A Salute to Sponges that don't wear Square Pants
Posted By Abigail Smigel on 15 December 2004
Sponges are the stars of the phylum Porifera (this having no bearing on the fact that they are the sole members). The body structures of sponges are characterized by numerous holes, known as pores, for which they are named. The Latin word Porifera translates to 'pore-bearing'. The approximately 5,000 species occur in all seas and oceans around the world. Though seemingly benign critters, sponges significantly contribute to water clarity and quality. Evolving approximately 550 million years ago, sponges are the most primitive of all multicellular animals.
Since the regenerative abilities are limited to one species, all other species of sponges posses necessary structural components. Spicules, tiny threads of silica, and spongin, stringy fibers of protein, are the two structural components. These two components combine to create a fibrous, internal skeleton for the body structure. The internal structural components add strength and aid in maintaining their shape. One large opening for exiting water flow is also a characteristic of the body structures of sponges. Though not all species are restricted to just one opening, most have a sole excurrent opening. The large opening of upright sponges is referred to as an osculum.
Most sponges are filter feeders. They feed by creating a current through their body structures. The surface of a sponge's body is covered by a skin, one cell thick. This skin is penetrated by numerous small pores and a few large openings. These are the entrances and exits, respectively, for a complex system of canals and chambers through which the sponge pumps a current of water. Water flowing through sponges provides food and oxygen, as well as a means for waste removal. This flow is actively generated by the beating of numerous small and hair-like flagella that line the interior canal walls of sponges. The volume of water passing through a sponge can be enormous, up to 20,000 times its body volume in a single 24-hour period. In addition to filter feeding, several sponges contain the symbiotic algae characteristic of corals, known as zooxanthellae, living in their tissues to provide the sponge with nutrients as a result of photosynthesis.
Variation in separate species of sponges occurs in terms of their shape, size and color. Typical body structure shapes include baskets, cups, tubes, vases, barrels, bowls, flattened and encrusting. Both color and shape are useful tools for distinguishing between various species of sponges. However, both characteristics are occasionally misleading as individual sponges vary even within a species. Therefore, a positive identification underwater based on color and shape alone is rarely accurate.
While most organisms are deterred by the noxious components of most sponge species, others are immune and feed on the sponges for nutrition. There are also some organisms that actually utilize the toxic components to their advantage, nudibranches and bristleworms are two examples. The noxious flavor is an important defense for sponges, as these sessile invertebrates do not have the ability to flee (at a reasonable speed) from potential predators.
Sponges reproduce by both asexual and sexual means. The species that reproduce sexually are typically hermaphroditic and produce eggs and sperm at different times. When sperm is broadcast into the water column, often it is so concentrated that the sponge appears to be 'smoking'. The sperm is released into the water column in the chance that it will be pulled through the canal system of an ovulating sponge of the same species.
Asexual sponge reproduction occurs by one of two ways, dividing into fragments that eventually develop into whole sponges or budding. In one form of budding, the buds form on the parent sponge, break off with the passing current and float away to propagate a new area. In other species, the buds remain attached to the parent sponge and give rise to the formation of a colony of sponges.
Though not all sponges live in a pineapple under the sea, or have a half hour of nautical nonsense devoted to them, they do have a bit of distinctiveness that somewhat sets them apart from the typical household sponge. Hope you absorbed all that. Yuk yuk yuk.
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