A new genus and species of deep-sea Coralliidae (Octocorallia) from the northeastern Atlantic, the first polymorphic octocoral genus with a membranous-stolonate colony growth form.

During bathyal samplings undertaken by the Spanish Institute of Oceanography, a new genus and species of trimorphic Coralliidae was collected from the Avilés Canyon System, the Le Danois Bank (Bay of Biscay) and the Galicia Bank (north and northwestern Iberian Peninsula, northeastern Atlantic). Mollecorallium aenigmaticum gen. nov., sp. nov. is described and illustrated, including a molecular study. It has retractile autozooids within small mounds, and an abundance of radiates and crosses morphologically very similar to those occurring typically in species from other genera of the family with a calcareous axis of cemented sclerites. However, the new species is exceptional in lacking an axis, growing instead as small membranous and stolonal colonies, and in having mesozooids in addition to siphonozooids. According to our molecular study, the new genus herein proposed is strongly supported by the phylogenetic distance from soft-coral and gorgonian growth form genera in the family. Despite the notable biodiversity of the octocoral bathyal fauna of the Bay of Biscay and nearby areas, new species are discovered whenever exceptional sites such as submarine canyons and seamounts are sampled. • A new octocoral species in a new genus is described from the northeastern Atlantic. • For our study we integrate morphological and molecular information. • The new species is trimorphic, having autozooids, mesozooids and siphonozooids. • It grows as small membranous and stolonal colonies; this kind of growth is unique in the family. • No other trimorphic membranous species are known in the subclass Octocorallia. [ABSTRACT FROM AUTHOR]