaceans found at hydrothermal vents lack image-forming optics and are specialized for the monochromatic detection of dim light. We studied the ontogeny of vision in the Pacific vent crab Bythograea thermydron. Adults of this species live at a depth of about 2500 m, but the larvae are probably pelagic, and the megalopae swim actively to settle at the vents. Eyes of four developmental stages (megalopa, juvenile 1 and 3, and adult), were examined with electron microscopy (TEM) and/or microspectrophotometry. B. thermydron megalopal visual pigment absorbs maximally at ~479 nm, and is thus well-suited for detection of downwelling light. Juvenile 1 (post-megalopa) eyes have rhabdoms with orthogonal microvilli like those of surface crabs with imaging eyes (e.g. Hemigrapsus), but lack a dioptric apparatus. Rhabdoms of juvenile 3 retinas are hypertrophied relative to those in retinas of either juvenile 1 or size-matched Hemigrapsus. Absorbance of adult visual pigment is red-shifted (peak ~489 nm) compared to that of the megalopae; however, TEM examination found no neural retina in adult eyes. Megalopal and juvenile 1 retinas may be adapted to a pelagic/mesopelagic existence, while juvenile 3 and adult retinas are better suited to detect the dim, long-wavelength light emitted by hydrothermal vents. Remodelling of structure and function at metamorphosis is probably a common feature in crustacean visual development.