Algae, horseshoe crabs, dead fish, coral and bryozoa -- recently I've described all these organisms that commonly wash onto the sandy beach near Hotel Reef. The most conspicuous beach item remaining to talk about is the sponges. This week as I walked along a tourist's-dream kind of beach I paid special attention to washed-up sponges. That's it below:
One thing not showing in that picture is the sponge's lightness and sponginess. Those features are to be expected because much of a sponge consists of open space. Looking at the sponge in the above picture, the basic concept is this: Water carrying tiny food particles and oxygen enters the sponge body through the body's thousands of very tiny, barely visible pores. The water then circulates through a system of channels inside the sponge body, finally exiting through any of the larger holes clearly visible in the picture. These larger holes are called oscula (singular osculum).
Facing into the sponge's internal cavities are tiny, special cells called choanocytes. Choanocytes bear microscopic, whiplike hairs known as flagella. Wiggling flagella keep the water circulating, plus they draw water-borne food particles into a collar- like structure on the choanocyte, where the particles get stuck in mucus at the base of the flagellum. Then the food is digested either by the choanocyte or a nearby amoeboid cell.
We've seen that when we hold a chunk of washed-up coral or bryozoa we're holding the remains of a colony of tiny individuals, not just one organism. In contrast, the sponge in my hand in the above picture is a single animal, albeit a very simple, loosely organized one. Though a sponge is little more than a mass of fairly simple cells embedded in a gelatinous matrix, the cells do recognize one another and thus more or less coordinate their behavior, plus different cells are specialized for different bodily functions. These features -- the cells coordinating their activities, and there being specialized cells for distinct bodily functions -- cause the sponge to be animal, not a colony of animals.
Still, sponges are so unlike all other animals that often they are placed into their own subkingdom, the Parazoa. They earn this distinction because they show no symmetry and lacks all tissue and organs.
You can see a diagram showing sponge anatomy here.