Invertebrate Feeding Tactics
It is not only fish that prey on reef invertebrates, nor do all these invertebrates live a life of grazing and filter feeding; there are some fearsome predators among the invertebrates. These hunters have adopted some innovative strategies to get past the sometimes quite considerable defenses of their fellow inverts, and some of these hunters are capable of taking on and subduing adult fish.
The "kill-or-be-killed" world of invertebrate hunters and their victims is like something from a science fiction novel, but as is often the case, truth is stranger than fiction. Some of the most remarkable predators can be found among these predatory reef invertebrates.
How do invertebrates get past tough physical defenses, such as shells?
Some starfish make their living by feeding on mollusks. The major barrier between them and a meal is their prey's shell, but this can be overcome if the right tactics are used. Starfish move using thousands of tube feet and these can be pressed into service for opening up a bivalve. Having located its bivalve prey, the starfish envelopes it and gradually exerts intense pressure in an attempt to gain access, applying an incredible force at the bivalve's hinge. Then it is a straightforward battle between the muscles of the protagonists, but few clams can stand up to the starfish. Certainly, any creature less than 4 inches (10 cm) long is in real trouble if a starfish gets hold of it. For this reason, clams can move if they see or smell the presence of a starfish in the area. If a shadow passes over their simple eyes, they take in a shellful of water and then rapidly contract their muscles, forcing a jet of water out of their exhalant siphon and jetting off at high speed.
Many octopuses have also faced the problem of shelled prey. Crabs are a favorite food among these super-intelligent predators and even large, aggressive crabs stand little chance when caught in the open by an octopus. The octopus -- which is itself a mollusk, a relative of clams and sea slugs -- simply wraps up the victim in its tentacles and brings its powerful central beak to bite on the crab's carapace.
How do invertebrates eat prey that is larger than themselves?
A problem for invertebrates such as shrimps is that their prey is often considerably larger than they are. One example is the harlequin shrimp, which takes on comparatively huge starfish as its prey. The first problem facing the shrimp when it comes across a starfish is how to tear its prey away from the substrate. It approaches this by nipping at the echinoderm's tube feet, but a starfish can have thousands of these and often it seems that as fast as the shrimp detaches some of the starfish's tube feet, more fasten on. If the shrimp persists, it may eventually manage to dislodge the starfish. However, better results are achieved with teamwork, and harlequins often approach the problem of such tenacious prey in pairs. Once the shrimp or shrimps have managed to detach the starfish from its hold, the battle is virtually over; the shrimps turn the starfish onto its back and carry it back to their home range, where they can eat it -- alive -- sometimes over a period of days.
How do predators overcome chemical defenses?
Potentially vulnerable animals on the reef often use powerful defenses to discourage their predators. However, the stinging tentacles of anemones are not always enough to discourage predatory nudibranchs. Having found their prey by following trails of chemical cues, the nudibranch approaches very carefully, avoiding touching the anemone and thereby triggering its defense response, which is to withdraw into itself. The nudibranch raises itself high above the unsuspecting anemone, keeping itself clear of the stinging tentacles. Finally, when it is above the center of the anemone, the nudibranch everts its mouthparts and plunges downward deep into its prey.
Although the anemone responds by withdrawing its tentacles, it is too late to save itself -- within its body, the nudibranch is already feeding. The stinging cells of the anemone are recycled by the nudibranchs that eat them and stored underneath their own skin, where they continue to provide protection.
Do any invertebrates prey on fish?
Fish make excellent prey for invertebrates, but they are extremely elusive and difficult to subdue. This is why fish-eating marine snakes have far more potent venom than their land cousins; marine snakes are under pressure to kill their prey quickly because a wounded fish can swim far out of range in just a few seconds.
The potency of marine snakes is matched by that of some cone shells. These animals can reach a considerable size -- up to 8 inches (20 cm) -- but like most snails, are extremely slow moving. To capture fish, they rely on a harpoonlike mechanism to carry deadly venom into their victim's tissues and paralyze them, in some cases almost instantly. Their venom is so powerful that some cone shells can cause serious injury to, or even kill, humans.
The "cigarette snail" is so-called because an attack by this animal is thought to leave little time for anything other than a final smoke!
What is the most dramatic invertebrate predator?
The ultra-versatile cephalopods (octopus, squid, etc.) are remarkably successful hunters, but in terms of their incredible weaponry, the mantis shrimps are hard to beat. They can mobilize their folded claws to strike their prey amazingly quickly, generating speeds approaching those of a bullet. In fact, the acceleration is so fast that it causes a cavitation bubble to form in the water, so that the victim is hit not only with the claw, but also with the even greater shock of the bubble. Some species of these shrimps spear their prey, impaling soft-bodied animals such as fish with spiny, barbed appendages. Other species smash shelled quarry using a more rounded clublike claw and then dismember their victim with a sharper, more versatile claw.