Thursday, January 24, 2008
Scalloped Hammerhead Sharks (Sphyrna lewini) are probably the most commonly found species of hammerheads located in coastal regions, appearing in very shallow waters such as estuaries and inlets. Their distribution in the water reaches from the surface down to a depth of approx. 275 m. The young, however, remain mostly in shallow waters along the shore to avoid the danger of falling into the mouths of predators. At certain times of the year and places, and during certain phases of their lives, scalloped hammerheads form very large schools, sometimes counting hundreds of individuals, but they also swim the oceans alone. Some populations remain stationary, others clearly wander, migrating in the direction of the poles in summer. Some sexually-related migrations have also been observed, e.g. females who undertake migrations during particular periods of their sexual development.
The scalloped hammerhead shark belongs to the large hammerhead species, and like all representatives of this family, has the typically formed "hammer" consisting of a central dent and an arched front edge (hence the name). Another typical characteristic is the free end tip of the second dorsal fin which almost reaches the tail fin. Their coloring is mainly olive, bronze or light brown with a white belly. The edges of the fins are usually darker on young animals but becomes lighter as they grow older.
Mature females can reach a length of more than 4 meters, the average length is, however, less. Males reach sexual maturity at a length of about 160 cm, females when they reach approx. 210 cm. The pups measure approx. 50 cm at birth.
This hammerhead species feeds mostly on fish such as sardines, herring and mackerels, occasionally also on invertebrates such as octopuses. Large scalloped hammerhead sharks also eat small-sized shark species such as the Atlantic sharpnose shark (Rhizoprionodon terraenovae) or the blacktip reef shark (Carcharhinus melanopterus).
As already mentioned, this shark species tends to form huge schools whose function is presumed to be manifold and may, among other things, concern feeding habits and reproduction. Although many studies also consider this behavior to be a group protective function, this is somewhat questionable since the animals have practically no natural enemies after reaching full maturity. Groups of scalloped hammerheads prefer staying in regions which have pinnacles or sea mounts which reach from great depths practically to the water's surface. Latest research also shows that these sharks can make use of the earth's magnetic field during their migrations.
The sawsharks or saw sharks are an order (Pristiophoriformes) of sharks bearing long blade-like snouts edged with teeth, which they use to slash and disable their prey. There are five described (and four undescribed) species known, in a single family Pristiophoridae of two genera. Most occur in waters from South Africa to Australia and Japan, at depths of 40 m and below; in 1960 the Bahamas sawshark was discovered in the deeper waters (640 m to 915 m) of the northwestern Caribbean.
Sawsharks also have a pair of long barbels about halfway along the snout. They have two dorsal fins, but lack anal fins, and range up to 170 cm in length. Genus Pliotrema has six gill slits, and Pristiophorus the more usual five. The teeth of the saw typically alternate between large and small.
The sharks typically feed on bony fish, shrimp, squids, and crustaceans, depending on species. They cruise the bottom, using the barbels and ampullae of Lorenzini on the saw to detect prey in mud or sand, then hit victims with side-to-side swipes of the saw, crippling them.
Most of the species are fished commercially, and their meat is considered to be of excellent quality. Japanese sawshark is used to make kamaboko, a traditional type of fishcake.
Although they are similar in appearance, sawsharks are distinct from sawfish. Sawfish have a much larger maximum size, lack barbels, have evenly sized rather than alternating sawteeth, and have gill slits on their undersurface rather than on the side of the head.