单项选择题
Hydrothermal vents along the mid-ocean ridges host ephemeral ecosystems of diverse fauna, including several crustacean species, some of which undergo development as larvae up to 1,000 meters above and 100 kilometers away from the vents. For example, vent crab Bythograea thermydron (at stage of larvae) possess image-forming compound eyes with a visual pigment sensitive to the blue light of upper oceanic waters. As these larvae metamorphose into adulthood and begin to descend to and settle at the vents, they lose their imageforming optics and develop highsensitivity naked-retina eyes. With maturation,the spectral absorbance of the visual pigment in these eyes also shifts towards longer wavelengths and this progressive visual metamorphosis trades imaging for increased sensitivity, and changes spectral sensitivity to the dim, longer wavelengths of light existing at the bottom of the ocean. As hydrothermal vents produce light, vision may supplement thermal and chemical senses to orient post-larval settlement at vent sites.
According to the passage, the optics of the vent crab Bythograea thermydron (line 4 ) serve all of the following purposes over the course of its life-cycle EXCEPT（）. A．They assist in the orientation process by helping the crab navigate toward post-larval settlements.
B．They assist the crabs visually by permitting imaging in the light-saturated upper oceanic depths.
C．They help the crabs navigate by providing thermal and chemical senses to the adult crab near post-larval settlements.
D．They increase the crabs’’ flexibility by allowing the crab to adapt to two different environments with different lighting conditions.
E．They permit the crabs to achieve increased sensitivity to the location of underwater thermal vents.