Mycale (Zygomycale) sibogae sp. nov. Figs 122 a–d, 123a–g Material examined. Holotype ZMA Por. 02901, Indonesia, Papua, Aru Islands, Pearl Banks, anchorage off Pulau Jedan, 5.4134°S 134.6677°E, depth 13 m, bottom sand and shells, coll. Siboga Expedition stat. 273, field nr. SE1599 IIA, 23 December 1899. Description (Fig. 122a). Bumpy, lobate mass of 2.5 x 2 x 2 cm. Surface optically smooth, but microhispid. No visible openings in preserved state, color beige-brown in alcohol. Consistency firm, compressible. Skeleton (Figs 122 b–d). Compact, with main choanosomal spicule tracts of 60–80 µm diameter running at right angles to the surface (Fig. 122b) at distances of 300–550 µm, frequently interconnected by thinner tracts of about 30 µm diameter. The reticulation is dense but irregular (Fig. 122c). Just below the surface the main tracts form brushes of single megascleres carrying the tangential surface skeleton (Figs 122 b–c), but also penetrating the surface membrane causing a short microhispidation. The ectosomal skeleton is of the aegogropila-type, intercrossing tracts of 20–30 µm diameter, tight-meshed, but rather irregular, also with some loose single spicules (Fig. 122d), with meshes of 100–150 µm in widest expansion. Rosettes of anisochelae I, 70–100 µm in diameter, occur on the intersections of the skeletal tracts, and in places clusters of toxodragmas (Fig. 122d arrow) are found. Spicules (Figs 123 a–g). Mycalostyles, two categories of anisochelae, isochelae, a single category of sigmas, toxas. Mycalostyles (Fig. 123a,a 1), often slightly curved, comparatively short and fat, with elongated heads and barely visible subterminal constriction, with opposite end sharply pointed, 186– 213.4 –233 x 4.5– 5.4 – 6.5 µm. Anisochelae I (Fig. 123b), well-developed, with free part of the shaft 35–40% of spicule length, with upper median alae extended outwards, lower alae comparatively small, 29– 34.1 – 37 µm. Anisochelae II (Fig. 123c), well-developed, narrow-shaped, free part of the shaft 25–30 % of spicule length, with upper median alae only slightly extended, almost parallel to the shaft, lower median alae with upward directed protrusion, 17– 19.2 – 22 µm. Isochelae (Figs 123d), well-developed, comparatively robust, with median alae slightly extended outwards, usually slightly asymmetrical, 10– 11.8 – 13 µm. Sigmas (Figs 123 e–f), thin (1 µm or less thick), mostly asymmetrical, but the smaller ones almost symmetrical, with sharply pointed endings, comparatively variable in size, 13– 22.0 – 28 µm, but not clearly distinguishable in size categories. Toxas (Figs 123g), thin, shallow-, evenly curved, occasionally single, but predominantly in dragmata, and very often in ‘double’ dragmata resembling those of Mycale (Kerasemna) humilis (cf. above.), 27– 32.4 –39 x 2– 3.4 – 7 µm. Distribution and ecology. Indonesia, reef and sand bottom in shallow-water. Etymology. Named after H.M.S. ‘Siboga’ to commemorate the 1899–1900 expedition carried out from the ship. Remarks. The new species is close to Mycale (Zygomycale) isochela Hentschel, 1911 in several aspects, e.g. it shares the absence of robust sigmas with that species. A difference is the apparent absence of anisochela I in M. (Z.) isochela, as it possesses only a single anisochela of 17–20 µm, which has the size of anisochelae II of our new species, and the presence of trichodragmata of 20 µm, absent in our new species. Further more detailed differences with our new species are the shape of the anisochelae, with upper median alae almost overlapping with the lower alae, no mention is made of rosettes, and the toxas are 45–50 µm, clearly in excess of those of our new species. The new species differs from M. (Z.) parishii in the absence of anisochelae III, absence of robust sigmas (sigma I) next to small thin sigmas (sigma II), and the absence of trichodragmata. From M. (Z.) pectinicola it differs in lacking micracanthoxeas, robust sigmas I, and possessing most toxas in dragmata. The anisochelae I of that species are 40–45 µm, clearly in excess of those of our new species. Addittional Mycale (Zygomycale) species from the region

Published as part of Van, Rob W. M., Aryasari, Ratih & De, Nicole J., 2021, Mycale species of the tropical Indo-West Pacific (Porifera, Demospongiae, Poecilosclerida), pp. 1-212 in Zootaxa 4912 (1) on pages 187-190, DOI: 10.11646/zootaxa.4912.1.1, http://zenodo.org/record/4450930