Batoid Size and Sexual Maturity
Responding to the question, "is it true for ALL rays that sexual maturity depends on size rather than age?", I wrote:
As a group, batoids are far too poorly known to make such generalizations.
There is some evidence from a mere handful of ray species to support a somewhat stronger link between size and maturity than age and maturity in these batoids. But maturation in general — and in elasmobranchs in particular — is a complex process that probably involves a great many factors, including (but by no means limited to): metabolic rate, activity levels, feeding success, social status, and sex — all of which are subject to change (with the possible exception of sex in elasmo's — although, given the incidence of hermaphrodism in these fishes, the possibility cannot as yet be ruled out completely). The dynamic balance of these factors (among others) in rays probably affects BOTH size and age at maturity to differing extents.
Consider the example of sex. A ray's sex very probably plays a role in determining its maturation size and age. The large, yolky eggs of elasmobranchs represent a substantial caloric investment on the part of an individual, and thus it seems likely that both the body size and time required to accumulate sufficient energy from the environment would be significantly greater for females of a given species than males. From those relatively few elasmobranch species for which basic age and growth data are available, this seems to generally be the case. But body size and age at maturity in elasmobranchs occur over a range of values in any given species and may not necessarily vary in direct proportion to one another. There is simply too little data to determine a general relationship between size or age and maturity in rays.
There is also the problem of observer bias: size is MUCH easier to measure accurately than age (especially in the field). As one of the most obvious life history parameters, size may mask or be associated with other, less obvious, life history parameters and thus be artificially correlated with maturity in rays. There is insufficient data to determine whether size is causally linked to maturity in rays. Offhand, it seems more likely that size and age are merely correlates of maturity, and the actual causal mechanism of maturation in rays has more to do with an as-yet poorly understood combination of genetic, endocrinological, energy storage, and possibly social factors. The simple truth is, we just don't know at present.
— R. Aidan Martin