Life on the Edge

Picky Eaters
September 2008
by Nancy Sefton

You and I (unless you’re a vegetarian) are “omnivores”. That means we chow down on both plants (love that broccoli!) and animal flesh (Col. Saunders thanks you!), whatever’s handy and tastes good.

But the underwater world has some very picky eaters, and different groups of creatures have different ways of ingesting their favorite foods. It’s easy to assume that all marine animals simply “grab it and gulp it”. But not all sea animals make their living with big mouths and sharp teeth.

One group, for instance, has earned the name “filter feeders”. These include clams, mussels, sponges, sea cucumbers and creatures with feathery appendages. They’re all designed to strain tiny drifting plants and animals (plankton) from the water, and they’re very good at it. The barnacle has feathery legs that emerge from the shell to act like a sieve. Clams and mussels draw water in through their siphons, filter out the goodies, and send the strained water back out again.

Clams filter plankton by pumping water through two siphons.

Marine snails, on the other hand, use their tongues; these have a rough texture like a nail file, capable of scraping seaweed and slurping up all that healthy vegetable matter. These mollusks are classified as herbivores, or eaters of plants. (If you ARE a vegetarian, you’re a herbivore too, but by choice. You weren’t born to it!)

Some marine animals can’t dash or creep around to get their food; they’re stuck in one place, perhaps for life. So the food comes to them. Sea anemones (and their jellyfish cousins) simply extend their tentacles into the water column and zap! –any passing organism accidentally touching a tentacle gets hit by a poison dart. Anemone tentacles are full of these microscopic weapons, and they’re deadly. The poison stuns the poor victim which is then drawn down into the anemone’s mouth. Gruesome but efficient.

Sea anemones snare their pray with stinging tentacles.

However, first prize for “gross eating habits” goes to the sea stars. Just lying prone on a rock, they may not look like predatory carnivores, but they can make short work of any bivalve in the neighborhood. An ochre star, for example, can wrap itself around a large mussel and pull the shells apart using its tube feet, which have suction cups at the ends. Once the mussel’s shell is pried partially open, the star regurgitates its own stomach and inserts it between the mussel shells in order to digest the soft parts. Ugh. But those tube feet get the job done. Not many sea stars go hungry.

“Deposit feeders” are the sanitary engineers of the marine world. Crabs, shrimps, lobsters and some snails keep the sea bottom clean by eating whatever falls from above, including dead and decaying organic matter.

Crabs feed on dead and decaying matter on the bottom.

So whether they’re herbivores or carnivores, or a combination of both (like us), marine creatures can be categorized according to the way they eat. Each group fills a niche in the oceanic food web, and as long as we humans don’t interfere with the way nature’s restaurant is managed, everybody leaves the table satisfied.