Presence of coral-killing sponges in Okinawan mesophotic coral ecosystems

Abstract

Shallow reefs worldwide are degrading due to multiple threats. Mesophotic coral ecosystems (MCEs; 30– 150 m depth) have been investigated for their diversity and potential to serve as a coral refuge and re-seed shallow reefs. However, recent studies showed that MCEs are also disturbed by various factors, including diseases, pollution, storms, and thermal-induced bleaching. Among the disturbances, coral-killing sponges are also known to decimate many Indo-Pacific shallow reefs but are rarely reported at mesophotic depths. Here, we report the first comprehensive assessment of the coral-killing sponges Chalinula nematifera (de Laubenfels, 1954) and Terpios hoshinota Rützler and Muzik, 1993 in MCEs. Specifically, we investigated (1) their prevalence, (2) sponge-affected corals, and (3) coral-sponge interaction types at 40 m depth in Okinawa, southern Japan. We found that C. nematifera and T. hoshinota were present but at low prevalences, with the highest mean cover of 3.8% and 1.8%, respectively. Although both species appeared to be substrate- generalists, they mainly affected the dominant coral taxa, such as branching corals Acropora tenella and Seriatopora sp. Both sponges mainly interacted with corals by overgrowth, followed by tissue and peripheral contact. Due to lack of prior knowledge, it is unclear whether the current observation of the coral-killing sponges captured their expansion, decline, or steady state in Okinawan MCEs. Thus, this study provides unprecedented insights about coral-killing sponges at mesophotic depths which highlights the need to monitor more MCEs globally, prioritizing regions with shallow reefs that are already threatened by these sponges.