Plasticity and Variation of Skeletal Cells and Tissues and the Evolutionary Development of Actinopterygian Fishes

Abstract

Plasticity of skeletal cells, tissues and organs is a general vertebrate character and rampant among teleost fishes. Plasticity is not restricted to early ontogeny but is evident throughout the entire life history. Epigenetic factors during development influence skeletal anatomy, mechanical properties of skeletal tissues and numbers of meristic characters such as vertebrae. Adult teleost skeletons demonstrate plasticity in functional adaptation, especially to mechanical forces and mechanical loading and to physiological factors such as temperature or the availability of minerals. Plasticity influences the character of skeletal tissues that can range from cartilage to acellular bone, as well as numbers of serial elements such as vertebrae, scales or fin rays. Variable numbers of vertebrae and the existence of vestiges, rudiments, atavisms and hyperostotic bones are discussed as examples of skeletal variation. Plasticity at the cellular level is best understood by recognising features of fish skeletal cells not seen in mammals, two of which are the absence of osteocytes in the acellular bone of most species of teleosts and the resorption of bone by mononucleated rather than multinucleated osteoclasts. A third is that mineral resorption in teleosts is phosphorous rather than calcium-based. The fourth is the variety of skeletal tissues that share features of bone and cartilage (chondroid bone, secondary cartilage) or between cartilage and notochord (chondroid) or of bone and dentine. Phenotypic plasticity of skeletal tissues is affected at the cellular level by modulation, transdifferentiation, metaplasia and remodelling. Plasticity of skeletal cells, tissues and organs has important consequences for skeletal development and evolution.