Environmental Engineering Reference
In-Depth Information
are thought to have evolved from hoofed land
mammals related to today
are capable of
intraspecies and interspecies
s horses and sheep,
whose descendants spend most of their time in
productive shallow waters searching for food.
Modern whales range in size from 2.8 m (6 ft) to
33 m (110 ft) in length and weigh up to
100,000 kg (110 tonnes). Their paddle-shaped
forelimbs are used primarily for steering, and
their hind limbs are reduced to vestigial bones
that do not protrude from the body. Mainly
horizontal tail
'
cooperation.
The term dolphin is generally used for small
cetaceans having a slender body and beak-like
snout, and porpoise for animals having a stocky
body and blunt snout. Dolphin and porpoise are
common names of two subtly different groups of
small odontocete whales. Porpoise, as a term,
refers to smaller members of the group, which
have spade-shaped teeth, a triangular dorsal
n
and a smooth front end tapering to a point. Black
ukes moving up and down by
powerful muscles at the animal
fl
'
s posterior end
)s
often sighted in the brackish waters of Indian
Sundarbans. Dolphins are usually larger and
have an extended bottlelike jaw
nless porpoise (
Neomeris phocaenoides
propel them.
Cetaceans are broadly divided into two sub-
orders. Suborder Odontoceti includes the
toothed whales , which are active predators and
possess teeth to subdue their prey. Toothed
whales have a high brain-weight-to-body-weight
ratio and much of their brain tissue is involved in
formulating and receiving the sounds on which
they depend for feeding and socializing. Smaller
whales in this group are the killer whales (the
largest of all dolphins). The largest toothed whale
is the sperm whale, which is 18 m in length and
can dive at least 1,140 m (3,740 ft). It feeds
mainly on large squids. Toothed whales search
for prey using echolocation , which is otherwise
the biological equivalent of sonar. Some com-
mon examples of toothed whales are beluga
whale, pilot whale, pygmy sperm whale and false
killer whale. The largest of the group is the sperm
whale, which feeds primarily on the squids. The
best-known toothed whales are the dolphins that
perform in shows at numerous aquariums and
oceanariums. The harbour porpoise is one of the
smallest cetaceans and is a familiar site to both
boaters and beach combers. The largest of the
dolphins is the killer whale or orca. These are the
only cetaceans to feed on warm-blooded animals
mainly seals and penguins.
The ears of toothed whales are modi
lled with sharp
round teeth. The small jumping whales in most
oceanarium shows are dolphins and killer
whales. However, this terminology has not been
strictly followed in naming the species.
Habitat wise, the cetaceans of the Indo-
Malayan region fall under three categories: (i) the
river dolphin, (ii) coastal and estuarine forms and
(iii) marine species which live in deep-waters.
The latter may be resident of tropical waters or
those, which mainly live in cold waters but
migrate seasonally towards the tropics such as
Balaenoptera musculus
.
The common species of dolphin found in the
Gangetic stretch adjacent to coastal Bay of
Bengal are Gangetic dolphin (
,
Balaenoptera physalus
Platanista gange-
tica
) and Irrawaddy dolphin (
Orcaella breviros-
tris
). Gangetic oolphin is restricted in fresh water
zone, whereas Irrawaddy dolphin is widely visi-
ble in the brackish water in and around the del-
taic Sundarbans. The identifying characters, food
preference, distribution and conservation status
of these species are listed in Table 2.17 .
The Ganges river (Hooghly estuary) is noted
for pouring and draining huge freshwater and silt
in the Bay of Bengal. This stretch is the homeland
of
ed to
receive a wide range of water vibrations. This
adaptation improves the animal
(Gangetic dolphin),
which is locally known as the
Platanista gangetica
. The species
was once common in many of the rivers in India,
Nepal and Bangladesh. However, it has disap-
peared from much of its former habitat over the
last 100 years and its future is uncertain. There
may be as few as 4,000
'
susu
'
'
s ability to hear
underwater and re
nes its hearing for the purpose
of echolocation. Echolocation is similar to sonar.
Dolphins are strongly social animals. They
exhibit problem-solving skills, have long periods
to mature with many learning experiences and
5,000 left. Dams and
barrages are major threats to the future of this
-
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