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

Acasta undulaterga sp. nov.

Figs 32 ̄38

Material examined. Holotype: ASIZCR000370, Longdong, Gongliao, New Taipei City, Taiwan (25°06′50.27″N, 121°55′13.40″E, water depth 17m), July 2011, coll. J.H.Y. Yu, on host sponge Jaspis splendens (de Laubenfels, 1954).

Paratype: CEL-SNE47-5—data same as for holotype.

Diagnosis. Shell white to pale yellow, tinged pinkish towards apex. Basis pentagonal, saucer-shaped, with 4 indistinct and shallow radial furrows extending from center to the crenate basal rim. Scutum strongly latticed, articular furrow distinct, articular ridge prominent, short. Tergum beaked, apex tinged with red, spur long, truncated, width about 1/3 of basal margin, basal margin deeply concave at spur base. Feeble armament of curved teeth on anterior ramus of cirrus IV.

Description. Shell white to pale yellow after extraction from sponge, becoming white and semitransparent (sometimes tinged with reddish patches towards apex) after bleach treatment (Figs 32 ĀD, 33ĀL). Basis pentagonal, saucer-shaped, with 4 external radial shallow furrows extending from center to crenate basal rim (Fig. 33 M, N). Parietes externally smooth with fine growth ridges; internally with longitudinal basal ribs not reaching sheath; sheath with horizontal striations; radii with inclined and horizontal striations; alae with horizontal striations, not reaching to base (Fig. 33 ĀL). Carinolaterals small, axe-shaped, about 1/5 of width of laterals (Fig. 33 G, H, I, J). Carina longest, with incurved apex (Figs 32 ĀD, 33K, L).

Scutum (Fig. 32 ĒF) externally latticed by strong growth and longitudinal ridges with prominent teeth on occludent margin. Basal margin slightly concave at basioccludent angle, basiscutal angle rounded. Internal surface smooth, with rudimentary adductor ridge, articular furrow distinct, articular ridge prominent, short, about 1/2 of length of tergal margin, adductor and depressor pits absent. Tergum (Fig. 32 G, H) thin, fragile, apex beaked, tinged with red, with fine growth ridges, spur narrow, truncated, width about 1/3 of basal margin, distinctly separated from basiscutal angle, spur furrow wide, shallow, basal margin deeply concave at spur base; internally scutal margin raised, articular ridge short, reddish, crests of depressor muscles absent.

Counts of cirral segments given in Table 1. Cirrus I with rami unequal, posterior margin with serrulate and plumose setae (Fig. 34 A), simple and bifid setae on distal margin of terminal segments of anterior and posterior rami, respectively (Fig. 34 B, C). Cirrus II rami unequal, protopod anterior and posterior margins with plumose and pappose setae, respectively (Fig. 34 D), terminal segments of anterior and posterior rami with bifid setae (Fig. 34 E, F). Cirrus III, rami subequal; protopod anterior and posterior margins with simple and plumose setae, respectively (Fig. 34 G), terminal segments of anterior and posterior rami with serrate and serrulate setae (Fig. 34 H, I), anterior and posterior rami with small, sharp denticles on intermediate segments (Fig. 34 J). Cirrus IV, rami unequal, protopod anterior and posterior margins with serrulate and plumose setae, respectively (Fig. 35 A), anterior margin of basis armed with few denticles (Fig. 35 B), proximal segments of anterior ramus bearing 1 curved tooth and upturned spines/denticles on anterior margin (Fig. 35 C̄E), intermediate segments with 2 pairs of long and short, serrulate setae (Fig. 35 F), terminal segments of anterior and posterior rami with simple and serrulate setae (Fig. 35 ḠH). Cirrus V, protopod anterior and posterior margins with serrulate and few short simple setae, respectively (Fig. 36 A), intermediate segments of both rami with 2 pairs of long and 1 pair of short serrulate setae, terminal segments of anterior and posterior rami with serrulate setae (Fig. 36 ĀC). Cirrus VI with rami unequal, protopod with few short simple and anterior serrulate setae (Fig. 36 D), intermediate segments with 2 pairs of long and 1 pair of short serrulate setae, terminal segments of anterior and posterior rami with serrulate setae (Fig. 36 D̄F). Penis finely annulated, without basidorsal point, gradually tapering to tip (Fig. 36 G), tip broken (Fig. 36 H).

Maxilla bilobed, oblong (Fig. 37 A), distal lobe with serrulate setae (Fig. 37 B), outer edge without long setae, inner edge straight, basal lobe with serrulate setae (Fig. 37 C, D). Maxillule cutting margin straight, without notch, with 8 large cuspidate setae, upper and lower pairs largest (Fig. 37 E, F), upper margin with 5 pairs of simple setae, lower margin with numerous simple setae (Fig. 37 G, H). Mandible with 5 teeth (Fig. 38 A), second bifid (Fig. 38 B), inferior angle with 3 denticles and stout setae, lower margin bearing simple setae (Fig. 38 C). Mandibular palp broadly truncated (Fig. 38 D), outer margin distinctly concave (Fig. 38 E), with dense serrulate setae (Fig. 38 G), basal margin with prominent process (Fig. 38 F). Labrum bilobed, with deep, V-shaped notch (Fig. 38 H), 3 small teeth on each crest (Fig. 38 I).

Remarks. In our specimens, the morphological characters are consistent between the holotype and paratype. Although this species shares some morphological characters with A. sulcata, including a basis with a crenate rim and a beaked tergum with a truncated spur, it differs from A. sulcata in having i) a completely, strongly latticed scutum, ii) a narrower spur of the tergum, and iii) a deeply concave basal margin of the tergum at the base of the spur. From the 34 extant species of Acasta, only two (Acasta japonica Pilsbry, 1911 and Acasta newmani Van Syoc & Winther, 1999) are characterized by the possession of a latticed scutum. However, the present new species may be easily distinguished from A. japonica and A. newmani by the longer and narrower tergal spur and the deeply concave basal margin of the tergum. The latticed scutum is characteristic of several species of the genera Archiacasta, Neoacasta and Pectinoacasta. The new species differs from species of the genus Archiacasta in having a saucer-shaped calcareous basis, the internal longitudinal ribs of the shell plates, and the teeth on cirrus IV. It also differs from Neoacasta in having internal longitudinal ribs of the shell plates and recurved teeth on cirrus IV, and by the absence of six internal ribs on the basis. It differs from Pectinoacasta due to the absence of a latticed or longitudinally striated tergum. On the basis of these differences, these specimens are described as a new species.

Etymology. The word ‘ undulaterga ’ refers to the concave basal margin of the tergum and is derived from the Latin words tergum and undulatis.