Pacific plaque shark

The Austrian ichthyologist Victor Pitchman for the first time scientifically described the Pacific plaque shark as a separate species in two publications in 1928. He assigned her a specific name in honor of the conhiologist Charles Montag Cook ^{[3] [4]} . However, Pacific plaque sharks were constantly confused with plaque sharks , referring to them in the scientific literature, until in 1960 Jack Garrick described this species again. Since the original holotype stored in Kauai , Hawaii , was lost, Garrick made a description based on a 2 m long male caught in Palliser Bay , New Zealand .

The name of the family and clan comes from the words Greek. αχινός - “ sea ​​urchin ” and Greek. ῥινός - “nose” ^[5] .

Pacific plaque-sharks are found in the Pacific off the coast of Japan , Taiwan , Victoria and Queensland ( Australia ), as well as off the islands of Palau , New Caledonia , Tonga , Hawaii and, possibly, Gilbert . They are common from Oregon to El Salvador , including the Gulf of California , around the Cocos and Galapagos Islands , off the coast of Peru and Chile . In general, this is a rather rare species, with the exception of Monterey Bay, where males and females of Pacific plaque-sharks are found in abundance all year round ^{[6] [7] [8]} .

Pacific plaque-sharks prefer a temperature range of 5.5–11 ° C. Therefore, especially in the tropics, they are kept at a depth of 100-200 m ^{[9] [10]} . There is evidence of the presence of these sharks at a depth of 650 m and probably 1500 m ^{[6] [11]} . On the other hand, at higher latitudes they enter shallow water, for example, in the Monterey Canyon they are regularly met at a depth of 15–35 m, and one individual was caught at a depth of only 4 m ^{[11] [9]} . These bottom sharks live on the continental and island shelf and mainland slope. They prefer a muddy or sandy bottom ^[9] . They tolerate a low oxygen content in the water and inhabit underwater basins , where conditions are unacceptable for most sharks ^[12] .

Pacific plaque-sharks have a flabby, cylindrical body (adult sharks are much more massive than young ones), and a short, slightly flattened head. The nostrils are widely spaced from each other and framed by small leather folds. The third eyelid is absent. Behind the eyes are tiny splatter . At the corners of a wide curved mouth there are short furrows. On the upper jaw 21-25 are located, and on the lower 20-27 dentitions. Dagger-shaped teeth with a small central tip, on the sides there are up to 3 lateral teeth. In young sharks, lateral teeth are absent. Pacific plaque-sharks have five pairs of gill slits, of which the fifth pair is the longest ^{[11] [2] [9]} .

The lateral line looks like a noticeable groove. The pectoral fins are short, while the ventral fins are wide with a long base. Dorsal fins small, almost the same size. The base of the first dorsal fin is located at the base of the ventral fins. Anal fin missing. Caudal peduncle thick, precaudal notches absent. The caudal fin is asymmetrical, the lower lobe is poorly developed. Ventral notch at the tip of the upper lobe. The skin of the Pacific plaque-sharks is covered with non-overlapping placoid scales up to 0.4 cm in diameter, resembling a spine in shape. They are covered with protrusions that diverge from the center. Unlike plaque-shark sharks, Pacific plaque-shark sharks never grow together. In adult sharks, the lower part of the snout is covered with thin scales. Coloring even brown or gray, fins have a bright edging. The belly is paler.

The maximum recorded length is 4 m, and the weight is 266 kg (female 3.1 m long) ^{[11] [2] [9] [7] [8]} .

Pacific plaque-sharks are rather slow; they often hover above the bottom ^[13] . A tagging study in the Monterey Canyon showed that these sharks perform daily vertical migrations. In the afternoon, they rest in a certain shelter at the bottom of the sea, at dusk they become active and go to the shore and rise up. These migrations are associated with hunting for schooling fish. They lead a sedentary lifestyle and rarely leave a limited territory, the area of ​​which does not exceed 2.2 km². In the Monterey Canyon, they regularly gather in groups of up to 30 individuals ^[11] .

The size and structure of the mouth and throat of Pacific plaque-sharks suggests that they suck prey. Their diet consists of a variety of benthic and pelagic bony fish , including hake , flatfish , sea ​​bass , atherinops , mackerel and herring , as well as cartilaginous fish such as chimeras , short-finned spiny sharks , juvenile six-shark sharks and egg capsules of black cat sharks ^{[2] [9]} . In addition, Pacific plaque-sharks prey on octopuses and squids, including Dosidicus gigas ^[11] . In turn, juveniles of these sharks can become prey for six-branch sharks, while adults in the natural environment have practically no enemies ^[9] . This species is propagated by egg production , embryos feed on yolk . So far, only one pregnant female has caught 114 embryos; this is the most numerous litter that has only been found in sharks. Probably, the length of newborns is about 40 cm ^[11] . The age of puberty is unknown, it is believed that males mature at a length of about 2 m, and females 2.5-3.8 m ^[14] .

During contact with divers, Pacific plaque-sharks do not show aggression if they do not tolerate them or swim away ^{[11] [9]} . As by- catch, they sometimes end up in commercial bottom trawls, gillnets and longlines. They do not represent commercial value, because they have soft and tasteless meat ^[9] . The International Union for Conservation of Nature has assigned this species a conservation status of “Close to Vulnerability” ^[6] .