Sponges are the simplest animals. A sponge has no nerves, bones, or other tissues and no organs, such as a stomach or heart. The body of a sponge functions a bit like a loose colony of cells, where each cell is capable of living independently. The basic anatomy of a sponge is elementary: Its body is made up of two layers sandwiched around a jellylike matrix called the mesoglea.
As shown in Figure below, the outermost cell layer, the epidermis, acts as a protective skin, while the inner layer, or gastrodermis, collects and processes food. The gastrodermis contains specialized collar cells, also called chaonocytes, that are equipped with flagella. The mesoglea is interspersed with small skeletal needles termed spicules. Made of calcium carbonate, spicules look like slivers of glass when viewed under the microscope. Some species lack spicules, possessing instead fibers made of tough, rubbery protein called spongin. The job of both types of fibers is to brace up and support the sponge. Amoebalike cells move around in the matrix carrying out a variety of jobs, including arranging spicules, collecting and delivering food, and removing waste.
Sponges grow well in the unpolluted, shallow waters of the reef where they take on an endless array of shapes. Some have irregular forms, while others are shaped like flowers, vases, branches, or leaves. Most attach to the reef, a rock, or other hard surface. Sponges compete fiercely with other organisms for space. If a section of coral is damaged by a predator, a young sponge may settle in and take over the spot before the coral can heal itself. Unlike many of the algae, sponges also thrive in the very deep sections of the reef where
fewer organisms are found. Since they do not require sunlight, water that is too deep for light to penetrate is not a problem for sponges.
Sponges get their food by filter-feeding, a method of trapping tiny food particles that are suspended in the water. To ensure that plenty of food-laden water travels past them, chaonocytes inside the sponge lash their flagella back and forth. This action draws seawater through the thousands of tiny pores in the epidermis and into the gastrovascular cavity. Food particles suspended in the water adhere to the chaonocytes, which absorb the particles and digest them.
Amoebalike cells then pick up the molecules of nutrients produced by digestion and carry them to all of the sponge cells. The water, minus its load of food, travels out of the sponge through the osculum, a large opening that works very much like the chimney in a fireplace.
To meet its nutritional needs, a sponge takes in a tremendous amount of water, handling up to five times its own volume every minute. Studies show that about 60 percent of the plankton around a reef is consumed by sponges. Much of what these animals eat is too small to be used by other filter feeders and would simply be lost to the ecosystem. As a sponge metabolizes all of this food, it produces and excretes waste products that serve as nutrient-rich fertilizers for many of the other reef organisms.
Sponges have two distinctly different phases of life. In the adult form, they are sessile, or attached to one place. When it is time to reproduce, some of the cells in each adult sponge transform into eggs and motile sperm. The sperm are released into water so that they can travel to another sponge where they are captured by collar cells and transported to the waiting eggs. The eggs and sperm unite to produce zygotes.
In some species, both eggs and sperm are released, and they unite in the water instead of inside a sponge. In either case, the zygotes develop into flagellated larvae. After swimming in the plankton for a while, larvae settle and begin growing. Sponges can also reproduce asexually. Small sponges grow from the base of an adult sponge by budding, a process in which a new organism grows from the body wall of the adult. Buds eventually break off and attach somewhere else. If a sponge is torn apart by waves, each part can develop into a new organism.
Because adult sponges are fixed in one place, they cannot run for safety when threatened by predators. For this reason, many sponges produce chemicals that discourage other animals from eating them. They also receive protection from their needle-like spicules that deliver painful sticks to nibblers.
Many species of sponges avoid predators altogether by growing in crevices of coral where nothing can reach them. A common and highly destructive type of reef sponge, Cliona, is capable of boring holes into the limestone skeletons of coral, causing the death of the coral animals inside. Cliona does not occupy the coral to feed from it; rather, its purpose is to live in the coral’s space. Once established, these sponges can spread across a reef until all of the coral animals are dead. Cliona and other boring sponges are responsible for much of the bioerosion on reefs.
Other sponges play roles in helping build the coral reef community and providing shelter for several types of animals. For many small reef residents, a sponge is an ideal hangout. As predators avoid sponges that are armed with foul chemicals, any houseguest is also relatively safe. And because a sponge maintains a steady stream of food-laden water traveling through its body, the guest receives a constant supply of oxygen and nutrients.
The reef supports a variety of sponges, several of which resemble tubes or vases, as those shown in the upper color insert on page C-1. The giant tube sponge (Aplysina lacunosa) may be yellow, blue, or purple and usually grows in water 65 to 165 feet (20 and 50 m) deep. It forms long, hollow cylinders that are about 3 feet (1 m) tall and 3.9 inches (10 cm) in diameter. The strawberry vase sponge (Mycale laxissima), covered with fleshy, thick-walled cylinders that look like tiny urns, is usually less than 8 inches (20 cm) tall and found in waters below 33 feet (10 m). The beautiful iridescent tube sponge (Spinosella plicifera) gives off a glow in daylight.