Almost all of us are familiar with commercial sponge, used for various purposes, like cleaning. There are certain living sponges that are found in both sea water as well as freshwater. These living ones are not plants, but are classified as animals that belong to the phylum Porifera. The name of this phylum is derived from the pores on the body of the sponges, and it means pore bearer in Greek. This phylum is subdivided into three classes - Calcarea, Hexactinellida, and Demospongia. It is believed that, there are around 5000 to 10,000 species of sponges, and most of them are found in sea water. So, sponges are unique aquatic animals with some interesting characteristics.
Sponges are primarily found as a part of the marine biome; but, around 100 to 150 species can be found in freshwater. They may resemble plants, but are actually animals that are sessile (not capable of movements). Sponges are often found attached to rocks and coral reefs. You may find them in different forms. While some of them are tube-like and straight, some others have a fan-like body. Some are found as crusts over rocks. You may also find sponges in the shape of cups or vases, or like bushy plants and trees. There are some sponges that have finger-like growths on them. Even the colors vary with different types. The color of sponges ranges from white and beige, to shades of green, brown, yellow, red, purple, lavender, and blue. They may have a width of around one inch to more than 1.5 meters. While some of them are soft, some are really hard. They can be brittle or flexible.
- The body of sponges can be described as an arrangement of different types of cells. These cells do not form tissues, but are loosely arranged to form the body. Each type of cell has a specific function.
- While one end of the body of a sponge is attached to the substrate (like rocks), the other end is an opening, which is termed osculum. In short, sponges are asymmetrical (sometimes displays radial symmetry) hollow structures with a body wall.
- The body wall of a sponge consists of two thin layers that are separated with a thin gelatinous layer called mesohyl. The outer skin is made of epidermal cells. This layer has numerous pores that facilitate water to travel in and out of the body. The cells that are responsible for the formation of pores are called porocytes.
- After the outer layer with epidermal cells and porocytes, the gel-like inner layer called mesohyl exists. In this layer, there are certain cells, called amoebocytes that are responsible for transport of nutrients and digestion of food. They are also responsible for the formation of spicules, which are considered the skeletal fibers of this animal. They play an important role in the sexual reproduction of sponges.
- The innermost layer is made of collar cells or choanocytes that are flagellated. These cells use the flagella to form a continuous current of water inside the sponge. This helps in transporting oxygen and nutrients to all parts of the body. They aid in digestion of food as well as removal of waste, through the osculum.
- Sponges have specialized cells with the ability to change to other cell types. So, sponges are made of these specialized cells, and they do not have any head, trunk, bodily organs, or appendages. They are simple multicellular organisms.
- Sponges are filter feeders, who absorb nutrients as well as oxygen from the water, that enters its body through the pores. Usually, they feed on bacteria and food particles present in the water; but, some species have a symbiotic relation with certain microorganisms. These microorganisms that are capable of conducting photosynthesis produce more food and oxygen, that will be shared with the sponges. Some are carnivorous, and feed on small aquatic animals, like crustaceans.
- Sponges can reproduce both sexually and asexually. Even though, both male and female parts are found in the same individual, fertilization happens only between the sperm and ovum produced by different sponges. Even if the sperm released by a sponge enters the same organism through water, it will be discarded through the osculum. The sperms travel with water currents, enter other individuals, and fertilize the ova in mesohyl. After fertilization, the ciliated larvae swim and find a substrate for growth.
- In case of asexual reproduction, the sponge may undergo regeneration, in which a broken piece or even a single cell may develop to a new individual. Another method is budding, in which new sponge will be formed with a few cells on the exterior surface of the parent. This new sponge breaks off and grows to a new individual.
- Another sponge characteristic is the formation of a bud called gemmule, inside its body. This gemmule is tough and hardy, and if the sponge is killed in adverse conditions, the gemmule will develop into a new individual.