Common names
Pacific bonnethead

Short Description
Diagnosis after Springer, 1940 [2259]: A small species (a 938-mm individual is apparently sexually mature), body moderately compressed;. head flattened, spade-shaped, its greatest width about 1. 5 times the greatest length of the oculonarial expanse in full-grown individuals, the front margin not lobed, no indentation in the front margin before the nasal apertures, front and side margins of head not clearly confluent in half-grown and mature individuals; length of snout in internasal distance about 1.75 times for large specimens; mouth well arched, a line through its angles in advance of the posterior edges of the, oculonarial expanse; teeth in 12 2 12/12 1 12 to 14 1 14/13 1 13 rows, upper teeth low·, the cusps angled toward the outer margins of the jaws, progressively shorter toward the angles of the jaws and nearly or entirely absent on the last row; teeth of the lower jaw entire with slender cusps, cusps low and progressively shorter toward the angles of the jaws, indistinct or absent on the last 4 or 5 rows; first dorsal high, its origin a little behind the end of the pectoral bases; second dorsal with the posterior lobe not greatly produced, length of lobe less than 1.5 times length of base of fin; eyes close to nasal apertures, separated from them by a distance equal to about 1.5 times the diameter of the eye. This species may be distinguished from others of the genus by characters given in the accompanying key. It is closely allied to Sphyrna tiburo of the Caribbean, Gulf of Mexico, and the Atlantic coast of the Americas. Direct comparison of specimens of vespertina and tiburo shown that the former has a broader and shorter head, greater internasal distance, higher first dorsal, shorter pectoral, greater body compression, and longer caudal fin. The area of the oculonarial expanse has an important relation to the swimming equilibrium of the sharks of this genus, and its development is accompanied by changes in the shape of the body and the relative sizes of the fins. Thus, the reduced pectorals, the relatively high first dorsal fin, the compressed body, and the large caudal fin which are characteristic of the genus, reflect the degree of development of the flattened head. Data from Englewood specimens of tiburo and tudes indicate that growth is accompanied by a proportionate increase in the width and decrease in the length of the head, by a reduction in the relative size of the pectoral fins, and by an increase in the relative height of the first dorsal fin. In adults of tiburo from Englewood there is a measurable difference in proportions of the head in the sexes, the females having slightly wider and shorter heads. It is probable that the maturity factor is important in the determination of any hammerhead.

Remarks
shark-references Species-ID=6592;