Acasta clausa, Yu & Chan & Achituv & Kolbasov, 2017

Yu, Meng-Chen, Chan, Benny K. K., Achituv, Yair & Kolbasov, Gregory A., 2017, Four new sponge-inhabiting barnacles of the genus Acasta (Thoracica: Archaeobalanidae: Acastinae) from the Indo-Pacific, Raffles Bulletin of Zoology 65, pp. 585-615 : 599-606

Acasta clausa , new species

( Figs. 15–21 )

Material examined. Ten specimens (holotype and paratypes) in the sponge Halichondria (Halichondria) sp. Fleming, 1828, southwest part of Green Island , Taitung, Taiwan , 22°38′20″N, 121°28′46″E, SCUBA diving, depth 21 m. Three specimens (paratypes) in the sponge Petrosia aff. pellasarca (de Laubenfels, 1934) , Indonesia , Java Sea, Kepulauan Seribu ( Thousand Islands ), off Jakarta, Semak Daun Island , NW-side, 5°43′56″S, 106°34′23″E, SCUBA diving, depth 20m, 17.09.2005. The holotype (dry shell compartments, mouth parts and cirri mounted in glycerol on glass slide) and paratype (shell in EtOH) are deposited in the Zoological Museum of Moscow State University under registration numbers Mg –1224 (holotype) and Mg –1225 (paratype). The other paratypes ( ASIZCR000376 and ASIZCR000377 ) are deposited in the Biodiversity Research Museum, Biodiversity Research Center, Academia Sinica, Taiwan and ( RMNH. CRUS.C.10244) Naturalis Biodiversity Center ( NCB), Leiden , The Netherlands .

Diagnosis. Shell white, semitransparent, lilyform in adults, orifice large; opercular valves exposed; wall plates and basis thin, fragile; parietes consist of axial striated part and 2 thinner, smooth lateral extensions; radii and alae ~1/3–1/4 length of parietes; basis cup-shaped; scutum with radial striation; tergal spur very short, ~2/3 width of basal margin; anterior ramus of cirrus IV with small, sharp denticles; basidorsal point of penis absent. Embedded in sponge Halichondria (Halichondria) sp. Fleming, 1828 and Petrosia aff. pellasarca (de Laubenfels, 1934) .

Description. (Based on holotype).Shell white,semitransparent, lilyform in adults, without slits and windows, ~ 5.4 mm maximum height and 2.3 mm rostro-carinal basal diameter; orifice large, toothed, 5.0 mm maximum length; opercular valves exposed; areas corresponding to 6 large windows filled by smooth, thin lateral calcareous extensions of parietes of wall plates ( Fig. 15A–C ). Basis ( Figs. 15C , 16A ) deep moderately, cup-shaped, ~1.0 mm high, with growth lines concentric, margin smooth. Parietes ( Fig. 15D–K ) consist of axial part with fine growth lines and 2 thinner, smooth lateral extensions; external surface without projections; sheath short, ~1/4 total length, with horizontal striations; inner lamina beneath sheath smooth. Radii and alae with summits oblique; ~1/3–1/4 length of parietes; radii with fine, oblique, external striations. Carina longest, recurved outwards slightly ( Fig. 15A–C, F, G ). Carinolatus 2 with developed axial parts of paries, ~1/2 width of carinolatus 1 ( Fig. 15A–C, J, K ).

Scutum ( Fig. 16B–E ) white-rose, semitransparent, apex tinged slightly with dark violet; height longer than width; external growth ridges fine, crossed by radial striation; occludent margin dentate. Articular ridge ~1/2–2/3 length of tergal margin, not prominent, lower end smooth, not truncated; adductor ridge feeble, pit for adductor muscle shallow, pit for depressor muscle not developed.

Tergum ( Fig. 16F–I ) semitransparent, white rose, growth lines fine; apex beak-shaped, tinged slightly with dark violet; scutal margin almost straight; spur very short, truncated, ~2/3 width of basal margin, not separated from basiscutal angle; spur furrow rudimentary. Articular ridge feeble, ~1/3 length of scutal margin, crests of depressor muscle rudimentary.

Labrum ( Fig. 19A, B, D ) with deep medial notch, 2 small teeth on each crest. Crests of labrum with thin, simple setae and rows of ctenoid scales ( Fig. 19A, D ); massive ctenoid scales in proximal area and at base of mandibular palps ( Fig. 19A, B ). Mandibular palps ( Fig. 19A, C ) club-shaped; with dense, long, setulate setae, small, thin. simple setae on distal half; lateral surfaces with dense, small, simple setae and rows of ctenoid scales. Mandibles ( Figs. 17A, B , 19E, F ) cutting edge normally with 5 teeth decreasing in size from upper to lower; one mandible of holotype with additional teeth between 3 rd and 4 th teeth ( Fig. 17B ); teeth 1–3 well separated from each other; teeth 2, 3 bifid; inferior angle sharp, with group of sharp denticles and setae; inner margin with long, simple setae; outer margin and upper part of blade with long, thin, omniserrate setae; lateral surface of blade with dense, small, biserrate setae ( Fig. 19F ). Maxillules ( Figs. 17C, D , 19G–I ) with 7–8 cuspidate setae of different lengths along straight cutting edge; notch absent; upper and lower pairs of cuspidate setae largest; small seta inserted beneath upper pair of cuspidate setae; tuft of small, sharp, cuspidate setae on inferior angle; long simple and serrate setae along outer and inner margins; dense, biserrate setae on lateral surface. Maxillae ( Fig. 20A–E ), bilobed; dense, long simple and serrate setae distally and along inner margins of upper and lower lobes; lateral surfaces of lobes with small, simple setae and sparse ctenoid scales; field of stub setae in upper-lateral part of lower lobe; dense carpet of sharp, biserrate setae and fine, small setae in fused basal parts.

Cirrus I with rami unequal; anterior ramus (14 annuli) twice longer than posterior ramus (7 annuli); both rami covered with dense setae. Cirrus II ( Fig. 21A, B ) with rami unequal; anterior ramus (8 annuli) longer than posterior ramus (6 annuli); annuli of rami with long, dense, simple and serrate setae on anterior edge and along upper margin; tuft of serrate setae in posterio-distal corner; lateral surfaces with ctenoid scales; protopod with long, serrate and plumose setae. Cirrus III ( Fig. 18A ) with anterior ramus (9 annuli) slightly longer than posterior (9 annuli); ramal annuli without denticles, with long, simple and serrate setae on anterior edges and lateral surfaces; tuft of serrate setae in posterio-distal corner. Cirrus IV ( Fig. 21C–E ) with rami unequal; anterior ramus with 20 annuli, posterior ramus with 26; basal annuli of anterior ramus with 3–5 upward denticles and combs of sharp small denticles on upper half of anterior edge; basal and middle annuli of anterior ramus with 2–4 long and middle serrate setae on upper part of anterior edge; tuft of thin, simple setae on posterior-diatal corner; basal and middle annuli of posterior ramus with 3 pairs of setae on upper part of anterior edge, tuft of setae on posterio-distal corner. Cirrus V with 26 and 28 annuli in anterior and posterior rami respectively; annuli of anterior ramus with 3 pairs of long, middle and short serrulate setae on anterior edge; tuft of setae on posterio-distal corner; annuli of posterior ramus with 2 pairs of long and short anterior setae; tuft of setae on posterio-distal corner. Cirrus VI with 29 and 31 annuli on anterior and posterior rami respectively; intermediate annuli of both rami with 3 pairs of long, middle and short, serrulate setae.

Penis ( Figs. 18B, C , 20F–H ) annulated; basidorsal point absent; gradually tapering distally; tip with few thin, simple setae.

Penis of one specimen has abnormal bifurcated tip, both tips may function, because have terminal openings ( Fig. 20G, H ). Remarks. The new species Acasta clausa differs from other congeners in having unique semitransparent, thin lateral calcareous extensions of parietes of the wall plates filled windows areas and very short, almost rudimentary spur of tergum.

Etymology. The specific name is derived from Latin ‘clausa’, meaning ‘closed’, referring to the areas corresponding to six large windows filled by smooth, thin lateral calcareous extensions of parietes of the wall plates.

Figures

Fig. 15. Acasta clausa sp. nov., shell, general view and compartments (holotype: A, B, D–K; paratype: C from Taiwan). A, B, Complete shell, left and right side; C, Complete shell, left side; D, E, Rostrum, external and internal surface; F, G, Carina, external and internal surface; H, I, Carinolatus 1, external and internal surface; J, K, Carinolatus 2, external and internal surface. Abbreviations: ba = basis, ca = carina, cl1 = carinolatus 1, cl2 = carinolatus 2, r = rostrum, sc = scutum, te = tergum. Scale bars in µm.

Fig. 16. Acasta clausa sp. nov., opercular plates and basis (holotype). A, Basis, internal side (partially destroyed); B, C, Right scutum, external and internal view; D, E, Left scutum, external and internal view; F, G, Right and left terga, external view; H, I, Right and left terga, internal view. Scale bars in µm.

Fig. 17. Acasta clausa sp. nov., mouth parts (holotype). A, B, Mandibles; C, D, Maxillules. Scale bars in µm.

Fig. 18. Acasta clausa sp. nov., cirri and penis (holotype). A, Cirrus III, proximal part; B, Basal part of penis; C, Distal part of penis. Abbreviations: c6 = coxa of cirrus VI, pe = penis. Scale bars in µm.

Fig. 19. Acasta clausa sp. nov., mouth parts (SEM, paratype, from Taiwan). A, Labrum and mandibular palps; B, Ctenoid scales at base of mandibular palp; C, Upper half of mandibular palp; D, Enlarged crests of labrum (denticles indicated by arrows); E, Mandible; F, Biserrate setae on lateral surface of mandible; G, H, Maxillules; I, Lower margin of maxillule. Abbreviations: mdp = mandibular palp. Scale bars in µm.

Fig. 20. Acasta clausa sp. nov., maxillae and penis (SEM, paratype, from Taiwan). A, Maxillae, internal surface; B, Distal lobes of maxillae; C, Proximal lobe of maxilla; D, Stub setae on proximal lobe of maxilla (groups of stub setae indicated by arrowheads); E, Small setae on basal part of maxilla; F, Basal part of penis; G, H, Abnormal bifurcated tip of penis. Abbreviations: c5 and c6 = cirri V and cirri VI, pe = penis. Scale bars in µm.

Fig. 21. Acasta clausa sp. nov., cirri (SEM, paratype, from Taiwan). A, Cirrus II; B, Cirrus II, posterior ramus, annuli 1–3; C, Cirrus IV, fifth annulus of anterior ramus; D, Cirrus IV; E, Cirrus IV, basal annuli of anterior ramus. Scale bars in µm.