Comparing Seamounts and Coral Reefs with eDNA and BRUVS Reveals Oases and Refuges on Shallow Seamounts

Abstract

Seamounts are the least known ocean biome. Considered biodiversity hotspots, biomass oases, and refuges for megafauna, large gaps exist in their real diversity relative to other ecosystems like coral reefs. Using environmental DNA metabarcoding (eDNA) and baited video (BRUVS), we compared fish assemblages across five environments of different depths: coral reefs (15 m), shallow seamounts (50 m), continental slopes (150 m), intermediate seamounts (250 m), and deep seamounts (500 m). We modeled assemblages using 12 environmental variables and found depth to be the main driver of fish diversity and biomass, although other variables like human accessibility were important. Boosted Regression Trees (BRT) revealed a strong negative effect of depth on species richness, segregating coral reefs from deep-sea environments. Surprisingly, BRT showed a hump-shaped effect of depth on fish biomass, with significantly lower biomass on coral reefs than in shallowest deep-sea environments. Biomass of large predators like sharks was three times higher on shallow seamounts (50 m) than on coral reefs. The five studied environments showed quite distinct assemblages. However, species shared between coral reefs and deeper-sea environments were dominated by highly mobile large predators. Our results suggest that seamounts are no diversity hotspots for fish. However, we show that shallower seamounts form biomass oases and refuges for threatened megafauna, suggesting that priority should be given to their protection.

Keywords: biodiversity; biomass; conservation; coral reefs; hotspot; mesophotic slope.

Figure 1

Sampling design in the five environmental strata. BRUVS and eDNA samples were collected on seven barrier coral reefs (15 m deep), four shallow seamounts (50 m summit depth), four continental slopes (150 m deep), four intermediate-depth seamounts (250 m summit depth), and three deep seamounts (500 m summit depth). Bathymetry data were derived from [50]. See Table S1 for more details on sampling design.

Figure 2

Violin plots and superimposed boxplots showing species richness observed on BRUVS (A) and MOTU richness in eDNA samples (B) for coral reef, seamounts (50, 250, 500 m), and continental slopes (150 m). The mean is represented by the red lozenge. Grey dots represent individual sample values scattered around each distribution. Significant differences at p < 0.05 are highlighted by grouping letters (PERMANOVAs and permutational t-tests with 9999 permutations). (C) Rarefaction curves of species richness from BRUVS and (D) MOTU richness from environmental DNA across coral reefs, seamounts, and continental slopes environments. The samples were rarefied (solid line) and extrapolated (dashed line) using the Hill number method [78,79]. 95% confidence intervals (CI) are shown in each respective ribbon. Horizontal lines are asymptote estimates (γ-diversity).

Figure 3

Violin plots and superimposed boxplots of biomass (BRUVS) on (A) all observed species, (B) large predators, and (C) sharks across 5 environmental strata: coral reefs, seamounts of variable depths (50, 250, and 500 m), and the continental slope (150 m). The mean is represented by the red lozenge. Grey dots represent individual sample values scattered around each distribution. Significant differences at p < 0.05 are highlighted by grouping letters (PERMANOVAs and permutational t-tests with 9999 permutations).

Figure 4

Euler diagram of species identified on BRUVS between coral reefs, seamounts of variable summit depths, and the continental slopes. Coral reef species that were also observed in at least one deep-sea environment (black grouping line) are compared in the bar plot through two functional traits: species size class and trophic group.