Monanchora arbuscula (Duchassaing & Michelotti, 1864)

Figures 84 a–h

Restricted synonymy:

Pandaros arbusculum Duchassaing & Michelotti, 1864: 88, pl. XVIII fig. 6.

Echinostylinos unguiferus De Laubenfels, 1953: 528, fig. 6; Collette & Rützler 1977: 309.

Monanchora barbadensis Hechtel, 1969: 21, fig. 3; Van Soest 1984: 40, Pl. IV figs 3–5, text-fig. 12;

Monanchora unguifera; Zea 1987: 152, pls 7–9, text-figs 50–51;

Monanchora arbuscula; Kobluk & Van Soest 1989: 1217; Hajdu et al. 2011: 146–150; Moraes 2011: 131–133; Rützler et al. 2014: 64.

Material examined. RMNH Por. 9957, 10546, Suriname, ‘ Snellius O.C.P.S. ’ Guyana Shelf Expedition, station G7, 7.28°N 56.7933°W, depth 64 m, bottom sand, 7 May 1966 (2 specimens).

Description. (Fig. 84 a) Thin encrustations on a dead oyster shell. Two different patches were present, each less than 1 mm thick and 1–2 cm in lateral expansion. Surface optically smooth, no visible oscules. Color (alcohol) beige–orange. Consistency soft.

Skeleton. Hymedesmioid, with thick tylostyles erect on the substratum, with heads embedded in a thin spongin plate. Brushes of thin subtylostyles carry the organic skin. Microscleres numerously present throughout the tissues.

Spicules. (Figs 84 b–h) Slightly different in both specimens, in one (10546) there are tylostyles, subtylostyles, tridentate anchorate isochelae and sigmoid microscleres, in the other (9957) the sigmoids are lacking and the chelae are polydentate.

Tylostyles (Figs 84 b,b1 & 84f,f1), robust, curved, with prominent tyles, similar in shape in both specimens, but those of 10546 (Figs 84 b,b1) are slightly longer, 237–438 µm, those of 9957 (Figs 84 f,f1) are 141–321 µm, combined the sizes in the two specimens are 141– 274 –438 x 8.5– 11.6 –16 µm.

Subtylostyles (Figs 84 c,c1 & 84g,g1), thinner than the tylostyles, straight, with elongate-oval tyles, similar in shape and size in both specimens, 204– 247 –291 x 3 – 4.6 –6 µm.

Anchorate isochelae (Figs 84 d & 84h), with fimbriae along the entire shaft, with unguiferous-spatulate alae, three in 10546 (Fig. 84 d), five in 9957 (Fig. 84 h), but similar in size, 21– 24.8 –27 µm.

Sigmoid microscleres (Fig. 84 e), tiny, strongly and evenly curved with finely pointed apices, only found in 10546, 6.5– 7.4 –9 µm.

Distribution and ecology. Guyana Shelf, Greater Caribbean, NE Brazil, on reefs and other hard substrates, down to 80 m depth (Guyana Shelf 64 m).

Remarks. This is interpreted as a variable species, both in growth form (arborescent to encrusting) and in spicule complement (presence or absence of microscleres, megasclere morphologies). The two specimens reported here occurring in the same station on dead oysters testify of the spicule variability, shown in the plate (upper row of spicules are from 10546, lower row from 9957): they are distinct in the number of alae in the anchorate chelae and the presence or absence of sigmoid microscleres.

No spined microxeas such as reported for this species by e.g. Moraes (2011) (p. 131) have been observed in the present specimens.