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Characteristics, distribution and ecology
Taxonomische classification
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Author: (Linnaeus, 1758)

Field Marks:
A small hammerhead with a unique, very narrow, shovelshaped head without indentations on its anterior edge, enlarged, molariform posterior teeth, first dorsal rear tip in front of pelvic origins, and shallowly concave posterior anal margin.

Diagnostic Features:
Expanded prebranchial head shovel-shaped and rather narrow but longitudinally elongated, its width 18 to 25% of total length (mostly below 21%); distance from tip of snout to rear insertions of posterior margins of expanded blades over half of head width; anterior margin of head broadly arched or somewhat angular, without indentations; posterior margins of head short, transverse, or angled posterolaterally, and generally narrower than mouth width; prenarial grooves not present anteromedial to nostrils; preoral snout about 2/5 of head width; rear ends of eyes slightly anterior to about opposite upper symphysis of mouth; mouth rather broadly arched; anterior teeth with short, stout cusps, not serrated, posterior teeth cuspless, keeled, somewhat expanded, and resembling the molariform teeth of Heterodontus species. First dorsal moderately falcate, its origin over inner margins of pectoral fins and well behind their insertions, its free rear tip usually somewhat anterior to pelvic origins; second dorsal fin moderately high, about as high as anal, with a strongly concave posterior margin; its inner margin moderately long, less than twice fin height, and ending well ahead of upper caudal origin; pelvic fins not falciform, with posterior margins straight or nearly so; anal fin larger than second dorsal fin and rather long, its base 6.4 to 8.5% of total length, its origin well in front of second dorsal origin, its posterior margin shallowly concave to nearly straight. Total vertebral centra 142 to 173. A small hammerhead, to about 1.5 m. Colour grey or grey-brown above, light below, often with small dark spots on sides of body.

Geographical Distribution:
Western Atlantic: From North Carolina and exceptionally Rhode Island, USA, to southern Brazil, also Cuba and the Bahamas. Eastern Pacific: Southern California, USA to Ecuador.

Habitat and Biology:
An abundant, inshore, coastal, continental and insular shelf species, in shallow water over mud and sand bottoms, also on coral reefs; commonly found in eastuaries, shallow bays and channels, at depths between 10 and 25 m, but down to at least 80 m and into the surf zone and the intertidal. Off Florida it fluctuates in numbers with the seasons, being virtually absent in summer but present in numbers in spring and autumn; large schools have been seen in the autumn there. Along the Atlantic coast of the USA it is a common summer visitor as far north as New England, but it apparently migrates southward with decreasing water temperatures in autumn and winter. Considerable sexual segregation occurs in this species as in many others, and adult females often predominate in the shallows during the pupping season. This shark usually occurs in small groups of 3 to 15 individuals, and seldom is found alone.

In a pioneering six-month behavioural study of a colony of ten bonnetheads in a semi-natural enclosure in Florida, Myrberg NA Gruber (1974) were able to elucidate the complex and subtle behaviour of this shark. Some eighteen postures and action patterns were discovered, along with a diel rhythm of activity peaking in the late afternoon and a definite dominance hierarchy at least partially based on size and sex. About half of the action patterns had a social content, and some agonistic behaviour was observed, though the sharks had a low level of intraspecific aggression and never fought. The sharks were very active and in seemingly constant motion day and night: they normally engaged in "patrolling" in a straight line just above the bottom, with larger sharks moving faster than smaller. These sharks might suddenly engage in "manouvering", whipping around in sharp lateral turns apparently to orient to a given spot or a prey item; and "explosive-glide", suddenly swimming rapidly followed by a long glide and sometimes a darkening in colour. Other action patterns by single sharks included "head-shake", lateral shaking of the head to left and right; "head-snaps", rolling of the shark followed by a slight upward and rapid and long downward displacement of its head, in a diagonal plane; "jaw-snap", opening and closing the mouth rapidly in succession once or twice, occurring during feeding sessions or when "patrolling"; "chafe", suddently rolling with the body coming in minimum contact with the bottom, possibly to remove parasites; "gill-puff", momentary expansion of the gill area often seen after a shark ingested something or after tight "manouvering" that disturbed the substrate, possibly to clear the pharynx; and two patterns by males, simple "clasper-flexion", flexing a clasper anteriorly while "patrolling", and "clasper-flexion-with thrust", rolling to one side, flexing a clasper, and then accelarating at speed for a few metres. Action patterns with a social context include "circlinghead-to-tail", where two sharks tightly circle each other head to tail; "approach-over-the-body", sudden overtaking of a shark by another from the rear, placing the approaching shark with its head about opposite the predorsal back of the other; "hit", an "approach-over-the-body" culminating in a ventral flicking of the head by the approaching shark onto the interdorsal back of the approached shark, which accelerates off and often shows a contused area where the first shark struck it; "hunch", arching the back, displacing the pectoral fins downward, dropping the caudal fin and raising the head, done in the presence of other bonnetheads and human observers; "turnback", one shark going in the opposite direction reversescourse and follows a second; "follow", one shark closely following another and repeating its movements; "follow-formation", three to six sharks following a leader in a single line, and varying their course after the leader's movements; and "give-away", with two sharks on a head-on collision course, having one deflecting to either side of the other. "Approaches" and "hits" were often scored by resident sharks on newcomers to the containment, especially by small males and females. The "hunch" is similar in many of its components to the spectacular threat display of the grey reef shark (Carcharhinus amblyrhynchos) and probably is also a threat display. "Turn-back", "follow", and the rare "follow-formation" were usually seen in the context of males following females, but sometimes the reverse occurred. "Give-away" data pointed to the existence of a definite social hierarchy, in which the largest shark, a female, was dominant and never gave way to any of the others in head-on approaches, but in which larger males may have been more dominant than equal-sized females. Territoriality, either by individuals or by the group, was not apparent in Myrberg and Gruber's experimental bonnethead colony.

Viviparous, with a yolk-sac placenta; number of young 4 to 16 per litter. Off Florida there may be a spring and autumn mating season in the bonnethead, or alternatively mating the year round. Off Brazil it apparently mates in the spring.

The bonnethead is primarily a crustacean feeder that eats crabs, shrimp, manis shrimp, isopods, and even barnacles, but also bivalves, octopiand small fish.

Size:
Maximum about 150 cm, males maturing between 52 and 75 cm and reaching at least 124 cm, females mature at 84 cm or less and reaching at least 130 cm; size at birth about 35 to 40 cm.

Interest to Fisheries:
An abundant inshore shark, commonly taken by smallscale fisheries; caught with shrimp trawls, trammel nets, bottom longlines, and hook-and-line, and utilized fresh, fresh frozen, or dried salted for human consumption; also processed into fishmeal.

Type material:
Holotype: None. Type Locality: "Habitat in America".

Bonnethead (Sphyrna tiburo)