AbstractThe primary objective of the research reported here was to decipher information on the relationship between size and age from the keratin plates of the carapace of hawksbill turtles, Eretmochelys imbricata. This required a detailed description of the development and differentiation of the keratin plates, and the formulation of a morphological model, consistent with all past observations, in which integrated changes in structure and pigmentation. Particular attention here was applied to estimating somatic growth rates from the spatial arrangement of the colour pattern.
During the course of the study, advances in egg incubation, embryology and captiveraising were made, which are reported. They have implications for efforts being made to produce E. imbricata for conservation and/or commercial purposes, through captive breeding and raising. Similarly, a morphometric analysis was carried out so that any shape changes likely to affect the utility of any morphological age indicator found could be accounted for.
The keratin plates of E. imbricata have a laminate structure, with each lamina being a corrugated band of -keratin, separated by spaces that reflect light differently and are a zone of weakness. These laminae are not annual structures, and at least 3 to 5 are laid down each year.
Growth arrest lines across the dorsal surface of a keratin plate form in consecutive winters, and they separate short shelves of keratin at the margin of imbrication, on the ventral surface of the keratin plate: both indicate annual growth and if present have opportunistic utility for aging. However, they are difficult to read in most individuals. The colour pattern within the keratin plates is also an index of annual growth. It results from isolated colonies of melanocytes in the dermis, secreting melanin that becomes trapped within the keratin laminae. The pigmentation tends to form in zigzag rows, across the growing edge of the C1 plates, and gives the appearance of moving backwards across the keratin plate when in reality, the plate is growing forward, surrounding the pigment layer.
In E. imbricata greater than 400-500 mm SCL these rows of pigmentation are highly correlated with successive winters, and they can be used to quantify the size-age vii relationship in captive-raised and wild E. imbricata. In animals less than 400-500 mm SCL, in addition to winter bands there are supernumerary bands that need to be corrected for.
When used in conjunction with a mark-recapture study, the assessment of colour bands can provide a wealth of new information on annual growth rates not otherwise available. When applied to a stockpile of E. imbricata shell plates, a size-age relationship can be derived for the wild population from which the shell plates came, although corrections for individuals up to 400-500 mm SCL may be needed.
|Date of Award
|Grahame Webb (Supervisor) & Corey Bradshaw (Supervisor)