MEER home Marine biology home Table of Contents Index References Links Fish Body Forms and Lifestyles Body form affects the mechanics and adaptations of fish to swimming and moving in the water. When you see a fish - whether you are diving, fishing, watching an aquarium, nature program, or preparing one for dinner (provided you bought the whole fish, a rarity today), you may be able to classify it into one of six broad categories of design (or body forms): Rover predators Ambush predators Surface-oriented fishes Bottom fishes Deep-bodied fishes Eel-like fishes These are not taxonomic categories, but simply general descriptions of body shapes and structural characteristics. Different groups of fishes evolved independently to fit most of these forms. Instead, think of these categories as adaptations to particular functions required by different habitats and lifestyles. In short, you can tell a lot about the natural history and general way of life of a fish just by looking at its physical appearance. Let's look briefly at these basic types of fish forms. Rover Predator This is probably the most generalized fish form and has the classic fish shape that most people think of as fishlike (for example, salmon , tuna , mackerel , minnows , etc.). These fish are streamlined and fusiform , with a pointed head, forked tail, and with the fins distributed rather evenly around the body (providing both stability and maneuverability). Fishes with this body shape are constantly moving and searching out prey, and when they locate it, they capture it after a short pursuit. Of course, within this general design there is a lot of variation. The faster species tend to have more deeply forked tails and narrow caudal peduncles with keels (e.g., tuna and swordfish ), which are adaptations to high speed in the open ocean. Slower species, such as salmon , tend to have less deeply forked tails, etc., because their prey are typically smaller and slower, and maneuverability instead of high speed may be relatively more important. Epinephalus tauvina from Roughly, 1916. Ambush Predators These fish, although fusiform , also have very long bodies (torpedolike or arrowlike): for example, barracuda and needlefish , in addition to the gars and pikes in freshwater environments, are ambush predators. These fish typically lie relatively still, waiting for prey (usually fish) to swim by. They then ambush the prey with a fast strike. In order to provide fast acceleration or thrust from a stopped (or slow speed) position, the caudal fin is usually large and often rounded, and the dorsal and anal fins are placed far back on the body (often in-line with each other) for extra thrust from a low speed. In addition, the head may be somewhat flattened, often with a large mouth in a long pointed snout filled with teeth. Viewed from the front, these fish have a small cross section and are not readily obvious as "big fish" to their potential prey. This allows them to remain less visible as they wait for or slowly sneak up on their prey. They also tend to have cryptic coloration that allows them to blend in with their background. Sphyraena sp. (Barracuda) from Fitzinger, 1862 Surface-Oriented Fishes These species are usually smaller, have mouths that point up, flattened heads, large eyes, and a dorsal fin placed posteriorly. These are adaptations to feeding on small insects , plankton , or smaller fish at the water's surface. Although the best examples of this type are freshwater or estuarine forms, marine species with some of these traits include the four-eyed fish, flying fish, and halfbeaks . (Exocoetus volitans). Exocoetus volitans (flying fish), from Tomiyama et. al., 1970. Bottom Fishes Many fishes live on or near the bottom. These actually have a variety of body shapes, but they are all adapted for nearly constant contact with the bottom. Subcategories of bottom feeders include: Bottom rovers, which tend to have rover-predator-like shapes, but they may also have barbels , flattened heads, and protrusible mouths (for sucking up prey from bottom sediments). Marine examples include goatfish , catfish , and some sharks . Bottom clingers are fish that usually have flattened heads, large pectoral fins, pelvic fins modified to cling (or as suction-cup-like structures) to the substrate. These fish typically lie under rocks, in crevices, or quietly on a substrate. Marine examples include gobies , sculpins , clingfishes , and blennies ..(Pictured is a blenny ). Gobius cobitis, modified from Tomiyama et. al., 1970 Bottom Fishes (Cont'd) Flatfishes are the ultimate modification for bottom life. These are laterally flattened fish that lie on one side (flounders , halibut , etc.), and whose mouth and eyes have changed position to accommodate their peculiar orientation. flatfish, from Fitzinger, 1862. Bottom Fishes (Cont'd) The skates and rays are also flatfishes , but they are dorso-ventrally flattened. In a manner similar to the bony flatfish , skates and rays tend to roam about with one side in nearly constant contact with the bottom as they search for prey within or near the bottom sediments, often burying themselves within the sediment itself. Torpedo sp., from Fitzinger, 1862. Bottom Fishes (Cont'd) The rat-tail shape is found in some deep-sea bottom species and has independently evolved in both the chondrichthyes and osteichthyes . These species have long pointed tails, with little or no caudal fin, with large pointed heads and large pectoral fins. These species tend to be scavengers and predators of benthic invertebrates. Why the rat-tail shape is so common in these deep-sea fish is still something of a mystery. Chimaera sp. Unesco, 1984a. Deep-Bodied Fish These species are laterally compressed (flattened from side to side) and have deep bodies (dorso-ventrally elongated). The dorsal and anal fins are often relatively long, the pectoral fins are placed high up on the body, and the pelvic fins have moved forward to a position beneath the pectoral fins. These fish are designed for rapid turning and maneuvering in tight quarters, such as coral reefs, rocks, eelgrass or seaweed beds, etc.; however, there are also some open-water species. Although they can change direction quickly, they are not extremely fast; they often have stiff spines in their fins for additional protection against predators. Deep-bodied forms include the perch species and many coral reef fishes. The open-water species tend to be epipelagic plankton feeders, and the deep but laterally compressed body, as well as a sharp ventral keel and silvery body, serves to reduce their visibility to predators approaching from below. Chaetodon lunula (Raccoon butterflyfish), from Fitzinger, 1862. Eel-like Fish These species have the classic long eel or snakelike form, blunt or rounded heads, small or absent pectoral and pelvic fins, and reduced (or rounded and small) caudal fins. If present, the dorsal and anal fins may be elongated with the body. These fish are good at hiding or foraging for prey in tunnels, crevices, tight spaces between rocks or coral , or burrowing in soft bottoms. Marine examples include many types of eels , gunnels , etc. Anguilla marmorata (eel), Tomiyama et. al.,1970. These categories of body shape and form are useful generalizations. However, many fish have forms or combinations of characteristics, and others are difficult to place into any grouping. Experienced biologists can also tell a lot about a fish's lifestyle and habitat by studying the mouth, teeth, scales, fins, spines, eyes, and other structures that have specific characteristics that are adaptations for different lifestyles. References: Tomiyama et. al., 1970 Fitzinger, 1862 Moyle & Cech, 1982 Nelson, 1994 Roughly, 1916 Unesco, 1984c