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Adaptations

The content and activities in this topic will work towards building an understanding of evolutionary adaptations that have enabled fishes to thrive and diversify throughout the world ocean.

Fishes have been swimming and reproducing for over 450 million years on Earth. During this time, fishes have undergone changes that help them live better in some water habitats than in others. This is a common theme in biology. Organisms change over time as some features are favored more than others for survival. With so many different types of environments to live in, fish have diversified into many different body plans.

For example, snappers (Fig. 4.73) are generalized predators, adapted for swimming and hunting. A snapper’s body is streamlined. Its paired fins are placed for maneuvering. The caudal fin is used to move the fish quickly through the water. Its eyes are looking forward, and it has a large, forward facing mouth with prominent teeth.

<p><strong>Fig. 4.73.</strong> Red Snapper (Lutjanus campechanus) caught in the Gulf of Mexico, about 20 miles south of Port Fourchon, LA</p><br />

Walking Fish

The snapper represents a kind of generalized fish body form, but many fish are far more specialized. The frogfish is one example of a very different type of fish (Fig. 4.74). Frogfish often look like seaweed, coral, or rock, which gives them good camouflage. The frogfish eye is particularly hard to find. Many fish camouflage their eyes because it makes it harder for other fish to spot them, judge their size, or tell which end is the front.

Frogfish use their arm-like pectoral fins to walk around on the bottom. Since they aren’t fast swimmers, frogfish must lure their prey to them. They use an illicium, which is a modification of the first dorsal ray, as a fishing pole. The lure at the end of the illicium is called an esca. Frogfish use the esca to draw small fish near.

<p><strong>Fig. 4.74. (A)</strong> A giant frogfish hidding on the bottom (Antennarius commersonii).</p><br />
<p><strong>(B) </strong>A frogfish hiding in a coral (Antennarius sp.).</p><br />

Speedy Fish

Tunas are built for speed (Fig. 4.75). Tunas have a streamlined, torpedo-shaped body, sometimes called fusiform. The tail of a tuna is cresent-shaped, and the base is narrow and stiffened, maximizing thrust. The pectoral fins are tiny, and probably not used for much turning.

Since tuna swim quickly through the open water and do not have to maneuver through a high-relief environment like a coral reef, the dorsal fin is broken up into a series of finlets that make tuna more hydrodynamic. The caudal peduncle keel along the sides of the body also reduce drag.

The mouth of a tuna is highly specialized. Most fish have protusible jaws—jaws that can be protruded or extended. But tuna strike their prey at such high speeds that a protrusible jaw would be torn apart. Thus, tunas are some of the few fish to have a fused jaw.

<p><strong>Fig. 4.75. (A) </strong>Skijack tuna (Katsuwonus pelamis) swimming in a loose school, called a shoal.</p><br />
<p><strong>(B)</strong> A drawing of a blackfin tuna (Thunnus atlanticus) showing fin placement and shape (</p><br />

Colorful Fish

The body of the butterflyfish (Fig. 4.76) is very different from that of a tuna. Butterflyfish are built for maneuverability. Their pectoral fins are broad and used to maneuver the narrow, steep body. Their dorsal and anal fins are high and reinforced to provide stabilization.

The coloration of butterflyfish is also very important. They have a substantial eyespot near the tail, which directs the eye of a predator to the rear of the body, so that predators are likely to miss catching them, (since the fish escapes in a forward direction). The large eyespot also makes the butterflyfish appear larger. The real eye is hidden by a stripe, making it difficult to locate.

<p><strong>Fig. 4.76. </strong>Copperband butterflyfish (Chelmon rostratus), with a large eyespot, tall and thin body, and maneuverable fins</p><br />

Spiny Fish

A porcupine fish is slow-moving and highly specialized to avoid predation (Fig. 4.77). It is able to gulp water or air, puffing up its body, and pushing out its spines, making it a formidable mouthful for any predator. The spines themselves are modified scales. In fact, many apparently complex features of fish (and other animals) are actually modifications of existing structures.

<p><strong>Fig. 4.77. (A) </strong>An uninflated porcupine fish</p><br />
<p><strong>(B) </strong>An inflated porcupine fish</p><br />

Finless and Scaleless Fish

Some modifications to fish bodies include the loss of structures. For example, moray eels use their long, narrow body shape to fit tightly into rock and coral crevices (Fig. 4.78). Structures that could get caught up on the rocks (like pelvic fins, pectoral fins, and scales) have been eliminated. Even the strong operculum has been reduced to a small pinhole opening.

<p><strong>Fig. 4.78.</strong> A Ribbon Moray (Rhinomuraena quaesita) inside a coral crevice in Indonesia</p><br />

Eyeless Fish

Another example of structure loss is the loss of eyes. Eyes take a lot of energy to produce, so if they aren’t needed, gradual modifications over time can reduce or eliminate the eyes. The energy that was used for eyes can be used elsewhere, for example, to grow larger or to produce more sperm or eggs for reproduction.

Blind cavefish are an example of fish that have lost the ability to see. Pigmentation and eyes are not very useful in dark cave environments. In fact, coloring and eyesight can even be harmful because they both require energy to maintain. The loss of body coloration and eyesight is complete in some cave fish, where their body is clear and there are holes in the place that eyes should be (Fig. 4.79). Or, eyes may be reduced to such an extent that they are light sensitive but not able to help a fish actually see. Non-fish examples of organisms that have lost their eyes due to selective pressure include blind cave spiders and blind lizards.

<p><strong>Fig. 4.79. </strong>Phreatichthys andruzzii is one of many colorless and eyeless fish species broadly described as cavefish; they are missing eyes, which allows the red flesh beneath to show through</p><br />

Similarities in Shape

The response of fishes’ body forms to the selective pressure of the environment is remarkable. Pikes and barracudas are very similar in form and in behavior even though they are not close relatives (Fig. 4.80). Barracuda and pikes are an excellent example of convergence, a process by which natural selection has arrived at identical or almost identical solutions to the same problem in distantly related species. Pikes are often called the freshwater barracudas because the two hunters look so similar.

Both pikes and barracudas are diurnal, visual predators. They stalk and then ambush their food with quick, powerful bursts of speed. The pike even has a special sighting groove on its snout that increases its visual acuity.

<p><strong>Fig. 4.80.&nbsp; (A)</strong> Great barracuda, a streamlined predator in tropical saltwater habitats</p><br />
<p><strong>(B) </strong>Muskellunge pike (Esox masquinongy), a streamlined predator in North American freshwater lake habitats</p><br />

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.