Sunday, October 13, 2002		Nature

How aquatic animals sieve food from water
Nutan Shukla

OCEANS are full of nutrients in the form of chemical substances washed into them by rivers. Animals cannot make direct use of all these substances. But in the upper layers of the sea, there live millions of single-celled plant organisms which can photosynthesise and build the nutrients into proteins, fats and carbohydrates. There are several kinds of these organisms, which are called ‘phytoplankton’. Despite their small size, these creatures are the main source of food for other larger creatures in the plankton. The phytoplankton are also known as the ‘pasture of the sea’.

There are many small animals in the plankton; some of the young creatures which grow to be adults who live elsewhere, feed on the phytoplankton. The whole collection of small animals and tiny plants are an almost limitless source of food for animals that can collect them. But to be able to sieve these forms of food out of the water requires special adaptation quite different from the jaws and teeth of animals that take food in large pieces. For example, the whalebone whales, unlike sperm whales and porpoises, have no teeth. In their place, they have horny strips with frayed edges hanging down the sides of their mouths. After taking a mouthful of seawater, the whale uses its tongue like a piston to drive water out of its mouth through the whalebone sieve which traps small shrimp-like creatures called krill on the inner sides of the strips. Krill occur in swarms in the Antarctic seas and provide the food for these huge mammals. To provide enough energy and proteins, the intake of food must be as continuous as in the feeding habits of many land herbivores.

Filter-feeding involves straining food from large quantities of water, and is found only in aquatic animals. Since most invertebrates live in water, filter-feeding is well developed in many of them. A good example is the peacock worm. It builds an erect tube which stands six to twelve inches high on the seabed. The worm lives inside the tube, moving to the top to display a fan of tentacular filaments which give the worm its name. Each filament sticks out radically from the head-end of the worm and the worms use them to catch the tiny food particles which swim by or come raining down on them.

The filaments of he peacock worm are arranged like a funnel, with the mouth of the worm at its base. Each filament has a row of tiny branches called ‘pinnules’ sticking out on two sides. The rows of pinnules on neighbouring filaments interlock near the bases, but higher up they are free from each other. hair-like ‘cilia’, are borne on these pinnules and produce a water current which carried food to the worm. The cilia beat towards the tips of the pinnules, bringing water into the funnel. Other cilia on the upper and lower sides of the pinnules beat cross-ways at right angles to the cilia on the outside. This action creates eddies in the water on the inner side of each pinnule and also deposits food particles inside the worm’s ‘funnel’. More cilia, beating towards the base of the funnel, convey food particles downwards and towards the mouth.

Notice that this mechanism catches particles and conveys them regardless of their size or of their usefulness as food. Some of the particles may be silt, without any food value at all, and others may be small diatoms and so on which can be used as food. It is typical of filter-feeding that at first the creature makes a random collection of particles. There then follows a period of sorting when some particles are retained and others are rejected. Generally, the choice is based on size, and this is the case with the peacock worm which retains only the smaller particles as food. The largest particles are rejected, and those of intermediate size used as building material for the worm’s tube.