Plankton communities often consist of cosmopolitan species with an extensive gene flow
between populations. Nevertheless, populations of some plankton species are genetically structured,
owing to various barriers such as ocean currents, hydrological fronts, and continents. Drivers that
could explain the genetic structures of most mesopelagic species remain unknown on an ocean-basin
scale, and our study aims to analyze the genetic and morphological differences between populations
of a cosmopolitan mesopelagic shrimp, Systellaspis debilis, from the Southern and Northern Atlantic
Ocean, and the Southwest Indian Ocean. We analyzed the ITS-1 and COI markers of 75 specimens
and assessed the genetic integrity and within-species variability of these genes. We also coded
32 morphological characteristics in 73 specimens, analyzed their variability, and assessed the correlation
between morphological and genetic characteristics using a Redundancy analysis and Mantel test.
Systellaspis debilis was genetically cohesive across the whole Atlantic and Southwest Indian Oceans,
which is possibly a result of an intensive gene flow through ecological barriers, the resistance of
species to hydrological gradients, a purifying selection of mitochondrial genes, etc. In contrast, we
found significant morphological differences between populations from different regions, which mirrors
morphological diversification and calls for further genomic approaches in order to understand
the basis of these variations and uncover potential local adaptations.