Unravelling the effects of ecology and evolutionary history in the phenotypic convergence of fishes

Understanding the ecological drivers and limitations of adaptive
convergence is a fundamental challenge. Here, we explore how adaptive
convergence of planktivorous fishes has been influenced by multiple
ecological factors, evolutionary history, and chance. Using
ecomorphological data for over 1600 marine species, we integrate
pattern-based metrics of convergence with evolutionary model fitting to
test whether phenotypic similarities among specialist planktivores exceed
expectations under null models and whether ecology, evolutionary history,
or their combined effects best explain trait evolution. We find that
planktivores are significantly more similar in phenotype than expected.
Traits with functional relevance for prey detection and capture, such as
eye diameter and lower jaw length, are strongly convergent, while general
body size and shape are constrained by deep divisions between clades where
the effects of evolutionary history are most pronounced. Since not all
traits undergo strong selection toward a convergent ecomorph, their
evolutionary trajectories have not entirely overcome ancestral differences
in the multivariate trait space, resulting in a specific form of
convergence termed conservatism. We show how adaptive responses to feeding
ecology intertwine with other ecological pressures (i.e., light
environment) and historical contingency to shape fish phenotype evolution
over deep time, offering key insights into the generality of phenotypic
evolution.