Phylum Proifera

A diver surveying orange finger sponges on a section of Grey's Reef National Marine Sanctuary off the Georgia coast.

One of the uncommon freshwater sponges

The name Porifera is Latin in origin and means "pore bearing." The use of this term for the phylum name is quite appropriate as these filter feeders are, indeed, covered in pores. They are aquatic, with the vast majority being marine. These are considered to be the simplest of animals; so simple in fact that they don't even have organs. They are quite unusual among animals since they lack symmetry, being amorphous with little rhyme or reason to their shape. Despite their simplicity, these animals are capable of some rather interesting feats. One experiment shows a sponge's ability to differentiate between its own cells and cells of another sponge. This was true even if the sponges were of the same species. Also, if the sponge is hacked apart or even put in a blender it is quite capable of reforming itself into a single sponge. Sponges play an important part in aquatic communities and are a common habitat for other animals, as well as other life such as bacteria. Though many sponges are dull, others can be quite brilliant in color.

A sponge with many oscula

These organisms have several specialized cell types. Choanocytes, also known as collar cells, are flagellated cells which move water through the body of the sponge and collect what food particles it can. Flat pinacocytes form the closest thing sponges have to skin: the pinacoderm. Archaeocytes are amoeba-like and digest food and perform whatever other task the sponge needs. This includes differentiating into other cell types, such as spongocytes, sclerocytes, and even sperm and egg. Porocytes are also seen, which are tube-shaped cells that surround the pores which serve as the entry point of water into the sponge. Oscula (singular: osculum), which are usually surrounded by pinacoderm rather than porocytes, serve as the exit point for the current of water created by the sponge.

A microscope view of silica spicules

Classification within the phylum is based on structural composition and morphology. All sponges produce protein fibers called spongin (produced by spongocytes), but not all produce the harder elements known as spicules (produced by sclerocytes). Spicules can be made up of either calcium carbonate or silica, and vary in shape and size. Some spicules are simple rods, while others may have several sections or be covered in tiny spikes. Spicules form an important protective layer in the dormant form that some sponges are able to produce, known as gemmules.

The three sponge body types

The three body types seen in sponges, listed in order of increasing complexity, are asconoid, syconoid,  and leuconoid. The addition of more complex twists and turns to the path that water has to travel through the sponge allows for much more efficient filtration. This is likely why the simple asconoid sponges are also the smallest and the complex leuconoid sponges are the largest. A large body requires more food, and more efficient filtering allows for the collection of more food.

Hexactinellid Venus's flower basket

The three classes in the Phylum Proifera are Class Calcarea, Class Demospongiae, and Class Hexactinellida. Calcarea includes all of the species with calcium carbonate spicules, as well as all of the species with the asconoid body type along with some synconoid and leuconoid sponges. Demospongiae is the largest group, consisting of species that are mostly leuconoid. Not all of the species in this class have spicules, and it is the only class to include freshwater species. The class name means "people's sponge" since bath sponges are included in the grouping. This group also contains the strange cladorhizid sponges which do not resemble other sponges and feed as carnivores. Hexactinellida is composed of species that always have complex silica spicules and as such they are also known as "glass sponges." They have many unique properties that have made some propose putting them in their own phylum.

It is commonly assumed that sponges form what could be called an "evolutionary dead end," since there are not any animals that appear to have come from sponges. Despite this, there is some molecular evidence that suggests the sponges with calcium carbonate spicules may have given rise to other animals.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under a Creative Commons license or are copyright free: one, two, three, four, five, six.