Racodiscula radixa, Schuster & Cárdenas & Pisera & Pomponi & Kelly & Wörheide & Erpenbeck, 2018

RACODISCULA RADIXA View in CoL SP. NOV.

( FIGS 8, 9; TABLE 3)

Diagnosis: Racodiscula with a deep, apical, cavity/ osculum, into which subdermal canals converge from the margins; desmas are irregular, loosely zygosed tetraclone desmas with root-like protrusions.

Type material: Holotype: HBOM 003 View Materials :02006, Coll. JSL-I dive 3913. [22 October 1995, Galápagos, N-Side 1.25 NM W of point Vincente Roca NW Coast, Isabela Island, 0°02 ′ 55 ″ S, 91°35 ′ 16 ″ W, 378 m]. GoogleMaps

Comparative material: Racodiscula seymoura sp. nov. HBOM 003 View Materials :02014 (396 m) (see below), Galápagos , Seymour Island, 0°22 ′ 51 ″ S, 90°16 ′ 06 ″ W, Coll GoogleMaps . JSL-I dive 3529; HBOM 003 View Materials :02007 (319 m), Galápagos , Isabela Island, 0°14 ′ 31 ″ S, 91°24 ′ 56 ″ W, Coll GoogleMaps . JSL-I dive 3015. Racodiscula borisi sp. nov. HBOM 003 View Materials :02011 (242 m) (see below), Galápagos Island , Tortuga Island, 1°4 ′ 59 ″ S, 90° 51 ′ 56 ″ W, Coll GoogleMaps . JSL-I dive 3923; HBOM 003 View Materials :02001 (215 m), Galápagos Island , San Cristobal Island, 0°43 ′ 08 ″ S, 89°27 ′ 14 ″ W, Coll GoogleMaps . JSL-I dive 3905.

Type locality: Isabela Island , Galápagos Islands (378 m) ( Fig. 1) .

Distribution: Only known from type locality.

Habitat: Attached to hard substratum at a depth of 378 m.

Description: Morphology, massive subspherical folded with a deep, apical, cavity/osculum, about 1 cm longest diameter, situated on the elevated apex ( Fig. 8A, B); dimension of the holotype, about 7 cm diameter, about 5 cm high ( Fig. 8A, B). Texture, medium hard. Surface, smooth, with branched, meandering, subdermal canals, converging from the margins ( Fig. 8B) towards the apical cavity; lateral inhalant surface with pores, 450–500 µ m diameter. Colour, white in cream ( Fig. 8A), dark cream in ethanol preservative ( Fig. 8B). Ectosomal skeleton, detachable in places due to large subdermal canals, composed of phyllotriaene cladomes, stacked and overlapping, ( Figs 8C–F, 9G), interspersed with microrhabds and other microscleres ( Fig. 9B, E) forming a thick outer layer; tylostyles are located at the subectosomal region ( Fig. 8D–F). Choanosomal skeleton contains loosely zygosed tetraclone desmas ( Fig. 8D), loose bundles of oxeas and scattered microscleres ( Fig. 9C, D). Megascleres, tetraclone desmas, phyllotriaenes and oxeas: tetraclone desmas, relately smooth, from which emerge thin, root-like branches, 405–532 × 254–276 µ m (N5) ( Fig. 8C–F); phyllotriaenes with a leafy cladome, 305– 389 – 494 µ m (N20), short, irregular rhabdome, 93– 116 – 160 × 32– 42 –55 µ m (N20) ( Fig. 9A, B, G); tylostyles, 450– 315 –605 µ m (N15) ( Fig. 9A). Microscleres, acanthose microrhabds with irregularly distributed spined tubercles, 21– 25 –30× 4 µ m (N30); metasters with long rays, 15.6– 18.5 –20.2 × 11.3– 14.7 –19.2 µ m (N20).

Etymology: Named for the root-like ornamentation of the tetraclone desmas in this species (radix, root; Latin).

DNA barcodes: we sequenced COI, 28S (C1-D2) and 18S (1,623 bp) of the holotype. GenBank accession number: COI KY652819 View Materials , 28S KY652788 View Materials , 18 KY652836 View Materials . SBD record no. 1720.

Remarks: Racodiscula radixa sp. nov. is differentiated from the two other new Racodiscula species from the Galápagos Islands, R. seymoura sp. nov. and R. borisi sp. nov. (see species description below), in having the cavity/oscula at an elevated centre with vein-like furrows to the margins, while R. seymoura sp. nov. have the oscula in a centred depression and R. borisi sp. nov. has a perfectly circular and centred oscula on the surface without an elevated/compressed centre. The oscula of all three new Racodiscula species black: comparative samples). Bayesian posterior probabilities and ML bootstrap support values are indicated for clades>0.75/60, otherwise marked with a dash. Numbers following the taxon names are collection numbers or NCBI Genbank accession numbers. Markers sequenced are indicated in square brackets. Three letter code in dark blue indicates the biogeographical location. described are different to Racodiscula asteroides Zittel, 1878 in having only one opening, while R. asteroides has multiple oscules in a shallow depression ( Pisera & Lévi, 2002b). However, intraspecific variations of this character cannot be excluded. Thick strongyles as found in R. seymoura sp. nov. and R. borisi sp. nov. (see below) could not be detected in any of the spicule preparations for R. radixa sp. nov., which may indicate that this second type of microsclere was secondarily lost. ( Table 3). The cladome of phyllo- to discotriaenes is smaller than in R. seymoura sp. nov. and R. borisi sp. nov. The rhabdome is also slightly shorter than in R. seymoura sp. nov. but larger than in R. borisi sp. nov. ( Table 3). R. radixa sp. nov. has irregularly rather loose root-like desmas with shorter branches than R. seymoura sp. nov. In contrast, R. borisi sp. nov. shows strong tuberculated tetraclone desmas similar to R. asteroides . Differences were also found in our molecular analyses ( Fig. 5). R. radixa sp. nov. differ to R. seymoura sp. nov. with 3.3% (COI), 0.3% (28S) and 1.8% (18S) and to R. borisi sp. nov. with 4.8% (COI), 0.5% (28S), 2.3% (18S). The sister-group relationship of R. radixa sp. nov. and R. seymoura sp. nov. is supported [posterior probability (PP) = 0.97 and bootstrap support (BS) = 80]. The clade with the three new Galápagos Racodiscula species is well supported by PP = 0.90, but not by BS. The sister-group relationship of this clade with R. asteroides and Racodiscula sp. from the Caribbean is well supported ( Fig. 5).