The IUCN/SSC Shark Specialist Group
Shark News 5: October 1995
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Manta Rays in the Yaeyama Islands
Manta ray Manta birostris, showing omega-shaped swimming style.
Pohnpei Island. Photo: Kimiya Homma.
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Hajime Ishihara and Kimiya Homma, Suido-sha and Kyowo Concrete
Industry
Mr Takashi Itoh, a professional diver, has been living on Obama Island
(near Iriomote Island) for 18 years, and observing the manta rays Manta
birostris which occur nearby and are abundant in the Yonara Channel
between Obama and Iriomote Islands. Using an identification method
based on a combination of patterns of ventral markings and signs of
shark bites, he has been able to distinguish more than one hundred and
thirty individual manta rays during this 18-year period. As a result,
migration, schools, age, feeding, predators, reproduction and behaviour
have been described.
Migration
Manta rays are abundant in two kinds of locale; at feeding stations and
cleaning stations. At their feeding stations, manta rays swim slowly in the
surface layer and are absorbed in feeding. At their cleaning station, rays
hover at a depth of 15-20 m and wait to be cleaned by a wrasse, Labroides
dimidiatus, or small shrimps. Rays migrate daily between feeding and
cleaning stations. While many manta rays stay near the Yaeyama Islands all
year round, some migrate annually to other islands such as Kerama Island,
which is about 350 km east of the Yaeyama Islands.
Schooling
Mr Itoh once observed a school of
about 50 manta rays some 17 years ago, and one of about 30 rays about
seven years ago. Recently, schools have numbered 14 to 15 fish at most,
although the total number of manta rays in the area may not have
decreased. Young rays and pregnant females are also represented in a
school. No other fish, including no other mobulids, are involved in the
group.
Swimming style
S-shaped, U-shaped and omega-shaped swimming styles (viewed from
in front) have been recorded.
Mating behaviour
Copulation has not been seen, but behaviour which appears to be
related to mating activity is often observed, with male and female manta
rays somersaulting together in the water column. This behaviour is
observed in spring and autumn and lasts for as long as one month.
Reproduction
One female was seen to bear a single pup three times during a six to
seven year period. Thus the pregnancy may last up to two or three years.
Parturition was not observed, although this has been recorded in the
magazine Skin Diving, in a volume of about 1975 to 1977. Nursing
behaviour is not seen, the new-born pup is left in the water after birth.
Age at maturity for females may be about six years.
Jumping behaviour
Jumping behaviour is often observed. It appears that the rays do this
for fun, and that this behaviour does not seem to be related to
parturition or the removal of parasites and remoras. Three types of
jumping are observed: jumping forwards and landing head-first, jumping
forwards and landing tail-first, and backward somersaults.
Age
One male ray, which appeared to be some four to five years old when
first sighted, has been observed for 15 years. It is, therefore, considered
that the ray's life span is more than 19-20 years.
Predators
Sharks may be the most common predator of manta rays, because the
marks of shark bites are often seen. However, it is uncertain whether
sharks eat the whole body of manta rays. Killer whales are not seen near
the Yaeyama Islands.
Black manta
Manta rays whose ventral surface is dark are usually called 'black
manta' at Pohnpei Island, where about 50% of all manta rays are black.
At Yaeyama Island only two black manta rays have been observed. Both
were male and the first of these has not been seen for several years.
Mr Itoh intends to continue to observe the behaviour of manta rays in
the Yaeyama Islands and we will continue to have a fair relationship
with him. We ourselves will be conducting a survey of manta rays at
Pohnpei Island, in the Caroline Islands.
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