Taxonomy and diversity of the sponge fauna from Walters Shoal, a shallow seamount in the Western Indian Ocean region
Seamounts are poorly understood ubiquitous undersea features, with less than 4% sampled for scientific purposes globally. Consequently, the fauna associated with seamounts in the Indian Ocean remains largely unknown, with less than 300 species recorded. One such feature within this region is Walters Shoal, a shallow seamount located on the South Madagascar Ridge, which is situated approximately 400 nautical miles south of Madagascar and 600 nautical miles east of South Africa. Even though it penetrates the euphotic zone (summit is 15 m below the sea surface) and is protected by the Southern Indian Ocean Deep-Sea Fishers Association, there is a paucity of biodiversity and oceanographic data. Thus, a multidisciplinary cruise was initiated in May 2014 on the FRS Algoa as a component of the African Coelacanth Ecosystem Programme. The research presented here focuses exclusively on the diversity, bathymetric distribution patterns and biogeographic affiliations of the sponge fauna of this seamount. Sponges were sampled using SCUBA and a roughed epibenthic sled, from the peak and down two opposing slopes of the seamount, to a depth of 500 m. Two hundred and fifty-five sponge specimens were collected, comprising 78 operational taxonomic units (OTU’s), 23 of which are known to science, 26 which are possibly new, 16 that could only be identified to higher taxonomic levels and 13 that could only be designated as morphospecies. Thirteen OTU’s are formally described here, four which are known, and nine possibly new to science. Sponge assemblages demonstrated no significant difference according to location on the shoal, with several species shared by both the western and eastern flanks. In contrast, sponge assemblages differed significantly according to depth, with the mesophotic zone (31 – 150 m) acting as a transition between the shallow (15 – 30 m) and submesophotic (> 150 m) zones. Species richness and the number of putative new species was highest in the submesophotic zone. Biogeographical affiliations were found with both the Western Indo-Pacific and Temperate Southern African realms based on the 23 known species recorded. No affiliations were found with the West Wind Drift Island Province, as has been documented previously for the fish fauna of this seamount, possibly due to the incomplete nature of the online database (World Porifera Database) used to assess affinities. Thirty-nine percent of the known sponge species found at Walters Shoal Seamount are widely distributed in the Indian Ocean, 35% are found exclusively within the Western Indian Ocean region, with this study representing the southernmost distribution record for several of these, and 26% have a restricted distribution around South Africa.