Thresher Shark

The common thresher is a fairly robust shark with a torpedo-shaped trunk and a short, broad head. The dorsal profile of the head curves evenly down to the pointed, conical snout. The eyes are moderately large and lack nictitating membranes. The small mouth is arched and, unlike in other thresher sharks, has furrows at the corners. The species has 32–53 upper and 25–50 lower tooth rows; the teeth are small, triangular, and smooth-edged, lacking lateral cusplets. The five pairs of gill slits are short, with the fourth and fifth pairs located over the pectoral fin bases. The long, falcate (sickle-shaped) pectoral fins taper to narrowly pointed tips. The first dorsal fin is tall and positioned slightly closer to the pectoral fins than the pelvic fins. The pelvic fins are almost as large as the first dorsal fin and bear long, thin claspers in males. The second dorsal and anal fins are tiny, with the former positioned ahead of the latter. Crescent-shaped notches occur on the caudal peduncle at the upper and lower origins of the caudal fin. …

The range of the common thresher encompasses tropical and cold-temperate waters worldwide. In the western Atlantic, it is found from Newfoundland to the Gulf of Mexico, though it is rare north of New England, and from Venezuela to Argentina. In the eastern Atlantic, it has been reported from the North Sea and the British Isles to Ghana (including Madeira, the Azores, and the Mediterranean and Black Seas), as well as from Angola to South Africa. In the Indo-Pacific, this species is known from Tanzania to India and the Maldives, Japan, and Korea to southeastern China, Sumatra, eastern Australia, and New Zealand; it also occurs around a number of Pacific islands including New Caledonia, the Society Islands, Tabuaeran, and the Hawaiian Islands. In the eastern Pacific, it has been recorded from British Columbia to Chile, including the Gulf of California. The common thresher is migratory, moving to higher latitudes following warm-water masses. In the eastern Pacific, males travel further than females, reaching as far as Vancouver Island in the late summer and early fall. Juveniles tend to remain in warm nursery areas. In New Zealand waters juveniles can be found over the inner shelf around the North Island and around the upper South Island. …

Common threshers are active, strong swimmers, with infrequent reports of them leaping completely out of the water. Like the fast-swimming sharks of the family Lamnidae, the common thresher has a strip of aerobic red muscle along its flank that is able to contract powerfully and efficiently for long periods of time. In addition, they have slow-oxidative muscles centrally located within their bodies and a blood vessel countercurrent exchange system called the rete mirabile ("wonderful net"), allowing them to generate and retain body heat. The temperature inside the red muscles of a common thresher averages 2 °C (3.6 °F) above that of the ambient seawater, though significant individual variation is seen. Unlike the pelagic and bigeye threshers, the common thresher lacks an orbital rete mirabile to protect its eyes and brain from temperature changes. Immature common threshers fall prey to larger sharks. Aside from observations of killer whales feeding on common threshers off New Zealand, adults have no known natural predators. Parasites documented from the common thresher include the protozoan Giardia intestinalis, the trematodes Campula oblonga (not usual host) and Paronatrema vaginicola, the tapeworms Acanthobothrium coronatum, Anthobothrium laciniatum, Crossobothrium angustum, Hepatoxylon trichiuri, Molicola uncinatus, Paraorygmatobothrium exiguum, P. …