Thyroid hormones, retinoids and steroid hormones require
essential components for their synthesis that the animals that
use them cannot always synthesize internally. In fact, the hormones themselves may be acquired through trophic interactions.
Despite the inherent requirement to either produce or obtain such
essential components, these hormones play critical roles in animal
development, metabolism, behavior and life history evolution.
Sterols are the ‘‘essential components” in arthropod ecdysteroid
signaling and iodine is the essential component of echinoid TH signaling. These hormone components, the hormone precursor or
even the active hormone itself may have been available to ances