The Structure and some Functional Aspects of the Eggshell of the Broad Shelled River Tortoise Chelodina expansa ( Testudinata: Chelidae)

Abstract

C. expansa eggs have a brittle calcareous shell composed of needle-like aragonite crystals which radiate from a central core to form conical shell units. The shell is attached to a multi-layered fibrous shell- membrane, the layers of which separate to form an air space at one end of the egg. Funnel-shaped pores occur at the junctions of some shell units. The shell is similar in thickness to that of comparable birds' eggs but the shell membrane is three times thicker. Pores are less dense at the poles of the egg than at equatorial regions but even here they are much less dense than in birds' eggs. Pore diameter is larger than in birds'eggs but this is largely due to the funnel-shape of the pores. Water vapour conductance is six times higher in C. expansa eggs than in comparable birds' eggs. This increase cannot be explained by a larger pore area but may result from water vapour conductance through cracks in the shell, which are frequently observed. The thicker shell-membrane may compensate for these cracks and the partial loss of the shell.