Giant Manta Ray, Mobula birostris (Walbaum, 1792)

Introduction

The Giant Manta Ray is the largest species of ray in the world. Disc widths have been reliably measured up to 6.7 m, but this species possibly grows up to 9.1 m disc width.

Identification

The Giant Manta Ray is the largest species of ray in the world. The disc of the Giant Manta Ray is wider than it is long. The species has one dorsal fin and a whip-like tail which lacks a sting. The surface of the body is rough to touch. They have a very broad mouth, on either side of which are prominent fleshy extensions called cephalic lobes. Giant Manta Rays are grey-blue to green-brown above.

Two species of Manta are known from Australian waters.

Long thought to have been a single wide-ranging species, Mobula birostris, the genus is now known to comprise two species. The Giant Manta Ray and Alfred Manta, Manta alfredi, can be distinguised by a number of measurements but also by size, dentition and spine morphology.

The easiest way to distinguish live fish is by the presence of dark spots on the ventral surface medially between the gill slits of the Alfred Manta. The Giant Manta Ray has no spots between the gill slits. The shape of the whitish-coloured shoulder patches is also a good character - those of the Alfred Manta "eminate from spiracle before curving medially" but those of the Giant Manta Ray are "very distinct and approximately triangular in shape".

Dr Mike Bennett (see Marshall et.al, 2009 in References, below) stated that "The dorsal views are much more difficult to resolve as there is pretty large variability….but look for the lump at the tail base and that can be your quick check…..if it is there = birostris, absent = alfredi."

Distribution

The Giant Manta Ray lives in tropical, marine waters worldwide, but is also found occasionally in temperate seas. In Australia it is recorded from south-western Western Australia, around the tropical north of the country and south to the southern coast of New South Wales.

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

Feeding and diet

Divers sometimes see Giant Manta Rays swimming gracefully through the water feeding on plankton. The planktonic organisms are filtered from the water by the gills.

Other behaviours and adaptations

Despite most individuals being seen swimming slowly, the Giant Manta Ray is capable of swimming at rapid speed. They are sometimes observed leaping out of the water and landing back on the surface with a loud slap.

References

1. Allen, G.R. 1997. Marine Fishes of Tropical Australia and South-east Asia. Western Australian Museum. Pp. 292.
2. Hutchins, B. & R. Swainston. 1986. Sea Fishes of Southern Australia. Complete Field Guide for Anglers and Divers. Swainston Publishing. Pp. 180.
3. Last, P.R. & J.D. Stevens. 1994 Sharks and Rays of Australia. CSIRO. Pp. 513.
4. Marshall, A., Compagno, L.J.V. and Bennett, M.B. 2009. Redescription of the genus Manta with resurrection of Manta alfredi (Krefft, 1868) (Chondrichthyes; Myliobatoidei; Mobulidae). Zootaxa. 2301: 1-28.
5. Randall, J.E., Allen, G.R. & R.C. Steene. 1997. Fishes of the Great Barrier Reef and Coral Sea. Crawford House Press. Pp. 557.