Ray-finned fishes generally avoid predators in two ways, through behavioral adaptation and physical structures, such as spines, camouflage and scents. Usually, several behavioral and structural tactics are integrated because it is advantageous for fishes to break the predation cycle (1-4) in as many places as possible, and the earlier the better. For instance, (1) the primary goal of most fish is to avoid detection, or avoid being exposed during certain times of the day. If detected, (2) a fish might try to hide very quickly, blend in with the surroundings, or school; (3) if the fish is about to be attacked then it must try to deflect the attack, and if attack is unavoidable (4) the fish will try to avoid being handled and possibly escape. Therefore, many fishes avoid even the chance of attack through particular cycles of activity, shading (or lighting, see below) and camouflage, mimicking, and warning coloration.
For example, fishes usually avoid dusk because predators often take advantage of quickly changing light conditions that make it difficult for prey to see predators. (Species that feed at dusk are termed crepuscular and include jacks snappers , tarpon , cornetfishes and groupers). Most ray-finned fishes feed during daylight hours (diurnal), when they can see predators. Zooplanktivores, cleaner fishes, and many herbivores are abundant and conspicuous by day but hide within the reef at night. Several wrasses and parrotfishes even secrete a foul-smelling mucous tent or bury themselves in the sediment for protection. Shoaling, which is common among many groups (found in sticklebacks , bluegills , gobies and many others), provides many benefits as a daytime defense. Some predators actually mistake shoals for large fish and avoid attacking. Also, when shoals detect predators they form a tight, polarized group, or school, that is able make synchronous motions. Attacking predators may find it difficult to isolate individuals as the school morphs around them, and some groups (snappers , goatfishes , butterflyfishes , damselfishes , etc.) even mob the predator, nipping and displaying, to thwart an attack.
Because many larger species of zooplankton and other invertebrates come out at night, several groups have developed nighttime feeding patterns (nocturnal) and associated defense mechanisms. Many of these groups, including flashlight fishes , ponyfishes , pineapple fishes and some cardinalfishes , have luminescent organs. While luminescence is likely used for communication (shoaling and mating) and catching prey (via luminescent eyes, which can be turned on and off (!), and baits), several species use luminescence for defense. Rows of lights along the bottom of the body make these fishes indistinguishable to benthic (living at the bottom) predators because they match the intensity of moonlight or dim sunlight shining down. This peculiar method of invisibility is similar to countershading, which is common in several other pelagic ray-finned fishes (as well as sharks and rays). Countershaded fishes are graded in color from dark on top to light on bottom, rendering them invisible from nearly any angle because their coloring is opposite that of downwelling light; the light reflected is equivalent to the background (as above). Two other methods by which pelagic fishes remain invisible are by having a shiny coating (mirror-sided), as in anchovies , minnows , smelts , herrings and silversides ; or by having transparent bodies, like glassfishes , African glass catfishes and Asian glass catfishes.
Benthic ray-finned fishes also utilize numerous methods of camouflage (for both hunting and predator avoidance). A common and elaborate method in tropical seas is mimicking the background of the habitat (protective resemblance), which involves variable color patterns as well as peculiar growths of the skin that may resemble pieces of dead vegetation, corals , and a variety of bottom types (e.g. flatfishes). There are numerous examples of this type of crypticity, from sargassumfishes and leafy seadragons that mimic the seaweed among which they hover, to clingfishes , shrimp fishes and cardinalfishes that have black stripes resembling the sea urchins they use for cover. Another method of camouflage is to look and behave like something inedible, but remain conspicuous. Juvenile sweetlips and batfishes mimic certain types of flatworms and nudibranchs that have toxins in their skin and associated bright coloration, making possible predators wary.
Bold or bright coloration in ray-finned fishes (termed aposematic) usually means that the species posses a structural or chemical defense, such as poisonous spines, or toxic chemicals in the skin and internal organs. Surgeonfishes and lionfishes , for instance, have bold coloration to match scalpel-like and poisonous spines, respectively. Aposematic fishes also advertise their inedibility by moving slowly, instead of darting away when predators are present. However, displays of aggression back up this behavior. When disturbed, weevers erect a dark-colored and highly venomous dorsal spine, while pufferfishes , also poisonous, puff up into a ball of spikes.
- fish (Actinopterygii)
- sharks Chondrichthyes
- aquatic invertebrates
- birds (Aves)
- mammals (Mammalia)
- reptiles (Lepidosauria)
- amphibians (Amphibia)
Anti-predator Adaptations: mimic; aposematic ; cryptic