Metabarcoding as a tool to examine cryptic algae in the diets of two common grazing surgeonfishes, Acanthurustriostegus and A. nigrofuscus

Abstract

Surgeonfishes (Acanthuridae) are an important group of herbivores that are abundant on reefs globally. Acanthurids consume macroalgae that can compete with corals for space, turf algae that can proliferate on degraded reefs, and detritus that may smother adult corals or inhibit settlement. For these reasons, they are of particular interest at present to resource managers seeking to restore and conserve reefs that are facing a myriad of stressors. To contribute to our understanding of the diet breadth and potential vulnerability of these important fishes, we employed metabarcoding to identify and compare the algal diets of two common Hawaiian surgeonfishes, the convict tang or manini (Acanthurus triostegus) and the brown surgeonfish or maʻiʻiʻi (A. nigrofuscus). These species serve important biological and cultural roles in Hawaiian coral reef ecosystems, and manini, in particular, constitute an essential component of traditional Hawaiian diets. Although diet richness was not significantly different between the species, with 64 unique taxa identified from the gut contents of 89 A. triostegus and 57 taxa from 73 A. nigrofuscus, A. nigrofuscus exhibited greater diet diversity as measured by the Simpson's Diversity Index. A. nigrofuscus had a greater relative read abundance of brown algae, cyanobacteria, and material from the epilithic algal matrix. A. triostegus had a taxonomically more specialized diet, consuming primarily red algae. A. triostegus showed greater variability between sites in diet composition than A. nigrofuscus, and when grouped at the site, shore, or species level, A. triostegus tended to have greater diet overlap among individuals. Conversely, A. nigrofuscus exhibited greater variation in diet among individuals, with less diet variability between sites. Grazing surgeonfishes such as these are critical components of existing and future herbivore management plans, making their diet breadth and consequent vulnerability to habitat loss or disturbance of great interest to resource managers.

Improving stable isotope assessments of inter‐ and intra‐species variation in coral reef fish trophic strategies

“…For example, highly diverse pomacentrid assemblages at Dongsha Atoll displayed a similar isotopic niche space to pomacentrids in Moorea (French Polynesia), despite much lower species richness in the latter (Gajdzik et al, 2018 ). Similarly, and contrary to expectations, surgeonfish with different morphologies have a similar dietary range (Brandl et al, 2015 ; Nalley, Donahue, & Toonen, 2022 ), suggesting species may differ across other axes of their trophic niches which are not reflected in the isotopic ratios (Schoener, 1974 ). It is clear from these examples, and our data, that only analyzing δ 15 N and δ 13 C misses important ecological information that could help explain unexpected niche overlaps.…”

“…This suggests that using a single tracer, such as only δ 15 N, δ 13 C, or δ 34 S, may hide important dietary variation, and lead to false conclusions regarding shared resources among groups. Multivariate analyses more accurately capture the extent of the isotopic niche space, representing a more comprehensive view of a species' trophic ecology, which is likely to be inherently variable (Nalley, Donahue, & Toonen, 2022 ; Skinner, Mill, et al, 2019 ).…”

“…Species‐level analyses may, therefore, mask individual differences in dietary variation and limit the ability to identify more complex energy flows and trophodynamics within a specific study area or differences in species' ecological roles. For example, using DNA metabarcoding, Nalley, Donahue, and Toonen ( 2022 ) showed that the convict surgeonfish ( Acanthurus triostegus ) had little variability in diet composition among individuals at a site, but that diet composition varied among sites. In contrast, the conspecific brown surgeonfish ( A. nigrofuscus ) exhibited greater variation in diet among individuals, but individuals had a similar diet composition across all sites.…”

Differences in the behavior and diet between shoaling and solitary surgeonfish (Acanthurus triostegus)

“…Acanthurus triostegus are common throughout coral reefs in the tropical Indo‐Pacific Ocean and it is a grazing reef herbivore that feeds primarily on filamentous algae and cylindrical algae, as well as some cyanobacteria, foliose algae, and calcareous algae (Nalley et al, 2021; Randall, 1961). Metabarcoding of diet contents of A. triostegus in Hawai'i identified 64 unique diet items, with Rhodophyta dominating the abundance, followed by ochrophyta and cyanobacteria (Kelly et al, 2016; Nalley et al, 2021). These abundant coral reef fish exhibit both shoaling and solitary behavior on both islands (Guerra et al, 2022), providing an excellent opportunity to explore the ecology of shoaling behavior.…”

Fingerprinting Arctic and North Atlantic Macroalgae with eDNA – Application and perspectives

“…Most eDNA studies focus on metabarcoding of animals or land plants (Deiner et al, 2017), while only a few studies have explored the use of eDNA to track the export of macroalgae to the open ocean (Ortega et al, 2019) or the contribution of marine plants and macroalgae to sediment pools or food webs, with various success (del Carmen Gomez Cabrera et al, 2019; Nalley et al, 2021; Ortega et al, 2020; Queirós et al, 2019; Reef et al, 2017). The use of eDNA combined with sediment chronologies would also enable the identification of community changes over time (Bálint et al, 2018), as demonstrated by documented invasion of seagrass species in the Eastern Mediterranean (Wesselmann et al, 2021) and community changes in coral reefs of Australia (del Carmen Gomez Cabrera et al, 2019).…”