Sharks The Animal Answer Guide

118 Sharks: The Animal Answer Guide

renew the water supply inside the egg, embryonic skates lash their tails

back and forth, moving water in and out of the case via small slits in its side.

This movement in the semitransparent egg case and the resulting water

currents can attract predators such as other skates as well as sharks, bony

fishes, some snails, and marine mammals. If something bumps into an egg

case, or if the egg case is exposed to a weak electric discharge such as the

ones produced by other skates, the embryo stops moving its tail.

Chimaeras are also egg layers. A female produces relatively large eggs

(15–20 cm long by 5–7 cm wide, or 6–8 in by 2–3.5 in) surrounded by a

horny shell, one embryo per egg case. Chimaera egg cases are rounder and

more elongate than skate and shark egg cases. Eggs are laid on the bottom,

usually two at a time, one from each oviduct. Several pairs of eggs may be

laid over the course of a reproductive season, with egg-laying bouts occur-

ring every one or two weeks. Small slits in the posterior end of the egg case

allow water to enter and metabolic wastes to be carried away.

How long do female sharks hold young in their body?

Gestation is the period from egg fertilization to birth. It’s another way

of discussing embryo development. The human gestation period is nine

Egg case, or mermaid’s purse, from
a Big Skate, Raja binoculata. This
North Pacific species is the largest
skate in North America, reaching a
length of up to 2. 4 m ( 7. 9 ft) and a
weight of over 90 kg ( 200 lb). A sec-
tion of the shell has been cut away
to reveal several developing embryos
inside the case. Some large skates
are overfished because they are born
at a size that is already too large to
escape through the mesh of fishing
nets. Photo by Gene Helfman

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