Biology Reference
In-Depth Information
1. INTRODUCTION
Most people, at least those living in temperate zones, are familiar with a
typical tadpole frog life history. Frog eggs are laid in water, where they develop
into tadpoles. Tadpoles feed for weeks or months, and then metamorphose to
frogs. At metamorphosis, the animal's body and its lifestyle change completely.
The tadpole's tail is resorbed, and limbs develop. Rather than swimming inwater
and eating plant material or detritus, the frog jumps around on land and eats live
insects and other animals. Thismetamorphosis is controlled by thyroid hormone.
It often comes as a surprise to learn that this typical life history is not found in
many parts of the world, particularly in tropical areas. Frogs have evolved several
strategies for keeping their young out of the dangerous water environment. One
strategy is to keep the embryos and tadpoles hidden somewhere in the body of
the father or themother. Body sites include the vocal pouch of themale, a pouch
on the back of the female, or the uterine-like ovisac of the female (
Duellman &
Trueb, 1986; Elinson & del Pino, 2012
). A second strategy is to delete the tad-
pole from the life history and to have the embryos develop to frogs on land. This
strategy, known as direct development, is quite common. It has evolved inde-
pendently at least a dozen times (
Duellman &Trueb, 1986; Hanken, 1999
), and
it is a remarkably successful strategy. One large group of direct developers is
the Terrarana, mainly in Central and South America and the Caribbean. It con-
tains about 900 species in four families (
Hedges,Duellman,&Heinicke,2008;
Heinicke,Duellman,&Hedges,2007;Schmidetal.,2010
). Another group is
from Sri Lanka, which contains more than 100 species so far (
Meegaskumbura
et al., 2002; Pethiyagoda & Manamendra-Arachchi, 1998
).
The lack of a tadpole in these direct developing frogs raises the question
as to whether they undergo a thyroid hormone-dependent metamorphosis.
This question has been investigated in only one species of direct developer,
the Puerto Rican tree frog
Eleutherodactylus coqui
, a member of the Terrarana.
These frogs, indeed, have a cryptic thyroid-dependent metamorphosis
(
Callery & Elinson, 2000a; Elinson & del Pino, 2012
), and these results form
the basis of this chapter.
2. LOSS OF TADPOLE STRUCTURES IN FROG DIRECT
DEVELOPMENT
Despite the dozen independent origins of frog direct development,
the different groups tend to converge on a common embryo appearance.
This is due in large part to the lack of a feeding tadpole, leading to the
