Filter Feeders and Anemones

By far the majority of individual marine invertebrates on the coral reef feed on microscopic prey borne on the ocean currents, sifting it from the water with a variety of net- and mesh-like body parts. These filter-feeding animals are usually sessile, or at the least slow moving, so they rely on their food to come to them, rather than the other way around.

Which invertebrates are filter feeders?

As well as the coral polyps, animals such as tubeworms, basket stars, clams, sponges, and even some kinds of crabs feed by filtering plankton from the water as it moves past them. Most of these animals feed at night, thereby avoiding the risk of exposing vulnerable body parts during the day, when so many fish predators are active. By concentrating their foraging activity into the night-time period, filter feeders can also benefit from the migration of unimaginable numbers of zooplankton as they rise to the surface waters at night to feed on microscopic plants.

How do filter feeders capture their prey?

A prerequisite for a filter feeder is to be situated in at least some water current so that it can feed on the tiny animals that flow past it as if on a conveyor belt After this, there are a few different strategies, Filter-feeding crabs have claws like mesh baskets that they sweep through the water Anything they catch is then directed towards the mouth.

Other animals, such as sea squirts and sponges, have millions of tiny hairs, known as cilia, which beat in waves and increase the flow of water and plankton into them. This food is either directed straight into the mouth or caught in a mucus-lined trap that catches all the particles in the water before the animal ingests them, like fly paper.

The idea of a sticky trap is also used by sea cucumbers, which extend branched and slightly sticky tentacles into the current to capture their food. However; not all sea cucumbers feed like this; some push their feeding tentacles out across the substrate to feed on organic particles that sink out of the water column.

Bivalves also feed by filtering the water they draw into themselves to breathe. Like corals, giant clams have symbiotic algae living in the exposed portion of their mantle, which also supply them with food. Some sponge species enter the same relationship with algae to help them satisfy all their nutritional requirements.

How do polyps and anemones deal with larger prey?

Plankton is a term that describes a huge variety of small, unrelated animals and plants. They vary enormously in size; some zooplankton may be hundreds of times larger than phytoplankton. Larger members of the plankton are more nutritious, but also more mobile and likely to resist capture by basic mechanical means. Such prey may need to be subdued, and anemones and their relatives achieve this using nematocysts. When
triggered, these cells release tiny harpoonlike structures that are attached to the anemone or polyp by threads. They drive into the flesh of the prey, spinning counterclockwise to go deeper and delivering toxins to paralyze.

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