Morphological Evolution Of Skull And Gill Bones Of Eight Tailed Tadpoles In Qinling Area

Tadpoles are the larvae in the life cycle of anurans. They are usually aquatic. In the larval stage, anuran species have the cartilaginous skeletal structures of chondrocranium and hyobranchial skeleton. The chondracranium in anuran tadpole is mainly constructed of upper and lower jaws, cornua trabeculae, braincase, palatoquadrates, and otic capsules. The two major components in hyobranchial skeleton of anuran tadpole are ceratohyals and branchial baskets. Basic elements of the chondrocranium and hyobranchial skeleton are similar in tadpoles, but species differ in morphologies of chondrocranium and hyobranchial skeleton. These differences in morphology reflect the phylogeny and habitat of tadpoles. The characters of chondrocranium and hyobranchial skeleton can be used to establish anuran relationships among genera and families, and provide ecological data which are valuable for identifying adaptational trends among larval anuran ecotypes. In the present study, the chondrocranium and hyobranchial skeleton of eight anuran tadpoles inhabiting in Qinling Mountains, including Feirana quadrana, Pelophylax nigromaculata and Rana chensinensis (Ranidae), Bufo gargarizans and B. tuberculatus (Bufonidae), Oreolalax sp. and Megophrys minor (Megophryidae), Microhyla ornata (Microhylidae), were investigated by double-stained methods, morphological comparison using the stereoscope and phylogenetic analysis, in order to search for correlations of these skeletal variations with ecology and evolution. The main results as follows:1. The suprarostrals in dorsoventral direction in F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus, and Oreolalax sp. tadpoles may be capable of cutting foods by keratinized jaws sheaths. By contrast, the suprarostrals in anteroposterior direction of M. minor and M. ornata tadpoles may be capable of filtering small food particles by non-keratinized jaws sheaths. In addition, the processus muscularis in rasping tadpoles including F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus, and Oreolalax sp. are more developed than that in filtering feeders including M. minor and M. ornata. Because the width of the processus muscularis is positively correlated with the size of the orbitohyoideus, the rasping tadpoles possess the developed processus muscularis in order to graze food easily, in compare with the filtering feeders. 2. The broad commissura quadratocranialis anterior of the palatoquadrate in the lotic benthic tadpoles including F. quadrana and Oreolalax sp. could enhance the adhesive power to strongly attach to the substrate compared with that in semiflowing or lentic tadpoles including P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus, M. minor and M. ornata. Thus, we found that the broad commissura quadratocranialis anterior in tadpoles is strongly related to their lotic benthic environment.3. The adrostrale is present in the Megophryidae including Oreolalax sp. and M. minor. The presence of the adrostrals occur only in Pelobatoidea and seem to have evolved in the common ancestor to the group. In addition, the absence of the processus dorsomedialis and the processus ventromedialis in Meckel’s cartilages is a synapomorphy in species of Microhylidae.4. The medial parts of ceratohyals in stream dwelling tadpoles are much more broader than that in semiflowing or pond dwelling tadpoles. Because the broad medial parts of ceratohyals is associated with the buccal volume. Moreover, the long lateral lever arm of ceratohyale in benthic larvae including F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus and Oreolalax sp. could generate powerful negative pressure contribute to adhere to the bottom easily. In contrast, the short lateral portion of ceratohyale in the M. minor as surface feeder and M. ornata as midwater suspension feeder may produce the weak suction. In addition, it can be concluded that the thick branchial rays of F. quadrana, Oreolalax sp. and M. ornata is often associated with capturing the relatively small food particles. By contrast, the thin branchial rays of P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus and M. minor appears to be an adaptation for the relatively large food particles. Thus, the morphological variations of ceratohyals and branchial rays in tadpoles can reflect their feeding and habitat environment. In addition, the distinct processus anterolateralis hyalis is present in F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans and B. tuberculatus. By contrast, the processus anterolateralis hyalis is present but inconspicuous in Oreolalax sp. and M. minor. The presence of the processus anterolateralis hyalis of ceratohyale is an advanced character for anurans.5. The basibranchiale of the transitional species including Oreolalax sp. and M. minor is long but does not extend to separate the plana hypobranchiales. In the advanced species including F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus and M. ornata, the basibranchiale is short and connect the plana hypobranchiales. Three commissurae proximales are missing in Neobatrachia tadpoles including F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans, B. tuberculatus and M. ornata, and the absence of the commissurae proximales may be an apomorphic character for anurans. Four well-developed spicula are found commonly in Neobatrachia including F. quadrana, P. nigromaculata, R. chensinensis, B. gargarizans, and B. tuberculatus, and the presence of spicula may be an advanced character for anurans. It can be concluded that basibranchiale, commissurae proximales and spicula are correlated with phylogenetic relationships among anurans. In addition, the two processus branchialis fuse and form a cartilaginous bridge between the ceratobranchials ("closed" condition) in P. nigromaculata and R. chensinensis. The character of possessing a "closed" processus branchialis may represent a synapomorphy for the Ranidae.6. There is significant variation in larval external morphology, chondrocranium and hyobranchial skeleton that provides characters amenable to phylogenetic analysis. The larval features play an important role in reconstructing the plylogeny and evolutionary history of the major clades of anurans.7. Our phylogeny based on larval characters showed a close relationship between the Microhylidae and Megophryidae, indicating a basal position for the Microhylidae. However, in the trees obtained by combining adult and larval characters, the Microhylidae is closely related to the Bufonidae which represent advanced anurans. Therefore, we can found the incongruence between the larval and adult evolution in the Microhylidae. The clade formed by Microhyla and Megophrys may be the result of functional constraints associated with larval feeding. The morphological resemblances between microhylid larvae and Megophrys forms may be due to convergences.