The spiny dogfish (Squalus acanthias), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfishes) family of sharks, which is part of the Squaliformes order. While these common names may apply to several species, Squalus acanthias is distinguished by two spines (one anterior to each dorsal fin) and no anal fin. It lives in shallow waters and further offshore in most parts of the world, especially in temperate waters. Those in the northern Pacific Ocean were reevaluated in 2010 and found to constitute a separate species, now called the Pacific spiny dogfish (Squalus suckleyi).
Description and behaviour
The spiny dogfish has dorsal fins, no anal fin, and white spots along its back. The caudal fin has asymmetrical lobes, forming a heterocercal tail. The species name acanthias refers to the shark's two spines. These are used defensively. If captured, the shark can arch its back to pierce its captor with spines near the dorsal fins that secrete a mild venom into its predator. This shark is known to hunt in packs that can range up into the thousands. They are aggressive hunters and have a sizable diet that can range from squid, fish, crab, jellyfish, sea cucumber, shrimp and other invertebrates. Dogfish sharks experience one of the longest gestation periods of any organism, which can last from around 18 to 24 months. During spawning season, which occurs during the colder months of winter, females can lay anywhere from 2 to 12 eggs, which develop ovoviviparously and the pups are birthed as live young, with about 5 to 6 in each litter of pups, mainly depending on the size of the female. Reproduction occurs in the winter in offshore waters, while pups are normally born in the warmer and deeper offshore waters where it is harder for humans and predators to reach them. The reproductive cycle begins when females produce several large eggs of yellow coloring, which become fertilized once they pass through the shell gland and are wrapped in what is called a "candle", or a kind of reproductive capsule. One can determine if an egg is fertilized when the blastoderm is visible. The candle passes through the rest of the reproductive tract until it reaches the uterus. Attached to the gill region of the pup is a yolk sac which provides nutrients for them as they develop, which they absorb as they grow. Even after fully absorbing the yolk sac, the pups may live in the uterus for a period of time afterwards during the gestation period. Both sexes are greyish brown in color and are countershaded. Males are identified by a pair of pelvic fins modified as sperm-transfer organs, or "claspers". The male inserts one clasper into the female cloaca during copulation. In females the size of ova and the state of uteri determines whether sexual maturity has been reached, while in males, clasper length and calcification is the determination factor. Many of the growth patterns show an isometric growth pattern, as the pups have very similar features to the adults. Sexual maturity does not match body size development, they mature quite earlier than one would expect. Most males reach sexual maturity at an earlier ages of around 10 years, growing to be about 60–65 cm (24-26 in) in length. Females take a longer period of time, around 16 years, to reach sexual maturity, and can grow to about 80-84 cm (31-33 in) at that time. Some dogfish have been seen to live nearly 70 years, but the average lifespan is around 30–40 years. Male Spiny dogfish use their claspers to impregnate females, which take time to develop properly, and before reaching sexual maturity, tend to be longer than their pelvic fins. During this time, the testes develop, but no sperm will exist inside until later stages are reached, and genital ducts are deformed compared to adult ducts. Once the adult stage is reached, claspers have elongated, and are now hardened with calcium and are still slightly longer than their pelvic fin. At this point testes are fully developed, and sperm exist within the seminal vesicles, and genital ducts conform to a twisted shape we commonly see in adults. Spiny dogfish are bottom-dwelling boreal-temperate sharks. They are commonly found within the epipelagic zone at depths of around 50–149 m (160–490 ft), but have been found deeper than 700 m (2,300 ft). Life span estimates based on analysis of vertebral centra and annuli in the dorsal spines range from 35 to 54 years.
Commercial use
Spiny dogfish are sold as food in Europe, the United States, Canada, New Zealand, Venezuela and Chile. The meat is primarily eaten in England, France, Italy, the Benelux countries, Germany, and as an unwitting houndshark in Greece. The fins and tails are processed into fin needles for cheaper versions of shark fin soup in Chinese cuisine. In England, dogfish are bought in fish and chip shops as "huss", and previously as "rock salmon" until the term was outlawed. Commercial harvests were banned in the UK from 2010 until 2023. In France, they are sold as "small salmon" (saumonette) and in Belgium and Germany as "sea eel" (zeepaling and Seeaal, respectively). In Greece, all small dogfish are commercially called galéos, after the critically endangered Galeorhinus galeus that produces a seasonal dish; other small sharks such as the spiny dogfish substitute it. Spiny dogfish bodies are ground into fertilizer, liver oil and pet food. Because of their availability, cartilaginous skulls and small sizes, they are popular vertebrate dissection specimens in high schools and universities. Reported catches varied between 31,700 tonnes in 2000 and 13,800 tonnes in 2008. Bottom trawlers and sink gillnets are the primary tools. In Mid-Atlantic and Southern New England fisheries, they are often caught with larger groundfish, classified as bycatch, and discarded. Recreational fishing accounts for an insignificant portion of the spiny dogfish catch. The Cape Cod Commercial Fishermen's Alliance promotes sustainable use of the dogfish in restaurants and fish markets in the Cape Cod area of Massachusetts, as of 2017, paid for by the National Oceanographic and Atmospheric Administration. It aims to persuade diners to buy less-popular fish.
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