Guest post from graduate student Kia Walcott

A reconstruction of what a female crab may see of a displaying male when her lenses are off.

Female blue crabs (Callinectes sapidus) are literally blind when they choose their mates, according to new research from Duke biologists Jamie Baldwin and Sönke Johnsen.

Blue crabs are one of many crustacean species that undergo molting and mating at the same time.  Because the multi-faceted lenses that make up the crab’s eyes are part of the exoskeleton, they too are shed.  So it’s like a molting female has taken out her contact lenses.

Baldwin and Johnsen put the crabs in an eye exam of sorts, with a rotating black-and-white striped drum. When the crab can see, she will move her eyes in the same direction as the rotating stripes. When she can’t see, she will not perform this behavior. By finding the width between the stripes that the female no longer moves her eyes, Baldwin and Johnsen were able to measure visual acuity.

They found that a female’s vision can be blurry from 3 days prior to molting until 3-6 days after molting.

This means that during the critical time of mating, when these female crabs should be experiencing all of the romantic courtship behaviors displayed by male blue crabs like claw waving, standing tall on the walking legs, and rhythmic waving of swim paddles, these single ladies can’t see a thing.

The males, on the other hand, can see perfectly, and in fact, use their color vision to choose females with red claws versus those with claws of other hues. Baldwin and Johnsen say all hope is not lost for female blue crabs however. They believe that chemical cues, what we would call smell, may help overcome her blurred vision.

Other studies have found that visual sexual cues are nearly non-existent or at least not documented in species that mate and molt simultaneously like this. These findings may explain why, at least for one species, looks aren’t everything.

CITATION: Baldwin and Johnsen. (2011) Effects of molting on the visual acuity of the blue crab, Callinctes sapidus. J Exp Biol. 214: 3055-61. <>