Most jellyfish are passive feeders, this means that species such as moon jellyfish don’t need proper eyes, and instead possess simple, light-sensitive ‘eye-spots’ or ‘ocelli’ that detect light and dark to help them orientate themselves and find food. But the box jellyfish, which has 24 eyes, is an exception.
Unlike open-ocean jellies, box jellyfish hunt actively in shallow, obstacle-strewn waters such as mangrove swamps. Their eyes are arranged in four clusters, called rhopalia, that hang from the corners of their cuboid bodies. Each of these rhopalia contains six eyes of four different types. Some are simple light detectors but others are sophisticated.
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Some of their upper eyes have retinas, corneas and lenses that allow them to look through the water’s surface. Researchers believe these can pick out the silhouettes of overhanging mangrove trees, to help the jellyfish stay near their feeding grounds.
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Other eyes with lenses look out for obstacles, while their simpler eyes are thought to measure light levels and estimate depth. Their vision is also thought to help with transferring sperm packets during courtship.
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