DOI: 10.5061/dryad.0k6djhb4k
title: Emigration patterns of motile cryptofauna and their implications for trophic functioning in coral reefs

description: Patterns of movement of marine species can reflect strategies of
 reproduction and dispersal, species’ interactions, trophodynamics, and
 susceptibility to change, and thus critically inform how we manage
 populations and ecosystems. On coral reefs, the density and diversity of
 metazoan taxa is greatest in dead coral and rubble, which is suggested to
 fuel food webs from the bottom-up. Yet, biomass and secondary productivity
 in rubble is predominantly available in some of the smallest individuals,
 limiting how accessible this energy is to higher trophic levels. We
 address the bioavailability of motile coral reef cryptofauna based on
 small-scale patterns of emigration in rubble. We deployed modified RUbble
 Biodiversity Samplers (RUBS) and emergence traps in a shallow rubble patch
 at Heron Island, Great Barrier Reef, to detect community-level differences
 in the directional influx of motile cryptofauna under five habitat
 accessibility regimes. The mean density (0.13–4.5 ind.cm-3) and biomass
 (0.14–5.2 mg.cm-3) of cryptofauna were high and varied depending on
 microhabitat accessibility. Emergent zooplankton represented a distinct
 community (dominated by the Appendicularia and Calanoida) with the lowest
 density and biomass, indicating constraints on nocturnal resource
 availability. Mean cryptofauna density and biomass were greatest when
 interstitial access within rubble was blocked, driven by the rapid
 proliferation of small harpacticoid copepods from the rubble surface,
 leading to trophic simplification. Individuals with high biomass (e.g.,
 decapods, gobies, and echinoderms) were greatest when interstitial access
 within rubble was unrestricted. Treatments with a closed rubble surface
 did not differ from those completely open, suggesting that top-down
 predation does not diminish rubble-derived resources. Our results show
 that conspecific cues and species’ interactions (e.g., competition and
 predation) within rubble are most critical in shaping ecological outcomes
 within the cryptobiome. These findings have implications for prey
 accessibility through trophic and community size structuring in rubble,
 which may become increasingly relevant as benthic reef complexity shifts
 in the Anthropocene.
description: We address the bioavailability of coral reef cryptofauna in
 rubble based on small-scale patterns of emigration. We adapted the
 accessibility of Rubble Biodiversity Samplers (RUBS), models used to
 standardise biodiversity sampling in rubble (Wolfe and Mumby 2020), to
 explore the local movement patterns of rubble-dwelling fauna, with
 inference to predation processes within and beyond the cryptobenthos. Five
 treatments were developed to detect community-level differences in the
 directional influx of motile cryptofauna under various habitat
 accessibility regimes. Four of these treatments were developed by
 modifying accessibility into RUBS (https://www.thingiverse.com/thing:4176644/files) to understand limitations on the directional influx and movement of cryptofauna within coral rubble patches using four treatments; (1) open (completely accessible), (2) interstitial access (top closed), (3) surficial access (sides and bottom closed), and (4) raised (above rubble substratum). The fifth treatment involved a series of emergence plankton traps, designed to target demersal cryptofauna that vertically migrate from within the rubble benthos at night, given emergent zooplankton biomass and diversity are greatest at night. Fieldwork was conducted over several weeks (11th September to 5th October 2021) in a shallow (~3–5 m depth) reef slope site on the southern margin of Heron Island (-23˚26.845’ S, 151˚54.732’ E), Great Barrier Reef, Australia (Fig. 1). All collections were conducted under the Great Barrier Reef Marine Park Authority permit G20/44613.1.