Osteology of the posterior vertebral column and caudal skeleton of marine amphibious gobies (mudskippers) (Teleostei: Gobioidei)

Abstract

Mudskippers are amphibious gobies (Teleostei: Gobioidei, Oxudercinae) that have served as models for the specialised physiology and behaviour of fishes out of water. In this study, a comparative analysis of the posterior vertebral column and the caudal skeleton of ten mudskipper species was conducted on the basis of X-ray imaging. The species considered were Apocryptes bato, Apocryptodon madurensis, Boleophthalmus dussumieri, Oxuderces dentatus, Periophthalmodon freycineti, Pn. schlosseri, Periophthalmus novemradiatus, Ps. waltoni, Pseudapocryptes borneensis, and Scartelaos tenuis. For the osteological description the new term 'modified caudal vertebra' is used for all those vertebrae that display visibly modified neural and/or haemal spines compared to the spines of a 'usual' caudal vertebra, but are not involved in the support of caudal rays. The results reveal that the most terrestrial forms (Pn. freycineti, Pn. schlosseri, Ps. novemradiatus, Ps. waltoni) possess distinct traits that are seldom found in the other species. Among these features are (a) the existence of at least two modified caudal vertebrae (also present in S. tenuis), (b) a particularly close, dovetailing association between the neural spines of the preural vertebrae two and three (restricted to Ps. novemradiatus and Ps. waltoni), and (c) thickening and shortening of the ventralmost principal caudal rays (also present in B. dussumieri and S. tenuis). These findings support the idea that the posterior caudal vertebrae and caudal skeleton of the mentioned species are modified to enhance locomotion on land. Moreover, a relationship between character development and degree of terrestrial adaptation is probable, as all three traits are most pronounced in Ps. waltoni, which correlates with its strikingly high level of adaptation to amphibious life. A further aspect of this study is that the newly recognized skeletal structures have good fossilization potential and could therefore facilitate recognition of fossil species of mudskippers, which are currently unknown.