There are as many as 15,000 species of sea sponge (or porifera, to use its scientific name). The many varieties of sea sponge are often brilliantly colored, and the skeletons of some are actually used as (expensive) commercial sponges. Porifera means “pore-bearing”--all over the body of the sponge are tiny pores, through which it gets water and, with it, food and oxygen. As the simplest multi-cellular animal, sponges do things differently than most other animals, including breathing.
Life as a Sponge
There are a lot of limitations to being a sponge. As sessile creatures, they are permanently fixed to one spot and cannot go looking for food. Sponges have to make do with whatever is around--which happens to be water. The anatomy of the sponge is designed to allow them to get the nutrients they need to live from the water passing through them and the organisms in the water. There are further limitations to being a sponge, though. Sea sponges have no organs and no true tissue. According to the Maui Ocean Center, “On the scale of evolution, a sponge is only one step above an amoeba.” With no respiratory organs or system, sponges have to find another way to exchange gases with their environment, which is necessary for all living organisms.
Definition of Terms
“Breathing” and “respiration” are terms that get confused a lot. "Breathing" is often used to refer to external respiration or the process of drawing air into the body to get oxygen and expelling it to get rid of carbon dioxide. Internal respiration refers to what happens inside the body, or the exchange of oxygen and carbon dioxide across a respiratory membrane. This process is often simply called "gas exchange." The sponge is so simple that it does not have a special area of its body where gas exchange takes place, nor is there any distinction between internal and external respiration.
First, oxygen-containing water needs to get distributed throughout the body of the sponge. The small pores, called ostia, of the sponge draw water into them, and the water is circulated throughout its body by the action of cells called choanocytes. The choanocyte cells are equipped with flagella, whiplike structures that move around and push water through the sponge. As water is driven through and out of the sponge, food and oxygen are brought to the sponge and waste and carbon dioxide removed.
Gas exchange occurs in a sponge by simple diffusion across each cell membrane. Gas exchange always takes place by diffusion, in which the gases move from where they are most concentrated to where they are least concentrated, carbon dioxide moving in one direction and oxygen in the other. In human beings this occurs across the alveolar-capillary membrane in the lungs.
Human beings cannot "breathe" the way the sponge does, because diffusion is too slow for the needs of the human body. To speed things up, human beings have developed a special respiratory surface that increases the surface area for gas exchange. The circulatory system also speeds things up by transporting the gases between the respiratory surface and cells deep within the body. The sponge, though, fulfills the requirements for respiration by diffusion alone: a large, moist area for gas exchange in the form of cells that are never more than 1mm away from the site of exchange.