Anableps’ bifurcated vision system is an advanced application of the same principle employed by the box jellyfish Tripedalia cystophora. This 1-cm jellyfish lives swims around by vigorously expanding and contracting its bell-shaped body.
Cystophora is fond of feeding on tiny copepods which it finds in sunny spots among mangrove roots. At night, a jellyfish takes its ease by moving away from the mangroves and sinking to the bottom of the shallow lagoon where it lives.
To see, cystophora has 6 eyes of 4 types. 2 eyes have lenses which give it a decent view. The uppermost lensed eye looks up. The lowest looks down. The lower eye spots obstacles; mostly the mangrove roots that the jellyfish swims among. The upper one keeps its eye on the prize.
If the top eye sees starkly bright light, it means that the jellyfish has strayed into open water, and risks starving. If instead the light gives the jellyfish the sense that it is within the mangrove canopy, lunch is in sight.
The box jellyfish is a brainless blob, but it has the wits to get around, find food, and get a good night’s sleep.