Description
The marbled stingray has a thin, oval-shaped pectoral fin
disc longer than wide. The snout is long and triangular, with the pointed tip
projecting from the disc. The eyes are small, and immediately followed by
spiracles over twice their diameter. There is a curtain of skin between the
nares with a fringed trailing margin. The mouth is gently arched and contains
an anterior row of four and posterior row of two papillae across the floor,
which are followed by a seventh papilla in larger individuals. There are 40–42
tooth rows in the upper jaw and 42–46 tooth rows in the lower jaw; the teeth
are arranged with a quincunx pattern into pavement-like surfaces. The tail
measures three times as long as the disc and bears two long stinging spines on
top; after the spine the tail becomes thin and whip-like, without any fin
folds.
There are numerous flattened, heart-shaped dermal denticles
on the back, arranged in a dense central band reaching the base of the tail,
and becoming smaller and sparser on the outer portions of the disc. Larger,
heart-shaped denticles are scattered over the disc, especially around the
"shoulders" and the middle of the back. Two pearl spines are present.
There is a line of 40–41 flat tubercles running down the dorsal midline, from
between the eyes to the tail spines; adult individuals also have two lines of
spiny denticles running along the sides of the tail from the spine to the tip.
The dorsal coloration is white to light gray, with brownish hexagonal blotches
forming a reticulated pattern that fades towards the disc margin. Smaller
individuals are covered by many irregular dark spots. The underside is entirely
light-colored. This ray attains a disc width of 36 cm (14 in).
Taxonomy
French zoologist Henri-Emilé Sauvage originally described
the marbled whipray as Trygon oxyrhynchus based on a female specimen caught
near Saigon, Vietnam, in an 1878 volume of the scientific journal Bulletin de
la Société philomathique de Paris. In 1913, Samuel Garman synonymized this
species with Himantura uarnak, a judgment that remained unquestioned in
subsequent literature until Maurice Kottelat referenced the name in his 1984 review
of Indochinese fishes.This species may also be referred to as the longnose
marbled whipray or the marbled freshwater stingray.
Distribution and habitat
Among the few members of its family restricted to fresh
water, the marbled whipray has been reported from Saigon in Vietnam, the Mekong
River near Tonle Sap and Phnom Penh in Cambodia, the lower Nan and Chao Phraya
Rivers in Thailand, and the Mahakam River in Kalimantan, Indonesia. The
subpopulations inhabiting these rivers are likely isolated from one another.This
bottom-dwelling species favors a sandy substrate in which it can bury itself.
Description
The marbled stingray has a thin, oval-shaped pectoral fin
disc longer than wide. The snout is long and triangular, with the pointed tip
projecting from the disc. The eyes are small, and immediately followed by
spiracles over twice their diameter. There is a curtain of skin between the
nares with a fringed trailing margin. The mouth is gently arched and contains
an anterior row of four and posterior row of two papillae across the floor,
which are followed by a seventh papilla in larger individuals. There are 40–42
tooth rows in the upper jaw and 42–46 tooth rows in the lower jaw; the teeth
are arranged with a quincunx pattern into pavement-like surfaces. The tail
measures three times as long as the disc and bears two long stinging spines on
top; after the spine the tail becomes thin and whip-like, without any fin
folds.
There are numerous flattened, heart-shaped dermal denticles
on the back, arranged in a dense central band reaching the base of the tail,
and becoming smaller and sparser on the outer portions of the disc. Larger,
heart-shaped denticles are scattered over the disc, especially around the
"shoulders" and the middle of the back. Two pearl spines are present.
There is a line of 40–41 flat tubercles running down the dorsal midline, from
between the eyes to the tail spines; adult individuals also have two lines of
spiny denticles running along the sides of the tail from the spine to the tip.
The dorsal coloration is white to light gray, with brownish hexagonal blotches
forming a reticulated pattern that fades towards the disc margin. Smaller
individuals are covered by many irregular dark spots. The underside is entirely
light-colored. This ray attains a disc width of 36 cm (14 in).
Biology and ecology
The natural history of the marbled whipray is poorly
understood. It likely feeds on benthic organisms such as small crustaceans and
molluscs. Reproduction is aplacental viviparous with females supplying their
unborn young with histotroph ("uterine milk"), as in other stingrays.
Human interactions
Only five specimens of the marbled whipray have been
deposited in museums. However, it is reportedly locally common in some rivers
and streams. It is an infrequent incidental catch of intense fishery activities
taking place across much of its range, using demersal tangle nets, fish traps,
and hook-and-line. These attractively colored rays, especially the young, are
valued by the aquarium trade. Another major threat to the marbled whipray is
habitat loss and degradation, from pollution, logging, and dam construction. In
Thailand, dams on the Chao Phraya have prevented stingrays in different
stretches of the river from intermingling, with a negative effect on genetic
diversity. Citing these threats, the International Union for Conservation of
Nature (IUCN) has assessed this species as Endangered. During the 1990s, the
Thai government initiated a captive breeding program for this and other
endangered freshwater stingrays at Chai Nat, but by 1996 the program had been
placed on hold.
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