Phylum Porifera: the Sponges

Colorful Sponge

For years, scientists argued about whether sponges should be considered plants or animals. It wasn’t until the mid-1800s, when stronger microscopes were made available, that scientists were able to determine, based on cell structure and their development from zygotes to mature organism, that they were in fact animals.

Porifera means “pore bearing.” Sponges act as a pump, drawing water into their bodies through tiny openings called ostia and then expelling it through larger openings called osculum.

Sponges range in size from 1 cm to 2 m across. There are more tan 10,000 different species of sponges, with only 150 species being found in freshwater.

Structure of Sponges

• The walls of sponges generally have two layers of cells that are separated by a thin, jellylike layer.
• The first layer of cells is called the epidermis. This protects the sponge from it’s environment.
• The inner layer of cells lines the cavity of the sponge.
• The jellylike layer is called the mesenchyme. Even though the mesenchyme is not composed of cells, it contains some structures that are very important for the survival and function of the sponge.
• Amoebocytes move freely through the mesenchyme and help to transport food and waste. They are also responsible for the production of spicules which are mineral deposits that provide structure for the sponge. The spicules range in shape from hooks, needles, and barbs. 

Life Processes of Sponges

• The sponge consumes microscopic algae, bacteria, and organic debris. Because the sponge cannot move, it must move it’s environment around it in order to bring the food to it. 
• Collar cells, which line the cavities of the sponge, have flagella that beat the water around it to create a current. The water then enters the ostia, leaving through the oscumlum. 
• The collar cells then engulf the food particles and start digesting it. The food is then transported to the amoebocytes to complete the digestion and transport food through the sponge. 
• Sponges perform excretion and respiration at a cellular level by diffusion. This is possible because the wall of the sponge is very thing, therefor all the cells are close to the environment. 

Reproduction of Sponges

• Asexual reproduction: there are various forms of reproduction in sponges. 
• Budding: a group of cells enlarge and separate from the parent to form a new individual. This is the most common type of asexual reproduction in sponges.
• During periods of cold weather or drought, many freshwater sponges form gemmules. These are cells that are encased and protected in a tough, spicule-reinforced coat. Once the harsh conditions kill the parent, the gemmules survive in a dormant state. When favorable conditions return, the gemmules open and new sponges form.

Gemmule

• Regeneration: a small piece of a sponge can regenerate into a new, complete adult. 
• Commercial sponge growers exploit this remarkable ability. By breaking a living sponge into several pieces and spreading them in beds, they cultivate a large number of sponges in the same area.
• Sexual reproduction: this occurs primarily in the spring.
• Sperm released by one sponge can swim and are carried to another sponge. 
• A sperm enters a collar cell, and an amoeboycyte transfers it to an ovum in the mesenchyme. 
• The zygote develops into a larva, leaving the parent sponge to find a suitable place to attach itself and grow.

Sexual Reproduction