Instruct: Given a question, retrieve passages that can help answer the question.
Query: Can you eat a dusky shark?
The dusky shark is a generalist that takes a wide variety of prey from all levels of the water column, though it favors hunting near the bottom. A large individual can consume over a tenth of its body weight at a single sitting. The bite force exerted by a long dusky shark has been measured at over the area at the tip of a tooth. This is the highest figure thus far measured from any shark, though it also reflects the concentration of force at the tooth tip. Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.
Stocks off the eastern United States are severely overfished; a 2006 stock assessment survey by the U.S. National Marine Fisheries Service (NMFS) showed that its population had dropped to 15–20% of 1970s levels. In 1997, the dusky shark was identified as a Species of Concern by the NMFS, meaning that it warranted conservation concern but there was insufficient information for listing on the U.S. Endangered Species Act (ESA). Commercial and recreational retention of dusky sharks was prohibited in 1998, but this has been of limited effectiveness due to high bycatch mortality on multi-species gear. In addition, some 2,000 dusky sharks were caught by recreational fishers in 2003 despite the ban. In 2005, North Carolina implemented a time/area closure to reduce the impact of recreational fishing. To aid conservation efforts, molecular techniques using polymerase chain reaction (PCR) have been developed that can identify whether marketed shark parts (e.g. fins) are from prohibited species like the dusky shark, versus similar allowed species such as the sandbar shark.
The great white shark has a robust, large, conical snout. The upper and lower lobes on the tail fin are approximately the same size which is similar to some mackerel sharks. A great white displays countershading, by having a white underside and a grey dorsal area (sometimes in a brown or blue shade) that gives an overall mottled appearance. The coloration makes it difficult for prey to spot the shark because it breaks up the shark's outline when seen from the side. From above, the darker shade blends with the sea and from below it exposes a minimal silhouette against the sunlight. Leucism is extremely rare in this species, but has been documented in one great white shark (a pup that washed ashore in Australia and died). Great white sharks, like many other sharks, have rows of serrated teeth behind the main ones, ready to replace any that break off. When the shark bites, it shakes its head side-to-side, helping the teeth saw off large chunks of flesh. Great white sharks, like other mackerel sharks, have larger eyes than other shark species in proportion to their body size. The iris of the eye is a deep blue instead of black.
This rail forages in shallow water near cover and eats mainly aquatic insects and crustaceans. It forages by probing the mud while moving around in shallow water.
With a gestation period estimated at up to 22–24 months and a one-year resting period between pregnancies, female dusky sharks bear at most one litter of young every three years. The litter size ranges from 3 to 16, with 6 to 12 being typical, and does not correlate with female size. Sharks in the western Atlantic tend to produce slightly smaller litters than those from the southeastern Atlantic (averaging 8 versus 10 pups per litter). Depending on region, birthing may occur throughout the year or over a span of several months: newborn sharks have been reported from late winter to summer in the northwestern Atlantic, in summer and fall off Western Australia, and throughout the year with a peak in fall off southern Africa. Females move into shallow inshore habitats such as lagoons to give birth, as such areas offer their pups rich food supplies and shelter from predation (including from their own species), and leave immediately afterward. These nursery areas are known along the coasts of KwaZulu-Natal, southwestern Australia, western Baja California, and the eastern United States from New Jersey to North Carolina.
At present, hang baits are illegal off Isla Guadalupe and reputable dive operators do not use them. Operators in South Africa and Australia continue to use hang baits and pinniped decoys. In South Australia, playing rock music recordings underwater, including the AC/DC album "Back in Black" has also been used experimentally to attract sharks.
The Bahamas sawsharks ("Pristiophorus schroeder") have very little information on them. Studies are being done daily to learn more about the deep sea dweller. They are located near Cuba, Florida, and the Bahamas (hence their name) where they dwell in the depths of 400–1000 m. As far as their appearance they can be identified by their snouts with teeth which appear as a saw, as well as their length, they are averaged at 80 cm in length.
Saw sharks and sawfish are cartilaginous fish possessing large saws. These are the only two fish that have a long blade-like snout. Although they are similar in appearances, saw sharks are distinct from sawfish. Sawfish are not sharks, but a type of ray. The gill slits of the sawfishes are positioned on the underside like a ray, but the gill slits of the saw shark are positioned on the side like a shark. Sawfish can have a much larger size, lack barbels, and have evenly sized teeth rather than alternating teeth of the saw shark. Clear difference is that a sawfish has no barbels and a saw shark has a prominent pair halfway along the saw. The saw shark uses these like other bottom fish, as a kind of antennae, feeling the way along the ocean bottom until it finds some prey of interest. Both the saw shark and the sawfish utilize the electroreceptors on the saw, ampullae of Lorenzini, to detect the electric field given off by buried prey.
Juvenile Caribbean reef sharks are preyed upon by larger sharks such as the tiger shark ("Galeocerdo cuvier") and the bull shark ("C. leucas"). Few parasites are known for this species; one is a dark variegated leech often seen trailing from its first dorsal fin. Off northern Brazil, juveniles seek out cleaning stations occupied by yellownose gobies ("Elacatinus randalli"), which clean the sharks of parasites while they lie still on the bottom. Horse-eye jacks ("Caranx latus") and bar jacks ("Carangoides ruber") routinely school around Caribbean reef sharks.
Shrimp and other shellfish are among the most common food allergens. The Jewish laws of Kashrut forbid the eating of shrimp. According to the King James version of the Old Testament, it is acceptable to eat finfish, but shrimp are an abomination and should not be eaten. In Islam, the Shafi'i, Maliki, Hanbali and Ja'fari schools allow the eating of shrimp, while the Hanafi school does not allow it.
For the most part Islamic dietary laws allow the eating of seafood, though the Hanbali forbid eels, the Shafi forbid frogs and crocodiles, and the Hanafi forbid bottom feeders such as shellfish and carp. The Jewish laws of Kashrut forbid the eating of shellfish and eels. In the Old Testament, the Mosaic Covenant allowed the Israelites to eat finfish, but shellfish and eels were an abomination and not allowed. In ancient and medieval times, the Catholic Church forbade the practice of eating meat, eggs and dairy products during Lent. Thomas Aquinas argued that these "afford greater pleasure as food [than fish], and greater nourishment to the human body, so that from their consumption there results a greater surplus available for seminal matter, which when abundant becomes a great incentive to lust." In the United States, the Catholic practice of abstaining from meat on Fridays during Lent has popularized the Friday fish fry, and parishes often sponsor a fish fry during Lent. In predominantly Roman Catholic areas, restaurants may adjust their menus during Lent by adding seafood items to the menu.
The diet of "Pteranodon" is known to have included fish; fossilized fish bones have been found in the stomach area of one "Pteranodon", and a fossilized fish bolus has been found between the jaws of another "Pteranodon", specimen AMNH 5098. Numerous other specimens also preserve fragments of fish scales and vertebrae near the torso, indicating that fish made up a majority of the diet of "Pteranodon" (though they may also have taken invertebrates).
The American oystercatcher feeds almost exclusively on shellfish and other marine invertebrates. Oysters are a staple of their diet, as their name suggests, but they also eat mussels, clams, limpets, sea urchins, starfish, crabs, and worms. In general, they use their bills to catch shellfish. As they walk across a shellfish bed, they look for a mollusk with a partially opened shell. When they find one, they jab their bill into the shell and sever the muscle that causes the shell to clamp shut. This can be dangerous, however, as they are sometimes drowned when they don't completely severe this muscle and the shell clamps down on their bill. A strong, tightly rooted mollusk can hold the bird in place until the tide comes in. They also feed by carrying loose shellfish out of the water and hammering at the shell or by probing the sand for soft-shell or razor clams.
The knifetooth sawfish ("Anoxypristis cuspidata"), also known as the pointed sawfish or narrow sawfish, is a species of sawfish in the family Pristidae, part of the Batoidea, a superorder of cartilaginous fish that includes the rays and skates. Sawfish display a circumglobal distribution in warm marine and freshwater habitats. Their extant biodiversity is limited to five species belonging to two genera (Pristis and Anoxypristis). The sawfishes are characterised by the long, narrow, flattened rostrum or extension on their snout. This is lined with sharp transverse teeth, arranged in a way that resembles the teeth of a saw. It is found in the shallow coastal waters and estuaries of the Indo-West Pacific, ranging from the Red Sea and the Persian Gulf to southern Japan, Papua New Guinea and northern Australia. It is the only living member of the genus Anoxypristis, but was previously included in the genus "Pristis". Compared to that genus, "Anoxypristis" has a narrower rostral saw with numerous teeth on the distal part and no teeth on the basal one-quarter (toothless section about one-sixth in juveniles). This endangered species reaches a length of up to .
Atlantic wolffish use their strong jaws to eat hardshell molluscs, crustaceans, and echinoderms. They do not eat other fish. They are known to frequently eat large whelks ("Buccinum"), cockles ("Polynices", "Chrysodomus" and "Sipho"), sea clams ("Mactra"), large hermit crabs, starfish, sea urchins. They are an important predator of sea urchins and green crabs, whose populations escalate rapidly and can negatively affect the health of a marine system.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish), although no species has yet been documented in the deepest 25% of the ocean. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates.
Sea urchins or urchins () are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to . Their tests (hard shells) are round and spiny, typically from across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish.
Marine biota in the region have representatives of both the Indian and Pacific oceans which were caught in the Caribbean before the emergence of the Isthmus of Panama four million years ago. In the Caribbean Sea there are around 1,000 documented species of fish, including sharks (bull shark, tiger shark, silky shark and Caribbean reef shark), flying fish, giant oceanic manta ray, angel fish, spotfin butterflyfish, parrotfish, Atlantic Goliath grouper, tarpon and moray eels. Throughout the Caribbean there is industrial catching of lobster and sardines (off the coast of Yucatán Peninsula).
The Tana lungfish has a diet not unlike other lungfish, consisting of various mollusks, crabs, prawn, and small fish within its distribution. It can also go for up to 3 1/2 years without any food intake whatsoever. During this time period it behaves much like an estivating fish in that it buries itself in the mud and does not move until more favorable conditions occur.
Some species of sea urchins, such as the slate pencil urchin ("Eucidaris tribuloides"), are commonly sold in aquarium stores. Some species are effective at controlling filamentous algae, and they make good additions to an invertebrate tank.
There has been some research on the freshwater drum's impact on the invasive Zebra mussel in northern lakes and rivers. Zebra mussels are consumed by freshwater drum once they reach a length of , but drum under in length only eat small mussels and reject the larger ones. The fish larger than exhibit less selectivity and consume mussels relative to their availability in lakes. These larger fish are not restricted by their ability to crush the zebra mussels, but they are restricted by the size of the clumps that they can remove. Though the drum do eat zebra mussels, they are not having an impact on the spread of this invasive species. Though they do not control the population of zebra mussels, they do contribute to a high mortality in the zebra mussels.
Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths. Some species, such as "Cidaris abyssicola", can live at depths of several thousands metres. Many genera are only found in the abyssal zone, including many cidaroids, most of the genera in the Echinothuriidae family, and the "cactus urchins" "Dermechinus". One of the deepest-living families is the "Pourtalesiidae", strange bottle-shaped irregular sea urchins that live only in the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench. Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars, starfish and crinoids that remain abundant below and sea cucumbers which have been recorded from .
As an apex predator positioned at the highest level of the trophic web, the dusky shark is generally less abundant than other sharks that share its range. However, high concentrations of individuals, especially juveniles, can be found at particular locations. Adults are often found following ships far from land, such as in the Agulhas Current. A tracking study off the mouth of the Cape Fear River in North Carolina reported an average swimming speed of . The dusky shark is one of the hosts of the sharksucker ("Echeneis naucrates"). Known parasites of this species include the tapeworms "Anthobothrium laciniatum", "Dasyrhynchus pacificus", "Platybothrium kirstenae", "Floriceps saccatus", "Tentacularia coryphaenae", and "Triloculatum triloculatum," the monogeneans "Dermophthirius carcharhini" and "Loimos salpinggoides", the leech "Stibarobdella macrothela", the copepods "Alebion" sp., "Pandarus cranchii", "P. sinuatus", and "P. smithii", the praniza larvae of gnathiid isopods, and the sea lamprey ("Petromyzon marinus").
The dusky shark ("Carcharhinus obscurus") is a species of requiem shark, in the family Carcharhinidae, occurring in tropical and warm-temperate continental seas worldwide. A generalist apex predator, the dusky shark can be found from the coast to the outer continental shelf and adjacent pelagic waters, and has been recorded from a depth of 400 m (1,300 ft). Populations migrate seasonally towards the poles in the summer and towards the equator in the winter, traveling hundreds to thousands of kilometers. One of the largest members of its genus, the dusky shark reaches 4.2 m (14 ft) in length and 347 kg (765 lb) in weight. It has a slender, streamlined body and can be identified by its short round snout, long sickle-shaped pectoral fins, ridge between the first and second dorsal fins, and faintly marked fins.
Newborn dusky sharks measure long; pup size increases with female size, and decreases with litter size. There is evidence that females can determine the size at which their pups are born, so as to improve their chances of survival across better or worse environmental conditions. Females also provision their young with energy reserves, stored in a liver that comprises one-fifth of the pup's weight, which sustains the newborn until it learns to hunt for itself. The dusky shark is one of the slowest-growing shark species, reaching sexual maturity only at a substantial size and age (see table). Various studies have found growth rates to be largely similar across geographical regions and between sexes. The annual growth rate is over the first five years of life. The maximum lifespan is believed to be 40–50 years or more.
The dusky shark is considered to be potentially dangerous to humans because of its large size, though little is known of how it behaves towards people underwater. As of 2009, the International Shark Attack File lists it as responsible for six attacks on people and boats, three of them unprovoked and one fatal. However, attacks attributed to this species off Bermuda and other islands were probably in reality caused by Galapagos sharks. Shark nets used to protect beaches in South Africa and Australia entangle adult and larger juvenile dusky sharks in some numbers. From 1978 to 1999, an average of 256 individuals were caught annually in nets off KwaZulu-Natal; species-specific data is not available for nets off Australia. Young dusky sharks adapt well to display in public aquariums.
The dusky shark is one of the most sought-after species for shark fin trade, as its fins are large and contain a high number of internal rays (ceratotrichia). In addition, the meat is sold fresh, frozen, dried and salted, or smoked, the skin is made into leather, and the liver oil is processed for vitamins. Dusky sharks are taken by targeted commercial fisheries operating off eastern North America, southwestern Australia, and eastern South Africa using multi-species longlines and gillnets. The southwestern Australian fishery began in the 1940s and expanded in the 1970s to yield 500–600 tons per year. The fishery utilizes selective demersal gillnets that take almost exclusively young sharks under three years old, with 18–28% of all newborns captured in their first year. Demographic models suggest that the fishery is sustainable, provided that the mortality rate of sharks over 2 m (6.6 ft) long is under 4%.