Sycettusa hirsutissima sp.nov.

Figs 71a–d, 72a–g

Material examined. Holotype, RMNH Por. 10004, Saudi Arabia, Jeddah, near Thuwal, Tahlah, 22.25725°N 38.880917°E, scuba, coll. N.J. de Voogd, field nr. THU14/JED211, 13 November 2014.

Paratype, RMNH Por. 9588, Saudi Arabia, Jeddah, near Thuwal, Abu Gishaa, 22.255194°N 39.025639°E, depth 12 m, scuba, coll. N.J. de Voogd, field nr. THU06/JED083, 9 November 2014.

Description. Densely hairy tubes (Figs 71a–b), hanging down, from reef walls. Color yellow or orange below a thick pelt of diactines. In preservation they become dark grey. Size up to 8 cm high, 0.5–1.5 cm in diameter. Consistency firm.

Aquiferous system. Syconoid.

Skeleton. (Figs 71c–d) Inarticulate with cortex of triactines, carried by the unpaired and shorter actines of pseudosagittal triactines. Choanosomal skeleton consists of the longer actines of the subcortical pseudosagittal triactines and the unpaired actines of sagittal subatrial triactines. Atrial skeleton consists of smaller sagittal triactines.

Spicules. (Figs 72a–g) Diactines, trichoxeas, cortical triactines, subcortical pseudoaagittal triactines, subatrial triactines, atrial triactines.

Diactines (Figs 72a–b), consisting of two types, long thin with sharp points (Fig. 72a), often fragmented, 1200– 2213 –3200 x 15 – 21.1 –33 µm, and a shorter type with lance head endings of variable length and thickness (Figs 72b,b 1), 336– 1179 –2110 x 12– 20.3 –30 µm.

Trichoxeas (Fig. 72c), invariably broken into smaller pieces, 540– 1413 –2940 x 2– 2.9 –3.5 µm.

Cortical triactines (Fig. 72d), almost regular or slightly sagittal, 171– 202 –261 x 10 – 16.0 –18 µm.

Pseudosagittal triactines (Figs 72e), longest actine 420– 582 –738 x 24 –31.3–43 µm, shorter actine 225– 294 –383 x 23 – 29.4 –40 µm, unpaired actine 162– 207 –299 x 21 – 28.8 –42 µm.

Subatrial triactines (Figs 72f), variably with longer and shorter unpaired actines and flat-angled or acuteangled paired actines, unpaired actines 282– 490 –530 x 282– 36.6 –52.5 µm, paired actines 162– 258 –375 x 14 – 31.6 –46 µm.

Atrial triactines (Fig. 72g), slightly sagittal, 81– 158 –231 x 7 – 13.1 –16 µm.

Distribution and ecology. Saudi Arabian Red Sea, on overhangs in shallow water.

Etymology. Hirsutus (L.) means hairy, rough, referring to the ‘pelt’ of long protruding bundles of diactines. We use the superlative of hirsutus to emphasize the exteme hairiness.

Remarks. The new species is close to S. hastifera in overall appearance and in spicule complement. However, the two specimens are extremely and densely hairy, unlike the dozen S. hastifera specimens we had at our disposal, which have the protruding diactines much less dense. In the new species, the body wall underneath the pelt of diactines is barely visible and has a yellow color, while it is always clearly visible and greyish or greenish white in S. hastifera . In contrast to S. hastifera the sizes (length and thickness) of the longest actine of the pseudodosagittal triactines of our new species is usually larger than that of the subatrial triactines, although this may not be of great importance. At first, we were inclined to consider the new species an extreme form of S. hastifera . However, we obtained partial 28S sequences of the two specimens (GenBank acc.nrs MF872774, MF872775, and MF872776) and in the phylogeny of Fig. 3 they did not group into the same clade with Sycettusa hastifera (GenBank acc.nrs MF872777 of RMNH Por. 9659, and MF872778 of ZMA Por. 13443) and S. simplex (Jenkin, 1908) (GenBank acc. nrs MF872779 of RMNH Por. 10154 and MF872781 of ZMA Por. 12446), supporting a separation at the species level. To investigate the molecular differences between S. hastifera, S. hirsutissima sp.nov. and S. simplex, we separately aligned and trimmed the available Sycettusa 28S sequences of these three species yielding a length of 403 sites. The sequences of S. hirsitussima sp.nov. were identical. They shared shared three site differences with S.hastifera and four with S. simplex . The latter two species differed only in a single site, despite being clearly distinct in habitus as S. simplex has an almost smooth surface (cf. below).