Sponges

Sponges are members of the phylum Porifera. They are a group of extremely primitive multicellular organisms characterized by the lack of proper. All members of this phylum live permanently attached to surfaces such as rocks, corals, or shells. More than 10,000 species of sponges have been described. Although some species occur in freshwater, the vast majority are marine, living mainly in shallow tropical waters. A wide range of sponge body forms occur and these are characterized by both shape and composition: some are tall, extending well into the water column, while others are low encrusting forms that spread out over a surface. Others, such as the Venus flower basket (Euplectella sp.), are tall and comprise an intricately-formed latticework arrangement, while many leuconoid sponges are goblet shaped. All sponges have a skeleton, which provides a framework that supports the animal. Skeletons may be composed of either calcium carbonate, silica or a softer spongy material known as spongin. In all species the skeleton is made up of a complex arrangement of spicules, which are spiny strengthening rods with a crystalline appearance. The spicules are an important character for classification of sponge species.

In cross section, most sponges consist of a convoluted outer wall that is liberally dotted with pores or openings of different sizes. These holes allow the passage of water into the central part of the body, called the atrium or spongocoel. Although water enters the body through a large number of openings, it always leaves through a single opening, the osculum. Some species, like the asconid sponges, are usually small and have a simple skeleton with a relatively large atrium. Others, however, have developed highly convoluted skeletons that not only maximize water intake, but also allows for a maximum amount of oxygen and food particles to be absorbed. Each of the individual chambers in the spongocoel is lined with specialized collar cells or choanocytes, which consist of a flagellum encircled by a collar. These cells are responsible for producing the current of water through the sponge.

Sponges rely on large volumes of water passing through their bodies every day for food supply, gas exchange and waste removal. All sponges feed by filtering tiny plankton from the water current. This same water also provides the animals with a continuous supply of oxygen and removes all body wastes as it leaves the sponge. In some ways, a sponge resembles a powerful water pump, drawing in water through its numerous pores and passing it through the body. Some of the

larger species have been estimated to pump more than 5.3 gal (20 l) of water per day. The regular current is assured by countless numbers of tiny flagella that line the many chambers throughout the body wall.

Sponges reproduce both sexually and asexually. In the latter, the simplest manner of producing new offspring is through the process known as branching or budding off, whereby the parent sponge produces a large number of tiny cells called gemmules, each of which is capable of developing into a new sponge. A gemmule consists of a mass of food-filled cells surrounded by a protective coat strengthened by spicules. In such a state the cells are able to withstand periods of drought or food scarcity; when conditions improve once again the cells become active, break through the outer coat, and develop into a new sponge. Other forms of asexual reproduction are similar to the production of runners in grasses. For example, Leucosolenia sp. sprouts horizontal branches which spread out over nearby rocks and give rise to a large colony of upright, vase-shaped individuals.

The process of sexual reproduction involves the production of large numbers of male sperm cells that are often released in dense clouds. As these gametes are transported by the water currents, some enter other sponges of the same species. Here they are trapped by the choanocytes and transported to the special egg chambers where fertilization occurs. The fertilized egg then goes through a process of division and transformation, developing eventually into a flagellated larvae. There are two main types of larvae in sponges; the amphiblastula and the parenchymula. Once developed, the larvae is released from the parent sponge and carried away by the water current. Eventually it settles out of the water column onto a suitable substrate, where it metamorphoses into an adult sponge form.

In their natural environment, sponges face a wide range of predators. Fish and sea slugs feed on a wide range of marine sponges. Some of the larger species have been harvested by humans for resale as ornamental souvenirs, while in some parts of the world the large sponge species have been collected for domestic purposes, the skeletons being used as bath sponges. Sponge fishing is a major industry in many countries.

As they are dependent on clean, clear water, many sponges suffer as a result of sedimentation caused by land-based activities such as agriculture and deforestation. Coastal development often results in high levels of particulate runoff after rainfalls, clogging up pores in the sponges. Aquatic and terrestrial-based pollutants also represent a threat to these species.

David Stone