Marine organisms are not uniformly distributed in the ocean realm, a phenomenon resulting from the effect of evolutionary and ecological mechanisms (Costello and Chaudhary 2017). Biogeography studies the distribution of living organisms and the mechanisms influencing their distribution, often aiming to identify “hotspots” - geographic locations with high levels of biodiversity - given the insight they can provide regarding the processes that generate and maintain biodiversity, as well as the value in protecting such places in the face of the ongoing biodiversity loss (Norman 2003). We currently have an advanced understanding of the geographic distribution of the most diverse terrestrial (Myers et al. 2000) and marine (Roberts et al. 2002) regions, as well as hotspots within these regions that can be defined either by their number of total species (Reid 1998) or of range-restricted species only (Myers 1990).

Ray-finned fishes (Actinopterygii) belong to one of the largest taxonomic groups in the animal kingdom with over 36,900 valid species and 5,283 valid genera (Fricke et al. 2024). Actinopterygians are evenly distributed between freshwater and marine environments (Seehausen and Wagner 2014), with marine ray-finned fishes (hereby “marine fishes”) spreading from tropical to polar latitudes (Lin et al. 2021) and reaching depths up to 8,336 m (Jamieson et al. 2023). Marine fishes are fundamental for ecosystem function, occupying a wide array of trophic positions and dietary niches (Hayden et al. 2019) and for human consumption (Natale et al. 2013). Given that highly diverse marine fish communities seem more resilient to current climatic pressures (Duffy et al. 2016), marine fishes represent an interesting model to understand how marine biodiversity originates and persists and how future biodiversity patterns might be determined.

In the face of biodiversity loss in the sea, intense anthropogenic pressure on fish communities and the immediate changes in biodiversity patterns due to climate change, it is imperative to keep paying attention to the spatial patterns of marine fishes and to understand the causal processes that have played a role in their varied distributions (Cowman et al. 2017). The Pacific Ocean, the world’s largest marine basin, is an actively studied area that includes regions of distinct biogeographic features (Springer 1982; Spalding et al. 2007; Kulbicki et al. 2013; Bowen et al. 2016). While the western tropical region that sits between the Indian and the Pacific Oceans represents a major area for biogeographic research (Lohman et al. 2011), few investigations have focused on the Southwest Pacific, a region characterised by a mixture of major landmasses and oceanic islands that cross subtropical, temperate and subantarctic latitudes (McClymont et al. 2016). As evidence shows that climate change drives latitudinal, distributional shifts of marine biodiversity (Poloczanska et al. 2016), a potentially higher climatic vulnerability and significant distributional changes for marine organisms are expected in regions that span multiple climatic latitudes, such as the Southwest Pacific.

This review summarises the state of knowledge for the biogeography of shallow (up to 500 metres) marine ray-finned fishes in the Southwest Pacific, based on the latest contributions of historical biogeographic studies in the region. Here, (i) we describe general patterns of species richness and endemism for marine biodiversity; (ii) we provide a concise review of the biogeographic patterns and the search for underlying historical causes for actinopterygians in the broader Pacific Ocean; (iii) we present the major geological, oceanographic and biological features of the Southwest Pacific; (iv) we outline the emerging biogeographic setting for marine actinopterygians in the region, based on recent studies and (v) we propose future lines of work to fill current knowledge gaps towards a better understanding of the geographic distribution of marine fish biodiversity in the Southwest Pacific.

In this review, the “Southwest Pacific” is defined as the south-western quadrant of the Pacific Ocean, bounded by latitudes 20°S-55°S and longitudes 150°E-165°W (Fig. 1). It is a geologically active region (Neall and Trewick 2008), characterised by two continental fragments of Gondwanan origin, Australia and Zealandia and by the Australian-Pacific plate boundary that crosses Aotearoa New Zealand between the North and South Islands. Consequently, major landmasses include the continental islands of mainland Australia (eastern segment) and the aerial sections of Zealandia that comprise New Zealand (North Island, South Island, Stewart Island, the Chatham Islands and several smaller islands) and New Caledonia (Trewick et al. 2007) (Fig. 2). Additionally, the Southwest Pacific harbours recently emerged oceanic islands of volcanic origin, with estimated ages of 23.8–1 Ma for subantarctic islands (Quilty 2007; Scott and Turnbull 2019) and 6.9–2.58 Ma for subtropical islands (Jones and McDougall 1973; McDougall et al. 1981; Brook 1998b). Most outlying islands sit on Zealandia (Mortimer et al. 2017), except the Kermadec Archipelago/Rangitāhua on the Kermadec Ridge (Brook 1998b) and Macquarie Island on the Macquarie Ridge (Quilty 2007). Politically, Australia administers the subtropical Lord Howe Island and Norfolk Island and the subantarctic Macquarie Island. New Zealand’s outlying territories include subtropical Rangitāhua, the eastern Chatham Islands/Rēkohu/Wharekauri, as well as the subantarctic Bounty Islands/Moutere Hauriri, Antipodes Islands/Moutere Mahue, Snares Islands/Tini Heke, Auckland Islands/Motu Maha and Campbell Island/Motu Ihupuku.

Surface currents in the Southwest Pacific follow the anti-clockwise movement of the South Pacific Gyre (Ganachaud et al. 2014): the main flow moves west at low latitudes as the South Equatorial Current, reaches mainland Australia through the Coral Sea, becomes the East Australian Current at 15°S to then flow southwards along the eastern Australian coastline (Fig. 2), branches eastwards between 32°S-34°S as the Tasman Front, crosses Norfolk and reaches the North Island of New Zealand, becoming the East Auckland Current. Additionally, two independent currents from the East Australian Current flow east across the high latitudes of the Southwest Pacific (Nelson and Cooke 2001): the Subtropical Front (ca. 43°S-45°S) that follows a convoluted path around the southern tip of New Zealand’s South Island to then reach the Chatham Islands; and the Subantarctic Front (ca. 52°S) that flows around the southernmost continental segment of Zealandia to meet the Subtropical Front south of the Chathams. The three major eastward currents of the Southwest Pacific are distinguished by their physicochemical characteristics (Lorrey et al. 2012): the Tasman Front carries highly saline warm water which is poor in nutrients; the Subantarctic Front is a nutrient-rich, cold-water mass with lower salinity; and the Subtropical Front separates the Tasman and Subantarctic fronts.

The Southwest Pacific hosts an exceptionally rich marine biodiversity resulting from the high species richness and endemism in New Zealand and to a lesser extent, Australia (Briggs and Bowen 2012; Costello et al. 2017). Both countries are considered the best taxonomically studied regions in the West Pacific (Costello et al. 2010), exemplified by the elevated completeness of New Zealand’s inventory of marine species (Mora et al. 2008) which include 17,135 taxa and 4,315 undescribed species according to Gordon et al. (2010). For fish diversity, Roberts et al. (2020) report 1,296 species in New Zealand, of which 1,085 are marine actinopterygians. As high marine fish biodiversity seems linked to habitat complexity (Leprieur et al. 2016), New Zealand’s marine richness might result from the extensive Zealandian continental shelf which likely provides abundant habitats for diversification (Gordon et al. 2010). Within the Southwest Pacific, taxonomic data point to the subtropical islands of Lord Howe, Norfolk and Rangitāhua as centres of endemism, with an overall 4.6% endemism rate for coastal marine fishes (Francis 1993), although more recent surveys report the same figure for Rangitāhua alone (Trnski et al. 2015). The marine fish communities in the three island groups are a mixture of tropical, subtropical and temperate species (Francis and Duffy 2015), a similarity likely explained by the predominant eastward oceanic currents of the region (Trnski and de Lange 2015). The recent compilation of marine fish taxa occurring in Lord Howe Island, Norfolk Island and Rangitāhua (Francis and Hoese 2025) illustrates the significance of marine fish biodiversity around the islands, as well as the relevance of distributional data to understand biogeographic patterns in the region.

Overall, the contemporary spatial patterns of marine fish biodiversity and endemism are broadly understood in the Southwest Pacific. However, the evolutionary value of regional actinopterygians and key biogeographic locations, have just started to be revealed (Delrieu-Trottin et al. 2018; Eme et al. 2020; Liggins et al. 2022). A comprehensive historical framework is needed to inform biogeographers, evolutionary biologists, ecologists and conservationists about the relevance of the processes shaping the biogeography of marine fishes in the Southwest Pacific, if we are to determine how marine biodiversity will evolve in times of heavy biodiversity loss and intense anthropogenic pressure.

Constructive research relies on the conscious identification of what we know and what we do not know (Hortal et al. 2015). Here, we identify five major grounds of investigation that would enrich the inferences of recent biogeographic studies on marine fish biodiversity in the Southwest Pacific.

First, phylogenetic inferences would improve by adding taxa within poorly-sampled genera, clarifying the evolutionary relationships amongst all extant congeners and avoiding the overestimation of divergence timings (Hodge and Bellwood 2016). Such addition would derive from an improvement of specimen preservation for DNA analysis and field surveys. Furthermore, the generation and inclusion of novel sequences for additional gene regions would improve topological resolution (Rubinoff and Holland 2005) and benefit species identification (Dupuis et al. 2012). The exploration of phylogenomics is a promising, relatively inexpensive alternative to molecular phylogenetics by using genome-scale data to infer evolutionary relationships amongst taxa, estimate divergence times and reconstruct ancestral ranges in biogeography (Young and Gillung 2020). The inference of molecular phylogenies at the tips of the Fish Tree of Life is a necessary approach that also addresses biological shortfalls (Hortal et al. 2015). For instance, Samayoa et al. (2022) clarified the evolutionary relationships within 21 genera, eight of which were fully sampled and four missed only one taxon. Moreover, the investigators built the first comprehensively sampled phylogenies for four genera (Enneapterygius, Lepidoperca, Hypoplectrodes, and Upeneus) and confirmed the species status for 34 endemic taxa, 11 of which are positioned for the first time within a published molecular phylogeny, including three undescribed taxa (Hypoplectrodes sp. A, Hypoplectrodes sp. B and Nemadactylus n.sp.). Additionally, novel molecular sequences for six endemics were added to open-access repositories.

Second, the full extension of the phylogenetic landscape of marine fishes in the shallow waters around New Zealand remains to be examined. Recent investigations focused on richness and endemism metrics, which are a small proportion of the numerous indices measurable with a phylogenetic tree (Tucker et al. 2017). Future investigations of measures of distance and evenness could provide complementary understanding (e.g. Eme et al. (2020)). Furthermore, the estimation of divergence rates, based on the phylogenies from Samayoa et al. (2022), would enhance our understanding of the evolutionary and ecological processes behind the biogeographic patterns of marine fish fauna, as diversification rates bridge questions raised in macroevolution and macroecology (McGill et al. 2019). These lines of investigation will benefit from the ongoing molecular extension of New Zealand’s marine fish phylogeny (Liggins unpublished results).

Third, the ecological aspects of the actinopterygian biogeography in the Southwest Pacific are yet to be fully explored. Fieldwork in subtropical islands, particularly Lord Howe and Norfolk, would improve taxonomic inventories and provide biological/ecological information on endemic taxa (e.g. dispersal capacities and reproductive success), offering an ecological background to contrast with the evolutionary inferences presented in this review. A short-term objective is the formal description of undescribed endemic taxa included in Samayoa et al. (2022), an ongoing work for Nemadactylus and Hypoplectrodes (Roberts unpublished results).

Fourth, the continuing curation of open-access repositories is a substantial work that provides a valuable amount of biological, ecological and biogeographic data. The biogeographic studies presented in this review exemplify the capacity of using contemporary analytical tools to synthesise an ever-growing amount of data and to improve our understanding of natural processes. Similar works should provide constructive inferences if web-based datasets (e.g. occurrence and range information) are maintained and updated by expert opinions, as this is useful information for evolutionary, ecological and biogeographic research.

Finally, the present review emphasises the relevance of phylogenetic methods in clarifying biogeographic patterns and inferring underlying evolutionary and ecological processes. We confirm the usefulness of fossils to calibrate molecular phylogenies and to estimate divergence timings (Powell et al. 2020), despite taxonomic and age uncertainties of some fossils (Heads 2005). Additionally, we highight the efficacy of probabilistic models in tracing back the evolutionary trajectory of biodiversity, particularly those that incorporate a jump-dispersal parameter (Matzke 2014). Lastly, we stress the value of phylogenetic metrics in determining evolutionary processes uncaptured by species richness alone (Faith 2013) and in quantitatively delineating centres of endemism (Mishler et al. 2014). We believe that the development and application of novel phylogenetic-based tools represent a suitable approach to better understand the historical biogeography of marine fishes in the Pacific.

Climate change is impacting the current geographic distribution of marine biodiversity at a global scale (Chaudhary et al. 2021) and is expected to aggravate the loss of marine life already stressed by anthropogenic pressure (Beaugrand et al. 2015). Areas of high biodiversity have been recognised as conservation priorities (Myers 1988; Myers et al. 2000; Roberts et al. 2002), with recent research showing that the evolutionary processes that shape biodiversity “hotspots” are equally valuable if we are to understand how life will cope with the current environmental pressures (Sgrò et al. 2011; Munday et al. 2013; Nielsen et al. 2017). As marine actinopterygians are highly diverse (Fricke et al. 2024), play critical ecological functions (Hayden et al. 2019), are economically valuable (Natale et al. 2013) and display responsiveness to climate change (Duffy et al. 2016), marine fishes are a good model to understand how marine biodiversity has evolved through time and space. To enhance our understanding of the evolutionary role that regions at the periphery of the Pacific Ocean play in the generation and maintenance of marine biodiversity, we focused on the Southwest Pacific, a highly diverse region with significant marine fish endemism at a global scale (Costello et al. 2017), but where evolutionary research has been in its infancy (Delrieu-Trottin et al. 2018; Eme et al. 2020; Liggins et al. 2022). To inform and better understand the evolutionary processes that shaped contemporary patterns of marine fish endemism and richness in the Southwest Pacific, we reviewed the latest generated empirical data, based on a variety of historical biogeographic approaches.

Overall, the current state of knowledge highlights a historical biogeographic scenario for marine fishes of the Southwest Pacific where centres of endemism are not concordant with richness hotspots, suggesting that the patterns originated from distinct historical processes. Biodiversity richness peaks at 35°S to decrease with increasing latitude, a pattern likely driven by the range overlap of tropical, subtropical and temperate species and expected to intensify with current poleward range shifts of tropical marine fish taxa as detected along northern New Zealand (e.g. Middleton et al. (2023)) and south-eastern Australia (e.g. Fowler et al. (2018); Gervais et al. (2021)). Endemism concentrates in subtropical oceanic islands, which act as peripheral islands that generate neoendemism in the Pacific and originates principally from mainland Australia lineages that occupied the recently emerged islands through rare colonisation events – following the predominant eastward oceanographic currents – to eventually diverge into endemic taxa. To conclude, it is considered that achieving a complete understanding of any natural process is highly unlikely (Ladle and Hortal 2013). However, we believe that the latest evolutionary and biogeographic research in the Southwest Pacific contributes to a better understanding of the historical biogeography of marine fishes in the Southwest Pacific, representing a knowledge bridge for future work within the marine evolutionary ecology field.