Glass sponge reefs are only found on the continental shelf of the Northeast Pacific. The reefs support diverse communities of fish and invertebrates. The abundance and distribution of live sponges likely contributes to the reefs’ productivity as the sponges provide structured habitat and trap energy and nutrients in the benthic system through their efficient filter-feeding. Here we investigated the relative importance of live sponges and the spatial distribution of structured habitat for the associated megafauna found on glass sponge reefs. In 2012 and 2013 we collected georeferenced video and imagery along 78 transects in nine glass sponge reef complexes, recently protected via bottom-contact fisheries closures in the Strait of Georgia, British Columbia, Canada. First, we developed and tested quantitative methods for assessing reef-building sponge cover and mapped the distribution of live and standing dead reef-building sponges. We then compared megafaunal communities associated with four habitat types: live sponge dominated, mixed live and dead sponges, dead sponge dominated, and no visible reef structure using indicator species analysis of video- and still image-based datasets.
We found that the distribution of structured habitat was highly patchy on all reefs. Habitat types defined by the presence of live or standing dead sponge structure supported significantly more diverse and abundant megafaunal communities. Taxonomic groups differed in their affinity for live versus dead sponge structure. Quillback rockfish (Sebastes maliger) and the longhorn decorator crab (Chorilia longipes) were associated with live sponge dominated habitat; boot sponges (Rhabdocalyptus dawsoni), spot prawns (Pandalus platyceros), and squat lobsters (Munida quadrispina) were associated with structured habitat (i.e. live, mixed live and dead, and dead sponge dominated habitats); and finally, sea pens (family Pennatulacea) and brittle stars (class Ophiuroidea) were associated with no visible reef structure. These associations were consistent between video- and still image-based based datasets. These results show that the abundance and distributions of sponges influences the community structure and abundance of megafaunal organisms. Further, our results suggest that human activities which damage the structure provided by both live and dead reef-building sponges reduce the ability of these ecosystems to support fish and invertebrate communities. Finally, we show that mapping sponges may be used to assess current health of glass sponge reef ecosystems and to monitor reef recovery.