The sea star is an unlikely candidate for the Ruthless Predator Awards, yet of the approximately 1600 species found worldwide nearly every one of them is a carnivore. While a few are extremely specific in their diets, most will eat pretty much anything made out of meat. And some, like the Northern Sea Star (Asterias forbesi) pictured above, have ways of devouring even some of the most protected animals you can think of.
You can usually tell when a sea star is dining because its body is hunched up like the one above. And when it's not actually eating it is most likely searching for its next meal. These guys are as voracious as they are creative when it comes to feeding. Although the favorite food by far consists of bivalve mollusks such as oysters and mussels, this species will also feed on other animals that would at first seem to be dauntingly protected from predation by such a slow-moving and harmless looking animal. Barnacles, crabs, snails, and spine-covered sea urchins are all defenseless against this predator.
The mouth of the starfish is on the underside of the body where the base of all the arms meet. The opening is very small, maybe a quarter of an inch in diameter on a full grown adult. That it could possibly eat anything of any substantial size seems improbable. There are no mouthparts or claws that could be used to tear flesh into bite-sized chunks. You would look at this animal and think even if it managed to capture its prey, there is simply no way it could get the food into the stomach.
Which is true, so rather than bring the food to the stomach, the sea star brings its stomach to the food. Once the prey is captured the jelly-like stomach actually oozes out of the tiny mouth in a process known as eversion. When the everted stomach is outside of the sea star’s body it is literally turned inside out, exposing the digestive enzymes to the prey and partially digesting the meal outside of the body. The liquified food is then absorbed through the stomach lining and into the body and is transferred to organs in the arms known as pyloric ceca where the digestive process is completed. When it’s all done feeding the stomach muscles contract, pulling it back inside the body again. Anything inedible, such as shell fragments or spines, are ejected back out of the mouth by muscle contractions.
The everted stomach of a sea star.
That’s how asteroids solve the size problem of devouring prey, but there is still the issue of overcoming the defenses of its favorite foods. And yes, sea stars as a group are called "asteroids". If the food is unprotected by a hard outer covering, like say sea cucumbers or dead fish, the sea star simply lays on top of the prey, everts the stomach and digests away. Animals with shells pose a larger problem. How it deals with this usually involves the powerful suction cups on the bottom surface of the arms known as tube feet. For a detailed description of how tube feet work see the diary "How Tube Feet Work".
Snails are covered by a hard impenetrable shell. But this shell has one weakness, which is an opening that is needed for the foot to protrude from. This opening, the aperture, is protected by a hard trap-door structure called an operculum. This is ample protection against crabs, fish and other lesser predators, but is no match for Asterias. The starfish simply grabs hold of the shell with the tube feet and spins the snail around so that the shell opening is lined up with the predator’s mouth. Then the stomach is everted, covers the opening, and the digestive enzymes seep into the minute gap between the operculum and the shell. The tissues that holds the operculum in place dissolves until the trap door falls off and the sea star is then free to digest the rest of the body.
Sea star feeding on a snail.
In general, bivalves are better protected than snails because they don’t have an aperture in the shell. They have two shells that are held tightly together with a pair of powerful adductor muscles. The challenge for the starfish is to get these two shells open in order to expose the meat inside. Again the tube feet come into play, but not as usually described in texts, which often portray the sea star-bivalve struggle as some kind of hours long tug-of-war battle. The starfish trying to pry the shells apart and the bivalve desperately trying to keep the shells closed. This is misleading since the key weapon used by the sea star is again the stomach, not the suction cups.
While it’s true the shells of most bivalves close tightly together, there is not a complete seal formed. Like when feeding on a snail, the sea star uses the tube feet to manipulate the prey so that the opening (or in this case the crack where the two shells meet) is lined up with the mouth. The stomach everts and as it is pushed against the prey’s shell will seep slowly into the crack. As tiny amounts of the digestive juices come into contact with the body inside the shell, the bivalve weakens enough where the tube feet are finally able to pull the two valves apart, allowing the rest of the stomach to enter. The bivalve is digested inside its own shell.
The stomach is amazingly effective at getting between the shells. Experiments done in Japan a while back showed this pretty dramatically when researchers wired the shells of mussels shut and introduced some sea stars to the prey. A few days later the wires were removed and they were shocked to discover that all the mussels were empty.
Tube feet of a sea star.
The predatory effeciency of this animal is well known among local fishermen as well. All my life I’ve watched as quahoggers, those fishermen that make their living digging for hard shell clams, killed sea stars by the thousands by leaving captured ones on their boat decks to dry out. The idea, of course, is that by ridding the bay of these awesome predators they are increasing the numbers of clams for them to catch. The irony here is two-fold. First, sea stars don’t feed on quahogs. Like all clams, quahogs live their entire lives underground and northern sea stars cannot dig. Even if they could, they would be unable to open the two shells because, unlike mussels and oysters, the inside edges of a clam’s shell is lined with tiny little interlocking ridges. As effective as the everted stomach is at squeezing through tiny cracks, there is simply no room at all for the stomach to penetrate the shells.
Second, the sea star may very well be the quahogger’s best friend. To see why you need to understand how the clams are harvested. The quahogger uses a tool called a bullrake, which is basically a metal basket with several sharp teeth that can dig into the bottom. This is attached to a long metal pole that is held by the fisherman back on the deck of his boat. As the bullrake is is dug into the sand anything it digs up, clams, rocks, sea stars, is trapped in the basket and hauled up.
Bullrakes don’t work unless the bottom is relatively free of rocks and other obstructions that would otherwise interfere with the rake entering the sandy bottom. Rocky bottoms are not suitable for bullraking. Neither are places where large numbers of mussels congregate. A mussel bed may be made up of hundreds of thousands of individuals that can cover the bottom and make large areas of potential clamming grounds unavailable to the harvester. But, sea stars also live in large colonies called swarms. When a swarm travels across the ocean bottom it leaves practically nothing alive in its wake. Everything edible is consumed, including the mussels. An entire mussel bed can be reduced to nothing but a bunch of empty, fragile shells, opening up the area once again for the quahoggers to dig up the buried clams.
Other diaries in this series can be found here.