126 Sharks: The Animal Answer Guide
provided by most chondrichthyans involves carrying a developing embryo
inside its body, safe from predators. The mother shark also provides ox-
ygen, water, and minerals for a developing embryo, while carrying away
metabolic waste products in her bloodstream. But pre-birth or pre-laying
care mostly involves providing nutrition to the embryo (see “Do sharks lay
eggs or do they give birth to live young?” above).
If we include egg shape and egg-laying behavior, we can expand the
list of care-giving species. Some chondrichthyan eggs have shapes and
structures that increase their invulnerability, or are deposited in places that
predators can’t reach. Horn Sharks have especially complex egg shapes, the
leathery shell forming a spiral that the mother pushes into cracks, basically
corkscrewing it down and out of harm’s way and making it very hard to ex-
tract from a crevice. Some skates in the genus Sympterygia (S. acutus and S.
bonapartei) along the Brazilian coast produce egg capsules with very long
tendrils. Females lay their eggs in bunches, the tendrils attached to debris
and intertwined by the female’s egg-laying movements, forming a “nest”
that looks like a bouquet of flowers and could help protect the developing
young.
How is the sex of a shark determined?
The sex, or gender, of an animal can be fixed at fertilization by inherited
genes or can be affected instead by environmental factors such as tempera-
ture. In most “higher” vertebrates (birds and mammals), sex is controlled by
genes from the start, whereas in many lower vertebrates (some bony fishes,
turtles, alligators), environmental and even social factors such as crowding
can cause an animal to be one sex or another and even to change sex during
its lifetime. It appears that sharks follow the higher vertebrate plan, their
sex being decided genetically when sperm and egg fuse. Males and females
in even the smallest newborn elasmobranchs can be told apart by definite
(albeit small) claspers on males and by their absence in females. Internally,
immature males and females both already have identifiable structures that
will turn into testes and ovaries. Sex chromosomes, usually similar to the
XY male and XX female pattern of humans, have been found in skates, gui-
tarfishes, stingrays, and many but not all sharks. Sex-determining factors
probably reside on these chromosomes.
Exceptions do arise occasionally, suggesting that the “instructions”
coming from sex-determining genes can be misunderstood. A Little Skate
captured off the New York coast possessed a mixture of male and female
reproductive parts. On its left side it had a large, well-developed testis, vas
deferens, and functional clasper. On its right side, the clasper was tiny, and
internally the skate possessed an ovary, a shell gland, and an oviduct.
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