The upper and lower jaws are covered with sensory pits, which encase bundles of nerve fibers
that respond to the slightest disturbance in surface water. Thus they can detect vibrations
and small pressure changes in water, making it possible for them to detect prey, danger and
intruders even in total darkness.
Turtles
Turtles are reptiles of the order Testudines, most of whose body is shielded by a special
bony or cartilaginous shell developed from their ribs. About 300 species are alive today and
some are highly endangered. Turtles cannot breathe in water, but can hold their breath for
various periods of time. Like other reptiles, turtles arepoikilothermic(or “of varying tem-
perature”). Like other amniotes, they breathe air and don’t lay eggs underwater, although
many species live in or around water.
The largest chelonian (all living species) is the great leatherback sea turtle (Figure13.28),
which reaches a shell length of 200 cm (7 ft) and can reach a weight of over 900 kg (2,000
lb). Freshwater turtles are generally smaller, but the largest species, the Asian softshell
turtle, has been reported up to 200 cm (7 ft). The only surviving giant tortoises are on the
Seychelles and Galapagos Islands and can grow to over 130 cm (4 ft) in length and weigh
about 300 kg (670 lb) (Figure13.29).
Figure 13.28: The largest living chelonian, the leatherback turtle, which can reach up to 7
ft in length and over 2,000 lb. ( 15 )
The smallest turtle is the speckled padloper tortoise of South Africa, measuring no more
than 8 cm (3 in) in length, and weighing about 140 g (5 oz). Turtles are broken down into
two groups, according to how they evolved a solution to the problem of withdrawing their
neck into the shell: the Cryptodira, which can draw their neck in while contracting it under
their spine, and the Pleurodira, which contract their neck to the side.