A Fishy Defense
Though lovely to behold, coral reefs are far from a placid environment for the creatures that call these places home. It’s a world of eat or be eaten, of stealthy hunts and lethal attacks that must be countered by equally sophisticated defensive strategies. For most, simply fleeing the scene won’t work. Instead, many reef dwellers have come up with unique methods for thwarting would-be predators that involve everything from chemical cloaks to poisoned spikes. Follow along as we explore some of the more unusual defensive adaptations these animals employ.
Eyes in the back of the head
Flounders spend most of their time lying on the bottom, camouflaged and either eluding prey or waiting for a meal to swim by. To give them a broader range of vision, adult flounders have both eyes set on the same side of their head—the top side. They aren’t born this way, and while they are free-swimming juveniles their eyes sit side-to-side like any other fish. Once they mature and settle in, however, one of the eyes actually migrates across the body to sit next to the other.
“To ready themselves for a hazardous life on the open ocean bottom, flounders pull off one of the animal kingdom’s most astonishing feats of developmental biology. Muscles, skin, blood vessels and bones inexorably shift into a flattened shape. The thin horizontal shape presents a low profile to predators and allows the flounder to dive into the sand, leaving only its stalked, independently functioning eyes – each capable of rotating 180 degrees – exposed.” Ned and Anna Deloach
They have the unique ability to move each eye separately in all directions, allowing them to keep tabs on things that are both coming and going. Another trick that helps keep the sedentary adults safe is the ability to change skin color almost instantaneously to match the sea bottom on which they are hiding.
That big mouth
Unlike humans, who sometimes let their big mouths get them in trouble, the jawfish depends on its enlarged and highly flexible mandibles both to create a zone of personal safety and to ensure the security of its unhatched offspring. The ability to open their mouths extra wide serves jawfish well when it comes time to dig protective hidey-holes in the sand. They open wide to scoop sand and move rubble as they excavate a tunnel, into which they will then disappear at the first sign of a threat. These same big mouths that are used to build a home will later become nurseries. Jawfish are mouth brooders, meaning they carry fertilized eggs around in their mouths to keep them safe until hatched. This tasks falls to the males, who end up as stay-at-home dads. living about a quarter of their adult lives with a mouth full of eggs.
Head in a bubble
Fish don’t sleep in the same way that most land animals do. But some like parrot fish do go into an energy saving mode that checks them out for the night. One of their greatest dangers during these resting periods is the chance of being discovered by something like a moray eel, which roams around in the darkness using its keen sense of smell to sniff out a meal. In addition to tucking themselves into likely cubby holes in the reef to lessen the chances of detection, certain species of parrotfish will envelope themselves in a transparent cocoon made of mucous secreted from an organ on their head. It is believed that the cocoon masks their scent, making them harder for nocturnal predators to find.
Pufferfish are poor swimmers, and would have a hard time trying to escape many of the hungry critters that might pursue them in hopes of an easy meal. Instead of attempting escape, they make themselves less desirable by becoming more than a mouthful, and by adding a prickly surprise. They do this by quickly ingesting huge amounts of water to expand and turn themselves into a virtually inedible ball several times their normal size, while at the same time raising sharp spines all along their body. And should a seemingly lucky predator manage to snag a puffer before it inflates, it won’t feel lucky for long. Almost all pufferfish contain tetrodotoxin, a substance that makes them foul tasting and often lethal to other fish.
In order to ward off potential enemies, pufferfish, like this juvenile Blackspotted puffer, can inflate their bodies by swallowing air or water. But even more dangerous to predators is the puffer’s ability to secrete tetrodotoxin, a poisonous substance that can be lethal to other fish. Photo by Paula Butler
Most octopuses can eject a thick, blackish ink in a large cloud to aid in escaping from predators. The main coloring agent of the ink is melanin, which is the same chemical that gives humans their hair and skin color. Photo by Mark Snyder
Slink and ink
Octopuses have relatively big brains, and the most highly developed nervous systems of any invertebrate. These smarts help them anticipate and elude threats, but they also have several more trick in their defensive arsenal. Their ability to morph their skin color can create instant concealment, while a talent for shape morphing allows them to elongate their bodies and slip into tight crevices or through small openings.
“Octopuses escape from predators not just by hiding quickly but by deceit. One of the most impressive examples of this deception is what marine biologist Roger Hanlon calls the moving-rock trick. An octopus morphs into the shape of a rock and then inches across an open space. Even though it’s in plain view, predators don’t attack it. They can’t detect its motion because the octopus matches its speed to the motion of the light in the surrounding water.” Carl Zimmer
If caught in open water, they may transform into a dart-like shape and expel water to create the equivalent of jet propulsion. If none of these tactics works, the octopus has a final line of defense: confusion. When threatened, it will expel a black ink-like fluid that not only clouds the water, but also releases chemicals that dull their attackers sense of smell.
More than mooning
Though it might look defenseless, the sea cucumber has come up with a novel way to discourage and sometimes even immobilize a would-be attacker. When the cucumber senses a threat it turns to face away from the attacker, presenting its anus to the approaching predator. Showing its backside isn’t a gesture of disrespect, but rather a strategic positioning that allows it to engage in a bit of biological warfare. The cucumber ejects a portion of its intestinal tract, which often contains elongated stands of gooey material formed from expanded tube-like portions of this primitive animal’s respiratory system. The tubules expand up to 20 times their original length when expelled, and become quite sticky when they make contact. The effect is to entangle would-be attackers such as small fish or crabs in web of sticky filaments that will immobilize them while the cucumber slowly crawls to safety.
Though it might seem like they just lay about, seastars actually do a good bit of crawling—just not very fast. This means they aren’t very good at escaping predators, so their primary means of defense is to never stray too far from shelter. Even if they could move faster, they wouldn’t be able to see would-be attackers the same way other reef creatures can, because they lack fully developed eyes. At the end of each arm, the sea star has a simple eyespot, which can sense light and rudimentary shapes that are stationary or slow moving. It’s not much, but with these low-resolution sensors pointed in all direction, the sea star can determine which way cover lies, and avoid open areas where they are more vulnerable. To supplement their rudimentary vision, sea stars do have a highly developed sense of smell, and they can sense friends and foe based on minute traces of scent chemicals in the water column.
On the reefs of Wakatobi, you will find animals deploying all of these defensive strategies, and more. The longer you study their fishy habits, and the closer you observe their behaviours, the more some seemingly unusual adaptations begin to make sense. In the end, these fish are just doing whatever it takes to stay alive.
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