Functional beta diversity of New Zealand fishes: characterising morphological turnover along depth and latitude gradients, with derivation of functional bioregions

Changes in the functional structures of communities are rarely examined
along multiple large-scale environmental gradients. Here, we describe
patterns in functional beta diversity for New Zealand marine
fishes vs depth and latitude, including broad-scale
delineation of functional bioregions. We derived eight functional traits
related to food acquisition and locomotion and calculated complementary
indices of functional beta diversity for 144 species of marine ray-finned
fishes occurring along large-scale depth (50 - 1200 m) and latitudinal
gradients (29° - 51° S) in the New Zealand Exclusive Economic Zone. We
focused on a suite of morphological traits calculated directly
from in situ Baited Remote Underwater Stereo-Video
(stereo-BRUV) footage and museum specimens. We found that functional
changes were primarily structured by depth followed by latitude, and that
latitudinal functional turnover decreased with increasing depth.
Functional turnover among cells increased with increasing depth distance,
but this relationship plateaued for greater depth distances (> 750
m). In contrast, functional turnover did not change significantly with
increasing latitudinal distance at 700 - 1200 m depths. Shallow functional
bioregions (50 - 100 m) were distinct at different latitudes, whereas
deeper bioregions extended across broad latitudinal ranges. Fishes in
shallow depths had a body shape conducive to efficient propulsion, while
fishes in deeper depths were more elongated, enabling slow,
energy-efficient locomotion, and had large eyes to enhance vision.
Environmental filtering may be a primary driver of broad-scale patterns of
functional beta diversity in the deep sea. Greater environmental
homogeneity may lead to greater functional homogeneity across latitudinal
gradients at deeper depths (700 - 1200 m). We suggest that communities
living at depth may follow a ‘functional village hypothesis’, whereby
similar key functional niches in fish communities may be maintained over
large spatial scales.