Comparative Morphology of the Mechanosensory Lateral Line System in a Clade of New Zealand Triplefin Fishes

The mechanoreceptive lateral line system in fishes detects hydrodynamic stimuli and plays a critical role in many fundamental behaviours, including orientation to water currents and the detection of stationary objects, prey and predators. Interspecific variation in lateral line structure may result from a process of functional adaptation, with the background level of hydrodynamic activity proposed as an important selective pressure. Here we use the eight species of the ecologically diverse New Zealand marine triplefin fish of the genus Forsterygion and one species from the sister genus Notoclinops to investigate interspecific differences in lateral line morphology and to assess the relationship between lateral line characteristics and exposure to wave energy (fetch/depth ratio). Overall, the results show that lateral line traits are divergent between species, and these differences could in part be related to the wave exposure of the habitats that the species occupy. Specifically, numbers of canal neuromasts differed significantly between species, and most canal groupings increased in neuromast number with fetch/depth ratio, while the number and area of some superficial neuromast groupings decreased significantly with exposure. Distribution of superficial neuromasts along the trunk in the semi-pelagic and paedomorphic species F. maryannae differed from the other, demersal species, which may be associated with the unique lifestyle of this species and/or developmental processes. Canal architecture also differed considerably between species, but displayed no relationship with fetch/depth ratio. The results from this study indicate that some interspecific differences in lateral line organs may be a by-product of selection for habitat divergence. Future work should explore additional causal factors that might have influenced the evolution of lateral morphology in these species, including phylogenetic and allometric effects.