Tawny nurse shark

Appearance

The tawny nurse shark grows to a maximum length of 3.2 m . It has a robust, cylindrical body with a broadly rounded and flattened head. The eyes are small and face laterally, with prominent ridges over them and smaller spiracles behind. There are a pair of long, slender barbels in front of the nostrils. The mouth is small, with the lower lip divided into three lobes. There are 29–33 tooth rows in the upper jaw and 26–28 tooth rows in the lower jaw, arranged in an imbricate pattern with the outermost 2–4 functional rows separated from the rest by a narrow space. Each tooth resembles a fan, with a broad base rising to a small, sharp central point flanked by 3 or more smaller cusps on both sides. As the shark ages, the teeth become relatively taller and thicker. The fourth and fifth pairs of gill slits are placed much closer together than the others.

The dorsal and pelvic fins are angular, with the first dorsal fin larger than the second. The pectoral fins are narrow, pointed, and falcate ; their shape separates this species from the similar-looking nurse shark. The origin of the first dorsal fin is about even with the origin of the pelvic fins, while the origin of the anal fin is even with or somewhat behind the origin of the second dorsal fin. The caudal fin has a shallow upper lobe and barely present lower lobe, comprising about a quarter of the total length in adults. The dermal denticles are diamond-shaped, bearing 4–5 faint ridges radiating from a blunt point. Tawny nurse sharks are yellowish, reddish, or grayish brown above and off-white below, and are capable of slowly changing their color to better blend with the environment. Young sharks have starkly white lower eyelids.

Many tawny nurse sharks found off the coasts of Japan, Taiwan, and the Ryukyu Islands lack a second dorsal fin. This physical abnormality has been speculated to result from pregnant females being exposed to water of unusually high salinity and/or temperature, possibly from human activity. In 1986, a 2.9 m long adult male with both a missing dorsal fin and partial albinism was captured off Wakayama Prefecture, Japan. This anomalous individual is the largest albino shark known to date, having survived for a long time in the wild despite its lack of camouflage.

Distribution

The tawny nurse shark is widely distributed in the Indo-Pacific region. In the Indian Ocean, it is found from KwaZulu-Natal, South Africa northward to the Red Sea, Persian Gulf and India, including Madagascar, Mauritius, the Chagos Archipelago, the Seychelles, and the Maldives. In the western Pacific, it occurs from southern Japan and the coast of China to the Philippines, Southeast Asia, and Indonesia, to as far south as the northern coast of Australia. In the central Pacific, it has been reported from off New Caledonia, Samoa, Palau, the Marshall Islands, and Tahiti. Fossil teeth belonging to this species have been found in the Pirabas Formation of northern Brazil, dating back to the Lower Miocene . The presence of these fossils indicates that the range of the tawny nurse shark once extended to the tropical Atlantic Ocean, prior to the formation of the Isthmus of Panama.

An inshore species, the tawny nurse shark inhabits continental and insular shelves over sandy flats or beds of seagrass, as well as along the outer edges of coral or rocky reefs. This shark may be found from the surf zone, often in water barely deep enough to cover its body, to a maximum depth of 70 m on coral reefs; it is most common at a depth of 5–30 m . Young sharks are generally found in the shallow areas of lagoons, while adults may be encountered across a variety of habitats.

Behavior

With a more streamlined form than other nurse sharks, the tawny nurse shark is believed to be a less benthic, more active swimmer. The characteristics of its body, head, fins, and teeth are comparable to other active reef sharks sharing its range, such as the sicklefin lemon shark . Tawny nurse sharks are primarily nocturnal, though they are said to be active at all hours off Madagascar, and in captivity they will become diurnally active if presented with food. During the day, groups of two dozen or more sharks can be found resting inside caves and under ledges, often stacked atop one another. Individual sharks have small home ranges that they consistently return to each day.

The tawny nurse shark has few natural predators; attacks on this species have been reported from bull sharks and great hammerheads , while the related nurse shark has been known to fall prey to tiger sharks and lemon sharks . Known parasites of this species include five species of tapeworms in the genus ''Pedibothrium'', which infest the shark's spiral intestine.

Habitat

The tawny nurse shark is widely distributed in the Indo-Pacific region. In the Indian Ocean, it is found from KwaZulu-Natal, South Africa northward to the Red Sea, Persian Gulf and India, including Madagascar, Mauritius, the Chagos Archipelago, the Seychelles, and the Maldives. In the western Pacific, it occurs from southern Japan and the coast of China to the Philippines, Southeast Asia, and Indonesia, to as far south as the northern coast of Australia. In the central Pacific, it has been reported from off New Caledonia, Samoa, Palau, the Marshall Islands, and Tahiti. Fossil teeth belonging to this species have been found in the Pirabas Formation of northern Brazil, dating back to the Lower Miocene . The presence of these fossils indicates that the range of the tawny nurse shark once extended to the tropical Atlantic Ocean, prior to the formation of the Isthmus of Panama.

An inshore species, the tawny nurse shark inhabits continental and insular shelves over sandy flats or beds of seagrass, as well as along the outer edges of coral or rocky reefs. This shark may be found from the surf zone, often in water barely deep enough to cover its body, to a maximum depth of 70 m on coral reefs; it is most common at a depth of 5–30 m . Young sharks are generally found in the shallow areas of lagoons, while adults may be encountered across a variety of habitats.With a more streamlined form than other nurse sharks, the tawny nurse shark is believed to be a less benthic, more active swimmer. The characteristics of its body, head, fins, and teeth are comparable to other active reef sharks sharing its range, such as the sicklefin lemon shark . Tawny nurse sharks are primarily nocturnal, though they are said to be active at all hours off Madagascar, and in captivity they will become diurnally active if presented with food. During the day, groups of two dozen or more sharks can be found resting inside caves and under ledges, often stacked atop one another. Individual sharks have small home ranges that they consistently return to each day.

The tawny nurse shark has few natural predators; attacks on this species have been reported from bull sharks and great hammerheads , while the related nurse shark has been known to fall prey to tiger sharks and lemon sharks . Known parasites of this species include five species of tapeworms in the genus ''Pedibothrium'', which infest the shark's spiral intestine.

Food

The tawny nurse shark may be one of the few fishes specializing in preying on octopus. Other known food items include corals, sea urchins, crustaceans , squid, small fishes , and the occasional sea snake. Hunting tawny nurse sharks swim slowly just above the sea floor, poking their heads into depressions and holes. When a prey item is found, the shark forcefully expands its large, muscular pharynx, creating a powerful negative pressure that sucks the prey into its mouth.

Evolution

Mating in the tawny nurse shark is known to occur from July to August off Madagascar. Adult females have one functional ovary and two functional uteruses. The mode of reproduction is aplacental viviparity, meaning that the embryos hatch inside the uterus; females in captivity have been documented depositing up to 52 non-viable egg capsules, which has led to erroneous reports of this shark being oviparous. The egg capsules of this species are onion-shaped, with thin, brown, translucent shells. The tawny nurse shark is the only carpet shark in which there is oophagy: once the developing embryos exhaust their supply of yolk, they gorge on eggs produced by the mother and acquire the distended abdomen characteristic of such oophagous embryos. Unlike in mackerel sharks, the eggs consumed by the embryos are large and shelled rather than small and undeveloped. There is no evidence of sibling cannibalism as in the sand tiger shark .

Various authors have reported the length at birth anywhere from 40 to 80 cm , with the discrepancy possibly reflecting geographic variation. Although females release up to four fertilized eggs into each uterus, the very large size of the newborns suggest that the litter size may be as few as one or two. In one examined female that had two embryos sharing a single uterus, one embryo was much smaller and thinner than the other, implying that competition may eliminate the additional siblings. Males attain sexual maturity at a length of 2.5 m , and females at a length of 2.3–2.9 m .