The historical biogeography of marine ray-finned fishes (Actinopterygii) in the Southwest Pacific: species richness and endemism patterns

Inferring how biodiversity is generated and how it responded to past events is fundamental to understanding how biodiversity may respond to future changes. As the largest ubiquitous taxonomic group amongst marine vertebrates, ray-finned fishes (Actinopterygii) represent a good model to understand the generation of biodiversity and the processes that shaped contemporary biogeographic patterns in the sea. Evidence shows that hotspots of marine fish richness and endemism do not always overlap, due to the distinct processes that contribute to them. In the Pacific Ocean, the tropical western region concentrates the highest records of species richness. In contrast, the peripheral oceanic islands are major endemism centres. Although the evolutionary setting for marine fish endemism in parts of the Central Pacific has been described, processes that generate and maintain biodiversity in other peripheral regions remain unknown. The Southwest Pacific (20°S-55°S and 150°E-165°W) spans subtropical, temperate and subantarctic climates, containing major landmasses such as eastern Australia and New Zealand, as well as oceanic islands. Due to its configuration, the region displays unique biogeographic patterns that can improve our understanding of marine biodiversity evolution. Here, we aim to address this topic by reviewing the latest biogeographic investigations that examine the origin, evolution, and processes shaping patterns of marine fish richness and endemism in the Southwest Pacific. We highlight the utility of specific methodological approaches used in previous investigations, including the inference of molecular phylogenies based on fossil data, probabilistic biogeographic modelling and statistical analysis of phylogenetic measures of richness and endemism. We propose a biogeographic scenario where the subtropical islands of the Southwest Pacific act as biodiversity cradles, with mainland Australia representing the main source of endemic lineages. Empirical data highlight the significance of both rare colonisation events and vicariance within formerly widely distributed lineages in shaping endemism patterns and reveal that the processes shaping patterns of endemism and richness differ at local scales. The Southwest Pacific is a geologically active region that harbours multiple oceanic islands of volcanic origin. Hotspots of marine fish richness are not geographically concordant with endemism centres along New Zealand, with richness peaking north of the North Island of Aotearoa New Zealand. Heightened richness in these locations likely originates from an overlap of fish fauna that includes older, distantly related taxa inhabiting warmer waters in the north and closely-related marine fish taxa in colder latitudes in the south. Within the Southwest Pacific, the subtropical oceanic islands of Lord Howe, Norfolk and Rangitāhua (Kermadec Islands) are characterised by the highest endemism rates in coastal marine ray-finned fishes. The geographic isolation and unique oceanic, subtropical climate of these islands have likely influenced the establishment of endemism.