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.