94 Sharks: The Animal Answer Guidewater that comes into its body by increasing its urine production 20-fold.
These and other physiological adjustments are quickly reversed when the
shark returns to the ocean.
People live, work, fish, play, and dump their trash in rivers. As a result,
freshwater elasmobranchs are more threatened than are chondrichthyans
in general (see “Are any sharks endangered?” in chapter 10).How far down in the ocean do sharks live?
Sharks do not live as deep in the ocean as bony fishes. The deepest liv-
ing bony fish is a cusk eel, Abyssobrotula galatheae, found at 8,370 m (27,460
ft). The depth records for sharks are held by a Great Lanternshark (Et-
mopterus princeps) at 4,500 m (14,763 ft), a Portuguese Dogfish at 3,675 m
(12,057 ft), and a Leafscale Gulper Shark (Centrophorus squamosus) at 3,280
m (10,761 ft), suggesting that sharks go only half as deep as bony fishes.
The record for skates is held by Bigelow’s Skate (Rajella bigelowi), at 4,156
m (13,635 ft), and the Pallid Skate (Bathyraja pallida), at 3,280 m (10,761
ft). Holocephalans appear to bottom out around 3,000 m (9,900 ft), repre-
sented by the Atlantic Chimaera (Hydrolagus affinis) at 2,909 m (9,544 ft)
and an unidentified member of the genus Harriota at 3,010 m (9,875 ft).
Chondrichthyan fishes as a group may avoid greater depths because of
their comparatively large size, which would be hard to maintain given the
general lack of food in the huge expanse of the ocean’s deepest waters.
This is not to say that sharks don’t live in fairly deep water. In fact,
about half of all shark, ray, and chimaera species occur predominantly in
water deeper than 200 m (660 ft), which is near the cutoff depth that ma-
rine biologists use to define the deep sea. Sharks occur on and just above
the bottom as well as in open water. Breaking deepwater sharks down by
groups, 254 sharks (53% of all sharks), 236 batoids (38%), and 40 holo-
cephalans (89%) occur below 200 m. Twenty-three families of sharks ap-
pear in deep water, but of these, only five families contain more than 10
species: scyliorhinid catsharks (102 species), etmopterid lanternsharks (42
species), squalid dogfish sharks (25 species), centrophorid gulper sharks (18
species), and somniosid sleeper sharks (16 species). The remaining families
contain only one or a few deep-sea members. Some of the world’s strangest
sharks are deep-sea denizens, with several monotypic (only one species in
the family) groups represented. These include the Frilled Shark (Chlamy-
doselachus anguineus), the Crocodile Shark (Pseudocarcharias kamoharai), the
Goblin Shark (Mitsukurina owstoni), and Bramble and Prickly sharks (Echi-
norhinus brucus and E. cookei). Carcharhinid sharks dominate shallow seas,
but only two species in this family, the Bignose Shark (Carcharhinus altimus)
and the Night Shark (C. signatus), spend much time in the deep.
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