Too See
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Story and Photos by Joanne Huemoeller
To see or not to see is the question, but the answer is not always as obvious as it seems. This is especially true in the world of marine invertebrates. Are image-making eyes necessary or would limited or no vision serve just as well? If eyes are present, where should they be located, and how many should there be? The answer to these questions is an unequivocal—it depends.
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It depends on lifestyle. Not all animals need to see well. Non-mobile bottom dwellers don’t need great eyesight. They feed not by hunting or scavenging but by simply filtering microscopic organisms out of the current with specialized appendages. It depends on the strength of the animal’s other senses. A strong sense of smell, chemoreception, or touch may be effective in detecting both food and predators. Among the invertebrates that see well, the octopus, equipped with camera-like eyes and a complex brain, is a standout. Crabs also see well. Eyes located on stalks see a full 360 degrees, probably explaining why shore crabs are able to avoid becoming an easy meal for birds flying overhead. Many species require only limited eyesight. Rock scallops just need to know when to close their thick shells. Two rows of beautiful blue eye sensors located near the rim of the shell detect movement and the scallop snaps shut.
A crab’s eyes are located on stalks, |
Above: Anemones don’t need great eyesight to filter microscopic organisms out of the current with specialized appendages.
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When feather duster worms and barnacles pull in their feeding appendages, it’s a sign they have detected changes in light intensity or a moving shadow. It does not mean that they can tell whether you are having a good-hair day.
The rims of a bell jelly’s medusa are lined with bright red photoreceptors. Bell jellies use their eyespots to help them find the darker bottom where they feed. Invertebrates completely without sight are numerous—or at least I thought they were. But in the exciting world of marine biology there are new discoveries around every corner. Questions about marine invertebrate vision abound. If some supposedly sightless organism only spawns at night when it’s dark, how does it know it’s dark, if it can’t see? Answers to this question and others are leading researchers to take another look at sight in the ocean. Before long, all my old standby sightless animals—sea urchins, sea anemones, sea cucumbers, sponges, and tunicates—may well be declared photosensitive. Perhaps, “To see or not to see?” is not the question after all. Perhaps, “To see well or to see just well enough?” is |
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One red, light-sensitive eyespot at the end of each sea star arm distinguishes the difference between light and dark. The six-rayed star uses this information to forage at night and remain protected under rocks and ledges during the day. If the sea star has 24 arms, it will have 24 eyes!