Defensive Armor in Freshwater Fish
Of course, not every fish darts away as a predator approaches. Some fish have evolved armor and other defenses, most often in the shape of spines, to make all but the meanest of predators seek their dinner elsewhere. But if it is so effective, why have not all fish evolved armor plating? The reason is that armor is heavy and cumbersome. For fish that live on the bottom, such as many catfish, this is not a problem, but for fish that need to hunt elusive prey such as insect larvae and copepods in the water column, the handicap of armor would be too great.
What is fish armor made of?
The armored plates, more correctly known as "scutes," found on many catfish are not scales. In fact, no catfishes have true scales as other fish do. Instead, the scutes are made up from the epidermis, or skin, of the animal and overlap to provide a tough barrier against predators. The strength of these scutes is provided by their constituent structural proteins, just as in animals such as lobsters and beetles. Providing there are enough minerals in the water to aid the growth of the scutes, the armor develops early in life. However, the smallest fish need speed to evade predators and so do not develop the full extent of the armor until later in life.
Who needs armor and where and when?
The physiological investment involved in making armor is considerable. As a result, differences exist between populations according to whether they live alongside predators or not. Populations that live alongside their hunters often develop thicker body armor than those that do not. This is partly controlled by the genes of the fish and partly develops in response to the presence of predator cues in the water during their early life.
Another factor is which parts of the body to protect; again this is a trade-off between security and the weight of the armor. Consequently, most armored fish have their thickest defenses around their heads and across their backs.The undersides of many armored catfish, which are usually pressed against the substrate, are often quite soft.
How else can a fish's body structure help it to overcome predators?
Spines are an excellent way of protecting oneself against predators. They fulfill two main purposes. Firstly, they can physically damage an aggressor; sharp tips and serrated edges are perfectly capable of wounding a foe, be it a rival or a predator. Secondly, you may have noticed how many species of fish lock out their fin rays when they are disturbed, especially the leading rays of their pectoral and dorsal fins. This has the effect of increasing the girth of the fish -- making it wider than the mouth of a piscivorous fish or a bird's throat and thus almost impossible to swallow. The spines can be locked into place using a specialized socket and offer an extremely stubborn defense. The effectiveness of the spines in saving the lives of catfishes was demonstrated by a study that reported that out of ten attacks made on catfish, where the predator actually managed to get the prey into its mouth, the presence of the spines meant that in all but one instance, the catfish were ejected unharmed from the predator's mouth.