Axial elongation in fishes: using morphological approaches to elucidate developmental mechanisms in studying body shape.

One of the most notable features in looking across fishes is their diversity of body shape and size. Extant actinopterygian fishes range in shape from nearly spheroidal in pufferfishes to extremely elongate in snipe eels with nearly every shape in-between. One extreme along the body-shape continuum is a highly elongate form, which has evolved multiple times independently in Actinopterygii. Thus, comparison of these separate (independent) radiations provides a unique opportunity for examining the anatomical traits underlying elongation as well as the similarities and differences in the evolutionary pathways followed. Body elongation generally evolves via an increase in region-specific vertebral number, although certain lineages elongate via an increase in vertebral length. In this study, we describe how anatomical characters related to feeding and locomotion are correlated with elongation of the body across Actinopterygii. In addition to modifications of the postcranial axial skeleton, elongation in fishes is often accompanied by an increase in head length, loss of the pelvic fins, reduction of the pectoral fins, and expansion of the median fins. Based on anatomical studies and on recent studies of developmental control of the body axis in different species, we hypothesize how an axial trait might change at the genetic level. Overall, we discuss the evolution of body elongation in fishes in light of an understanding of the underlying anatomical modifications, developmental control, ecology, and locomotion.