Adaptive Evolution of Deep-Sea Amphipods from the Superfamily Lysiassanoidea in the North Atlantic

In this study we reconstruct phylogenies for deep sea amphipods from the North Atlantic in order to test hypotheses about the evolutionary mechanisms driving speciation in the deep sea. We sequenced five genes for specimens representing 21 families. Phylogenetic analyses showed incongruence between the molecular data and morphological taxonomy, with some morphologically distinct taxa showing close molecular similarity. Approximate dating of nodes based on available calibration suggested adaptation to the deep sea around the Cretaceous-Palaeogene boundary, with three identified lineages within the deep-sea radiation dating to the Eocene-Oligocene transition. Two of those lineages contained species currently classified in multiple families. We reconstructed ancestral nodes based on the mouthpart characters that define trophic guilds (also used to establish the current taxonomy), and show a consistent transition at the earliest node defining the deep-sea lineage, together with increasing diversification at more recent nodes within the deep-sea lineage. The data suggest that the divergence of species was adaptive, with successive diversification from a non-scavenging ancestor to 'opportunistic', 'obligate' and 'specialised' scavengers. We propose that the North Atlantic species studied provide a strong case for adaptive evolution promoted by ecological opportunity in the deep sea.