Development of the bony skeleton in two salamander species (Mertensiella caucasica and Chioglossa lusitanica: Salamandridae) with partial embryonization

Abstract

168 The Caucasian salamander (Mertensiella cauca sica) and golden striped salamander (Chioglossa lusi tanica) are the most closely related taxa in the group of true salamanders belonging to the family Salaman dridae [1, 2]. In addition, they stand out among the other salamanders by a considerable similarity in their morphological constitution [3] and life style [3–5]. A study on the reproduction biology of these species has also demonstrated that they are very similar in a num ber of reproductive parameters, namely, both species reproduce in brooks and lay large (5–7 mm in diame ter) eggs in small clutches (on average, 17–18 eggs) [4–6]. These traits distinguish M. caucasica and C. lusitanica from the majority of the other true sala manders, characteristic of which is ovoviviparity or viviparity, and the remaining members of the family Salamandridae, newts, which lay considerably smaller eggs. On the other hand, the large size of the eggs and hatching of larvae at advanced development stages (as was demonstrated for M. caucasica [7]) suggest a pro nounced embryonization of development, i.e., transi tion of the initial stages in larval development to the embryonic period. Among amphibians, this evolutionary process leads to establishment of a direct development (lacking the larval stage), which is present only in members of the family Plethodontidae within the order Urodela. The effect of the degree of embryonization on the specific morphoge netic traits and, in particular, craniogenesis, has been also demonstrated for these amphibians [8, 9]. As we see it, a study of the skeletal development in the amphibian spe cies with partial embryonization of development will help to shed light onto the initial stages in the evolution path way that had led to direct development. For this purpose, we have studied the skeletal development of M. caucasica and C. lusitanica in the ontogenesis. Clutches of both salamander species were incubated under laboratory conditions; the embryos at late developmental stages (staring from limb differen tiation), larvae, and metamorphosing animals were regularly fixed with 10% formaldehyde and stained with alizarin red and alcian blue to detect bones and cartilages, respectively. The developmental stages were determined according to the table of M. caucasica normal development [7], which, except for some col oration traits, equally well described the C. lusitanica developmental stages. GENERAL BIOLOGY