The name of the spiny dogfish shark is derived from the sharp, venomous spines in front of each dorsal fin of the shark. They are generally small in size compared to other big sharks, averaging three to four feet in size in adulthood. Despite their small size, the spiny dogfish is known to be aggressive and are reputed for relentlessly pursuing their prey. These shark species are called dogfish because of their habit of feeding and hunting in packs, which at times may number several hundreds of individuals. Similar to the tiger sharks, dogfish have voracious appetites and will eat almost anything in front them. It is also known that newborn dogfish even attack fishes that are two to three times bigger than their size.
Dogfish are in popular demand in the international market specifically for their meat. Researchers have declared spiny dogfish as overfished. The spiny dogfish probably has the longest gestation period of any vertebrate ranging between 22 to 24 months. They are also known to be long distance travellers and can move across countries. Despite being tagged as overfished, they are considered among the most abundant living shark species in the ocean. Due to their range and habitat, they pose little or no threat to humans.
Taxonomy
The dogfish sharks have the second largest order among sharks, comprising about 119 species. They are relatives of the pygmy shark, and sleeper shark. They belong to the Squalidae family. They are known scientifically as Squalus acanthias.
Morphology
The spiny dogfish sharks are slim, have moderately large eye, somewhat flattened top head and a narrow pointed snout with characteristic white spots. They have a gray coloration on the dorsal side and white underbellies. They possess two dorsal fins with un-grooved large spines. The caudal fin is asymmetrical with a larger upper lobe. The smooth edged short and oblique teeth are identical in both the upper and lower jaw. The male dogfish can grow up to a length of 3.3 feet, while the females reach up to 4 feet. The shark has a rough skin resembling sandpaper, which can be used for making leather after processing.
Adaptation
The spiny dogfish sharks form a pack similar to dog packs during hunting, which enhance their success rate as well as make them formidable to possible predators. Also, the spiny dogfish defends themselves by injecting venom into predators through the two spines located close to their dorsal fins. Even though humans are not in the list of these sharks diet, if these sharks are not properly handled, it might lead to injury. Their caudal fin has an enlarged upper lobe, which is highly efficient in propelling the shark rapidly through the water
Feeding
The spiny dogfish sharks are opportunistic feeders that feed upon any abundant prey. Generally, their diet includes small fishes like capelin, herring, ratfish, menhaden and haddock. In addition to these, they feed on invertebrates such as squid, amphipods, jellyfish, krills, crabs and octopus. They can hunt singly or collectively (packs). In some cases, they ram their prey with their snouts.
Habitat
The spiny dogfish is abundant in both cold and warm temperate oceans between 0 and 15 degrees Celsius. These shark species tolerate a wide range of salinity and are found in estuaries and can tolerate brackish water. They are found at various depths in the ocean ranging from water surface to a depth of about 2,400 feet. Tagging studies have revealed that the spiny dogfish are long distance migratory species of sharks. Some species of spiny dogfish are known to migrate more than 5,000 miles. Due to the economic value of these sharks, The IUCN Red List of Threatened Species have reported that the population of the spiny dogfish once considered as the most abundant in the ocean to be decreasing. Therefore, they are considered as vulnerable due to intense fishing pressure. In the marine environment, the predators of the spiny dogfish include cod, red hake, larger species of sharks, seals, killer whales and even other spiny dogfish.
Reproduction
The male spiny dogfish sharks attain maturity at 11 years, while it takes 18 to 21 years for the female to mature. Mating among spiny dogfish occurs in offshore waters and the fertilization process is internal. The next stage of the reproductive process is an ovoviviparous development. After 4-6 month, the membrane providing nourishment to the embryo breaks down, leaving yolk-sac to provide nourishment during the remaining 17 to 19 months of gestation. The spiny dogfish is believed to have the longest gestation of any shark or ray species. The young are born alive with cartilaginous sheaths on the sharp spines to protect the mother from injury. The average sizes of the litter range from 6 to 7 inches, however, it can vary between one and 15. The sizes of the new pup range between 8 to 13 inches in length.
Lifespan
The majority of the spiny dogfish sharks are believed to live up to 20 to 24 years, while some species of this shark have lived up to 50 years.
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References
Atlantic State Marine Fisheries Commission (2019). Spiny dogfish. Retrieved from http://www.asmfc.org/species/spiny-dogfish
Chesapeake Bay Program (2019). Spiny Dogfish. Squalus achanthias. Retrieved from https://www.chesapeakebay.net/discover/field-guide/entry/spiny_dogfish
Fisheries and Oceans Canada (2019). Spiny dogfish. Squalus acanthias. Retrieved from http://www.dfo-mpo.gc.ca/species-especes/profiles-profils/spiny-dogfish-aiguillat-commun-atl-eng.html
Florida Museum (2019). Spiny Dogfish. Squalus acanthias. Retrieved from https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/squalus-acanthias/
IUCN red list (2016). Spiny Dogfish. Squalus acanthias. Retrieved from https://www.iucnredlist.org/species/91209505/2898271
Monterrey Bay Aquarium (2019). Spiny Dogfish. Retrieved from https://www.montereybayaquarium.org/animal-guide/fishes/spiny-dogfish
NOAA Fisheries (2019). Atlantic Spiny Dogfish. Retrieved from https://www.fisheries.noaa.gov/species/atlantic-spiny-dogfish
OCEANA (2019). Spiny Dogfish. Retrieved from https://oceana.org/marine-life/sharks-rays/spiny-dogfish
Slater museum of natural history (2019). Spiny Dogfish (Squalus acanthias). Retrieved from https://www.pugetsound.edu/academics/academic-resources/slater-museum/exhibits/marine-panel/spiny-dogfish/