128 Sharks: The Animal Answer Guide
the artificial conditions—including a lack of male companionship—that the
females experienced in captivity? Many skates and rays are kept in captivity,
but none have, as yet, reproduced asexually. Virgin births are another aspect
of our limited knowledge of the reproductive ecology of elasmobranchs.
Is there such a thing as a hybrid shark?
Hybrids are offspring that result when two different species mate. Until
2012, no one thought sharks or their relatives hybridized. (We can ignore
all the monster movies in which mad scientists mate sharks with a variety of
other animals to terrorize the world’s teenagers.) But a study of mitochon-
drial DNA in two blacktip species from the east coast of Australia—the
Australian Blacktip (Carcharhinus tilstoni) and the Common Blacktip (C.
limbatus)—has changed our thinking. Researchers showed that a large per-
centage of individuals identified as Australian Blacktips because of their out-
ward appearance were in fact hybrids between the two species. Aside from
being a spectacular “first,” this result has implications for fisheries conserva-
tion, our understanding of evolution in sharks, and the possible response of
species to global climate change. Importantly, if hybrids are misidentified as
Australian Blacktips and are less fit than full species, fishery quotas for the
real species could be set too high, potentially leading to overfishing.
Do male and female sharks live together?
Except during the breeding season, many sharks tend to segregate by
sex, males occurring predominantly with males and females with females.
Among the species known to form such relatively sex-specific aggregations
are Spiny Dogfish, Scalloped Hammerheads, Small-spotted Catsharks,
Oceanic Whitetips, School Sharks, Sand Tigers, Gray Reef Sharks, Bon-
netheads, Blue Sharks, Cownose Rays, and Thornback Rays (Raja clavata).
For example, Shortfin Mako males and females occupy different parts of
the Pacific off of Japan for most of the year. Females occur widely in sub-
tropical and temperate regions, whereas males are more narrowly distrib-
uted in southeastern portions, below 30° north latitude and east of the
International Dateline at 180° longitude. Females also show a seasonal dis-
tribution: when they are pregnant, their distribution shifts from southerly
latitudes to more northerly latitudes as their embryos grow.
Sexual segregation applies to other migratory species. We now believe
there is one wide-ranging world population of Whale Sharks, suggesting
that animals come from many places to breed together, perhaps in a few,
still-unknown sites. But at other times they separate by size and sex. Ag-
gregations of large females have been observed in only two locales: at the
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