When you go into a store to purchase a package of sponges, chances are they are made of cellulose. These commercially produced sponges have replaced the real sponges that were once collected by divers and sold. The real sponges are very primitive animals in the phylum Porifera which means “pore-bearing.” Though considered primitive by most zoologists, there are about 10,000 different species living today.
A sponge is a simple animal. It has no organs or organ systems. There are no digestive, excretory, nervous, or muscular systems. Instead, a typical sponge has a body wall with three layers. The first is the epidermis or outside layer that contains many pores. Internal to this layer is the mesoglea which contains a matrix of skeletal-like particles known as spicules, though there are exceptions. Finally there is an inner layer composed of specialized cells called choanocytes or collar cells which beat the water setting up small currents that flow into the sponge through the pores. Small food particles get trapped and are digested. Thus, a sponge’s morphology can best be described as at a cellular level of organization. They are also considered filter-feeders because of their method of obtaining nutrition.
There are three basic types of sponges. The first and simplest is the asconoid sponge. These animals are characterized by a central cavity with a single opening to the outside known as an osculum. They are usually very small sponges, just a few centimeters high. The second type is the syconoid sponge. These sponges demonstrate an increasing amount of folding, therefore increasing the surface area. This leads to improved feeding efficiency. Finally, there are the leuconoid sponges which are the largest and most common. There is a much greater amount of body wall folding in leuconoid sponges.
Sponges may reproduce asexually and sexually. In asexual reproduction a sponge can reproduce by budding off a new segment, by regenerating a whole sponge from small fragments, or by producing small cyst-like gemmules which are highly resistant to environmental stress conditions. The regeneration process is fascinating. A small piece of a sponge such as that of the redbeard sponge (Microciona prolifera) can be ground up into a new sponge. In sexual reproduction most sponges are hermaphroditic, that is they contain both male and female reproductive structures. Other sponges have separate sexes.
Sponges are mostly marine, yet there are several freshwater species. Several cold-water sponges can be found washed up on Cape Cod beaches, and on pilings, shells, and wharves. I’ve already mentioned the redbeard sponge. Oyster shells are often riddled with holes from the boring sponge (Cliona celata). Deadman’s fingers (Haliclona occulata) can be found on the beach after a storm. There are several less conspicuous, but common, species in this area. It may be difficult to imagine the sponges as animals, but they are highly successful in evolution and very well adapted to their environment.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.