Morphology of the elygium and developing umbraculum in the eye of amietia vertebralis tadpoles

Abstract

The elygium is a darkly pigmented projection over the pupil of the tadpoles of certain species that live mostly at high altitudes. It has been suggested that this structure shades the retina, protecting it from high UV levels. In post-metamorphic individuals, the elygium is replaced by a structure known as an umbraculum. Confusion arose in the past from the inconsistent use of terminology when referring to these two structures. While they may serve the same function, these structures differ fundamentally in structure and origin. Our investigation of the ultrastructure of the eye in Amietia vertebralis tadpoles, using electron and light microscopy, revealed that the elygium consists of an accumulation of melanophores situated within the inner cornea, whereas the umbraculum is a dorsal extension of the iris pigment epithelium, analogous to the ventral pupillary nodule, which is present in most frog species. In A. vertebralis, the umbraculum starts to develop on the iris of older tadpoles (Gosner [1960] stage 37 Herpetologica 16:183- 190), medial to the overlying elygium. A smaller, ventral elygium is also present in this species and is similar in structure to the dorsal elygium. The development of the elygium over a six-month period is also described.