Spiny dogfish
(Squalus acanthias)
General data
The spiny dogfish, spurdog, mud shark (Squalus acanthias) or piked dogfish, is one of the best known species of the Squalidae (dogfish) 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.
Males mature at around 11 years of age, growing to 80–100 cm (31–39 in) in length; females mature in 18–21 years and are slightly larger than males, reaching 98.5–159 cm (38.8–62.6 in). Both sexes are greyish brown in color and are counter-shaded. 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.
Reproduction is aplacental viviparous, which was once called ovoviviparity. Fertilization is internal. The male inserts one clasper into the female oviduct orifice and injects sperm along a groove on the clasper’s dorsal section. Immediately following fertilization, the eggs are surrounded by thin shells called candles with one candle usually surrounding several eggs. Mating takes place in the winter months with gestation lasting 22–24 months. Litters range between two and eleven, but average six or seven.
Spiny dogfish are bottom-dwellers. They are commonly found 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.
