Cookiecutter shark

Appearance

The cookiecutter shark has an elongated, cigar-shaped body with a short, bulbously rounded snout. The nostrils have a very short flap of skin in front. The large, oval, green eyes are placed forward on the head, though not so that binocular vision is extensive. Behind the eyes are large spiracles, positioned on the upper surface of the head.

The mouth is short, forming a nearly transverse line, and is surrounded by enlarged, fleshy, suctorial lips. About 30–37 tooth rows are in the upper jaw and 25–31 tooth rows are in the lower jaw, increasing with body size. The upper and lower teeth are extremely different; the upper teeth are small, narrow, and upright, tapering to a single, smooth-edged cusp. The lower teeth are also smooth-edged, but much larger, broader, and knife-like, with their bases interlocking to form a single saw-like cutting edge. The five pairs of gill slits are small.

The pectoral fins are short and roughly trapezoidal in shape. Two spineless dorsal fins are placed far back on the body, the first originating just ahead of the pelvic fins and the second located just behind. The second dorsal fin is slightly larger than the first, and the pelvic fins are larger than either. The anal fin is absent. The caudal fin is broad, with the lower lobe almost as large as the upper, which has a prominent ventral notch. The dermal denticles are squarish and flattened, with a slight central concavity and raised corners. The cookiecutter shark is chocolate brown in color, becoming subtly lighter below, and a dark "collar" wraps around the gill region.

The fins have translucent margins, except for the caudal fin, which has a darker margin. Complex, light-producing organs called photophores densely cover the entire underside, except for the collar, and produce a vivid green glow. The maximum recorded length for this species is 42 cm for males and 56 cm for females.

Naming

The name "cookiecutter shark" refers to its feeding habit of gouging round plugs, as if cut out with a cookie cutter, out of larger animals.

Distribution

Inhabiting all of the world's major tropical and warm-temperate oceanic basins, the cookiecutter shark is most common between the latitudes of 20°N and 20°S, where the surface water temperature is 18–26 °C . In the Atlantic, it has been reported off the Bahamas and southern Brazil in the west, Cape Verde, Guinea to Sierra Leone, southern Angola, and South Africa in the east, and Ascension Island in the south. In the Indo-Pacific region, it has been caught from Mauritius to New Guinea, Australia, and New Zealand, including Tasmania and Lord Howe Island, as well as off Japan. In the central and eastern Pacific, it occurs from Fiji north to the Hawaiian Islands, and east to the Galápagos, Easter, and Guadalupe Islands. Fresh wounds observed on marine mammals suggest this shark may range as far as California in warm years.

Based on catch records, the cookiecutter shark appears to conduct a diel vertical migration up to 3 km each way. It spends the day at a depth of 1–3.7 km, and at night it rises into the upper water column, usually remaining below 85 m, but on rare occasions venturing to the surface. This species may be more tolerant of low dissolved oxygen levels than sharks in the related genera ''Euprotomicrus'' and ''Squaliolus''. It is frequently found near islands, perhaps for reproductive purposes or because they hold congregations of large prey animals. In the northeastern Atlantic, most adults are found between 11°N and 16°N, with the smallest and largest individuals being found in lower and higher latitudes, respectively. There is no evidence of sex segregation.

Behavior

Best known for biting neat round chunks of tissue from marine mammals and large fish, the cookiecutter shark is considered a facultative ectoparasite, as it also wholly ingests smaller prey. It has a wide gape and a very strong bite, by virtue of heavily calcified cranial and labial cartilages. With small fins and weak muscles, this ambush predator spends much of its time hovering in the water column. To maintain neutral buoyancy, its liver, which can comprise some 35% of its weight, is rich in low-density lipids. As this species has higher skeletal density than ''Euprotomicrus'' or ''Squaliolus'', its body cavity and liver are proportionately much larger, and the oil content is much higher. Its large caudal fin allows for a quick burst of speed to catch larger, faster prey that come in range.

The cookiecutter shark regularly replaces its teeth like other sharks, but sheds its lower teeth in entire rows rather than one at a time. A cookiecutter shark 14 cm long has been calculated to have shed 15 sets of lower teeth by the time it is 50 cm long, totaling 435–465 teeth. This represents a significant investment of resources and is probably why the shark swallows its old sets of teeth, so that it can recycle the calcium content. Unlike other sharks, the retina of the cookiecutter shark has ganglion cells concentrated in a concentric area rather than in a horizontal streak across the visual field; this may help to focus on prey in front of the shark. This fat shark has been known to travel in schools, which may increase the effectiveness of its lure , as well as discourage counterattacks by much larger predators.

Food

Virtually every type of medium- to large-sized oceanic animal sharing the habitat of the cookiecutter shark is open to attack; bite scars have been found on cetaceans, pinnipeds, dugongs, sharks, stingrays, and bony fishes. The cookiecutter shark also regularly hunts and eats entire squid with a mantle length of 15–30 cm, comparable in size to the shark itself, as well as bristlemouths, copepods, and other prey of more modest dimensions.


Parasitic attacks by the cookiecutter shark leave a round "crater wound", averaging 5 cm across and 7 cm deep. The prevalence of these attacks can be high: off Hawaii, nearly every adult spinner dolphin bears scars from this species. Diseased or otherwise weakened animals appear to be more susceptible, and in the western Atlantic observations have been made of emaciated beached melon-headed whales with dozens to hundreds of recent and healing cookiecutter shark wounds, while such wounds are rare on nonemaciated beached whales. The impact of parasitism on prey species, in terms of resources diverted from growth or reproduction, is uncertain.

The cookiecutter shark exhibits a number of specializations to its mouth and pharynx for its parasitic lifestyle. The shark first secures itself to the body surface of its prey by closing its spiracles and retracting its basihyal to create pressure lower than that of the surroundings; its suctorial lips ensure a tight seal. It then bites, using its narrow upper teeth as anchors while its razor sharp lower teeth slice into the prey. Finally, the shark twists and rotates its body to complete a circular cut, quite possibly aided by the initial forward momentum and subsequent struggles of its prey. The action of the lower teeth may also be assisted by back-and-forth vibrations of the jaw, a mechanism akin to that of an electric carving knife. This shark's ability to create strong suction into its mouth is likely also of use in capturing smaller prey such as squid.

Evolution

Like other dogfish sharks, the cookiecutter shark is aplacental viviparous, with the developing embryos being sustained by yolk until birth. Females have two functional uteri and give birth to litters of 6 to 12 pups. A case has been recorded of a female carrying 9 embryos 12.4–13.7 cm long; though they were close to the birth size, they still had well-developed yolk sacs, suggesting a slow rate of yolk absorption and a long gestation period. The embryos had developed brown pigmentation, but not the dark collar or differentiated dentition. Newborn cookiecutter sharks measure 14–15 cm long. Males attain sexual maturity at a length of 36 cm, and females at a length of 39 cm.