12.13: Nekton

Nekton are animals that swim strongly enough to move independently of ocean currents. Most nekton are vertebrates, which are organisms that have an internal skeleton, spinal cord, and brain. Marine vertebrates include fishes, reptiles, birds, and marine mammals. Some invertebrates, such as squid, are also nekton. The nekton are dominated by the varied and abundant fish species. There are three types of fishes: primitive jawless fishes including hagfishes and lampreys; fishes that have cartilage skeletons, including sharks and rays; and bony fishes that have bone (largely calcium carbonate and calcium phosphate) skeletons. More than 95% of all living species of fishes are bony fishes.

Bony Fishes

Bony nektonic fishes have a wide variety of body forms and range in length from just a few centimeters to more than a meter. Such pelagic giants as tuna (Fig. 12-22a) or swordfish can exceed 3 m. Species of fishes that live on or near the seafloor are called “demersal” fishes, and those living predominantly in the water column are called “pelagic” fishes.

   

   

Figure 12-22. Representative species of fishes. (a) The dogtooth tuna (Gymnosarda unicolor, Papua New Guinea) is an oceanic predator. (b) A school of Pacific sardines (Sardinops sagax). (c) The Atlantic cod (Gadus morrhua) was once extremely abundant, but its populations have now been substantially reduced by overfishing. (d) The flowery flounder (Bothus mancus, Solomon Islands) spends much of its life lying camouflaged on the seafloor.

The most abundant fishes in coastal waters are various species of silver-bodied schooling fishes including herrings, anchovies, pilchards, sardines (Fig. 12-22b), and menhaden. Because these fishes generally do not exceed a few tens of centimeters in length and have oily tissues, they were historically undervalued in some cultures, especially for their use as fresh fish in the U.S. and Great Britain. However they are now highly valued for human consumption and industrial uses. Their numbers are so vast that the world’s most productive commercial fisheries target these species. Much of the catch is processed and used as animal feed, fish oils for paints and diet supplements, soaps, and lubricants. These species have each been overfished, and their historical populations have been decimated in many coastal regions (Chap. 16). Various species of cod (Fig. 12-22c) are extremely abundant in middle and high latitudes, especially in the Northern Hemisphere, and they are commercially very valuable. Cod are demersal, living most of their lives on or near the ocean floor, where they feed on invertebrates and small fishes.

The body shapes of fishes are determined primarily by their swimming habits (Chap. 14). Species that are members of the flatfish group have bodies modified to enable them to lie flat on the seafloor, where they remain well camouflaged (Fig. 12-22d). Juvenile flatfishes, like other fishes, have eyes on each side of their body. As they mature, one eye migrates across the body so that both eyes are on the same side of the head (Fig. 12-22d). If the eye migrates to the left side of the body, the species is usually called a “flounder.” If it migrates to the right side, the species is usually called a “sole.” However, some species, such as the starry flounder, have both left- and right-eyed forms.

Many fishes contain swim bladders filled with a gas that is usually a mixture of oxygen and carbon dioxide. Swim bladders are used to maintain buoyancy, and, at least in some species, they appear to be used in the fish’s “hearing” mechanism. Because gas expands or contracts as pressure changes with depth, fishes with swim bladders must add or remove gas from their swim bladders as they move vertically. Some fishes absorb the gas back into the bloodstream, and others, such as herrings, can release the gas as bubbles. Many fishes bloat and die if brought rapidly to the surface because they cannot purge their swim bladders fast enough. Fish species that migrate vertically each day often have no swim bladders, but maintain buoyancy through high concentrations of lighter-than-water oils.

Sharks and Rays

Sharks and rays are more primitive than other fishes, and many species have existed essentially unchanged for millions of years. Unlike bony fishes, sharks and rays have skeletons made of cartilage. Sharks and rays have reproductive cycles that require eggs to be fertilized inside the female. Most other fish species reproduce by the simultaneous release of eggs and sperm by female and male. In many cartilaginous fish species, the fertilized eggs are hatched and develop into the adult form within the mother before being released in a live birth.

The common perception of sharks is that they are fearsome predators that will hunt and kill anything, including people (Fig. 12-23a). Although many species are able to hunt and kill large fishes and marine mammals, only a few species are known to attack people. Because humans are not the natural prey of sharks, most shark attacks on humans are thought to be cases of mistaken identity. The fact that most carnivorous sharks are opportunistic hunters, honing in on weak and dying prey, belies their reputation.

   

   

   

   

Figure 12-23. Representative sharks and rays. (a) Great white shark (Carcharodon carcharias), attracted to a diver’s bait. (b) Reef whitetip shark (Triaenodon obesus, Papua New Guinea) with a sharksucker, or remora (Echeneis naucrates) attached. (c) Gray reef shark (Carcharhinus amblyrhynchos, Fiji), which is called a “bronze whaler” in Fiji. (d) Scalloped hammerhead shark (Sphyrna lewini, Papua New Guinea). (e) The whale shark (Rhincodon typus, Papua New Guinea) is the largest known shark species. This one was quite small, only about 8 to 10 m long. (f) The graceful manta ray (Manta birostris, Philippines). (g) Up close and personal with a nurse shark (Nebrius concolor, Philippines). (h) Blue-spotted rays (Taeniura lymma, Red Sea) have a dangerous barb on the tail. (i) The torpedo ray (family Torpedinidae, Red Sea) can inflict a powerful electric shock. The whale shark and manta ray are plankton feeders, the nurse shark eats benthic invertebrates, and the other sharks are scavengers and carnivores.

Species such as the reef whitetip, gray reef, and hammerhead (Fig. 12-23b-d) are regularly attracted to dead fishes set as bait by scuba divers. I have witnessed several occasions when these sharks ate such bait while swimming frantically among surrounding divers. Even when agitated, these sharks do not attack the divers and can almost always be chased away easily on the extremely rare occasions when they do attack. Other shark species, such as the tiger and great white, are more dangerous, and cautious divers will leave the water when they appear.

Although many sharks eat fishes and marine mammals, a number of shark species feed only on plankton. Plankton-eating sharks include basking sharks that grow to 12 m in length and whale sharks (Fig. 12-23e) that grow to more than 15 m. The beautiful and graceful manta ray is also a plankton eater (Fig. 12-23f). Plankton-eating sharks and rays swim with mouths wide open. Large volumes of water pass into the mouth and are filtered to capture the plankton before flowing out of the gills. The nurse shark (Fig. 12-23g) and many species of rays (Fig. 12-23h,i) feed only on mollusks and other benthic animals.

Many rays, such as the blue-spotted sting ray (Fig. 12-23h), have a barbed stinger embedded in the tail. They use the stinger as a defensive weapon against natural predators, such as sharks, and occasionally against a human foot. Torpedo rays (Fig. 12-23i) deliver a strong electric shock that can be lethal to humans, as much as 200 volts, from an organ in the head.

Squid and Their Relatives

Squid (Fig. 12-24a), nektonic mollusks of the class Cephalopoda, are extremely abundant in the oceans. Squid normally live in schools that can contain huge numbers of individuals. They are extremely fast swimmers (using their fins) and voracious predators of small fishes. They can also jet-propel themselves by ingesting water and forcibly squirting it out through a special cavity. They can even propel themselves several meters into the air. Most squid species live below the photic zone during the day and migrate to the mixed layer at night to feed. The largest squid species, the giant squid, can grow to 16 m or more in length (6 m of body and 10 m of tentacles). It lives in the deep ocean and is rarely caught or seen, but it is known to be a favorite food of sperm whales.

   

   

   

   

Figure 12-24. Cephalopod mollusks. (a) A common reef squid (Sepioteuthis lessoniana, Indonesia) photographed at night. (b,c) Broadclub cuttlefish (Sepia latimanus, Indonesia and Papua New Guinea). (d) Stumpy spined cuttlefish (Sepia bandanensis, Indonesia). (e) Nautilus (Nautilus pompilius, Papua New Guinea), captured at a depth of several hundred meters in a trap similar to a lobster pot, which was baited with fish. The Nautilus was released to swim back to its deepwater home after this photograph was taken. (f) This sectioned Nautilus shell shows the internal chambers that are filled with gas at low pressure to provide buoyancy. Because these chambers are incompressible, Nautilus can safely migrate great vertical distances.

Cuttlefish (Fig. 12-24b-d) are cephalopods that have an unusual internal single “bone” or shell (the “cuttlebone” that is typically hung in parakeet cages). Nautilus species (Fig. 12-24e) have a calcium carbonate shell with a series of internal gas-filled chambers (Fig. 12-24f). They are an ancient form of cephalopod that has changed little over millions of years. All cephalopods are believed to have had external shells at one time. Nautilus species, cuttlefish, and squid represent steps in cephalopod evolution in which the external shell, an excellent means of defense, has been discarded in favor of greater swimming speed, which is also an excellent means of defense, but more valuable for hunting prey.

Marine Mammals

Marine mammals, which include seals and sea lions, dolphins and other whales, and a number of other less familiar animals, are warm-blooded and breathe air. Their young are born live and nursed by their mothers, just as terrestrial mammals are.

The cetaceans, which include dolphins, porpoises, and other whales, live their entire lives in the water. The largest whales—the blue, finback, right, sei, and humpback (Fig. 12-25a-c)—are baleen whales that feed on plankton. The smaller gray whale, also a baleen whale, is unique because it feeds mainly on small crustaceans, mollusks, and worms that it stirs up from muddy sediments with its snout. Sperm whales feed primarily on squid, whereas most small whales, including the pilot and beluga whales, dolphins, and porpoises, eat mostly fishes. However, the killer whale (Fig. 12-25d) lives up to its name by hunting and eating fishes, seals, sea lions, and even other whales. Killer whales often hunt in packs like wolves. On several occasions, pods of killer whales have been filmed systematically harassing a California gray whale mother and calf until they were separated. If they successfully separate the mother and calf, the killer whales, as a pack, set upon the calf, which they then kill and eat.

   

   

   

Figure 12-25. Cetaceans. (a)The relative sizes of various toothed and baleen whales, compared to humans and the largest known dinosaur, Apatosaurus. (b) Blue whales (Balaenoptera musculus). (c) Humpback whales (Megaptera novaeangliae). (d) Killer Whales (Orcinus orca). (e) Pacific White Side dolphins (Lagenorhynchus obliquidens).

Manatees (Fig. 12-26) and dugongs, commonly called “sea cows,” are herbivorous sirenians that graze on vegetation in the shallow tropical coastal waters where they live. Sea cows are considered to be the source of the mermaid (or siren) legend.

Figure 12-26. Manatees, or sea cows, (Trichechus manatus) in the Crystal River, Florida.

Seals (Fig. 12-27a,b), sea lions (Fig. 12-27c), and walrus (Fig. 12-27d) are pinnipeds. Pinnipeds live and feed in the oceans for most of their lives, but they must haul themselves out of the water to breed and bear young and to rest and conserve body heat. The elephant seal (Fig. 12-27b), whose bizarre-looking adult males weigh 2 tonnes, is the largest pinniped. Most pinnipeds eat fishes, but walrus (Fig. 12-27d) eat clams and other shellfish from the sediment, and Antarctic leopard seals will eat invertebrates, seabirds, and other mammals.

   

   

   

Figure 12-27. Pinnipeds and otters. (a) Harbor seals (Phoca vitulina) haul out on the rocks near Monterey, California. (b) A scientist measures the length of an adult male elephant seal (Mirounga angustirostris) at the Año Nuevo State Reserve, California. This can be a dangerous job for the scientist because the elephant seal may weigh well over a tonne, can move surprisingly fast, and can be very aggressive when disturbed. (c) California sea lion (Zalophus californianus) populations are growing very quickly and becoming a nuisance in many harbors, where they aggressively compete for space when they haul out on any convenient platform, especially boat docks like this one in Monterey, California. (d) Walrus (Odobenus rosmarus) apparently use their tusks as the equivalent of sled runners that enable them to glide along the seafloor as they feed by sucking or slurping up sediments with their whiskered mouths. (e) Sea otters (Enhydra lutris, California) sometimes eat shellfish by resting the shellfish on their chests and pounding them open with a stone while floating on their backs.

The sea otter (Fig. 12-27e) is a unique shellfish-eating mammal that, unlike all other marine mammals, lacks an insulating layer of blubber. Instead, sea otters retain body heat with soft, thick fur that they must continuously groom if it is to retain its insulating properties.

Many marine mammals were once hunted intensively for their fur, meat, and oil. Because they have long life cycles, hunting quickly reduced some species to critically low population levels. Although certain species are still hunted, the hunting of marine mammals was stopped by most nations several decades ago. Most marine mammal populations that were decimated by hunting are now recovering slowly. However, some species are still threatened by the destruction of habitat (particularly pinniped breeding areas), by some fishing methods, such as the use of drift nets that kill mammals as well as targeted fishes, and by pollution (Chap. 16).

Reptiles

Only a small number of reptiles, which are air-breathing animals, live in the oceans. The best known are sea turtles, of which four large species are widely distributed in the oceans: the green (Fig. 12-28a), hawksbill (Fig. 12-28b), leatherback, and loggerhead. Green turtles are herbivorous and graze on sea grasses that grow in shallow lagoons. One common species of sea grass is known as “turtle grass” because it is a favorite food of these turtles. The hawksbill lives in tropical waters and eats mostly sponges. The loggerhead also eats sponges, but it adds crabs and mollusks to its diet. The biggest turtle, the leatherback, can weigh as much as 650 kg and is among the few large animals in the ocean that eat jellies as their preferred food.

   

   

   

   

Figure 12-28. Examples of the relatively few marine reptile species. (a) Green sea turtle (Chelonia mydas, Hawaii). (b) Hawksbill turtle (Eretmochelys imbricata, Fiji). (c,d) Banded sea snake (Laticauda colubrine, Indonesia). (e,f) Marine iguana (Amblyrhynchus cristatus), which lives only in the Galápagos Islands.

Turtles have been hunted for centuries. They are easy to catch and, if placed on their backs out of water, live for many weeks but cannot escape. For this reason, turtles were an ideal source of fresh meat on sailing vessels before refrigeration was developed. Turtles were common just 200 years ago, but all turtle species have been decimated by humans, and most are threatened or endangered by extinction. The Kemp’s ridley turtle, a relatively small species that lives only in the Gulf of Mexico and South Atlantic, is especially in danger of extinction.

Turtles lay their eggs in beach sands above the high-tide line. The eggs hatch in several weeks, and the young turtles immediately head to the sea. Turtle eggs are considered a delicacy in many areas, and the fact that they are easily dug out of the sand by human and other predators has contributed to the decline of turtle populations. In addition, turtles are easy for predators to catch when they climb onto the beach to lay their eggs and when they emerge as hatchlings.

Turtles use relatively few beaches throughout the world for breeding. They almost always return to lay eggs on the same beach where they hatched and usually will not lay eggs on any other beach. Because they are reluctant to lay eggs where human development has altered a beach, they have abandoned many traditional breeding beaches. The reduction in the number of breeding beaches that are suitably protected has seriously affected turtle populations.

The Pacific and Indian Oceans have about 50 species of sea snakes (Fig. 12-28c,d). Most are highly venomous, and their bite is generally lethal to people. Fortunately, sea snakes have small mouths and small fangs that cannot easily break human skin, unless they catch a finger or other small appendage. Sea snakes are also relatively shy and do not attack unless threatened. Some species live near the shore so that they can slither out of the water onto rocks to sun themselves, though most are fully aquatic. They eat small fishes, can dive as deep as 100 m, and can stay submerged for more than 2 h before they must come up for air.

In certain areas of Asia and the Pacific Islands, saltwater species of crocodiles feed on fishes in coastal waters. American alligators are also known to enter coastal-ocean waters, but normally they are confined to freshwater. The only marine species of lizard is the Galápagos marine iguana (Fig. 12-28e,f), which lives on land but frequently enters shallow coastal waters to graze on marine algae.

Birds

Penguins (Fig. 12-29a) are flightless birds that live in the oceans and feed on fishes. They leave the water only to lay eggs and brood their young. Most species are restricted to the cold waters near Antarctica, and none are present in the Northern Hemisphere.

   

   

Figure 12-29. Swimming seabirds that feed in the marine environment. (a) King penguins (Aptenodytes patagonicus), Fortuna Bay, South Georgia. Adults and molting juveniles. (b) Herring gull (Larus argentatus, California), one of the many species of seagulls. (c) Brown pelican (Pelecanus occidentalis, California). (d) Double-crested cormorants (Phalacrocorax auritus, Monterey, California), Cormorants are excellent underwater swimmers and dry their wings by holding them out in the sun.

Many species of seabirds feed exclusively on ocean fishes. Some, including most seagulls (Fig. 12-29b), feed at the ocean surface, but many, like the brown pelican (Fig. 12-29c), dive into the water to catch their prey. Others, such as the cormorant (Fig. 12-29d), are excellent swimmers that can dive tens of meters deep and stay submerged for as much as several minutes in pursuit of prey.