Multiple Defenses Some reef animals have evolved not one but often two or three lines of defense—often when any single one of them may seem entirely adequate In trying to understand the evolution of a seemingly defensive or protective mechanism in a reef animal, it is important not to underestimate the fierceness of the predatory pressure in such a crowded habitat At the same time, many defensive features typically have other functions in protecting or assisting the animal during its own food-procuring activities or against environmental stresses

Commensal Shrimp Have a protective exoskeleton, prominent pincers, coloration that effectively camouflages them with their habitat, and a habit of living out their lives among the spines of a highly protective sea urchin Crustaceans such as these put on lively displays involving threatening claw waving and scurrying about when defending themselves In this picture, a pair of commensal shrimp with their own protective exoskeletons (1) live safely in the stinging spines of a fire urchin (2), and it has pincer claws (3).

Puffer and Porcupinefishes Pufferfishes and porcupinefishes produce highly toxic substances that permeate their organs—they are repulsive to the taste of most predators Some have sharp, point spines, and their bite can be fierce These fishes also deter predators by inflating themselves like balloons so they appear to be larger than they actually are, and hopefully too large to fit in the mouth of a threatening predator With three lines of defenses, these fish make up for their slow-swimming nature Blackblotched Porcupinefish

Chemical Defenses The world of protective or defensive biochemical production among marine animals is far from being fully understood or explored Only is relatively recent years has it been demonstrated that many sessile reef animals, such as soft corals, sponges, sea squirts, and others have the capacity to generate very potent toxins Corals have several ways of repelling intruders that also function as effective predatory tactics

Chemical Defenses Long sweeper tentacles: tipped with especially potent, venom-packed nematocysts, to sting the intruder, forcing it to pull back before overgrowing the defender’s territory is one tactic Hard corals also extend mesenterial filaments: capable of digesting the tissues of an intruder, although they generally use their sweeper tentacles for defense Allelopathy = producing chemicals that alter the behavior or physiology of another animal Soft corals tend to use species-specific chemicals to repel intruders and slow the growth of competitors Biochemicals used include: terpenes, brominated phenols, polyphenolics, peptides, alkaloids, and many others

Coral Chemical Defenses Stony coral colonies, such as Acropora sp., can retract their soft polyps into a hard skeleton of calcium carbonate for protection When exposed, the tentacles have a line of self-defense in the nematocysts The colored tips indicate the presence of UV-screening pigments that protect delicate new coral growth Mushroom coral (bottom-left) delivers a burning message to a colony of anchor coral (top right) using its elongate sweeper tentacles—can result in a standoff, with colonies maintaining their distance or one coral may subdue and kill the other over time

Coral Chemical Defenses Turf war: brown sponge (1) overgrows the skeleton of a stony coral (2)—corals and sponges often compete for space on the reef, exuding bioactive chemicals through allelopathy Stony corals with sections that are diseased or physically damaged quickly become the site for sponges and algae that can overgrow the entire coral colony Fire coral is an aggressive competitor for substrate in many reef areas and is covered with fine, hairlike dactylozooids (1) that contain potent stinging structures They deliver a proteinaceous toxin that causes an immediate burning sensation on sensitive tissue Fire corals have hard skeletons but are more closely related to jellyfish than stony corals

Chemical Defenses Some sea squirts can concentrate the metallic element vanadium and combine it with sulfuric acid—a combination that deters predators and most epibionts (organisms that settle and live on the surface of other organisms Many boring animals use sulfuric acid in their boring activities and may use it defensively as well Tetrodotoxin = toxin produced by pufferfishes that works by cutting off nerve impulses to the brains of its victims

Tunicates (sea squirts) have gelatinous bodies but deter grazing by fishes and invertebrates by exuding sulfuric acid and other repellant substances. Unlike sponges, tunicates retract quickly when touched or threatened.

Epibionts Settling of organisms onto another often occurs because space is limited on coral reefs Anemone crabs are hermit crabs that decorate their shells exclusively with anemones, which they use to deter predators When confronted with danger, the crab retracts into its shell and pushes the stinging anemones at its predator When the hermit crab is ready to move into a bigger shell, it strokes the anemone body with its claw to encourage it to release its hold on the old shell—when the hold is released, the crab transfers its symbiont to the new shell

Epibionts A carrier crab (1) bears a heavy load of venomous protection in the form of a long-spined sea urchin (2) that it transports on its rear legs, just above the carapace In turn, tiny zebra crabs may live among the spines of the urchin Defended by stinging hydroids that it has attached to its carapace, the spider crab both camouflages its appearance and effectively signals inquisitive predators to keep their distance In its own hunting efforts, the crab may create feeding opportunities for the hydroids that are its involuntary partners

Epibionts Decorator crabs are an obvious group of animals that actually encourage the growth of epibionts A decorator crab cultivates its own bio-shield, having placed stinging corals and noxious sponges on its back The crab’s carapace is covered with tubercles that provide a textured surface and attachment points for the hijacked items it collects A decorator crab can also live under the umbrella of an upside-down jellyfish that it carries with two pairs of specially modified legs The flattened crab thrusts the jellyfish at attackers to chase them away

Epibionts Some of the sessile mollusks, such as the thorny oyster, have roughened shells that offer a perfect substrate for the growth of sponges, calcareous algae, and other epibionts All this live matter on the oyster’s shell tends to make the oyster disappear into its piece of the reef Some fishes also seem to use the same tactics—allowing algae to coat or encrust their skin, giving them a perfect camouflage for protection and a convincing disguise to prey These fishes seem to shed mucous as a protective barrier and sometime rid themselves of all epibionts 1-tiny blue eyes 2-epibiotic growth 3-bright orange mantle