Phorbas amaranthus Duchassaing & Michelotti, 1864

Figures 88 a–e

Restricted synonymy:

Phorbas amaranthus Duchassaing & Michelotti, 1864: 92, pl. XXI fig. 1; Van Soest 1984: 86, pl. VI fig. 7, text-fig. 34. Merriamium tortuganensis De Laubenfels, 1936: 83, pl. 11 fig. 1.

Material examined. RMNH Por. 10520, Suriname, ‘ Snellius O.C.P.S. ’ Guyana Shelf Expedition, station G56, 7.26°N 56.6667°W, depth 67–68 m, Agassiz trawl, 10 May 1966.

Description. Thin encrustation on a calcareous nodule (Fig. 88 a), red-brown in alcohol. Size 2 x 1.5 cm in lateral expansion, about 2 mm in thickness. Surface faintly areolate, but no visible oscules. Consistency soft.

Skeleton. The choanosomal skeleton is plumose, with columns of megascleres, consisting of tornotes and long acanthostyles, 60–100 µm in diameter, rising up from the substratum. Shorter acanthostyles echinate the columns. At the surface there are numerous areolate porefields the walls of which are supported by single vertically arranged tornotes. The inside of the porefields is strengthened by microscleres. Between the porefields, single tornotes are found in the surface membrane along with numerous microscleres.

Spicules. (Figs 88 b–e) Acanthostyles, tornotes, arcuate isochelae.

Acanthostyles, divisible in two distinct size categories, (1) large (Figs 88 b,b1), curved, spined all over but less densely towards the pointed end, rather uniform in size, 216– 249 –271 x 9 – 11.6 –13 µm, and (2) short (Figs 88 c,c1), straight, spined densely all over, similarly uniform in size, 104– 118 –126 x 8 – 10.4 –12 µm.

Tornotes (Figs 88 d,d1), strongylote or faintly tylote-like, subtly inequiended, shaft faintly polytylote, 231– 249 –262 x 3 – 4.2 –5.5 µm.

Arcuate isochelae (Fig. 88 e), shaft strongly convex, alae short and squarish in outline, 23– 27.6 –30 µm.

Distribution and ecology. Guyana Shelf, Greater Caribbean and NE Brazil, encrusting nodules on soft bottom, at 6–68 m depth (Guyana Shelf 67–68 m).

Remarks. The specimen conforms to Phorbas amaranthus (Duchassaing & Michelotti, 1864), widespread in the Western Atlantic, reported from the Virgin Islands, Florida and the Gulf of Mexico (as Merriamium tortuganensis De Laubenfels, 1936), Curaçao (Van Soest 1984), Colombian Caribbean (Zea 1987), and NE Brazil (Moraes 2011; Muricy et al. 2011). There are also list records (without descriptions) from Cuba (Alcolado 1976), Bonaire (Kobluk & Van Soest 1989), Jamaica (Lehnert & Van Soest 1998) and Belize (Rützler et al. 2000). This species has been repeatedly described, usually with mucronate tornote apices, whereas our specimen has distinctly strongylote endings. A review of the many slides present in the collections of the Naturalis Biodiversity Center (ZMA collection) revealed that tornote apices are variably more mucronate or more strongylote without a clear correspondence with shape, structure and other spicule shapes and sizes.

The present find at 67–68 m is the deepest record for the species.

Four further Phorbas species have been reported from the Central West Atlantic, P. fusifer (Ridley & Dendy, 1886) (originally Myxilla plumosa var. fusifera), P. hechteli Hajdu & Teixeira, 2011 (originally Anchinoe ramosa Hechtel, 1983), and P. capixaba Hajdu & Teixiera, 2011, all three from (North-) Eastern Brazil, and P. aurantiacus Rützler, Piantoni, Van Soest & Díaz, 2014 from Belize. The latter species differs clearly from the present material by its lack of chelae and the possession of stylote tornotes. P. fusifer is lobate, has only a single category of acanthostyles and oxeote tornotes. P. hechteli is an erect branch or digitation, and likewise has only a single category of small acanthostyles. P. capixaba is bushy, also erect, but more irregular; it has oxeote tornotes and two size categories of arcuate isochelae, unlike the present specimen.