1. 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

2. 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).

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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.

10. 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

11. 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

12. 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

13. 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