Trophic separation in planktivorous reef fishes: a new role for mucus?

Spatial and temporal variability in tropical off-reef zooplankton across broad spatial and temporal scales

Productivity of oligotrophic coral reefs is largely dependent on the constant influx of zooplankton. However, our understanding of how zooplankton communities in tropical reef-associated regions vary over large spatial and temporal scales is limited. Using the Australian continuous plankton recorder dataset, we explored if, and to what extent, the off-reef zooplankton community along the Queensland shelf (including most of the Great Barrier Reef lagoon) varied with latitude, month, and diel time. The zooplankton community was consistently dominated by copepods (∼60%) which, with appendicularians, chaetognaths, non-copepod crustaceans, and thaliaceans, comprised ∼98% of the zooplankton. However, the abundance of these taxonomic groups did not vary predictably across latitude, month, or diel time, with these gradients only explaining 5% of community variation. At the scales sampled herein the composition of zooplankton was highly predictable in terms of broad taxonomic groups but variation in the relative abundance of these groups was not predictable.

Spatial subsidies drive sweet spots of tropical marine biomass production

Spatial subsidies increase local productivity and boost consumer abundance beyond the limits imposed by local resources. In marine ecosystems, deeper water and open ocean subsidies promote animal aggregations and enhance biomass that is critical for human harvesting. However, the scale of this phenomenon in tropical marine systems remains unknown. Here, we integrate a detailed assessment of biomass production in 3 key locations, spanning a major biodiversity and abundance gradient, with an ocean-scale dataset of fish counts to predict the extent and magnitude of plankton subsidies to fishes on coral reefs. We show that planktivorous fish-mediated spatial subsidies are widespread across the Indian and Pacific oceans and drive local spikes in biomass production that can lead to extreme productivity, up to 30 kg ha-1 day-1. Plankton subsidies form the basis of productivity "sweet spots" where planktivores provide more than 50% of the total fish production, more than all other trophic groups combined. These sweet spots operate at regional, site, and smaller local scales. By harvesting oceanic productivity, planktivores bypass spatial constraints imposed by local primary productivity, creating "oases" of tropical fish biomass that are accessible to humans.

Planktivores as trophic drivers of global coral reef fish diversity patterns

One of the most prominent features of life on Earth is the uneven number of species across large spatial scales. Despite being inherently linked to energetic constraints, these gradients in species richness distribution have rarely been examined from a trophic perspective. Here we dissect the global diversity of over 3,600 coral reef fishes to reveal patterns across major trophic groups. By analyzing multiple nested spatial scales, we show that planktivores contribute disproportionally to the formation of the Indo-Australian Archipelago (IAA) marine biodiversity hotspot. Besides being "hotter" at the hotspot, planktivorous fishes display the steepest decline in species numbers with distance from the IAA when compared to other trophic groups. Surprisingly, we did not detect differences in diversification, transition, and dispersal rates in extant species phylogenies that would explain this remarkable gradient in planktivorous fish richness. Thus, we identify two potential complementary drivers for this pattern. First, exceptional levels of partitioning among planktivorous coral reef fishes were driven by temporally stable oceanographic conditions and abundant planktonic resources in the IAA. Second, extinctions of planktivores outside the IAA have been particularly pronounced during Quaternary climate fluctuations. Overall, our results highlight trophic ecology as an important component of global species richness gradients.

Phylogenomic Analysis of Concatenated Ultraconserved Elements Reveals the Recent Evolutionary Radiation of the Fairy Wrasses (Teleostei: Labridae: Cirrhilabrus)

The fairy wrasses (genus Cirrhilabrus) are among the most successful of the extant wrasse lineages (Teleostei: Labridae), with their 61 species accounting for nearly 10% of the family. Although species complexes within the genus have been diagnosed on the basis of coloration patterns and synapomorphies, attempts to resolve evolutionary relationships among these groups using molecular and morphological data have largely been unsuccessful. Here we use a phylogenomic approach with a data set comprising 991 ultraconserved elements (UCEs) and mitochondrial COI to uncover the evolutionary history and patterns of temporal and spatial diversification of the fairy wrasses. Our analyses of phylogenetic signal suggest that most gene-tree incongruence is caused by estimation error, leading to poor resolution in a summary-coalescent analysis of the data. In contrast, analyses of concatenated sequences are able to resolve the major relationships of Cirrhilabrus. We determine the placements of species that were previously regarded as incertae sedis and find evidence for the nesting of Conniella, an unusual, monotypic genus, within Cirrhilabrus. Our relaxed-clock dating analysis indicates that the major divergences within the genus occurred around the Miocene-Pliocene boundary, followed by extensive cladogenesis of species complexes in the Pliocene-Pleistocene. Biogeographic reconstruction suggests that the fairy wrasses emerged within the Coral Triangle, with episodic fluctuations of sea levels during glacial cycles coinciding with shallow divergence events but providing few opportunities for more widespread dispersal. Our study demonstrates both the resolving power and limitations of UCEs across shallow timescales where there is substantial estimation error in individual gene trees.