Looking closer at the determining of a phenotype? Compare by stages or size, not age

During ontogeny, individual fish larvae and juveniles will develop at different paces despite apparently identical age and environment, and even genetic background. The urgent need to elucidate mechanisms critical to juvenile production, recruitment or even lifelong growth patterns implies that measures and results are compared by a meaningful standard such as developmental stage of the individual fish. Characterizations of developmental stages based on relatively invariable characteristics such as the sequence of cranial ossification have proven to be not only commercially and scientifically applicable in Atlantic halibut, but also point to a practical use for a character which is preserved in most fishes examined. Once described, useful proxies for stage determinants may be size parameters such as standard length, or preferably myotome height (particularly for flatfishes). The worst proxy for development of a batch of fish is age, exacerbated by the expense of analyzing irrelevant intervals.