In the coastal marine environment countless species of algae and invertebrates use various methods of attaching to firm substrates to eke out their living. Think about anytime you’ve visited the seashore. Nearly everything is covered with weeds and unidentifiable animals. Buoys, docks, crab shells, oyster shells. Even other seaweeds are covered with hydroids, bryozoans and smaller seaweeds.
But I can guarantee you that there is one type of common marine animal you will never see anything growing on top of, even though its firm surface area would seem to be an ideal place for these biofouling organisms to grow on: Members of the class Asteroidea, or sea stars.
Sea stars don’t have a front or back, head or tail or even a left side or a right side. They are radially symmetrical.
They do, however, have a top and a bottom. The top side is referred to as the aboral surface, because it doesn’t contain the mouth parts, and the bottom surface is the oral side.
We’ve already covered aspects of the oral surface of a sea star in this series in two other diaries: How Tube Feet Work and How Sea Stars Eat. You’d think the aboral side would seem a bit boring in comparison. But you’d be wrong. On a microscopic level the surface of Asteroid is pretty incredible.
To prevent other animals, plants, dirt or detritus from settling on top of the sea star’s body, it has an armory of tiny tools to rid itself of these annoyances. Tiny claws for plucking, miniature hammers for clobbering, scissor-like appendages for snipping and spines for stabbing. The surface of an asteroid is like an echinodermal version of a Swiss army knife. There’s a specific tool present for nearly any contingency.
In the Tube Feet diary, I described the water vascular system of an echinoderm as a plumbing system designed to bring water to the suction cups of the tube feet. The entry point of the water needed to make this plumbing work is the madraporite, or sieve plate. This is a circular structure on the upper surface of the sea star that is usually mistaken for an eye. A close up of the sieve plate is shown above.
The sieve plate is the most prominent structure that can be seen by the naked eye, as are the spines. The spines are pretty simple. They are simply sharp protrusions of the endoskeleton that help to prevent predators from swallowing the animal whole. In the photo below, the spines are the little raised white bumps. Notice the sieve plate in this photo as well.
Now let’s focus in a bit more. In between the spines we find the various fouling prevention structures, which keep the sea star from being covered in weeds and whatnot. These structures are collectively known as pedicellaria. I tried to find the origin of the word, and the best I can come up with is a greek translation of "little forceps". I’m not sure if this is accurate, but if I could think of one term to describe them, it would be this.
Seaweeds spread by releasing spores into the water, and sessile (non-moving) animals have planktonic larval stages that seek out hard substrates to grow on. These sessile animals include tunicates, sea anemones, hydroids, corals and many, many other groups of animals. When a substance settles on top of the sea star the pedicellaria go into action, snipping, clubbing and grabbing the foreign object and ejecting it from the animal’s upper surface. Sometimes a larger particle will actually be passed from forcep to forcep until it reaches the edge of the body, and then is discarded. Below is a microscopic view of one of these forceps.
Other structures are used like tongs to grab an irritant and pass it along to a forcep. The pedicellaria are stalked, which means they are flexible enough to bend and twist once the substance is grabbed. This allows the sea star to toss the offending material off the body or pass it on to another appendage for dispatch. Often times simply crushing the larvae will suffice, while at other times the object needs to be ejected. One of these grabber pedicellaria is seen below.
Although less common, a sea star will also possess a handfull of club-like structures. Seen below, these globular pedicellaria, perched on the end of a flexible stalk, can be used as a hammer to crush a spore or larvae, which can then be dispatched with one of the forceps.
Ok, so why have these elaborate structures on such a primitive animal? The answer lies with how echinoderms respire. Like most marine animals, sea stars breathe using gills. Unlike fish, crustaceans and most other aquatic species, whose gills are kept within the body wall (see How Gills Work), an echinoderm’s gills are external. They protrude through the body wall on the upper surface of the animal. This is where gas exchange occurs, and if the animal were to be covered in weeds and anemones, this would interfere with breathing. On the close-up of the sea star arm below the gills are the round patches of purple.
Now, let’s see these forcep structures in action. Fortunately I have pretty hairy arms, so this little experiment should work out well. First I turn a sea star upside down and let it rest on my forearm.
My arm hairs are an irritant and trigger the pedicellaria to get into gear. The tiny forcep structures start grabbing onto individual hairs in an attempt at removing them from being in contact with the aboral surface. After a few seconds I slowly pull the sea star away from my arm, and as you can see, the pedicellaria are still clinging to my hairs.
That top image was photoshopped by my son Graham.
Other diaries in this series can be found here.