Sea cucumbers influence coral growth and alter sediment-associated prokaryotic communities on a Caribbean reef

Sea cucumbers (class Holothuroidea) are abundant reef bioturbators that influence sediment biogeochemistry and nutrient cycling, yet their function on coral reefs remains poorly resolved. To elucidate the effect of the donkey dung sea cucumber Holothuria mexicana on coral growth, sediment organic content, and sediment-associated prokaryotic assemblages, we conducted a six-week field enclosure experiment coupled with microbial community analyses on Little Cayman, Cayman Islands. Eighteen cages containing the staghorn coral Acropora cervicornis were deployed in a sandy habitat adjacent to patch reefs under three treatments: closed cage with H. mexicana , closed cage without H. mexicana , and open-frame controls. Model-estimated linear growth rates of A. cervicornis were significantly higher in the presence of H. mexicana compared with the other treatments. The specific growth rate of A. cervicornis was highest in cages containing H. mexicana and significantly exceeded the specific growth rate in open-frame controls, whereas growth in closed cages without H. mexicana was intermediate. Consequently, a potential cage effect could not be ruled out. Sediment organic content did not differ among treatments at the beginning or end of the experiment, indicating no detectable change in bulk organic matter over this timescale. We also characterised the prokaryotic communities of sediment and H. mexicana egesta using 16S rRNA amplicon sequencing to test whether digestion alters benthic microbial assemblages. Faecal communities exhibited lower alpha diversity and a distinct taxonomic composition relative to sediments, characterised by depletion of sediment-associated families and enrichment of a subset of taxa, including Vibrionaceae. Our results indicate that H. mexicana may enhance A. cervicornis growth while substantially restructuring sediment-derived microbial assemblages without altering bulk sediment organic content.