CAPTIVE GUMMY SHARKS, MUSTELUS-ANTARCTICUS, FORM HYPERMINERALISED BANDS IN THEIR VERTEBRAE DURING WINTER

The subjective interpretation of the growth increments in shark verteb rae and uncertainty over the temporal periodicity of their formation d ecrease confidence in the accuracy of ages estimated from increment co unts. To determine the periodicity with which growth increments are fo rmed in the vertebrae of gummy shark, Mustelus antarcticus, fluorochro me dyes were injected into captive sharks at quarterly intervals. Vari ation in the vertebral growth rate between fluorochrome labels was exa mined and compared with measurements of the density of mineralisation of vertebral sections. The slowest rates of vertebral growth and the h ighest radiographic densities were found in the vertebral tissues form ed during winter. The microradiographic method described here resolves individual hypermineralised increments that are indistinguishable usi ng surface reading methods. With this extra information, a reader may better count, group, or exclude certain growth increment patterns when making an age estimation. Counts of clusters of hypermineralised incr ements may be more useful in age determination than counts of individu al increments because individual increments form unpredictably and pos sibly result from brief, irregular episodes of slow vertebral growth.