Does morphological diversity in amphidromous Macrobrachium (Decapoda: Caridea: Palaemonidae) reflect species homology or habitat partitioning?

Abstract

The taxonomy of the genus Macrobrachium , a group of mostly amphidromous shrimps inhabiting unstable streams on Indo-Pacific tropical islands, is based on several characters repeatedly associated with specific environments and stream velocities. This allows the identification of morphological traits that improve an organism's stability in given flow conditions and an assessment of their monophyly. This study investigates whether the altitude-dependent distribution of certain Macrobrachium species - characterised by specific morphological traits - is phylogenetically constrained or a result of phenotypic plasticity. We first use an integrative taxonomy approach to assess species hypotheses based on ~200 individuals collected across the Indo-Pacific. We then construct multi-gene phylogenetic trees (65 individuals; 3-5 concatenated genes; 2711bp) to explore the evolutionary origins of morphological similarities among closely related species. Our results support the species status of 30 Macrobrachium species, with 16 species exhibiting distinct morphologies and 14 species forming 5 species complexes. Phylogenetic and distribution patterns of closely related species suggest habitat-driven speciation, with species differentiating in similar environments across vast distances. This indicates that diversification can occur over large distances, yet still in sympatry within a given environment. Lastly, our findings suggest that the Macrobrachium body shape is derived from evolutionarily conserved adaptations to flow velocity.