Aquatic Life: Sponges

Some aquatic organisms can tell you about oxygen levels, some are indicators of major pollution; all have a particular role that they play in the ecology of aquatic systems. The aquatic sponges vary in all shapes and sizes. Some are hard and rigid, while others are soft and pliable. Sponges are living organisms that are filter feeders. They do not breathe but do require oxygenated water to live. Sponges have a series of holes and pores. The pores, called ostia, take in water and microscopic organisms. The food is trapped by a series of cells that carry out digestion and the nutrients are then transported to other cells that carry out other functions associated with growth and reproduction. The water is then removed via the larger holes, called oscula. While most sponge species are marine, there are 150 freshwater species in North America, some of which are found in North Carolina streams and lakes. Sponges reproduce both sexually and asexually. While interesting, freshwater sponges are not particularly abundant. They are found in clear water attached to vertical surfaces such as docks, dams, and canal walls. They will also attach to boulders and logs. Very few things prey on sponges so their overall contribution to stream productivity is very low.

The hydroids are another interesting group of organisms. While rare in freshwater ecosystems, they do occur and like sponges, contribute very little to the overall productivity of the benthic community. Hydroids are very cool. Hydroids are in a group of organisms known as Cnidarians (pronounced NI-as in eye-DAR-IANS). Some that most folks would be familiar with are corals, jellyfish, and Portuguese Man-o-War. Cnidarians typically have two life stages: a medusa stage and a polyp stage. Some have only one (either polyp or medusa). The polyp stage is usually the adult stage and looks like a tree stump with branches. The stump is like an anchor that allows the organism to attach to things. The medusa stage is usually a larval stage and is what most people would call the jellyfish stage, although this is somewhat of a misnomer. Cnidarians can reproduce either sexually or asexually. All cnidarians have tentacles that possess nematocysts or stinging cells with which they attack and capture their prey.

The most common freshwater cnidarian is hydra that grows as a polyp. Hydra can only barely be seen with the naked eye but are incredible to watch under a microscope. They feed primarily on one-celled organisms, water fleas, and sometimes larvae of mosquitoes and midges (gnats). As the prey swims by, it gets tangled in the tentacles, stunned by the stinging cells, and then ingested by the hydra. A less common sighting is freshwater jellyfish. They look like normal jellyfish only they are about the size of a quarter. They actually float around and eat whatever happens to run into them just like marine jellyfish. Jellyfish have been seen in Mirror Lake in the Riverbend Development. They are of course harmless to humans. Hydroids are not necessarily indicators of water quality because they can live in some pretty polluted situations, as long as there are plenty of one-celled critters to eat.