Armor and Spikes for Defense
If all else fails -- if a predator cannot be discouraged by poisons or be confounded by a dash to safety -- then tough, armored skin or spines may keep it at bay.
How does a fish's morphology relate to the risks that it faces?
Most predators of reef fishes, such as groupers, lizardfish, and trumpetfish, are gape-limited. This means they can only take prey that they can subdue and fit into their mouths in one go; anything larger than their gape is likely to be passed over in favor of smaller prey. As fish mature and increase in size, the number of predatory species that can handle them steadily reduces. Not all fish are equal in this respect. A goby and a butterflyfish of the same body length present very different problems for a predator, the disk-shaped butterflyfish being much more difficult to engulf. This is one reason why reef fishes that live in the water column, and which are therefore more vulnerable to attack, usually have deeper bodies than, say, gobies that live on the substrate.
How do spines help against predatory fish?
Growing too large to be a meal for a predator is a slow process. For this reason, many small fishes develop
spines. These work by simply and effectively increasing the size of the prey fish, making them too large for the mouths of their smallest and most numerous predators. Larval reef fishes often have the largest, most elaborate spines. Although these are usually lost during metamorphosis and settlement on the reef, they help to neutralize the threat of many planktivorous predators.
Cowfish are an example of a fish that continues to use spines as an adult. The same principle is used by puffers and porcupinefish, who are able to inflate rapidly when threatened. Although initially faced with a bite-sized meal, any predator approaching these fishes is now confronted by something too large to swallow and is forced to withdraw.
How else can spines be used?
Some fish use their spines more aggressively against attackers. Surgeonfish, for example, are named for the sharp, scalpel-like spines towards their tails that may be used in territorial disputes and against potential predators. Scorpionfish may also use their spines defensively. Triggerfish use the first two rays of their dorsal fin, not only in defense, but also to lock themselves into place in a reef crevice, thus preventing predators from ejecting them.
Are any coral reef fish armored?
Armor, in the form of toughened scutes on fishes' bodies, is a common defense in freshwater catfish. It is a much less common trait in reef fishes, possibly because such heavy armor seriously limits the ability of fish to move freely, making armored fish far less effective at foraging in the highly competitive reef community. In addition to this, armor only deters a small number of predators; the largest reef hunters are very unlikely
to be deterred by a tough skin -- they simply eat the fish whole.
Nonetheless, many larval reef fishes are heavily armored but lose this thick dermal protection as they settle on the reef. Among the plankton, this armor protects against some of the danger, but has little benefit once the young fish joins the adult community. Some reef fish do use armor throughout their lives; for example, the slow-swimming boxfish is almost impregnable to attack behind the tough skin and bony plates on its flanks.