Oceanic islands of the Southwest Pacific: Tracing the history of marine fish endemism

The subtropical oceanic islands of Lord Howe and Norfolk (Australia), as well as Rangitāhua (Kermadec Islands, New Zealand) host marine fish species found nowhere else. But where do these endemics come from? How did they originate? We explored the history of marine ray-finned fish endemism in the region using biogeographic probabilistic models that integrate time-calibrated phylogenies and geographic distribution information.

Above: Twospot demoiselles (Chromis dispila) swimming above a reef in Rangitāhua (Kermadec Islands), New Zealand. Photo by: Dr. Irene Middleton, Seacologynz.com.

Global Biogeography article: (Free to read online for two months)
Samayoa, A. P., Aguirre, J. D., Delrieu‐Trottin, E., & Liggins, L. (2023). The origins of marine fishes endemic to subtropical islands of the Southwest Pacific. Journal of Biogeography. https://doi.org/10.1111/jbi.14579

During his five-year expedition aboard the HMS Beagle, Charles Darwin was amazed by the unique faunal composition found in isolated islands of volcanic origin. More than 180 years later, we are still fascinated by species that only occur in oceanic islands, but with the advantage of having suitable analytical tools to examine endemism patterns through space and time. We are now able to propose temporal frameworks for lineage evolution by inferring the evolutionary relationships among taxa using molecular and fossil data. Additionally, the development of biogeographic models that combine time-calibrated phylogenies with geographic distribution information has opened the possibility to infer the most likely geographic origin and evolutionary trajectory of taxonomic lineages. Modern-day techniques enable us to start answering where, when, and how patterns of endemism have emerged.

Nowadays, standard biogeographic models parameterize evolutionary processes likely to influence the splitting of taxonomic lineages over time. In the sea, these processes include jump-dispersal, the rare long-distance dispersal of organisms, and vicariance, the geographic separation of two populations by either land or marine barriers, such as landmasses, long stretches of oceanic distances, and oceanographic conditions. Consequently, geographic distance and climate difference between oceanic regions are seemingly relevant factors affecting the dispersal of marine organisms. With the availability of R packages, such as BioGeoBEARS, that apply biogeographic models which differ by the processes and factors they parametrize, we can test evolutionary hypotheses for endemism patterns. As the models incorporate phylogenetic and distributional information, we can also estimate the origin of biodiversity and the processes by which it has evolved around oceanic islands. As our research group focuses on the marine fish fauna of the Southwest Pacific, we were interested to follow this approach as a first step to determine the origins of marine fish endemism in an understudied region of the Pacific Ocean.

Coastal view along Denham Bay in Raoul Island, the northernmost oceanic island of Rangitāhua, New Zealand. Photo by: Assoc. Prof. Libby Liggins.

The coastal waters surrounding the oceanic islands of the Pacific are characterized by significant endemism rates in the case of marine ray-finned fishes. Although extensive literature is available to understand the evolutionary origins and underlying processes of marine fish endemism in islands such as Hawaii and Easter Island (Rapa Nui), few evolutionary investigations have addressed the history of endemism in the Southwest Pacific. The region comprises Aotearoa New Zealand, New Caledonia, and eastern coastal areas of Australia, as well as a handful of oceanic islands administered either by New Zealand or Australia. In recent years, taxonomic surveys have characterized the unique actinopterygian composition around the islands of Lord Howe (Australia), Norfolk (Australia), and Rangitāhua (Kermadec Islands, New Zealand), highlighting the presence of species restricted to at least one of these islands. The recent inference of time-calibrated phylogenies for taxa endemic to these subtropical oceanic islands of the Southwest Pacific represented an interesting opportunity to explore the origin and evolution of marine fish endemism in the region. In our study, we used these molecular phylogenies to test multiple biogeographic models that parametrize jump-dispersal and vicariance, as well as the influence of geographic distance and climate differences in determining dispersal probabilities.

[Left] Kermadec scalyfin (Parma kermadecensis) is endemic to the islands along the Kermadec Archipelago. [Right] Mado (Atypichthys latus) is found in coastal waters of the subtropical oceanic islands of Lord Howe, Norfolk, and Rangitāhua, as well as in areas of mainland New Zealand in the Southwest Pacific. Photos by: Dr. Irene Middleton, Seacologynz.com.

Our findings did not support a sole geographic origin, nor a dominant biogeographic process responsible for marine fishes endemic to the oceanic islands of the Southwest Pacific. Our analyses rather pointed to a complex but exciting scenario in which most endemic lineages likely originated from fauna occurring mainly in Australia, the closest major landmass, but with rare origins in northern warmer areas and in the East Pacific. Furthermore, the relevance of jump-dispersal and vicariance in range evolution differed, sometimes each one acting as the main underlying process tested, and sometimes acting together. Similarly, geographic distance and climate differences significantly affected dispersal in specific taxa, but not in others. Our study highlights the complexity of the history behind endemism patterns, representing a significant step toward a better understanding on how endemism originated and evolved in an understudied corner of the Pacific. Our approach exemplifies the usefulness of contemporary open-access data and modelling techniques to explore the causal mechanisms behind a biogeographic pattern that has fascinated explorers and naturalists during the last two centuries.

Written by:
André P. Samayoa, Doctoral researcher, Massey University, Auckland, New Zealand

Thanks to Libby Liggins, the main supervisor of this research

Published by jbiogeography

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