The pumping sponge
Sponges do not have nervous, digestive, or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies for their survival. One spectacular species is the Giant barrel sponge (Xestospongia testudinaria) found in the Philippines, Australia, western and central Indian Ocean, Indonesia and Malaya (picture left, taken at Sipadan). A related Caribbean giant barrel sponge is called Xestospongia muta.
The enormous barrel-shaped sponges help to improve water clarity, support reef regeneration, and provide a habitat for other invertebrates, benthic fish, and bacteria living inside or on the surface of the sponge. Its body is hollow and is held in shape by the mesothyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. The inner surface of most sponges is covered with collar cells or choanocytes, cells with cylindrical or conical collars surrounding one flagellum (‘whip’: see photo section above).
Pumping to survive The first task of the pumping system of the sponge is to create a flow of water and the second is to capture food items as they pass by these cells. Filter-feeding sponges filter a water volume six times or higher than their volume body per minute. On a daily basis, that is around 50,000 times their own volume in water. The flow generated by the currents pushes the seawater through the pores on the body (ostia) of the sponge, where the nutrients (mainly plankton) are captured, before being ejected from the top opening called osculum (see photo section above). In more detail, the mechanism works as follows. Water enters the sponge through numerous incurrent pores, which line the sponge surface, and then flows through a series of branching and successively narrowing incurrent canals to the water-pumping units, the choanocyte chambers. Choanocytes are versatile cells which extensions called flagellae (‘little whips’) that create the active pumping of water through the sponge, while the collars of the choanocytes are the primary areas that nutrients are absorbed into the sponge (see photo section above). The action of the choanocyte flagellum is to generate a low pressure to draw water through the collar. The basic mechanism that drives water through the sponge's body is believed to be the wave-like motion of the whip-like flagellae working in parallel. From here, water subsequently flows through a series of excurrent canals that merge and empty into the atrial cavity, the spongocoel, then to an excurrent osculum.
In sum: The choanocytes appear to work in parallel and therefore function as the basic pump units. They deliver the moderate pressure rise required to draw water through the inhalant openings (ostia) and to maintain flow through the rather short and open canals and the exhalant openings (oscula) of these species. The flow of water is initiated through the coordinated beating of flagella. Once water enters the sponge through the ostia, it passes through a canal system of lesser or greater complexity, depending on the species, until it reaches the choanocytes.