Estimates of sponge consumption rates on an Indo-Pacific reef

Determining predator diets is essential for understanding the strength of top-down processes and how they cascade through food webs. This is especially important for sponges, key members of benthic communities, whose dominance has increased in recent years on some coral reefs. However, the diversity of spongivorous fishes and the sponges they consume are relatively unknown. Here, we estimated sponge consumption by spongivorous fishes in the Wakatobi Marine National Park, Indonesia. We deployed cameras to identify fish biting at the dominant reef sponge Xestospongia spp. and then used gut content analysis and fish abundance estimates to quantify sponge consumption. In total, 33 species from 10 families of reef fish were identified taking bites from Xestospongia spp.; however, the 2 most prolific sponge-grazers, Ctenochaetus binotatus and Chaetodon kleinii, had no sponge in their guts, showing that for some fish, bites on sponge surfaces are not reliable evidence of sponge consumption. Gut contents indicated that Pygoplites diacanthus was an obligate spongivore, while Pomacanthus imperator, P. xanthometopon, Zanclus cornutus and Siganus punctatus regularly consumed sponges. Sponge consumption by these 5 spongivores was estimated at 46.6 +/- 18.3 g sponge 1000 m(-2) d(-1). Molecular approaches developed to sequence the 18S gene for sponges consumed by angelfishes led to the successful amplification of 14 consumed sponges representing 6 orders of Porifera. We provide the first estimate of sponge consumption in the Indo-Pacific and are the first to successfully sequence partially digested sponges from fish stomachs, identifying several sponges previously unknown to be consumed by spongivores.

Automated summary

Predator diets, especially those of spongivorous fishes on coral reefs, play a crucial role in understanding the dynamics of food webs. This study in the Wakatobi Marine National Park, Indonesia, estimated sponge consumption by spongivorous fishes using camera footage and gut content analysis. The research revealed that 5 spongivores consumed an estimated 46.6 +/- 18.3 g of sponge per 1000 m2 per day in the region, with successful molecular sequencing of consumed sponges from fish stomachs uncovering previously unknown sponge species consumed by these fishes.