Habitat-related specialization of lateral-line system morphology in a habitat-generalist and a habitat-specialist New Zealand eleotrid

Ecological and sexual roles of scale mechanoreceptors in two species of Neotropical freshwater snake (Dipsadinae: Helicops)

Understanding the roles of ecological and sexual selection in the variation of sensory systems may elucidate aspects of the natural history of organisms. Little is known about the evolution of mechanoreception in snakes and how the function and structure of mechanoreceptors vary between species or sexes. Here, we describe the internal and external morphology of cephalic mechanoreceptor sensilla and quantify inter- and intraspecific variation in four sensilla traits of two freshwater snake species that differ in their habitat and diet preferences, Helicops pastazae and Helicops angulatus, by combining scanning electron microscopy (SEM), histological techniques and image analyses. SEM showed sensilla as prominent evaginations of the epidermis surrounded by concentric rings, with H. pastazae having larger and more heterogeneous sensilla. In both species, histology showed a reduction in the outer epidermal layer above the sensilla with a grouping of dermally derived central cells below it. Higher values of sensilla traits were found in H. pastazae, except for the chin-shields. We also found that males of both species had significantly higher values of sensilla traits on all of the scales examined. We hypothesize that the variation in both qualitative and quantitative traits in scale sensilla might be a consequence of differences in foraging and/or reproductive strategies between species and sexes.

Spatial, sexual, and rapid temporal differentiation in neuromast expression on lateral plates of Haida Gwaii threespine stickleback (Gasterosteus aculeatus)

Lateral lines, a major sensory modality in fishes, are diverse among taxa, but their intraspecific variation has received limited attention. We examined numbers of superficial neuromasts on the buttressing lateral plates (LP) of 1910 threespine stickleback (Gasterosteus aculeatus Linnaeus, 1758) from 26 ecologically and morphologically diverse populations on the Haida Gwaii archipelago, western Canada. Extending from previous studies, we predicted that (i) highly stained dystrophic localities would have threespine stickleback with elevated numbers of neuromasts per plate due to a greater reliance on non-visual sensory modalities and (ii) that LP count and neuromast numbers per plate would functionally covary with predatory assemblage. We found that there were no differences in neuromast count across major habitats (marine, lake, stream), but clear-water populations and those with predatory fish had significantly more neuromasts per plate than most populations in highly stained dystrophic lakes, the effects being accentuated on the first buttressing plate (LP4). We also report the first evidence that neuromast counts per plate are sexually dimorphic, with males having a greater density of neuromasts in most populations. Two transplant experiments between ecologically opposite habitats indicate that within 12 generations, neuromast counts per plate can rapidly shift in response to a change in habitat.