14.5: Selected Adaptations in Fishes

Each marine species is adapted in different ways that enable it to survive. Fishes are adapted in many different ways to respond to such challenges as swimming in a manner that best supports their hunting and defensive strategies, the high osmotic pressure of seawater, the variability of osmotic pressure in estuarine waters, and control of their buoyancy.

Swimming Adaptations

Water is much denser than air, so it is much more difficult to travel through water than through air. The next time you go swimming, try running through knee-deep water and you will understand just how much more difficult it is to move through water. Fishes must overcome water resistance to swim. Therefore, the body shape of a fish must be optimized to facilitate its specific swimming habits.

While swimming, fishes must overcome three types of resistance or drag: surface drag, form drag, and turbulent drag. Surface drag is the friction between a fish’s body surface and the water. Form drag comes about because water must be pushed out of the way, and turbulent drag is related to the smoothness of water flow past the swimming object.

Surface drag increases as the surface area in contact with the water increases. Consequently, surface drag is minimized if the swimming object is spherical because a sphere has the smallest surface area per unit volume of any solid object. Form drag increases in proportion to cross-sectional area (Fig. 14-21). The perfect shape to minimize form drag is needlelike, but this shape has a high ratio of surface area to volume and is subject to increased surface drag. A few fish species are needle-shaped (Fig. 14-22a), and others have an extremely thin platelike body (Fig. 14-22b,c) to minimize form drag. Most fish shapes are a compromise: rounded or oval in cross section, but elongated in the swimming direction. This shape tends to minimize total (form plus surface) drag (Fig. 14-21).

Figure 14-21. Three types of drag slow an object as it moves through water: surface drag, form drag, and turbulence drag. For each type, the magnitude of the drag varies with the shape of the moving object. This figure shows the optimum shape required to minimize each type of drag. Body shapes of most fish species are a compromise between the need to minimize drag and other needs of the animal, such as acceleration and turning speed. Fishes that swim constantly at high speed, like the tuna in this figure (Thunnus sp., Fiji), have a body shape that provides the best compromise for reducing the sum of the three types of drag: a rounded body that is narrow at the tail and thicker near the front. Nuclear submarines also are built in this general shape.

 

 

 

Figure 14-22. Some fish shapes that minimize form drag. (a) The cornetfish (Fistularia commersonii, Red Sea) has a very long, needle-shaped body to minimize form drag. (b) Side view of Klein’s butterflyfish (Chaetodon kleinii, Papua New Guinea). (c) Front view of the same species of butterflyfish. Note the lateral compression of the body (narrower across than vertically), which reduces form drag and enables the fish to execute turns quickly.

The third form of drag, turbulent drag, dictates the final refinement in fish body shape. The turbulent flow of water around the fish is reduced if the front is rounded and blunt and the rear tapers to form a teardrop shape (Fig. 14-21). This shape is similar to the cross section of an airplane wing, or the form of a blimp or submarine, all of which are designed to minimize drag.

The fastest and most continuously swimming fishes, such as tuna (Fig. 14-21), have a generally teardrop shape to minimize total drag and thus minimize energy used in swimming. Eyes that are flush and smoothly contoured against the body and a slimy coating are other adaptations that reduce surface drag. Fishes that do not swim continuously and fishes that are adept at fast turns rather than high speed have less need to reduce drag than fast swimmers have. Thus, these fishes often have body shapes that are greatly modified from the teardrop form to conform with their own special habits. Fishes that specialize in short bursts of speed with quick accelerations generally are somewhat thickened in the middle (Fig. 14-23) by the heavy musculature necessary for such maneuvers (similar to the difference between long-distance runners and weight lifters). Fishes that specialize in quick turns generally are somewhat flattened (Fig. 14-22b,c) so that the flat sides of their body can be used in turning in much the same way that a boat’s rudder is used. Fishes that hunt mostly by stealth or feed on plankton and do not need to swim quickly often have bizarre body shapes (Fig. 14-16e). Fin shapes are also modified to accommodate different swimming habits.

Figure 14-23. The body shapes of fish species are related to their swimming habits. Efficient cruising with minimal energy expenditure, maximum maneuverability, and rapid acceleration are all desirable but require different body shapes. Because there is no perfect shape, each species tends to specialize or compromise in a different way, as shown here. The species shown are (a) Meyer’s butterflyfish (Chaetodon meyeri, Papua New Guinea), (b) sergeant fish (Abudefduf sp., Papua New Guinea), (c) cheeklined wrasse (Oxycheilinus digramma, Papua New Guinea), (d) yellowstreak fusilier (Pterocaesio lativittata, Vanuatu), (e) flounder (Bothus sp., Hawaii), (f) trevally (probably Carangoides sp., Fiji), and (g) chevron barracuda (Sphyraena putnamiae, Solomon Islands). The relative sizes of the species are not preserved in this figure. Although most fish species follow the general relationships between shapes and behaviors depicted in this figure, some do not fit this pattern, because their behavior is highly specialized. For example, frogfishes (Fig. 14-14b) are sedentary, except for short bursts of rapid acceleration. Their thick bodies provide the musculature for these quick bursts, but they cannot sustain the movement as barracudas can.

Adaptations of Fins

Fish fins are extremely important in swimming and executing turns, just as the tail fin, rudder, short rear wings, and wing flaps of an airplane are important in controlling course and stability. Fishes generally have a pair of pectoral fins (one on each side of the body behind the head), two dorsal fins along the center of the back, and an anal fin beneath the rear half (Fig. 14-24a). They also have a caudal fin at the posterior, and a pair of pelvic fins on either side of the lower body forward of the anal fin. These fins vary greatly among species in size, shape, and location, and some fins may be absent in certain species.

 

 

Figure 14-24. Fish fins and swimming motion. (a) Fishes have a dorsal fin or fins along the back, pectoral fins on either side of the body, a pair of pelvic fins under the forward half of the body, an anal fin under the rear half of the body, and a caudal fin at the tail. Each of these types of fins may be much modified for special purposes in different fish species. The fish shown here is a threadfin, or red-cheeked fairy basslet (Pseudanthias huchtii, Papua New Guinea). (b) Most fish species swim by using a series of undulating body motions. (c) Triggerfishes and their relatives, such as this barred filefish (also called an orange-fin filefish; Cantherhines dumerilii, Hawaii) swim not by undulating their bodies, but by back-and-forth motions of their specially adapted soft dorsal and anal fins. This movement gives them great maneuverability, but it is less efficient in providing acceleration than is the undulating body motion method of swimming.

The pairs of pelvic and pectoral fins are used primarily to execute maneuvers, including turns and stops, and usually can be folded flat against the body when not in use. The vertical dorsal and anal fins serve primarily as stabilizers during swimming and, in some species, can be folded against the body when not needed. The caudal fin is the primary provider of propulsion in most species. Most fishes also alternately contract and relax muscles along their body to create a wavelike motion that travels along the body and produces a forward thrust (Fig. 14-24b). The caudal fin that provides the final thrust is flared out vertically to provide a large surface area and, consequently, strong thrust as it is moved from side to side. You can experience such a thrust increase if you use swim fins.

Increasing the caudal fin surface area increases thrust but also increases surface drag. Therefore, the caudal fin is modified to reflect the swimming habits of the species. These modifications are shown by the aspect ratio of the fin, which is defined as

(fin height)² / (fin area)

Five major types of caudal fins are distinguished by different ranges of aspect ratio and fin shape (Fig. 14-25). Rounded fins (aspect ratio 1; Fig. 14-25a) are useful for maneuvering and quick acceleration by species such as butterflyfishes (Figs. 14-14e, 14-22b,c, 14-23a). Truncate fins (aspect ratio 3; Fig. 14-25b) and forked fins (aspect ratio 5; Fig. 14-25c) reduce drag in comparison with rounded fins, but they still provide substantial maneuvering assistance. Truncate and forked fins are used by many species that swim reasonably fast but also maneuver relatively quickly (Fig. 14-23). Lunate caudal fins (aspect ratio up to 10; Fig. 14-25d) are typical on fast and continuously swimming species, such as trevally (Fig. 14-17a,b, 14-23f), tuna (Figs. 12-22a, 14-21), marlin, and swordfish. Lunate caudal fins in these species are rigid and have little use in maneuvering, although they are very efficient in forward propulsion. Fast-swimming predatory species with lunate caudal fins can outrun other species with truncate, forked, or rounded caudal fins, but the prey species have an excellent chance of avoiding the predator by using their greater maneuverability.

Figure 14-25. The shape of the caudal fin in fish species is related to their swimming behavior. There are five basic types of caudal fins, characterized by different aspect ratios, although caudal fins of many species have characteristics intermediate between these categories. The caudal fins shown here belong to (a) a blue-girdled angelfish (Pomacanthus navarchus), (b) a redbreasted Maori wrasse (Cheilinus fasciatus), (c) a male black-spot angelfish (Genicanthus melanospilos), (d) a trevally (Carangoides sp.)—all photographed in Papua New Guinea—and (e) a gray reef shark (Carcharhinus amblyrhynchos), photographed in Palau. Their relative sizes are not preserved in this figure.

Heterocercal caudal fins are asymmetrical, the upper lobe being longer and taller than the lower lobe (Fig. 14-25e). This type of caudal fin is used primarily by sharks (Fig. 12-23). It is similar to the lunate caudal fin in that it provides very efficient forward thrust but little help in maneuvering. However, its asymmetrical shape also provides upward lift, which is important to sharks because they have no swim bladder and tend to sink if they stop swimming. Lift is also provided by the sharks’ pectoral fins.

Unlike those of most other fishes, sharks’ pectoral fins are large, flat, and relatively inflexible (Fig. 12-23), providing lift like aircraft wings. The pectorals are relatively far forward on the shark’s body (Fig. 12-23) and lift the front of the shark, while the caudal fin lifts the tail. Although sharks are powerful swimmers, their fins are designed poorly for maneuvering, and they are not adept at capturing prey that can anticipate their charge and perform evasive maneuvers. Unfortunately, this knowledge is of little use to swimmers who may be attacked by sharks, because human body shapes do not allow for quick maneuvers in the water.

Although most fish species use their fins for swimming, numerous species have fins adapted to perform highly specialized functions. A number of species have dorsal fins modified to act as defensive, and perhaps offensive, weapons. In these species, the individual rays or spines of the dorsal fins, or parts of the dorsal fins, are needle-sharp and may contain a venom that is injected into any predator that challenges the fish. Species with this type of dorsal fin include stonefishes and scorpionfishes (Fig. 14-16d), which lie on a reef or the seafloor and use their dorsal defenses to protect against attacks from above. They also include lionfishes (Fig. 14-26a), which swim close to a reef, turning their backs toward any approaching predator and their more exposed undersides toward the reef. In some species, such as frogfishes (Fig. 14-28c) and anglerfishes (Fig. 14-15), the forward-most ray or rays of the dorsal fin are adapted to become lures that can be dangled in front of the fish’s mouth to entice prey.

 

 

 

 

Figure 14-26. Examples of fin adaptations. (a) The zebra lionfish (Dendrochirus zebra, Papua New Guinea) has long, poisonous dorsal-fin spines to protect itself. (b) The clown triggerfish (Balistoides conspicillum, Philippines) has a strong, sharp spine that it can raise when threatened. Like many other reef fishes, clown triggers are territorial and will chase off any intruder of their own species. (c) The speckled sandperch (Parapercis hexophtalma, Fiji) uses its pectoral fins for support as it sits and waits for its prey. (d) Wrasses, like this moon wrasse (Thalassoma lunare, Papua New Guinea), use their pectoral fins like oars to propel themselves. (e) Many fishes, especially those that live in holes in the reef or seafloor, such as this Old Glory goby (Amblygobius rainfordi, Papua New Guinea) have long, fused dorsal and anal fins. (f) This darkspotted moray eel (Gymnothorax fimbriatus, Papua New Guinea), photographed while hunting at night, showed aggression when disturbed by the diver’s lights. Note the sinuous, snakelike appearance. (g) This remora, or sharksucker (Echeneis naucrates, Papua New Guinea), was attached to a scuba diver’s leg, where it stayed for some time before leaving to try to find another ride.

In triggerfishes (Fig. 14-26b), the forward dorsal fin consists primarily of a single strong, rigid spine that normally lies flush against the fish’s body. This spine can be extended from the body to become a fearsome weapon that gives the triggerfish its name. Certain triggerfish species may also use this spine defensively. When chased, they swim headfirst into holes in the reef and extend their trigger to lock themselves in. Because of the way the trigger is hinged, almost no amount of tugging by a predator can pull the triggerfish out. Once the predator leaves, the trigger can be relaxed, allowing the triggerfish to back out of its refuge. Triggerfishes also use modified fins for swimming. The anal and dorsal fins are enlarged and undulate back and forth in wavelike motions that replace the body undulations used by other fishes. These fins enable triggerfishes to hover, turn, and swim slowly forward or backward to enter holes in the reef just wide enough for them to fit through.

Many species that live or rest frequently on the seafloor have pectoral and sometimes caudal fins that are elongated and have strong rigid spines on which the fish can rest. Sandperches (Fig. 14-26c) have this type of adaptation. In certain deep-sea species that inhabit areas where currents are generally weak, both the pectoral fins and the lower lobe of the caudal fins are elongated to an extent that can exceed the fish’s body length. These fishes “walk” on the seafloor as if perched on a tripod.

Like triggerfishes, wrasses (Fig. 14-26d) do not normally swim by using body undulations for propulsion. Instead, they propel themselves with their pectoral fins, which they stroke back and forth in much the same way that an oar is used. The fin is moved backward while spread vertically to push the water back and the fish forward. Then it is rotated and moved forward while in a horizontal orientation that minimizes drag. Wrasses can also swim by using their caudal fin and body undulation in the same way that other fishes do, but the oar-like propulsion created by using the pectoral fins provides better control of movements at slow speeds. Such control is ideally suited to the wrasses’ feeding habit of picking small crustaceans, algae, and individual coral polyps from cracks and crevices in reefs.

Many fishes spend most of their lives concealed in holes in a reef and do very little swimming. These fishes generally have greatly reduced fins, and their dorsal, caudal, and anal fins are often fused into one continuous fin extending around the fish. In extreme cases, the dorsal and anal fins may be missing entirely. Such fishes usually have an elongated body and swim by using sinuous body undulations. Swimming without the use of dorsal and anal fins is slow and inefficient, but it is perfectly suited to the lifestyles of these fishes. Snakelike flexibility and the lack of protruding fins enable them to swim easily through the narrow, tortuous passages of holes in the reefs where they live. Species adapted in this way include gobies (Fig. 14-26e) and moray eels (Fig. 14-26f).

Flyingfishes have some of the most bizarre fin adaptations. These warm-water fishes have elongated pectoral (and, in some species, also pelvic) fins that can be spread out from the sides to resemble bird wings. When they sense danger, flyingfishes swim upward with a rapid burst of speed that carries them through the water surface and into the air. Once in the air, they spread their fins and, using these “wings,” sail a few tens of centimeters above the waves. They can glide for as long as 30 s, and some species can prolong their glide by flailing an elongated lower lobe of the caudal fin at the sea surface as they descend close to the water. Although flying is a very effective strategy to escape from some predators, flyingfishes expose themselves to predation by seabirds.

A number of species, such as clingfishes and remoras (Fig. 14-26g), use fins as suction devices with which they can hold onto the seafloor or another organism. Clingfishes use modified pelvic fins to cling tenaciously to rocks in coastal areas where wave action is intense. Remoras use a modified dorsal fin as a suction cup to attach the top of their head to sharks, manta rays, other large ocean animals, and even boats and scuba divers. The remora’s sucker looks so little like a fin that careful research was needed to identify its origin. Although remoras must swim rapidly to catch and attach to a host, they are then transported without having to expend energy. Remoras can detach themselves to feed on any nearby available food, particularly scraps of their host’s meal if they are riding on a shark or other predator.

Ghost pipefishes (Fig. 14-16e) and seahorses (Fig. 14-16h,i) have perhaps the most extreme adaptation of body form and fins. Some species swim in a vertical head-down or head-up position and propel themselves slowly by rapid back-and-forth oscillations of their small dorsal fins. Because the fish is oriented vertically, the thrust from these fins is oriented perpendicular to the fish’s body. Ghost pipefishes thus swim sideways, although they swim in a normal horizontal position when they need to swim rapidly to avoid danger.

Osmoregulation

The relative proportions of dissolved chemicals in body fluids of fishes, other vertebrates, and invertebrates are remarkably similar to their relative proportions in seawater. In most invertebrates, the salinity of internal fluids is also the same as the external seawater salinity. However, the internal fluids of fishes are less saline than seawater. The reason is that bony fishes are thought to have first evolved in freshwater. Because osmosis causes water to diffuse across cell membranes from lower salinity to higher salinity, fishes must be able to counteract osmosis.

Osmosis of water molecules across a semipermeable membrane (such as the cell surface) from lower to higher salinity is easy to understand. Because the “concentration” of water molecules is higher in the lower-salinity fluid, more water molecules are in contact with that side of the membrane. Thus, more water molecules diffuse through the membrane toward the higher-salinity fluid than diffuse in the opposite direction. In contrast, dissolved salt molecules would be more likely to diffuse from high to low salinity because of their higher concentrations, but ions of dissolved salts are generally much larger than water molecules and hence less likely to pass through the openings in a semipermeable membrane.

One way to counteract osmosis is to increase the pressure on the high-salinity side of the membrane, which forces more water molecules through the membrane to the lower-salinity fluid. The pressure needed to balance water migration across a membrane is called “osmotic pressure.” Osmotic pressure increases as the difference in salinity between the fluids on either side of the membrane increases.

Marine fishes must have a mechanism for counteracting osmosis or they would continuously lose water to their surroundings and dehydrate. Fishes cannot maintain a pressure difference across their external membrane, because they would be able to do so only if they had an impermeable body surface, like the pressure hull of a submarine. With an impermeable body surface, feeding and excretion of waste products would be extremely difficult because they would have to take place through the equivalent of a submarine airlock. In addition, all aquatic organisms must exchange oxygen, carbon dioxide, and nutrients with seawater through a porous membrane. Consequently, fishes have adapted methods called osmoregulation to counteract osmosis in seawater.

Fishes that live in seawater osmoregulate by drinking seawater to replace water lost from their internal fluids by osmosis. The excess salt ions ingested with the seawater are excreted through specially adapted cells in the gills (Fig. 14-27a). Freshwater fishes osmoregulate by drinking almost no water and excreting large volumes of very dilute urine to discharge the water that enters their bodies by osmosis. They also must take up dissolved salts through their gills (Fig. 14-27b). Certain estuarine fish species or species that live in environments of variable salinity, such as tide pools, must be able to osmoregulate in both directions. Because few fish species have such ability, most estuarine fishes migrate within the estuary as river and tidal flows vary to remain at the same approximate external salinity.

Figure 14-27. Osmoregulation. (a) Fishes that live in seawater, such as the Goldman’s sweetlips (Plectorhinchus goldmanni, Vanuatu) illustrated here, lose water continuously by osmosis across their body surfaces and must drink seawater and excrete salt through specially adapted cells to replace the lost water. (b) Fishes that live in freshwater, such as Rainbow trout (Oncorhynchus mykiss), absorb water continuously by osmosis and get rid of the excess by producing copious amounts of very dilute urine.

Two special categories of fishes have evolved life cycles that require them to cross the salinity gradient between freshwater and seawater twice during their life cycles. These are the anadromous and catadromous fishes (Chap. 13).

Swim Bladders and Buoyancy 

Some fish species that live most of the time on the ocean floor can afford to be negatively buoyant (dense enough to sink) because they expend relatively little energy swimming against gravity during their limited excursions above the seafloor. Examples include most species of frogfishes (Fig. 14-28c) and scorpionfishes (Fig. 14-16d). Most fishes live in the water column and cannot afford to expend energy by swimming continuously to counteract gravity and maintain their depth. Consequently, most pelagic fishes (and other vertebrates and some invertebrates) must find a way to adjust their buoyancy to be approximately equal to that of seawater. The two primary ways of doing this are to synthesize and retain low-density oils, or to develop a gas-filled bladder.

Many fish species that live in surface layers and mid depths achieve neutral buoyancy by filling an internal swim bladder with gas. The amount of gas within the swim bladder must be adjusted as the fish changes depth. Otherwise, the gas will expand or contract and change the buoyancy of the fish as it ascends or descends. Some fishes with swim bladders make only limited and slow vertical excursions because the exchange of gas between blood and swim bladders is slow. Such species often die if caught and brought rapidly to the surface, because the swim bladder expands faster than the fish can evacuate the gas (Fig. 14-15). Other species have a special duct that connects the swim bladder with the esophagus, and they can ascend rapidly by “burping” to release excess gas from the swim bladder. Moray eels (Fig. 14-26f) have this duct, allowing them to change depth rapidly while hunting prey.

In shallow-water fishes, the gases in the swim bladder are similar to the atmosphere in composition: about 20% oxygen and 80% nitrogen. Some shallow water fishes fill their swim bladder by surfacing and gulping air. Fishes that live at greater depths have higher oxygen concentrations in their swim bladders, in some species up to 90% oxygen. The oxygen concentration in swim bladder gas reflects the mechanism fishes employ to force gas from solution in their blood to a gas in the swim bladder which is more effective for oxygen than for other gases. 

At a depth of 7000 m, the pressure is so great and gases so compressed that their density is approximately the same as that of fats. Consequently, many deep-water fishes have swim bladders filled with oil or fat instead of gases. These fishes do not need to adjust the amount of gas in the swim bladder as they change depth.

The largest and most active swimmers, such as mackerel and tuna (Figs. 12-22a, 14-21), have no swim bladder. These fishes can afford the relatively small energy penalty required to maintain their depth against their negative buoyancy because they expend much greater amounts of energy in swimming. Many of these species, especially sharks, have fins and body shapes designed to counteract their negative buoyancy as they swim. Sharks also have large livers with high concentrations of lighter-than-water oils to provide some compensation for their negative buoyancy.