Changes in key traits versus depth and latitude suggest energy-efficient locomotion, opportunistic feeding and light lead to adaptive morphologies of marine fishes.

1. Understanding patterns and processes governing biodiversity along
broad-scale environmental gradients, such as depth or latitude, requires
an assessment of not just taxonomic richness, but also morphological and
functional traits of organisms. Studies of traits can help to identify
major selective forces acting on morphology. Currently, little is known
regarding patterns of variation in the traits of fishes at broad spatial
scales. 2. The aims of this study were: (i) to identify a suite of key
traits in marine fishes that would allow assessment of morphological
variability across broad-scale depth (50 – 1200 m) and latitudinal (29.15
– 50.91°S) gradients; and (ii) to characterise patterns in these traits
across depth and latitude for 144 species of ray-finned fishes in New
Zealand waters. 3. Here, we describe three new morphological traits:
namely, fin-base-to-perimeter ratio, jaw-length-to-mouth-width ratio, and
pectoral-fin-base-to-body-depth ratio. Four other morphological traits
essential for locomotion and food acquisition that are commonly measured
in fishes were also included in the study. Spatial ecological
distributions of individual fish species were characterised in response to
a standardised replicated sampling design and morphological measurements
were obtained for each species from preserved museum specimens. 4. With
increasing depth, fishes, on average, became larger and more elongate,
with higher fin-base-to-perimeter ratio and larger
jaw-length-to-mouth-width ratio, all of which translates into a more
eel-like anguilliform morphology. Variation in mean trait values along the
depth gradient was stronger at lower latitudes for fin-base-to-perimeter
ratio, elongation and total body length. Average eye size peaked at
intermediate depths (500-700 m) and increased with increasing latitude at
700 m. 5. These findings suggest that, in increasingly extreme
environments, fish morphology shifts towards a body shape that favours an
energy-efficient undulatory swimming style and an increase in jaw-length
versus mouth width for opportunistic feeding. Furthermore, increases in
eye size with both depth and latitude indicate that changes in both the
average ambient light conditions as well as seasonal variations in
day-length can act to select ecomorphological adaptations in fishes.
10-Oct-2019