How to Avoid Becoming "Snack Food"
Escape Mechanisms in Marine Animals
By Linda Lambert Litteral, Ed.D.
With all the animals in the sea, you might expect that an attack on a scuba diver would come from a ferocious animal. However, my only brush with potential disaster came not from a predatory fish of mammoth proportions, but from the seemingly harmless Spanish lobster (Scyllarides aequinoctialis).
My dive partner and I found this decidedly unattractive, armored tank of a lobster while on a night dive. Located in a large crevice, it seemed to be doing what lobsters normally do, lying quietly minding its own business. As we moved in for a closer look, the lobster became frightened and tried to escape.
In just a flash, the lobster slammed into my chest. Startled, I moved back, but not fast enough to avoid the second assault of the upward-swimming lobster. This managed to dislodge my regulator from my mouth. As I reinserted my regulator, the lobster disappeared into the night, not to be found again. It had escaped after successfully employing two of the three "Fs" of escape response, fright, flight or fight.
In the underwater world, practically every creature at one time or another is both predator and prey. Even the oceans' apex predator, the Great White shark (Carcharodon carcharias), is preyed upon. In this case, the predator is man.
In the game of predator vs. prey, the first goal is to avoid being eaten. It's a high-stakes, live-or-die game that is taken very seriously by all players. Many marine creatures rely on their strength, size and aggressive behavior to catch their prey without being preyed upon. Still others have elaborate escape mechanisms designed to help keep them out of the bellies of their adversaries.
"Avoiding" vs. "Escaping" Predators
Most animals live in fear of being preyed upon by another animal. But prey animals are not totally defenseless. Due to constantly evolving traits and behaviors, animals can often avoid or escape their predators.
Some prey animals avoid predators by lowering their chances of undesirable encounters. They may do this by living their entire lives sequestered in holes or shells.
Animals may also avoid their attackers by preventing detection. Camouflage, employed by peacock flounders, among others, is one way to do this. Although predators' eyes are keenly sensitive to motion, freezing prevents discovery of their camouflaged bodies.
Other animals do not have adaptations to avoid predators, but have evolved traits that increase their chances of escape. It could be the simple act of freezing in place or one of withdrawal, where an animal such as the feather duster worm retreats into its tube. Others, like the porcupine fish, may make themselves bigger in an attempt to bluff their way out of becoming a meal.
If these early defenses fail, the animal may flee the area, if possible. When all else fails, a cornered animal will attack the predator. As foolhardy as this may seem, it can be quite effective. The sudden change of behavior may confuse or startle the predator just long enough for the victim to get away. Additionally, the predator does not want to risk becoming injured. Because an injured predator may not be able to feed effectively, it may "think twice" about trying to capture a feisty victim.
The ways in which marine or freshwater animals escape are not unlike the escape responses of their land-dwelling cousins. Since scuba divers often look like big predators, escape responses are often easy to observe.
"I'm Outta Here"
The most obvious method of escape is rapid flight. Most animals that depend on flight for escape are remarkably quick. Snake eels, for example, can bury themselves faster than a human can dig.
Not unlike rabbits, some aquatic animals use a dash-and-hide technique. The flyingfish (Hirundichthys) does this with unusual flair by swimming near the surface, then hiding by leaping out of the water for considerable distances (up to 80 feet/25 m in some species).
As you dive or snorkel, you will no doubt see many examples of quick retreats. However, what might surprise you are the escape responses of animals you usually think about as being firmly rooted to the bottom.
For example, the swimming ane-mone (Stomphia coccinea) can escape its attacker, the leather star (Dermasterias imbricata). While foraging, the slowly creeping leather star wanders about looking for an anemone appetizer. Neither the anemone nor the sea star seems to be aware of the other. However, at first contact, the anemone forcefully bends over to release a battery of stinging cells (nematocysts) from its sweeping tentacles.
If the sea star continues to attack, the anemone will attempt to flee. Within seconds, the anemone loosens its grip on the bottom. It then bends its body into horseshoe shapes as it wildly flexes back and forth. It also utilizes the three-dimensional structure of the water by swimming upward rather than laterally just ahead of the sea star. Eventually, the anemone stops its gyrations and sinks to the sea floor, where it will take up a new place of residence.
Deflecting an Attack
Many animals escape by causing a predator to attack a less vulnerable part of their body. For instance, they may deflect attacks away from the head. The foureye butterflyfish (Chaetodon capistratus) makes its tail look more like a head by having a false eyespot at the base of the tail. To further attract attention to the tail, the false eyespot is larger than the real eye. Because a vertical dark stripe runs through the real eye, it is camouflaged and attracts less attention. Some butterflyfish with eyespots also begin swimming slowly backward at the first sign of danger. When an attack is imminent, it darts forward in a direction the predator is not expecting.
You may have heard that sea cucumbers are capable of expelling some or all of their internal organs so that they can then escape while the predator feeds on their eviscerated organs. Although it is true that sea cucumbers can expel their internal organs, scientists are not convinced that it is a true defensive mechanism.
Getting a sea cucumber to eviscerate is extremely difficult and is more likely to occur for some reason other than predatorial threat. For example, the sea cucumber (Sclerodactyla briareus) used for student dissections will eviscerate when water is polluted, but not when the sea cucumber is handled. Similarly, the familiar donkey dung sea cucumber (Holothuria mexicana) of the Caribbean will not eviscerate when handled by curious divers.
Sea cucumber anatomy is different from what you might expect. The tentacles you may see at the mouth are tentacles for feeding, not gills for breathing. Moreover, sea cucumbers do not breathe through their mouths. They breathe by pumping water through the anus a short distance to the breathing tree, where gases are exchanged.
Cuvierian tubules attach to the breathing tree in the area near the anus. They appear to be true defensive organs. The cotton spinner sea cucumber (Holothuria parvula) aims the anus at its predator, explosively releasing the white tubules. This sticky mass of threads ensnares the attacker, allowing the sea cucumber to escape. New tubules are regenerated for future use.
Some sea slugs, often called snails without shells, have evolved some interesting defenses to compensate for their shell-less condition. The story of their defense is complex and starts with what they eat. Some sea slugs eat tentacles from animals such as jellyfish and hydroids. Along with the tentacles, they take in numerous stinging cells, the nematocysts. Although these nematocysts do not bother sea slugs, they are effective in warding off other predators. Sea slugs incorporate these stinging cells into their own tissues by concentrating them within fingerlike projections, the cerata, along their back.
Waving in the current, the cerata are the first things a predator notices about a slug. As the predator nibbles, it is surprised by a bad-tasting stinging mouthful. The predator usually withdraws, and the sea slug lives to regenerate new cerata.
Sea slugs are not the only animals to use nematocysts and lose body parts for the purpose of defense. The juveniles of the blanket octopus (Tremoctopus) are known to collect the tentacles of the Portuguese man-of-war (Physalia) and hold them between the suckers on their arms. Older individuals have two arms longer than the others. They normally keep them coiled up so that they are about the same length as the other arms. When danger threatens, the octopus uncoils its longest arms. They consist of segments, each with its own white-rimmed, bright-red eyespot. The zone between segments is weak, allowing the octopus to break off the segments one at a time. While the predator gobbles up the noticeable morsels, the octopus escapes. Like the sea slug, the blanket octopus can regenerate its lost parts.
The octopus is probably most famous for using its cloud of ink to distract its predators. Presumably, the cloud of ink both distracts the predator and hides the octopus, giving the octopus a chance to flee. However, in a fascinating experiment, scientists discovered that the ink cloud may have another purpose. They placed an octopus in an aquarium with its predator, a moray eel. Moray eels have poor eyesight and typically hunt by smell.
When the eel got to within 1 or 2 feet/.5 m of its prey, the octopus squirted out an ink cloud. The eel diligently searched for the missing octopus in the small confines of the aquarium. Surprisingly, the eel put its nose directly on the octopus, but did not attack. Apparently, the ink cloud contained a chemical that deadened the olfactory sensors of the eel. The effect lasted between one and two hours before the eel was able to smell. Then the eel tried to attack the octopus again.
Sometimes it is difficult, as humans, to imagine that some of these escape responses work. After all, you have had success at following a flounder that was using the dash-and-hide technique, and you do know which end of a foureye butterflyfish is its head. However, you have definitely been fooled by distraction displays.
Can you remember trying to catch a brightly colored grasshopper or butterfly, only to find that it disappeared from view just when it was about to land? As it landed, the insect folded its wings, hiding its bright colors. Because you were focused on the bright color, you suddenly lost the whereabouts of the insect when only its drab coloration showed.
This "flash retreat" is also the method of escape employed by the spotted scorpionfish. The top of its brightly colored fins "flashes" and attracts attention when the fish is spooked and swimming. As it "retreats," its flashy fins close, and the fish blends in with its background.
A classic distraction display among birds is the broken wing display. This deceptive practice is utilized when a predator gets too close to an occupied nest. The parent bird draws the predator away from the nest by struggling and feigning a broken wing.
The bowfin Amia calva is a North American freshwater fish with a similar strategy. Found in the Mississippi River drainage and the Great Lakes, the drab olive male builds circular nests along the edges of the water. He guards his brood until the hatchlings are eight to 10 days old. He will lure menacing bass or sunfish away from his offspring by leaving the nest and thrashing about in the manner of a wounded fish. As the attacker catches up, the bowfin quickly swims away.
The balance among predators and prey is constantly evolving. As prey animals develop better defenses against their predators, the predators evolve more efficient hunting methods. Some prey will always get eaten, or there would be no predators. Some prey must always get away, or there would be no prey. As sure as there are fish in the sea, the multitude of unique escape responses will provide you with many fascinating hours of animal watching.