sponges [Porifera]

In lower intertidal zones and tide pools, sponges attach to submerged rocks, dock pilings, seaweed, shells and other hard surfaces. Their shapes vary from the tiny tubular clusters of an organ-pipe sponge, to the encrusting mass of the crumb of bread sponge. Some even leave behind field marks. The boring sponge is a yellow crust, which covers and dissolves shells, leaving them riddled with pockmarks.

Though their shapes vary, marine sponges share the same basic structures: an outer surface, an inner surface, and a jelly-like mass in between. These multicellular creatures are the simplest of all animals, since their cells are not organized into organs. Still, each cell, though acting as if alone, has a specialized function that benefits the colony.

Like our household swabs, marine sponges absorb lots of water through minute pores in their outer layers. This is accomplished by cells covering their inner surface. These “collar cells” create an incurrent of water flow with the rhythmic beating of their whip-like flagella. They also trap and digest microscopic food particles, like bacteria, plankton and detritus. Other cells, called amebocytes, move along the canals between the pores and inner layer also digesting food particles. The water exits through one or more excurrent openings.

A network of tiny spicules of varying shapes is contained within the gelatinous mass between the two layers. These spines provide shape for the sponge. Sponge classification is actually based on the chemical compositions of the spicules. They can include, alone or in combination, calcium carbonate, silica or spongin (elastic protein fibers).

Sponges can reproduce asexually, when one of their parts breaks off and regenerates into a new sponge. They also reproduce sexually, releasing sex cells that may be fertilized in the water flow.

The crumb of bread sponge (Halichondria panicea) in the picture gets its green color from microscopic algae living in the sponge colony.

Copyright © 2006 Barbara Driscoll.