Scalloped Hammerhead, Sphyrna lewini (Griffin & Smith, 1834)

Introduction

The Scalloped Hammerhead sometimes forms large migratory schools, dispersing at night to feed in deeper waters. The adult females are believed to live in deep water and only move onto the continental shelf to mate and when giving birth. After a period of 9-10 months they give birth to an average litter of 17 pups.

3D interactive model of a Scalloped Hammerhead

_{3D model made by cdmstudio.com.au}

Identification

The Scalloped Hammerhead has an undulating scalloped margin to the front of the head with an indentation medially. It has a low second dorsal fin and a relatively straight posterior pelvic fin margin.

The species is brownish-grey to olive or bronze above and pale below. Adults have dusky pectoral fin tips, but no other markings. Juveniles have dark pectoral, lower caudal and second dorsal fin tips.

Habitat

The species occurs in coastal and offshore waters down to 1000m Juveniles mainly in shallow inshore areas, subadults in deeper waters and adults further offshore and deep sea mounts.

Distribution

Scalloped Hammerheads occur in most tropical and warm temperate waters worldwide.

In Australia it is known from about Geographe Bay in north-western Western Australia, around the tropical north and south to the Sydney, New South Wales.

The map below shows the Australian distribution of the species based on public sightings and specimens in Australian Museums. Source: Atlas of Living Australia.

Feeding and diet

Often seen in large schools that aggregate around seamounts and offshore islands during the day, then feed at night alone or in small groups. Females have been known to move up to 700km between islands in the eastern Pacific. Their diet consists mostly of teleost fishes and cephalopods.

Breeding behaviours

Australian specimens of the Scalloped Hammerhead are born at approximately 45-50cm and males mature at ~140-160cm TL while females mature at ~200-220cm TL. They produce up to 17 pups per litter after a gestation period of up to 9-10 months.

Behaviours and adaptations

The Scalloped Hammerhead sometimes forms large migratory schools, dispersing at night to feed in deeper waters.

Sharks can detect weak electrical fields using electrical sense organs called the ampullae of Lorenzini. These are connected to the exterior via pores in the shark's snout. This ability allows some species to detect the electrical cues that emanate from prey animals and even the weak electrical currents flowing through the earth's magnetic fields. Some species can detect voltages as low one millionth of a volt.

Danger to humans

It is considered a potentially dangerous species that has occasionally displayed its threat posture to scuba divers who have approached too closely (Thomson et al, 1979).