Functional and phylogenetic responses of motile cryptofauna to habitat degradation

Abstract

1. Biodiversity of terrestrial and marine ecosystems, including coral reefs, is dominated by small, often cryptic, invertebrate taxa that play important roles in ecosystem structure and functioning. While cryptofauna community structure is determined by strong small-scale microhabitat associations, the extent to which ecological and environmental factors shape these communities are largely unknown, as is the relative importance of particular microhabitats in supporting reef trophodynamics from the bottom up.2. The goal of this study was to address these knowledge gaps, provided coral reefs are increasingly exposed to multiple disturbances and environmental gradients that influence habitat complexity, condition and ecosystem functioning. 3. We compared the density, biomass, size range, phylogenetic diversity and functional roles of motile cryptofauna in Palau, Western Micronesia, among four coral-derived microhabitats representing various states of degradation (live coral [Acropora and Pocillopora], dead coral and coral rubble) from reefs along a gradient of effluent exposure.4. In total, 122 families across ten phyla were identified, dominated by the Arthropoda (Crustacea) and Mollusca. Cryptofauna biomass was greatest in live Pocillopora, while coral rubble contained the greatest density and diversity. Size ranges were broader in live corals than both dead coral and rubble. From a bottom-up perspective, effluent exposure had mixed effects on cryptic communities including a decline in total biomass in rubble. From a top-down perspective, cryptofauna were generally unaffected by predator biomass. 5. Our data show that, as coral reef ecosystems continue to decline in response to more frequent and severe disturbances, habitats other than live coral may become increasingly important in supporting coral reef biodiversity and food webs.

Emigration patterns of motile cryptofauna and their implications for trophic functioning in coral reefs

“…Per individual biomass was greatest in treatments with open and interstitial access (see Figure S3 ), skewed by low numbers of larger‐bodied echinoderms (e.g., Linckia multifora , Amphiuridae), decapods (e.g., Galatheidae, Hippolytidae, Palaemonidae, Portunidae, and Xanthidae), and fishes (Gobiesocidae and Gobiidae; e.g., Eviota and Callogobius ), as common of rubble biomes (Stella et al, 2022 ; Wolfe et al, 2020 ; Wolfe, Kenyon, & Mumby, 2021 ). Interestingly, the mean density and biomass of cryptofauna did not differ between open and interstitial access (i.e., surface‐blocked) treatments, confirming that exposure to top‐down predation is not a limiting factor of the rubble‐dwelling cryptofauna (Fraser et al, 2020 ; Stella et al, 2022 ), despite being heavily preyed upon (Kamen, 2020 ). Invertivorous fishes frequently inspect RUBS when deployed (Wolfe, pers.…”

“…While external higher‐order predators may have little impact on rubble‐derived productivity (Fraser et al, 2020 ; Stella et al, 2022 ), there are clearly size limitations to life in rubble. Our data indicate that those required to move along or above the benthos to colonize sampling units were dominated by small‐bodied taxa with the capacity to swim and disperse, as found previously (Takada et al, 2016 ).…”

“…On coral reefs, the density and diversity of motile cryptofauna increase from live coral to dead coral and rubble (Fraser et al, 2021a ; Stella et al, 2022 ). As for cryptobenthic fishes, the productivity of motile invertebrate cryptofauna is high and can be up to 3 orders of magnitude greater in dead coral than in live coral (Fraser et al, 2021a ; Kramer et al, 2014 ).…”

“…The abundance of organisms typically decreases with increasing body size, with differences in this size spectra indicating variations in properties of ecosystem functioning such as predation and energy transfer (Currie, 1993 ; Damuth, 1981 ; Jennings & Mackinson, 2003 ; Jennings & Warr, 2003 ; Trebilco et al, 2013 ). High‐standing biomass and secondary production in rubble are therefore predominantly available in some of the smallest individuals, which limits how accessible this energy is to higher trophic levels (Fraser et al, 2021a ; Heather et al, 2021 ; Stella et al, 2022 ). Predators able to forage successfully in rubble represent a critical stepping‐stone in reef trophodynamics, transferring energy from a highly productive but complex and somewhat inaccessible microhabitat (Kamen, 2020 ; Wolfe, Kenyon, & Mumby, 2021 ).…”

“…Local population retention coupled with rapid growth and extreme mortality underpins the fundamental role of cryptobenthic fishes in trophic productivity (Brandl et al, 2018(Brandl et al, , 2019). Yet, cryptobenthic fishes represent a comparatively small portion of the total diversity and density of coral reef cryptofauna, which is dominated by invertebrates (Stella et al, 2022;Wolfe, Kenyon, & Mumby, 2021).…”