Sponges are one of the simplest multicellular organisms found in marine and freshwater environments worldwide. Sponges exhibit unique structures and feeding habits and are considered an independent branch in the early evolution of animals.
Morphology and Structure
The appearance of sponge varies greatly, with forms such as block, cushion, tube, goblet, sphere and branch. Some of them are radially symmetrical, but most are asymmetrical. Sponges are the oldest and simplest multicellular animals. They lack tissues, organs and nervous system. Instead, they are sessile aquatic animals and rely on a few different cells to perform life activities.
They consist of a central cavity called spongocoel. The outer surface of cavity is covered by closely packed epidermal cells, and the interior contains numerous flagellated choanocytes or collar cells. In between, there is a gelatinous region called mesohyl, which contains amoebocytes and skeleton. Many small pores perforate the cavity and serve as water channels. A collar cell has a flagellum surrounded by a collar-like structure and are responsible for filter-feeding. When they wave their flagella, water is drawn into the sponge cavity through the pores and then expelled through the osculum. Bacteria, algae, protozoans and organic debris in the water get trapped on the sticky mucous collars and are engulfed by the collar cells.
Amoebocytes are extremely crucial in sponges. They can capture food from the water or from the collar cells and then digest it. Their pseudopodia and flexible bodies enable them to move through the mesohyl and deliver nutrients to other cells. Additionally, they can differentiate into any type of cell, allowing sponges to regenerate even if they lose a portion of their body or get fragmented.
In their mesohyls, a simple skeleton provides support and protection. The skeleton can be composed of tiny needle-like structures called spicules made of calcium carbonate or silica, or it can be flexible and fibrous that composed of a protein called spongin. Some sponges have a combination of both spicules and spongin. Sponges can be categorized according to the structure and component of spicules.
Habitat and Distribution
Sponges are found in various aquatic environments, including oceans, seas, lakes, and rivers. They can inhabit both shallow and deep waters, and they occupy a range of habitats such as coral reefs, rocky surfaces and sandy bottoms. Sponges are sessile organisms, meaning they are permanently attached to a substrate like rocks, shells, or other hard surfaces. They can tolerate a wide range of temperatures and salinities to thrive in diverse habitats worldwide.
Sponges reproduce both sexually and asexually
Fragmentation: It is a special capacity for all species of sponge. If a sponge is damaged or broken apart, each fragment has the potential to regenerate and develop into a new individual. It is the animal with the strongest regenerative ability.
Budding: Some species of sponges reproduce asexually through budding. A small bud forms on the parent sponge's body and develops into a genetically identical clone. Eventually, it detaches to become a separate individual.
Gemmules: Mostly freshwater and some saltwater sponges produce gemmules during unfavorable environmental conditions, such as low temperatures or reduced food availability. They contain a group of dormant cells encased in a protective spicules coating. When conditions improve, the gemmules can hatch, and the cells inside develop into new sponges.
Sexual reproduction: Sponges are male and female within the same individual, though some species may be separate sexes. Generally, their sperms are produced by collar cells and eggs are formed by transformation of amoebocytes. During spawning, sperm are expelled through the osculum to fertilize the eggs of other sponges. A few species release fertilized eggs into the water, but most retain the fertilized eggs until they develop into flagellated larvae within the parents. Then larvae leave and attach to a suitable substrate where they transform into amoebocytes and develop a tiny sponge.
Humans and Sponges
Filter-feeding sponges can clarify seawater by removing excess algae, bacteria, and organic debris. The surface of sponges is typically inhabited by algae, and the cavities provide shelter for fish, shrimp and crabs. These organisms also bring abundant organic material to the sponges. The porous and soft body of sponges can absorb water. The spicules and spongin can generate great friction. Therefore, people near the Mediterranean used to collect sponges for bathing and cleaning since ancient times. Natural sponges are safer, more durable, and more expensive than synthetic sponges since they do not contain toxic synthetic organic compounds and are rich in collagen and calcium. Active substances within the sponge body mainly include alkaloids, fatty acid derivatives, amino acid derivatives, etc., which exhibit activities in terms of anti-cancer, antiviral, antifungal, and anti-inflammatory properties.