Shark Ecology 95
Skates live on the sea floor, with three families—rajid hardnose skates,
arhynchobatid softnose skates, and anacanthobatid legskates—making up
90% of the 236 deep-sea skate and ray species. Only five species in four
families of stingrays occur in the deep sea, of which two are monotypic: the
Sixgill Stingray (Hexatrygon bickelli) and the Deepwater Stingray (Plesio-
batis daviesi). As with the carcharhinid sharks, the whiptail stingray family
Dasyatidae, so successful in warm, shallow waters, has only one deep-sea
member, the Shorttail Stingray, which also occurs in shallow water. Ho-
locephalans are well represented in the deep sea; 40 of the world’s 46 chi-
maera species are deep-sea animals. As a group, chimaeras generally swim
just above the bottom.
A few of the deep-sea sharks are large: somniosid sleeper sharks can be 6
m (20 ft) long, and hexanchid sevengill and sixgill sharks grow to 3 m and 5
m (10 ft and 16.5 ft), respectively. Otherwise, deep-sea sharks are generally
small and include the world’s smallest sharks, maturing at only 17 to 25 cm
(7–11 in) in length, much smaller than the vast majority of shallow-water
sharks. These microsharks include the etmopterid lanternsharks, dalatiid
pygmy sharks, and proscyliid finback catsharks. (See “What are the largest
and smallest sharks alive today?” in chapter 1.)
Which geographic regions have the most species of sharks?
Zoologists divide the world’s oceans into four major zones, depending
mostly on temperature: tropical, warm temperate, cold temperate, and po-
lar. Each zone is further divided by oceans and continents, and oceans are
again divided according to depth.
Overall, the warm temperate regions house the greatest combined di-
versity of chondrichthyans (720 species). The tropics come next, with 654
combined species. Then come cold temperate regions (296 species); and
last are polar seas, with only six species, all in the north polar (Arctic) zone.
Many species, such as Tiger Sharks and Spiny Dogfish, occur in several
areas and are thus counted more than once in this distribution, so the num-
bers far exceed the world’s overall shark diversity of 520 or so species.
Exploring further, the North and South Pacific warm temperate re-
gions combined have more species than the North and South Atlantic re-
gions (340 versus 292 species), with a small region of high diversity around
South Africa (88 species). The Indo-West Pacific tropical area (which en-
compasses the western tropical Pacific plus Indian oceans) is home to about
1.5 times more species than the tropical eastern Pacific and tropical At-
lantic areas combined (423 versus 231 species). In the cold temperate seas,
the highest diversity is around Australia and New Zealand (128 species),