EGGSHELL FORMATION DURING PROLONGED GRAVIDITY OF THE TUATARA SPHENODON-PUNCTATUS

Scanning electron microscopy (SEM) was used to examine the process of shell formation in tuatara. Tuatara carry eggs in the oviducts for sim ilar to 7-8 mo before nesting, a period of gravidity more than three t imes as long as in any other oviparous reptile. Our aim was to determi ne whether shell formation occurred rapidly after ovulation, or whethe r it occurred gradually throughout gravidity. Eggs were obtained from females In early gravidity (May, similar to 1 mo after ovulation), mid gravidity (August and September, 4-5 mo after ovulation), and late gra vidity, immediately prior to nesting (December, 8 mo after ovulation). The shell membrane (fibrous layer) was well formed by May, but calcif ication of the outer surface had only just begun. Vertical columns of calcium carbonate were embedded in the shell membrane and appeared to erupt through the outer surface between early and midgravidity. Change s in the appearance of the outer calcareous layer were evident as grav idity progressed. In all shells, calcium carbonate was present as calc ite. The appearance of the inner boundary (innermost layer of eggshell ) was variable; some shells had a smooth and amorphous inner boundary as previously reported for tuatara and other reptiles, whereas other s hells had an inner boundary composed of small spherical granules on th e inner surface of which small calcareous spicules were scattered. A p reviously published model of the process of shell formation in tuatara eggshells is refined in light of our observations. We interpret the a bility of female tuatara to shell their eggs gradually during winter a s further evidence of their unusual physiological tolerance of cold co nditions. (C) 1996 Wiley-Liss, Inc.