Listen to part of a lecture in a biology class.

P: Okay, your reading homework for class today took a brief look at how life forms adapt to their environment, and I want to get deeper into that today. Yes, Mark?

S: Professor, I have a question about part of the reading. One of the chapters was about how seawater has influenced the evolution of marine organisms, right? Well, there was a section about filter feeders? P: Yes.

S: The book said that filter feeders eat by straining food particles from the water, right? P: Right.

S: Well, maybe this seems kind of obvious, but why aren't there any filter feeders on land? Or are there? The book didn't say.

P: No, it didn't. Does anyone want to guess? No? Well, maybe it's not so obvious. Okay, then, first, seawater is almost 1000 times more dense than air, so relatively large particles and organisms can stay buoyant or suspended. And because of this, there is a major marine ecosystem of small organisms called plankton that are perpetually suspended in seawater from the top to the bottom. Plankton can be plants, animals, or even certain kinds of bacteria.

In fact, any of the small drifting life forms in bodies of water, such as oceans, seas, and lakes, can be called plankton, and it's the primary food source for some of the smallest and some of the largest animals in the oceans, the so-called filter feeders. These filter feeders have adapted over time and have developed specialized structures to extract plankton and other food particles that float around in the ocean.

S: And that's where the baleen comes in, right? P: Oh, yes, baleen is what several species of whale have. They have baleen instead of teeth , and what a baleen is? Well, it's made of long plates, long plates that are composed of a tough protein called keratin. The edges of these keratin plates have fringes almost like hairs that hang down and act as a strainer. So what happens is that a baleen whale takes in a huge amount of water full of plankton and other small organisms into its mouth, and then pushes the water out through the baleen fringes, which filters out the plankton so the whale can eat it.

And then there are sessile filter feeders. Sessile filter feeders stay in one place. They don't swim around like whales do. So while baleen whales can go to places where there's a lot of food, sessile filter feeders, like sponges, stay in one place and thrive because the currents bring food to them.

S: So how are the sponges specially adapted to filter feed?

P: In a way, the whole sponge is a filter. Sponges are generally chimney shaped, and they have special cells that draw in water and food particles through small holes in their walls. They have other cells that trap and digest the food particles and yet others that move nutrients throughout the sponge. Filtered water and waste are expelled through the big hole in the top of the sponge.

S: Okay, so what it comes down to is that there is nothing like plankton living in the air. So no land animals have evolved that filter the air for food.

P: Right, now there are some birds, like flamingos, that are filter feeders, but flamingos filter their food from water, not the air. The closest thing to a filter feeder on land are spiders, but they're not actually filtering the air for food either.