Meeting Abstract
Morphological traits comprising fish body plans can tell us a lot about how species interact with their surroundings. However, much of our understanding of the distribution of morphological disparity across habitats and environments is geographically constrained and taxonomically limited. In this study, we investigate body shape evolution and disparity at a global scale, as it occurs with increasing ocean depth, an environmental dimension along which many biologically important factors vary, including light, temperature, pressure, and nutrients. We make use of a large morphological dataset of ray-finned fishes, evaluating body shape disparity for over 3,000 marine species. Fishes were categorized into four traditional depth zones, epipelagic (0-200m), mesopelagic (200-1,000m), bathypelagic (1,000-4,000m), and abyssopelagic (4,000-6,000m). Using eight size-corrected linear traits, we show that morphological disparity increases more than two-fold across ocean depth zones. Body length and caudle peduncle traits had the greatest variance and elevated rates of evolution at depth. We observe that deep-sea fishes readily diversify along a body elongation axis, accessing the full range found at shallow depths in addition to novel elongate morphotypes. We also find greater jaw size (but lower variance and rate of evolution) with depth, a pattern that is frequently offered anecdotally, but has never been shown in a comparative study of this magnitude. The relative constraint on jaw size in aphotic, deeper waters is consistent with opportunistic trophic strategies. In contrast, the large variance in body elongation in the deep-sea may reflect a release from selective pressures on locomotion compared to light-dominated surface waters.
