Reproduction and Development 117to facilitate passage, and the venomous barb is flexible and covered in a
sheath that is lost shortly after birth.
In egg-laying elasmobranchs, the developing embryo gets its nutrition
from the yolk that was part of the unfertilized egg. Aside from some of
the carcharhinid reef sharks, chondrichthyan eggs are not round like bird
eggs but are more often rectangular and flattened. The corners of the egg
shell often have horns and sticky, stringy extensions (“tendrils”) that help
anchor it to a structure. The usual situation is one embryo per egg case,
although in two skates (Big Skate and Mottled Skate, Raja pulchra), two
embryos occur frequently and as many as seven embryos have been found
in a single egg case. Chondrichthyan eggs range greatly in size and shape,
from the more common flat rectangles to complex, corkscrew-shaped cases
in heterodontid bull sharks. Skate egg cases can be as small as 4.6 cm long
by 2.7 cm wide (1.8 by 1 in) in the Little Skate, to as large as 31 by 19 cm
(12 by 7 in) in the Big Skate. The Blue Skate of the western Atlantic is the
largest skate in the world, but its egg capsules are no more than 24 cm long
by 14 cm wide (10 by 5.5 in). The empty egg cases of skates often wash up
on beaches and are referred to as “mermaid purses” by beachcombers. Egg
cases differ in size and structure depending on species, often allowing iden-
tification of the skate that produced it (see http://www.eggcase.org).
The developing embryo inside the egg is protected by a leathery or
horny shell that was produced by a shell gland in the mother’s reproduc-
tive tract. As the embryo grows and uses up its yolk supply, its metabolism
increases. Oxygen demand goes up, and waste products accumulate. ToPlacental connections in sharks. A newborn Atlantic Sharpnose Shark, showing the umbilical cord with the out-
growths that take up nutrients from the mother. The cord ends in a placenta that attaches to the uterine wall of the
mother. Photo by W. C. Hamlett; used with permission