by Doug Pemberton

The learning process involved in understanding our oceans and the life that inhabits them is painfully slow. Our comprehension of how even the simplest organisms work is usually the result of research by dozens, even hundreds of trained scientists and interested observers over an equal number of years. Case in point is the lowly sponge, one of the simplest organisms in the evolution of life in the oceans.

Over 2500 species of sponges are scattered throughout the temperate and tropical seas of the world, most living in relative obscurity while others have held historical importance. Sponges were once highly prized as a trade item, in fact it has been debated that some ancient civilizations actually rose and fell on the sponge trade. They were eminently regarded for their medicinal properties due to a high content of iodine and, for centuries, they have been a household necessity. But, as indispensable and widely used as they were, nobody seemed to know exactly what a sponge was.

Over 2000 years ago Aristotle claimed that sponges were animals, but this claim was discredited since animals had to possess the power of movement to be classified as such, and sponges are sessile. Since then, prominent scientists have come forth and proclaimed them to be everything from plants or homes of marine worms to the work of 'some kind of sea insect', and solidified sea foam. Not until the early 19th century was their rightful place in the Animal Kingdom confirmed.

Sponges are members of the phylum Porifera and have no mouth, nervous system or digestive sac. They are simply a giant filter composed of an intricate network of disparate cells, each with a special function that benefits the whole animal. The cells are meshed together, permeated by millions of pores called ostia (singular: ostium) through which water is drawn into a complex honeycomb of canals and chambers. Lining the interior of the sponge, millions of choanocytes or collar cells whip large flagella, creating the current that draws water into the sponge and circulates it before it is expelled through one or more large openings called osculum. Large sponges are able to circulate hundreds and even thousands of litres of water in a single day, and it has been calculated that a sponge must circulate one ton of water for a gain of one ounce in body weight.

The collar cells are also responsible for food gathering and the absorption of oxygen while other cells within the walls of the sponge disperse nutrients, collect waste for disposal or produce the silica or calcareous crystals and spicules that give the sponge it's semi-rigidity. The sponge is also protected by a network of defensive cells that house spear-like nematocysts capable of deterring most predators.

Most sponges are hermaphroditic and reproduce sexually while others reproduce by budding. The egg is fertilized inside the sponge and divers may see sponges 'smoking' at certain times of the year as clouds of motile larvae are expelled.

Of the 2500 species of sponge distributed throughout all temperate and tropical oceans of the world, only a few are harvestable, and this practice has all but disappeared with the introduction of synthetics. Today, the value of sponges comes more from their sheer beauty and diversity. Their variety of form, size, shape and colour test our imagination. Carpeting sponges lay down thick blankets of crimson or bright yellow, while others may be large, convoluted and as freeformed as clouds. And some species resemble familiar items such as vases, tennis balls, trumpets or barrels. A few are tiny and hardly noticeable while others, such as the huge barrel sponges of the Caribbean, command the reef. But for all their size they are fragile and break easily and, although sponges have some amazing powers of regeneration, most are very slow growers. Care should always be exercised when diving near sponges.

Man is not alone in his use of the sponge. Depending on the species, the osculum may be microscopic, or large enough to house other species of life. Large cloud sponges and boot sponges are living condominiums, home and haven to an assortment of rockfish, warbonnets, sculpins, crabs and shrimp.

Sponges also form symbiotic relationships with other animals. Swimming scallops, and some sedentary rock scallops often sport a thick layer of sponge on their shells. The sponge's porous nature and nematocysts may hinder potential predators such as sea stars from getting a good grip on the scallop. The swimming scallop in turn is also a mobile, filter feeder, able to seek out prime areas to feed.

A few crabs also have an affinity for sponge. The sponge hermit crab, either by destiny or desire, attracts the growth of a specific sponge on its shell. The sponge grows and eventually eats away most of the shell but secretions by the crab provide a smooth, hard barrier between itself and the sponge, which eventually becomes the crab's home. This relationship provides the hermit crab with defense and camouflage and the sponge with a mobile base. Over time though, the sponge grows faster than the crab and I have seen moderate sized hermit crabs lugging around gargantuan pieces of sponge.

Sponges have few predators but the most notorious is the nudibranch. They are immune to the stinging defenses of the sponge, and on occasion we may find a sponge hermit crab that is literally being eaten out of house and home by a hitchhiking nudibranch.

Decorator crabs may also use a growth of sponge on their backs, along with other organisms, in order to blend in with their surroundings.

Because of its basic simplicity, one might think of the lowly sponge as an evolutionary failure, but much more complex animals have fallen by the wayside while this successful experiment in cooperative living is abundant and widespread.