Members of Phylum Porifera ("pore-bearer") are more commonly known as sponges. The only member of subkingdom Parazoa, they are the least highly organized of the animals, lacking true tissues and tissue systems.
Classification, Phylogeny, and Evolution
Sponges were originally catgorized by Aristotle as plants; the organisms were not considered true animals at all until the latter half of the 18th century, when their feeding mechanism was first observed (3). Their heterotrophy and evidence of cellular organization places them in the animal kingdom.
Simple sponge fossils have been found in China that date back 570 million years. These fossils consist of a small (150-750 um) cell/spicule association, and are thought to be some of the first sponges (3). It is generally thought that these simple sponges evolved from colonial flagellated protists; in fact, the choanocytes very strikingly resemble certain choanoflagellates found in nature (5). Porifera are considered to be a very important, albeit simplistic, step between the single-celled organisms and the more complex tissue-oriented animals; this concept is supported with DNA evidence (4).
There are currently about 5000 identified species in the phylum. These species fall into three different classes, characterized mainly by the type and shape of their spicules: Calcarea or Calcispongiae (calcium-based spicules); Hexactinellida or Hyalospongiae (siliceous spicules); and Demospongiae (irregular spicules or no spicules).
Structure and Lifestyle
Sponges live aquatic lifestyles; most species are marine, but there are about 150 species of freshwater sponges. Sponges are also sessile, preferring to anchor themselves on solid surfaces (though there are some species that anchor on sandy ocean bottoms).
As previously mentioned, sponges lack tissue organization; in fact, they are little more than aggregations of specialized cells arranged in the most functional way possible. Therefore, while the different life functions are performed by different groups of cells, each is performed intracellularly, including absorption of minerals and nutrients, food uptake, digestion, and support. Gas exchange and waste excretion usually occurs by intracellular diffusion.
Most sponges are tubular in shape, or some variation on that theme. The outer layer of cells of a sponge are called the epidermal cells; these cells provide an outer support layer for the sponge. Specialized epidermal cells called porocytes form pores in the sponge that extend through the body of the sponge to the inner cavity; these porocytes regulate the flow of water into the sponge with contractions.
The inner cavity, or spongocoel (literally, "sponge space"), is lined with collar cells, or choanocytes. The "collar" of these cells is made of a ring of microvilli surrounding the single flagellum extending from the cell into the spongocoel. These cells produce a beating motion with their microvilli (and presumably the flagellum) which moves water through the spongocoel and out through the top of the sponge, called the osculum. This motion allows the choanocytes to bring nutrients into the sponge and expel wastes into the outflowing water. The choanocytes also perform ingestion of food; bacteria, plankton and algae are phagocytized (literally, "eaten by the cell") by the choanocytes.
Between the outer layer and the spongocoelic layer lies a gelatin-like layer called the mesohyl. Floating around in this layer are cells called amoeboid cells; these cells digest and transport food and nutrients to the rest of the sponge. Some of the amoeboid cells secrete spicules, which function in support and defense. Spicules are sharp, spiky spines made of calcium carbonate or silica, that function in structure and in defense of the sponge. Those members of Class Demospongiae that do not have spicules (and some that do) are support by a matrix of a substance called spongin, which is related to collagen.
Sponges are capable of reproducing asexually, through budding or breakage. Sponges are also usually hermaphroditic, meaning that they are capable of producing both eggs and sperm (although not necessarily at the same time). Certain amoeboid cells can develop into eggs or sperm; eggs are retained within the sponge, and sperm are released to find other sponges of the same species that harbor eggs.
These little critters show other unique characteristics as well. Sponges are capable of reassembling if their cellular organization is disrupted; cells can identify each other and replace themselves appropriately. Sponges come in all shapes and colors; some form symbiotic relationships with algae, giving them a food source and a color source. For all their simplicity, sponges are exceptionally ecologically successful, highly developed, and very elegant in their execution.
Natural Bath Sponges
Today, almost all sponges are synthetic; but we had to get the idea somewhere, right? Natural bath sponges are members of Class Demospongiae that do not have spicules, which is important if you don't want to shred your skin every time you take a bath. Very few bath sponges are still harvested today; those few that remain can be found off the shores of the Florida Keys and in the Caribbean.
- Solomon, E.P., L. Berg, and D. Martin. 2002. Biology, 6th edition. Brooks/Cole Thomson Learning, USA.