The evolution of fast-growing coral reef fishes

Individual growth is a fundamental life history trait^1-4, yet its macroevolutionary trajectories have rarely been investigated for entire animal assemblages. Here we analyse the evolution of growth in a highly diverse vertebrate assemblage-coral reef fishes. We combine state-of-the-art extreme gradient boosted regression trees with phylogenetic comparative methods to detect the timing, number, location and magnitude of shifts in the adaptive regime of somatic growth. We also explored the evolution of the allometric relationship between body size and growth. Our results show that the evolution of fast growth trajectories in reef fishes has been considerably more common than the evolution of slow growth trajectories. Many reef fish lineages shifted towards faster growth and smaller body size evolutionary optima in the Eocene (56-33.9 million years ago), pointing to a major expansion of life history strategies in this Epoch. Of all lineages examined, the small-bodied, high-turnover cryptobenthic fishes shifted most towards extremely high growth optima, even after accounting for body size allometry. These results suggest that the high global temperatures of the Eocene⁵ and subsequent habitat reconfigurations⁶ might have been critical for the rise and retention of the highly productive, high-turnover fish faunas that characterize modern coral reef ecosystems.

Surface slicks are pelagic nurseries for diverse ocean fauna

Most marine animals have a pelagic larval phase that develops in the coastal or open ocean. The fate of larvae has profound effects on replenishment of marine populations that are critical for human and ecosystem health. Larval ecology is expected to be tightly coupled to oceanic features, but for most taxa we know little about the interactions between larvae and the pelagic environment. Here, we provide evidence that surface slicks, a common coastal convergence feature, provide nursery habitat for diverse marine larvae, including > 100 species of commercially and ecologically important fishes. The vast majority of invertebrate and larval fish taxa sampled had mean densities 2-110 times higher in slicks than in ambient water. Combining in-situ surveys with remote sensing, we estimate that slicks contain 39% of neustonic larval fishes, 26% of surface-dwelling zooplankton (prey), and 75% of floating organic debris (shelter) in our 1000 km² study area in Hawai'i. Results indicate late-larval fishes actively select slick habitats to capitalize on concentrations of diverse prey and shelter. By providing these survival advantages, surface slicks enhance larval supply and replenishment of adult populations from coral reef, epipelagic, and deep-water ecosystems. Our findings suggest that slicks play a critically important role in enhancing productivity in tropical marine ecosystems.

Subconscious Biases in Coral Reef Fish Studies

In complex, diverse ecosystems, one is faced with an exceptionally challenging decision: which species to examine first and why? This raises the question: Is there evidence of subconscious biases in study species selection? Likewise, is there evidence of this bias in selecting methods, locations, and times? We addressed these questions by surveying the literature on the most diverse group of vertebrates (fishes) in an iconic high-diversity ecosystem (coral reefs). The evidence suggests that we select study species that are predominantly yellow. Reef fish studies also selectively examine fishes that are behaviorally bold and in warm, calm, attractive locations. Our findings call for a reevaluation of study species selection and methodological approaches, recognizing the potential for subconscious biases to drive selection for species that are attractive rather than important and for methods that give only a partial view of ecosystems. Given the challenges faced by high-diversity ecosystems, we may need to question our decision-making processes.

Demographic dynamics of the smallest marine vertebrates fuel coral reef ecosystem functioning

How coral reefs survive as oases of life in low-productivity oceans has puzzled scientists for centuries. The answer may lie in internal nutrient cycling and/or input from the pelagic zone. Integrating meta-analysis, field data, and population modeling, we show that the ocean’s smallest vertebrates, cryptobenthic reef fishes, promote internal reef fish biomass production through extensive larval supply from the pelagic environment. Specifically, cryptobenthics account for two-thirds of reef fish larvae in the near-reef pelagic zone despite limited adult reproductive outputs. This overwhelming abundance of cryptobenthic larvae fuels reef trophodynamics via rapid growth and extreme mortality, producing almost 60% of consumed reef fish biomass. Although cryptobenthics are often overlooked, their distinctive demographic dynamics may make them a cornerstone of ecosystem functioning on modern coral reefs.