Different Types of Sponges

The sponges do not have a definite form and their bodies are delicate and filled with a soft substance known as mesohyl, resembling jellies. Being aquatic creatures, their body allows them to absorb fluids, water, oxygen and food, with ease.

An interesting thing to note about sponges is that they don't have a digestive, circulatory or nervous system. They extract the food out of the water entering their body through pores. Since sponges need some kind of a substrate, such as rocks to remain stable in the sea, they are sessile - sedentary organisms. There are more than 7000 sponge species and possibly more of them would be discovered in the future. The different forms of sponges are encrusting-sheets, mounds, tubes and upright-sheets.

Sponges primarily feed on bacteria, but in some cases microbes live inside their bodies as endosymbiont - an organism living in symbiosis with its host, and inside its body. Some sponges have also turned into carnivores, where the food availability is scarce.

Types of Sponges
The sponges are primarily classified on the basis of the characteristic structures, for example spicules, used by them to reinforce the mesohyl. The different types of sponges or classes thus formed in the process of reinforcing, are explained below.

Calcarea or Calcareous Sponges
As stated earlier, the classification in sponges is based on the reinforcing structures used. Here the structures in case of calcareous sponges are called as spicules. They form the skeleton of calcareous sponges and are made up of calcium carbonate. These sponges are just about 10cm in length, however, they encompass all the different types of body plans in sponges, viz. the

• Asconoid (Indefinite shape)
• Syconoid (Tubular body)
• Leuconoid (Flagellated chambers)

Demosponges
These are made of spicules, like the calcareous sponges; but, in this type spicules are formed of fibers produced from a protein called 'spongin'. These are the biggest of sponges and form 90% of the total number. Demosponges live, both in the warmer and colder parts of the sea. They reproduce sexually as well as asexually. They have a huge variety in their forms and are much more diversified than any other class of sponge. Asexual reproduction takes place by means of gemmules and budding. Gemmules are internal buds capable of development in favorable conditions, they are otherwise dormant. According to Mr. Gordon Love, a scientist working in the California University, demosponges came into existence as early as 635 million years ago, in the Neoproterozoic era.

Glass Sponges
These are not very common, they are mostly found at great depths in the oceans. Their body is cup-shaped, and spicules are made up of siliceous compounds. Glass sponges live for 15,000 years and are the longest living creatures on the earth. These are very sensitive to external stimuli and are able to react quickly, because of the electric impulse they generate.

There is another criteria to classify the glass sponges further - the ability to build reefs. Thus, the sponges are classified as,

• Reef Building Sponges: These sponges include the funnel-shaped Heterochone calyx or Finger Goblet Sponge, growing 1.5m in height. It is found in shades of yellow and white color. Other two species of reef-builders are Aphrocallistes vastus also known as Cloud Sponge and the Farrea occa. The unique characteristic of Cloud sponges is, its mitten-like outgrowth. These are funnel-shaped and 2m in height. The Farrea occa is the biggest of the three and is 15m in height.
• Non-reef Building Sponges: There are four species of non-reef builders, and are white in color with their height being about 50cm. Staurocalyptus dowlingi, Acanthascus platei, Acanthascus cactus and Rabdocalyptus dawsoni are the different non-reef builders.

These sponge species create different forms of reefs, thus increasing the biological diversity in the ocean. The reefs are beneficial in maintaining the population of different species of fishes and other marine animals. The sponges live a long life, but are slow in growth. Efforts are being taken worldwide to save sponges. Experts suggest that the areas inhabited by sponges, should be declared as marine protected areas (MPA). It is our collective responsibility to preserve this neglected, but important form of life, for the betterment of our own future.