Acasta vivicava, Kolbasov, Gregory A., Chan, Benny K. K., Molodtsova, Tina N. & Achituv, Yair, 2016

Kolbasov, Gregory A., Chan, Benny K. K., Molodtsova, Tina N. & Achituv, Yair, 2016, Revision of the coral-inhabiting genus Conopea (Cirripedia: Archaeobalanidae) with description of two new species of the genera Conopea and Acasta, Zootaxa 4178 (2), pp. 182-208 : 192-196

Acasta vivicava sp.nov.

Figures 6–8 .

Type material. National Museum of Natural History (Naturalis), Leiden, Holland. Locality : ‘ Oceanic Reef’s Expedition’ 1992–93, Seychelles, Amirante Islands, 6o12’S, 53o02’E, SCUBA diving, 0 2.01.1993, one specimen covered by a white scleraxonian alcyonacean Briareum sp. ( Briareidae ) . Holotype RMNH. CRUS.C.10234 (shell compartments in ethanol, mouth parts and cirri mounted in glycerol on glass slide) is deposited in National Museum of Natural History (Naturalis), Leiden, The Netherlands .

Diagnosis. Shell white, with thin, feebly connected compartments, parietes with small dense external projections and developed basal longitudinal ribs on internal surface, carinolatus 2 narrow, basis saucer-shaped, with crenated margin, width of scutum equal to height, articular ridge prominent, tergum with long, sharp, beakshaped apex, basidorsal point of penis absent. Embedded in alcyonacean coenenchyme.

Description. Shell covered with host tissue forming gall ~ 8.8 mm long with small orifice ~ 0.9 mm long ( Fig. 6 A, B). Shell white, slightly elongated in rostro-carinal axis, rostro-carinal basal length 4.5 mm, basal width 3 mm, height 4.0 mm, opercular orifice dentate, ~ 1.5 mm maximum length, compartments and basis feebly connected ( Fig. 6 C, D). Basis saucer-shaped, with concentric growth lines, crenated margin, irregular depression in centre marking connection to axis of gorgonian, junction between basis and axis of host weak. Parietes ( Fig. 6 F–H) with small, dense, external projections and fine growth lines, sheath ~1/2 total length, with horizontal striations, internal basal longitudinal ribs distinct. Radii and alae with oblique summits, radii reaching bases of parities and with fine oblique external striations, alae with horizontal striations. Carinolatus 2 narrow ( Fig. 6 H), with parie only about 1/ 19 width of carinolatus 1.

Scutum ( Fig. 7 A–C) thin, semitransparent, broad, width and height equal, external growth ridges crossed with few short, feeble longitudinal ridges, occludent margin dentate. Articular ridge ~1/2 length of tergal margin, prominent, with smooth, not truncated lower end, articular furrow developed, adductor ridge feeble, pits for adductor and depressor muscles not developed.

Tergum ( Fig. 7 D–G) thin, semitransparent, with growth lines, apex long, sharp, beak-shaped, scutal margin concave, spur truncated, ~1/2 width of basal margin, distinctly separated from basiscutal angle, spur furrow shallow, wide. Articular ridge developed, ~1/2 total height, crests of depressor muscles absent.

Labrum ( Fig. 8 A) with deep medial notch, 2 small teeth on each crest. Mandibular palps ( Fig. 8 A) clubshaped, with long, dense setae distally. Mandible ( Fig. 8 B–E) cutting edge with 5 teeth decreasing in size from upper to lower, teeth 1–3 well separated from each other, teeth 2–5 with bifid, inferior angle serrated, with 2 sharp denticles, inner margin with long, dense setae, outer margin with rare long, paired setae, lateral surface of blade with dense, small setae. Maxillules ( Fig. 8 F, G) with 9–10 cuspidate setae of different lengths along straight cutting edge, notch absent, upper and lower pairs of cuspidate setae largest, long fine setae along outer and inner margins and on lateral surfaces of blade. Maxillae ( Fig. 8 H) bilobed, with long setae along inner margins of lobes and on tip.

Most of the cirri of the specimen studied were dried and rami broken. Cirrus I with rami unequal, anterior ramus (12 segments) twice as long as posterior ramus (6 segments), both rami covered with moderately dense setae. Cirrus II with rami unequal, anterior ramus (7 segments) longer than posterior (5 segments), segments with dense setae. Cirri III ( Fig. 8 I) with anterior ramus (8 segments) slightly longer than posterior (6 segments), several pairs of setae of different sizes along anterior edges of ramal segments, pair of stout setae at posterio-distal angle, segments of both rami without denticles or teeth. Cirri IV–VI with broken rami. Basal segments of cirrus IV without teeth and denticles. Penis ( Fig. 8 J) long, annulated, basidorsal point absent.

Remarks. Nine species of the genus Acasta were found in alcyonaceans— A. alcyonica Rosell, 1991 ; A. alcyonicola Utinomi, 1953 ; A. echinata Hiro, 1937 ( Fig. 13 H–K); and A. umitosaka Utinomi, 1962 from alcyoniinan alcyonaceans; A. antipathidis Broch, 1916 (this species was previously reported to have an antipatharian host; however, study of photographs of the syntypes revealed that the colony reported as a black coral in fact represents a denuded dead axis of an alcyonacean); A. gregaria Utinomi, 1959 ; A. hirsuta Broch, 1916 ; A. purpurata Darwin, 1854 ; and A. vipensis Van Syoc, Carrison-Stone, Madrona & Williams, 2014 from alcyonaceans (gorgonians). The new species, A. vivicava , differs from these species in having a narrower CL2, a scutum with a prominent articular ridge and a tergum with an elongated sharp apex. The new species can also be distinguished from A. alcyonica , A. purpurata and A. umitosaka by the absence of large slits or windows between the shell plates, and from A. alcionica , A. gregaria and A. hirsuta and A. vipensis by the absence of the longitudinal striation of scutum.

It should be noted that several archaeobalanid species that are epibionts of alcyonaceans and antipatharians also representing acastines were erroneously assigned to the genus Conopea , as demonstrated by the phylogenetic analyses (see below). These include C. canaliculata (Ren et Liu, 1978) ; C. dentifer ( Broch, 1922) ; C. folliculus ( Hiro, 1937) ; C. fragilis ( Broch, 1931) ; C. longibasis ( Hiro, 1937) ; C. navicula ( Darwin, 1854) ( Fig. 13 A–G); C. pygmaea ( Broch, 1931) ; and C. squamosa ( Rosell, 1991) . Acasta vivicava also differs from C. canaliculata , C. dentifer , C. fragilis , C. longibasis , C. navicula and C. pygmaea in having the scutum with equal basal and tergal margins and with a prominent articular ridge, the tergum with an elongated sharp apex and a narrower carinolatus 2. The new species has opercular valves slightly similar to C. follisculus , but differs by the scutum having a more prominent articular ridge and by the absence of a depressor muscle pit. Acasta vivicava differs from C. squamosa by possessing a narrower carinolatus 2 and by the elongated, sharp apex of the tergum. Acasta vivicava also can be distinguished from C. canaliculata , C. dentifer , C. longibasis , C. fragilis and C. navicula by the absence of developed longitudinal striation of the scutum.

Etymology. The specific name represents the similarity to a bird living in the tree hollow, from the Latin ‘ vivet ’ meaning live and ‘ cavae ’ hollow.

Figures

FIGURE 6. Acasta vivicava sp. nov., general appearance, shell compartments. (A), (B) Gall on gorgonian host, lateral and top view, orifice indicated by arrow. (C), (D) Shell, lateral and top view. (E) Margin of basis. (F), (G) Carina, inner and outer view. (H) Carinolatus 2, outer view. Abbreviations: ba — basis, ca — carina, cl 1 — carinolatus 1, cl 2 — carinolatus 2, r — rostrum. Abbreviations: bp—basidorsal point of penis. Scale bars in µm.

FIGURE 7. Acasta vivicava sp. nov., opercular plates. (A), (B) Left scutum, inner and outer view. (C) Right scutum, outer view. (D), (E) Left and right terga, outer view. (F), (G) Right and left terga, inner view. Scale bars in µm.

FIGURE 8. Acasta vivicava sp. nov., mouth parts, cirri and penis. (A) Labrum with palps. (B), (D) Mandibles. (C), (E) Lower parts of mandibles (inferior angles). (F), (G) Maxillules. (H) Maxilla. (I) Cirrus III. (J) Base of penis. Scale bars in µm.

FIGURE 13. Shell morphology of 2 species of genus Acasta from octocorals (Acasta (Conopea) navicula — A – G; A. echinata — H – K). (A), (H) Sell, general appearance, lateral view, host tissue removed. (B) Carinolatus 1, inner view. (C), (D) Left scutum, outer and inner surface. (E), (F) Left tergum, outer and inner surface. (G) Serrated margin of basis. (I) Tergum, outer surface. (J) Scutum, outer surface. (K) Serrated margin of basis. Abbreviations: ba — basis, ca — carina, cl 1 — carinolatus 1, cl 2 — carinolatus 2, r — rostrum. Scale bars in µm.