Hydrodynamic processes are important in all marine environments and on coral reefs drive patterns of habitat zonation, community structure, and biodiversity. Abrupt geomorphological features like pinnacles and seamounts often possess distinct localized currents and these habitats are also often characterized by high abundance and biomass of fishes. However, differences in fish community structure between pinnacles and emergent reefs, and their key drivers are poorly understood. In this study, we compared fish communities among emergent fringing and offshore coral reefs, and submerged pinnacle reefs in Papua New Guinea. Submerged pinnacles possessed higher fish biomass, abundance, and species richness than both fringing and offshore emergent reefs. We collected in‐situ current speed and temperature data over a full year at each reef and used random forest analysis to investigate the relative influence of hydrodynamics compared to other well‐established drivers of reef fish biodiversity, including habitat and biogeographic factors. Environmental variables explained 70%, 52%, and 5% of variability in models for species richness, abundance and biomass respectively. In all models, average current speed, current speed variability, and reef area were consistently among the most influential variables. Models examining relationships between fish biodiversity metrics and current speed did not yield conclusive results but did highlight the association of distinct hydrodynamic regimes on pinnacles with high fish richness, abundance, and biomass. Our study highlights the strong influence of reef‐scale hydrodynamics on fish biodiversity and demonstrates the ecological value of small, submerged coral reefs, which are globally numerous yet remain understudied in coral reef ecology.

Seamounts and remote oceanic islands serve as valuable natural laboratories in which to study patterns and processes in marine biodiversity. A central hypothesis arising from studies of these systems is the ecological function of seamounts as stepping-stones for dispersal and population connectivity. Evidence of this mechanism exists for a range of taxa, including coral reef fishes, but is still lacking from many tropical seamounts in remote regions. In this study, we used remotely operated vehicles and baited remote underwater video systems to survey fish and benthic communities between 1 and 100 m on seamounts in the Coral Sea Marine Park (CSMP), Australia. We found evidence to support the stepping-stone model of ecological connectivity from new observations of 16 coral reef fishes which have previously not been recorded by quantitative surveys in the region. The widespread distribution of many of these species throughout the full latitudinal extent of the CSMP suggests that there is greater connectivity between mesophotic habitats in the Coral Sea and surrounding biogeographic regions than previously known. We also found a wide variety of mesophotic habitats and recorded significant depth range extensions for 78 fishes in these habitats. This further highlights the potential role of increased habitat area and heterogeneity in a stepping-stone effect throughout the region. Four of the fish occurrence records represent significant range extensions into the Coral Sea from adjacent biogeographic regions, and 13 fishes recorded by this study in the CSMP are not known from the neighbouring Great Barrier Reef, despite its close proximity. Although the Coral Sea remains relatively understudied, these findings suggest that larger-scale models of marine biogeography are relevant to communities in the region, particularly at mesophotic depths. Given the extent and the spatial arrangement of seamounts in the Coral Sea, our findings emphasise that the region is an important link between the centre of marine biodiversity in the Coral Triangle and the Southwest Pacific. Greater mesophotic sampling effort and genetic studies are necessary to understand the nature of connectivity and to establish the role of regional seamount chains, like the Coral Sea reefs, in broader marine biogeographic processes.

Predation is a key ecological process regulating the structure and diversity of biological communities, yet predators do not exist homogeneously in nature. Coral reefs possess diverse assemblages of predatory fishes, the distribution and abundance of which is well documented for coastal and emergent reefs. However, for remote, isolated and submerged reefs, such as those found on pinnacles and seamounts, our understanding of predatory fish communities is limited. These features are ubiquitous in the world’s oceans and frequently targeted by fishers for their presumed fish aggregation properties. Here we describe communities of predatory fishes on a series of pinnacle reefs and contrast these to regional coastal and offshore emergent reefs. Pinnacles supported 2-4× higher abundance, biomass and diversity of predatory fishes compared to emergent reefs. They also supported unique communities, with 32 out of the 63 predator species in our study found only on pinnacle reefs. For species found on all 3 reef types, all were most abundant on pinnacles and the 6 taxa driving differences in community structure were most abundant on pinnacles. Together, our results show that predatory fishes are strongly attracted to pinnacles, although the mechanisms are still unclear. Prioritising the selection of these small ecological magnets in conservation planning would be an effective approach to target the protection of regional reef fish biodiversity.

With an area covering just under 1 million km², Australia’s Coral Sea Marine Park (CSMP) is among the largest marine protected areas globally. Yet, the demersal deep-reef fish fauna of the CSMP remains poorly described, due in part to its remote location, as well as its sheer size, much of which is inaccessible below conventional SCUBA diving depths. Recent remotely operated vehicle (ROV) explorations conducted by the Schmidt Ocean Institute provided an opportunity to explore the benthic ichthyofauna of deep mesophotic and rariphotic coral ecosystems. Contributions to the deep, demersal ichthyofauna of Australia’s CSMP are herein supplemented with an integrative dataset consisting of ROV imagery and specimen vouchers. Sixty-two new records of fishes from 26 families are reported from the CSMP. Of these, 45 are new Australian records, one of which is also a new family record for Australia. Four species are new records for the Southern Hemisphere, and 21 species are potentially new to science. We report new voucher material for 29 species, including several species previously identified as new for the CSMP with no voucher specimens.

Gradients in light, temperature and hydrodynamics associated with water depth are important determinants of ecological communities in marine environments. While depth specialism in coral reef fishes has been extensively studied in shallow (< 30 m) coastal reef systems, less is known about how depth‐associated drivers operate over the larger depth ranges on isolated pinnacle and seamount reef systems, which are known to support abundant assemblages of predatory fishes. Using remotely operated vehicles, we surveyed predatory fish assemblages across a 100 m depth gradient on three seamount reefs in the Coral Sea. We tested for declines in abundance and diversity, as well as differences in assemblage structure of predatory fishes among depth strata. Species richness and abundance decreased significantly with depth, with predator abundance declining fourfold between the shallowest (5 m) and deepest (95 m) depths surveyed, while species richness was halved. Despite this, compositional differences among depth zones were minimal, with most taxa spanning the full depth range, suggesting adaptations to the limited horizontal habitat available on seamounts. Depth‐associated shifts in taxonomic composition were primarily attributed to a single predator family, reef sharks (Carcharhinidae), which increased in abundance at mesophotic depths. The capacity of a large number of predatory fish taxa to utilize a wide range of depths allows these organisms to access favoured thermal environments and may be a potential resilience mechanism under future environmental change. Further studies are needed to assess the implications of depth use for predator behaviour, trophodynamics and conservation strategies.

Tropical seagrass meadows are important global marine ecosystems that provide critical ecosystem goods and services. The extent of global seagrass meadows is mostly mapped from shallow coastal regions and not well known or sampled from deeper offshore locations. Seagrasses can, however, form deep‐water meadows, which likely significantly increase the total area of global seagrass ecosystems and may contribute important ecological functions to offshore tropical seascapes. Here we report the first observation of a dense meadow of Thalassodendron ciliatum at a depth of 25 m using remotely operated vehicles (ROVs) from the Coral Sea Marine Park (CSMP). Despite significant survey effort in the region, to date there have only been three other observations of seagrass in the CSMP, all sparse and small patches of Halophila ovalis and Halophila decipiens. We discuss the significance of this newly discovered meadow within the context of current reef health monitoring of the CSMP, reef fish biodiversity and the ecological value of deep‐water seagrass habitats for offshore coral reef systems like the Coral Sea.