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Cover illustration: An eye for an eye

Light levels at mesopelagic depths (200–1000 metres) are limited, creating unique problems for predators and their prey living at such depths. Evolution has responded with interesting and perhaps profound adaptations.

The slender javelin spookfish (Bathylychnops exilis Cohen) is a translucent mesopelagic fish with evolutionary adaptations so unusual as to seem unbelievable. Specimens of this fish have been taken at depths from as shallow as 30 metres to the extreme of 1000 metres. Evidence suggests that this fish is capable of nocturnal upward migration especially among the juveniles, and that the adults may live at depths as deep as 600 metres. For this species, endemic to the northern Pacific Ocean, prey probably consists of small crustaceans or coelenterates. B exilis probably remains motionless until prey comes within range. Then, with short bursts of speed, this predator catches and crushes the prey within its mouth. This capture mechanism is predicated in part because of the lack of red muscle, thereby presuming a lack of myoglobin and/or haemoglobin, although this has not been determined. (Interestingly, this raises other questions regarding aerobic metabolism, and even retinal oxygenation, but we will save that for another time.) This suggests that the animal may not have much muscular reserve.

Figure

Unquestionably, the most startling anatomical aspect of this species is the second, smaller, ventrally located eye that buds directly off the limbus of the primary eye, as can be seen on the …

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