Rhizophyta Shen & Dohrmann & Zhang & Lu & Wang 2019, gen. nov

Genus Rhizophyta gen. nov. urn:lsid:zoobank.org:act: B71AF3A9-527F-454E-9AC7-0074CC189FE0 Type species. Rhizophyta yapensis gen. et sp. nov. Diagnosis. Body is fungus-like, pedunculate, rhizophytous with completely everted, laterally directed atrial cavity. Choanosomal spicules of the main body are diactins and hexactins. Dermalia and atrialia are pinular hexactins and rare pentactins. The internal spicules of the peduncle are diactins fused into a rigid skeleton by synapticulars. External spicules of the peduncle are loose pentactins forming a veil. Microscleres are toothed stellate discohexasters. Etymology. The new genus name, Rhizophyta, reflects the rhizophytous attachment method (fixation by rootlike outgrowths instead of spicule tuft or basal plate). Remarks. Due to the presence of a long peduncle and absence of hypodermal pentactins, the new specimens most closely match the diagnosis of Euplectellidae: Bolosominae. Although the surface pentactins veiling the peduncle resemble similar structures formed by hypodermalia in the Rossellidae, there are no smaller megascleres accompanying them, so they are best interpreted as homologous to dermalia rather than hypodermalia. Furthermore, our molecular evidence (see below) clearly rules out any affinity to Rossellidae. The rhizophytous method of fixation is quite rare among extant Hexactinellida and has to our knowledge never been reported for bolosomine euplectellids – as far as we are aware, only two rhizophytous euplectellids have been described before, namely Regadrella rhizophora and R. pedunculata, which are venus-flower-basket-like forms with a very short "peduncle", currently classified in Corbitellinae (see Tabachnick & Lévi 2004; Reiswig & Kelly 2018). The new species cannot be accommodated in any of the currently known ten genera of Bolosominae. The overall body shape combined with pinular dermal and atrial hexactins suggest affinities to the monotypic genus Neocaledoniella Tabachnick & Lévi, 2004. However, the new species is clearly distinguished from it by the presence of regular discohexaster microscleres rather than a combination of calycocomes and graphiocomes. Furthermore, recent investigation of a newly collected specimen of Neocaledoniella caulophacoides (Tabachnick, 2002) allowed for the first time examination of the base of its peduncle, showing that it has a basiphytous method of fixation (CS, unpublished observation). The presence of no other microscleres than discohexasters is uniquely shared with Bolosoma Ijima, 1904 among known pedunculate euplectellids. However, besides from differing in body shape (no everted, laterally directed atrial cavity and shorter, thicker stalk), Bolosoma species are characterized by the presence of anchorate discohexasters (codonhexasters) and their derivatives (missing in the new species). Our molecular evidence (see below) also does not support any affinity of the new species to Bolosoma.