Diversity and phylogeny of demosponge fauna in the abyssal nodule fields of the eastern Clarion‐Clipperton Zone, Pacific Ocean

Abstract

More than 7000 demosponge species have been described to date globally but <2% are known from the abyssal plains, which occupy some 50% of the Earth's surface. The demosponge fauna in the abyssal nodule fields at Clarion‐Clipperton Zone (CCZ) in the Pacific Ocean, a region being explored for potential deep‐sea mining, is a case in point. A total of 21 species belonging to 13 genera in nine families and seven orders were identified from the eastern region of the CCZ, of which most are new to science. They are small in size (<5 mm), with simple skeletons and poor spicule diversity. All ordinal representatives of Demospongiae were utilized to determine taxonomic position of the highly homoplasious tiny demosponges in our molecular phylogenetic analyses. Our results indicated Plenaster craigi, the most common and abundant species in the eastern CCZ, represents a new family, and possibly in a new order. Interestingly, P. craigi and members of the families Polymastiidae and Hamacanthidae, all filter‐feeding demosponge species, are far more abundant in nodule fields than the carnivorous sponges (Cladorhizidae) which were widely known to be the most dominant demosponge group in the abyssal depths. Lastly, it is highly likely that such tiny demosponges are present in other habitats. They might have been overlooked and/or ignored by sponge researchers in the past due to their tiny size and nondescript habitus. These demosponges could be distinct new species, not juveniles or indeterminates and warrant full taxonomic treatment.