Acasta huangi Yu & Kolbasov & Hosie & Lee & Chan 2017, sp. nov.

Acasta huangi sp. nov.

Figs 11 ̄17

Material examined. Holotype: ASIZCR000367, Longdong, Gongliao, New Taipei City, Taiwan (25°06′50.27″N, 121°55′13.40″E, water depth 17 m), July 2011, coll. J.H.Y. Yu, on host sponge Jaspis splendens (de Laubenfels, 1954).

Diagnosis. Shell white-yellow. Basis saucer-shaped, pentagonal, with 4 radial, indistinct, shallow furrows extending to crenate basal rim. Parietes with small, rare calcareous projections; sheath feeble, with horizontal striations, short internal longitudinal ribs in basal part only. Scutum with horizontal growth ridges crossed with few longitudinal ridges in lower half, latticed in upper half. Tergum with growth ridges, spur truncated, width about 1/2 of basal margin. Cirrus IV anterior ramus and basis with strong armament of curved teeth.

Description.―Shell white-yellow after extraction from sponge, becoming white after bleaching (Figs 11 ĀE, 12ĀL). Basis pentagonal, saucer-shaped, with 4 indistinct, shallow radial furrows extending from center of basis to tcrenated basal rim (Fig. 12 M, N). Parietes (Fig. 12 ĀL) with small, rare, external projections and fine growth lines, radii with inclined and horizontal striations, radii not reaching base; alae with strong horizontal ridges; sheath with horizontal striations, short longitudinal ribs on basal margin of parietes not reaching sheath. Inner and outer edges of alae white, thickened.

Scutum (Fig. 11 G, H), externally with horizontal growth ridges crossed with few longitudinal ridges resulting in only upper half being latticed, growth lines forming prominent teeth on occludent margin. Internal surface smooth, articular furrow feeble, articular ridge not prominent, about 2/3 of length of tergal margin, adductor ridge rudimentary, adductor and depressor muscle pits absent. Tergum (Fig. 11 I, J) with growth ridges, carinal margin perpendicular to basal margin, spur truncated, width about 1/2 of basal margin, separated from basiscutal angle, spur furrow wide, shallow, internally with scutal margin raised, crests of depressor muscles absent.

Counts of cirral segments given in Table 1. Cirrus I with rami unequal, protopod with only a few plumose setae on proximal posterior margin (Fig. 13 A), anterior surfaces of both rami with dense setae (Fig. 13 B), serrulate and bifid setae on distal ends of terminal segments of anterior and posterior rami (Fig. 13 C, D). Cirrus II with rami unequal, protopod anterior and posterior margins with serrulate setae (Fig. 13 E), distal segments of rami with bifid setae (Fig. 13 E, F). Cirrus III (Fig.13 G, H) protopod anterior and posterior margins with short and plumose setae, respectively, segments of anterior ramus without denticles, distal segments of posterior ramus with serrate and serrulate setae. Cirrus IV, rami unequal, protopod anterior and posterior margins with short and plumose setae, respectively (Fig. 14 A), armed with a row of 8 curved teeth on anterior margin of basis (Fig. 14 B), proximal segments of anterior ramus bearing 1–2 curved teeth and 1̄2 denticles on anterior margin (Fig. 14 C̄F), terminal segment of anterior ramus with serrulate setae (Fig. 14 H). Cirrus V, protopod with few short setae (Fig. 15 A), proximal segments of anterior ramus each bearing 1 curved tooth on anterior margin (Fig. 15 B̄C), distal segments of anterior ramus with serrulate setae (Fig. 15 D). Cirrus VI, protopod with few short setae (Fig. 15 E), distal segments of posterior ramus with serrulate setae (Fig. 15 F). Penis finely annulated, gradually tapering to tip (Fig. 15 G), tip with long setae (Fig. 15 I), basidorsal point vestigial (Fig. 15 H).

Maxilla bilobed, oblong (Fig. 16 A), distal lobe with serrulate setae (Fig. 16 B), outer edge without setae, inner edge straight, basal lobe with serrulate setae (Fig. 16 C, D). Maxillule cutting margin straight, without notch, with 9 large cuspidate setae, upper and lower pairs largest (Fig. 16 E, F), upper margin and lower margins with simple setae (Fig. 16 G, H). Mandible with 5 teeth (Fig. 17 A), inferior angle with 3 denticles and stout setae, lower margin bearing simple setae (Fig. 17 B), second tooth of mandible bifid (Fig. 17 C). Mandibular palp truncated (Fig. 17 D), outer margin concave (Fig. 17 E), with dense terminal setae (Fig. 17 F). Labrum bilobed, with V-shaped notch (Fig. 17 G), small tooth on each crest (Fig. 17 H).

Remarks. This species shares several common morphological characters of the sulcata species complex, including i) a basis with a crenate edge, ii) a truncated spur of the tergum, iii) he armament of curved teeth on the anterior rami of cirri IV and V, and iv) a rudimentary basidorsal point of the penis. However, this species can be distinguished from A. sulcata by the i) the saucer-shaped pentagonal basis, ii) a scutum with well-developed longitudinal ribs, and iii) the feeble sheath of parietes. This species is also similar to A. conica Hoek, 1913, and A. aspera sp. nov., but differs in having short ribs along the basal margin of the parietes and a latticed pattern formed by the longitudinal striations. This species differs from A. semota Hiro, 1933, and A. chejudoensis Kim and Kim, 1988, as these species have a prominent articular ridge of the scutum, a narrow tergal spur, a nearly straight basal margin of the tergum, and the protopod of cirrus IV with a row of curved teeth. On the basis of these differences, the specimen is considered as a new species.

Etymology. Named in honor of Dr. Huang Yu-Sheng from National Penghu Technology University of Taiwan, a specialist in sponge diversity in Taiwan, who helped us in sponge identifications.