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The Sponge - enigma of nature    First published in the 'Nassau Guardian'.

With the exception of the single - celled animals, the Protozoa, the sponges are the simplest animals in existence today. In fact, they are so unusual that scientists for hundreds of years were uncertain exactly what they were.

As long as 2,000 years ago Aristotle recognised the nature of sponges, but only in the last 200 years have all scientists come to accept this view. Like sea fans and other soft corals, sponges were thought to be plants by most people, a few even thought that they were solidified sea foam or the homes of marine worms!

In 1766 John Ellis discovered that they eject currents of water and must therefore be animals, however, as recently as 1841 some scientists were still arguing before the Royal Society that sponges were plants. Even today, in this age of enlightenment, many people do not really know what sponges are, considering them only in the context of the bathtub.

Much early misconception arose from the fact that studies were carried out on freshwater sponges, many of which have within their tissues single celled plants giving them a green colouration. Their shape is also, in many cases, plant-like. Despite the fact that sponges have in some cases muscle 'fibres' and even nerve cells, some scientists still do not think of them as true animals but place them in a separate sub-kingdom, the Parazoa.
Whatever their true relationship with the rest of the Animal Kingdom, sponges are nonetheless successful in their own right. In the Bahamian seas, sponges are one of the most dominant groups in terms of total mass (Biomass), probably running a close second to the corals.

All animals in the sea are ultimately dependant upon plants, for only plants can make organic food from simple substances like carbon dioxide and water by using the energy of the sun. The majority of such plants in the sea are the tiny single-celled members of the plankton, drifting by their countless millions in the upper layers of the sea. These provide food for the filter feeders like the corals and the sponges.

Sponges vary greatly in size, shape and structure. The largest sponge is the Loggerhead sponge, Spheciospongia vesparium, which may grow to the size of a small barrel. Such a large sponge may contain hundreds of tiny animals living within its tubes and chambers, including worms, shrimps, molluscs, starfish and even small fish.

Structurally, the simplest sponge is a tube with its upper end open. Hundreds of small pores in the walls of this tube lead in to what are known as flagellated chambers. Each of these chambers is lined by numerous tiny cells, each bearing a long whip which beats and creates the water current passing in toward the central chamber of the sponge. These cells also remove small food particles from the water, which are passed on to other cells.

Many of the cells within a sponge are capable of surviving independently. One classic experiment involves forcing a sponge through a fine silk, so breaking it up into the smallest parts. After this was done the sponge cells re-aggregated on the other side of the silk to produce new colonies.

Reproduction, as in other simple organisms, is by various methods. Usually a larva is released which swims for some 24 hours before settling on the bottom to begin the formation of a new sponge colony.

The skeleton of a sponge may consist of a complicated network of fibres, as in the commercial sponges, or of many small spicules embedded in the tissues. These spicules may be made of silica or of calcium carbonate, and may assume the form of forks, anchors, needles stars or a variety of other shapes. 

Frequently the identity of the sponge may only be determined by microscopic examination of these spicules. The common names of sponges are usually fairly descriptive. 

For instance, there is the bleeding sponge which gives off a blood red liquid when squeezed, the stinker sponge, which has a strong unpleasant odour; and the Do-Not-Touch-Me Sponge whose Latin specific name 'nonlitangere' means precisely that, on account of the fact that when handled it may cause severe burning and blistering. Sponges have a worldwide distribution, being found in both fresh and salt water, and even in lakes in the craters of extinct volcanoes. Nowhere, however, do the sponges attain the number and variety that they do here in the shallow warm waters of the Bahamas.


R. Attrill 2006