Imbricaverruca yamaguchii, Newman, 2000

Newman, William A., 2000, A new genus and species of barnacle (Cirripedia, Verrucomorpha) associated with vents of the Lau Back-Arc Basin: its gross morphology, inferred first juvenile stage and affinities, Zoosystema 22 (1), pp. 71-84 : 74-81

Imbricaverruca yamaguchii n. sp.

( Figs 1A ; 2 ; 3 ; 4C ; 5; 6)

MATERIAL. — Specimens were collected by the French submersible Nautile , BIOLAU Cruise of 1989 to the Lau Basin , Tonga (A.-M. Alayse-Danet, chief scientist), and they were received from the Centre national de tri d’Océanographie biologique ( CENTOB), IFREMER, Brest. Two adult specimens on round stones partially coated with sulfide deposits, plus several juveniles about to shed their cyprid shells, inferred to belong to this species (Jensen’s x-juveniles of HØeg & Newman 1997; see below), all from station 1 (Hine Hina, 22°32’S, 176°43’W, at 1900 m depth). Two of these samples ( BL-01 & BL-03 ) contained other vent barnacles and other vent organisms ( Newman & Yamaguchi 1995; Yamaguchi & Newman 1997; Southward & Newman 1998; cf. Desbruyères et al. 1994) .

ETYMOLOGY. — Named for Toshiyuki Yamaguchi in appreciation of his numerous and exacting works on the systematics and ecology of the living and fossil cirripeds, especially the vent barnacles.

DIAGNOSIS. — As for the genus.

COMPARATIVEDESCRIPTION

Hard parts

The low-conic profile of the new genus, Imbricaverruca , is similar to Neobrachylepas Newman & Yamaguchi, 1995 , whose representatives were relatively young, and to the juveniles of Neoverruca and Eochionelasmus Yamaguchi & Newman, 1990 . The operculum forms a flat lid, as in the Proverrucidae and Verrucidae , but the hinge line is curved rather than essentially straight as in Neoverruca , and in higher verrucomorphs (cf. Figs 1A ; 2A ). Therefore, in order for the operculum to open, there is likely some flexibility to the wall as well as some freedom in the arthrodial membrane forming the hinge, and it must be able to bend along the scuto-tergal articulation, an ability likely facilitated by the basal gap protected by the median latus ( Fig. 2C ). Like Neoverruca , the scutal adductor muscle inserts in a pocket below the apex of the movable plate ( Fig. 2E ), and its origin on the fixed scutum, near the centre of the plate ( Figs 2G ; 4C ), is supported by an adductor ridge or “myophore” as in Metaverruca Pilsbry, 1916 and Verruca Schumacher, 1817 (cf. Young 1998 for revision) ( Fig. 4 G-H).

The fixed scutum and tergum are much reduced in height ( Fig. 2 F-G, J-K) and there is a complex articulation between them. Together with the rostrum and carina, which are also much wider than high, they form a ring-like wall supporting the operculum. Therefore, the imbricating plates between this primary wall and the substratum form a substantially greater proportion of the body chamber than in Neoverruca . The principal three tiers of lateral imbricating plates, standing four plates high (rl 1-4, l 1-4 and cl 1-4), are fully represented in these two apparently adult Imbricaverruca ( Figs 1A ; 2A ; 4C ), as they are in brachylepadomorphs ( Newman 1987: fig. 4A) and in the juveniles of Neoverruca ( Newman & Hessler 1989; Newman 1989).

Soft parts

The trophi and cirri ( Fig. 3 ) are similar to those of other hydrothermal vent barnacles ( Scalpellomorpha, Brachylepadomorpha, Verrucomorpha and Balanomorpha ), reflecting adaptations to feeding on very fine particles, presumably small, suspended bacterial clumps and filaments (cf. Southward & Newman 1998). The slightly bullate, indented but unnotched labrum of I. yamaguchii ( Fig. 3F ) supports the usual row of fine teeth and relatively large palps provided with relatively fine, stiff setae. The mandible ( Fig. 3G , right exterior, H, I, left interior) is comparable to that of other hydrothermal vent forms, especially that of Neoverruca brachylepadoformis , in having one superior incisiform tooth above a setose area, followed by two low teeth each supporting a row of numerous, short, sharp spines along their superior curvatures, several of the uppermost of which roll over onto the inner or posterior surface ( Fig. 3I ), and the upper margin of the inferior angle supports a row of similar spines.

Maxillae I and II ( Fig. 3 J, K and L , respectively) each support a relatively uniform comb of slender setae along the cutting edge, but maxillae I of I. yamaguchii differs from Neoverruca in having a single strong spine among the ordinary setae inside and just below the superior angle ( Fig. 3K ).

What could be seen of the cirri ( Fig. 3 A-D) suggested they were typical of most vent barnacles, but unfortunately those of the paratype were badly damaged or missing. The right side was in the best condition and the cirral counts are as follows (a, anterior ramus; p, posterior ramus; –, ramus absent; +, terminal articles missing):

As can be observed ( Fig. 3A, B ), the first two pairs of cirri are modified as maxillipeds and at least the posterior ramus of the first as well as of the second are antenniform. The posterior four pairs of cirri are normal (ctenopod), but the lesser curvature of the intermediate articles of cirrus VI each have as many as five pairs of moderately long setae, fewer than Neoverruca , and their s/a ratios are markedly different, that for Imbricaverruca falling within the range for other vent barnacles while that for Neoverruca is the lowest known ( Table 1 ).

The caudal appendages, while multiarticulate in Eochionelasmus and minute and uniarticulate in Neolepas Newman, 1979 and Neobrachylepas , are absent in Imbricaverruca as well as Neoverruca ( Fig. 3C ). Both adults were hermaphroditic; penis long, conspicuously annulated, and sparsely clothed with short, stiff setae ( Fig. 3C, E ). While the generally well-developed penes of vent barnacles have a well-developed pedicel, there is no basidorsal point ( Fig. 3C ). Well-developed testes extend into the pedicels of cirrus I & II ( Fig. 3A, B ), and to a lesser degree into at least those of cirrus III & IV. Ovigerous frena were not seen, but no ovigerous lamellae were being incubated and in their absence frena may be easily overlooked.

The x-juvenile

A number of attached cyprids found on sulfide deposits associated with hydrothermal activity were included with the collection from the Lau Basin sent by CENTOB. Some were sent to Peter G. Jensen for the study of lattice organs (cf. Jensen et al. 1994) who removed the cyprid shell covering the first juvenile stage inside, when preparing them for SEM. Some of these were neolepadines but two were the first juvenile stage of an unusual thoracic cirriped. It was studied and dubbed “Jensen’s x-juvenile” by HØeg & Newman (1997) ( Fig. 5 ). Its capitulum, supported by a peduncle of comparable length, is armored by the five primordial valves commonly found in juvenile thoracicans, except iblomorphs.

The position and shape of the carinal primordium and a slight asymmetry between the scuta suggested the juvenile was a verrucomorph ( HØeg & Newman 1997).

The early juveniles of Neoverruca ( Newman, 1989) can be distinguished from the x-juvenile in being more generalized; e.g. in having a carinal primordium that is higher than wide extending up between the terga ( Fig. 6 ; cf. Table 2 ). The only first juvenile of a verrucid known is that of Verruca stroemia Müller, 1776 described by Runnström (1927) and it differs from the x-juvenile in the scutal primordia being grossly unequal rather than nearly equal in size. Therefore it was concluded the x-juvenile could represent 1) a verrucid more primitive than V. stroemia such as an Altiverruca , 2) an advanced neoverrucid such as the one described from Lau herein, or 3) a yetto-be discovered population of the presumably extinct proverrucid, and it was thought that, because of these uncertainties, determining the affinities of Jensen’s x-juvenile would require further sampling of the Lau Basin fauna ( HØeg & Newman 1997). However, the Lau neoverrucid, now known as Imbricaverruca , turned out not only to be more highly evolved than Neoverruca , but the adult has an autapomorphy, a carina that is much wider than high, which is the most notable characteristic of the x-juvenile. Therefore it seems highly likely that Jensen’s x-juvenile is the first juvenile of Imbricaverruca .

Figures

FIG. 1. — A, Imbricaverruca yamaguchii n. gen. and n. sp. (holotype, MNHN Ci2710); B, Verruca s.l., both viewed from above with the right scutum and tergum forming the operculum.Note that in the former the four plated wall appears to be covered largely by imbricating plates and the operculum includes a large median latus,characters that readily distinguish it from Neoverruca. Scale bar: 5 mm.

FIG. 2. — Plates of Imbricaverruca yamaguchii gen. et n. sp. (paratype, MNHN Ci2711); A, B, viewed from above and from the rostral end, respectively; A, note the operculum, comprising the plates of the right side (MS-L-MT), includes a large median latus; B, note the slightly open aperture to the mantle cavity between the occludent margins of the movable (MS & MT) and the fixed (FS & FT) scutum and tergum, respectively; C-M, variously disarticulated hard parts; C, interior view of wall (R-C-FS-FT) and operculum (MS-L- MT) with all of the imbricating plates, except the uppermost of the three principle lateral tiers of the right side (rl1-l1-cl1), stripped away [an approximation of the rostrum (R), which did not survive dissection, is indicated by the dashed line]; D, E, movable scutum (MS); external and internal views respectively; scutal adductor muscle, inserting from just below to up under the apical concavity in Fig. 2E, extends to its origin on the fixed scutum (FS, Fig. 2G; the relationship of MS to FS can be seen in Fig 2C); F, G, fixed scutum (FS); basal and internal views, respectively. Dashed line in G indicates origin of scutal adductor muscle but no scar is evident. The relationship of the two protuberances (best seen near the tergal margin in Fig. 2G) to the fixed tergum (FT, Fig. 2J, K), was not resolved; H, I, movable tergum (MT); external and internal views respectively (external corrosion in bleach and cleaning inadvertently obliterated growth line ornamentation in H); J, K, fixed tergum (FT); external and internal views respectively; L, M, carina from below and above respectively (the rostrum did not survive dissection and cleaning). Abbreviations: C, carina; cl1-4, l1-4, rl1-4, carinolateral, lateral and rostrolateral plates forming three tiers each four plates high, respectively; FS, FT, fixed scutum and tergum; L, median latus; MS, MT, movable scutum and tergum; R, rostrum. Scale bars: 1 mm.

FIG. 3. — Cirri (right side) and the trophi (appendages from the right side viewed from without, those of the left sides form within) of Imbricaverruca yamaguchii n. gen. and n. sp. (paratype, MNHN Ci2711); A, cirrus I, anterior ramus missing; B, cirrus II, setae omitted from the posterior ramus of the intact cirrus (left) are shown on the excised ramus (right); C, cirrus VI (setae of distal half omitted) and penis (annulations omitted); D, setation of an intermediate segment of outer ramus of cirrus VI (17th article from distal end); E, end of penis enlarged; F, labrum and mandibular palps; G, H, right and left mandible; I, spinous margin of H enlarged; J, K, right and left first maxillae; L, right second maxilla. Scale bars: A-C, 0.5 mm; D, E, 0.15 mm; F-L, 0.2 mm.

Fig. 4. — Schematic plan views and transverse sections of grades of skeletal organization from the most primitive sessile barnacles (A, Brachylepadomorpha) through the asymmetrical sessile barnacles (B-H, Verrucomorpha). Of the eight verrucid genera currently recognized (Young 1998) only the four most representative ones (Altiverruca, Newmaniverruca, Metaverruca and Verruca s.s.) are depicted here. Right sides are to the viewers right and rostral ends are downward in plan views or coming out of the page in transverse sections. The transverse sections are slightly rostral of the rostro-carinal gap or suture. While there are numerous whorls of imbricating plates standing in tiers four plates high in brachylepadomorphs, and on the movable side of neoverrucids, only those of the R-C gap are labeled. Abbreviations: C, R, carina and rostrum; cl1-4, l1-4, rl1-4, carinolateral, lateral and rostrolateral tiers of imbricating plates respectively; cl, rl, the pair of imbricating plates in proverrucids; FS, FT, fixed scutum and tergum; hl, hinge line; L, median latus (completely lost in proverrucids and verrucids); m, myophore; MS, MT, movable scutum and tergum; sam, scutal adductor muscle; S, T, normal terga and scuta in brachylepadomorphs; dashed horizontal lines: membranous basis (transverse sections F-H after Young 1998).

FIG. 5. — Jensen’s x-juvenile from the Lau Basin, Tonga (Høeg & Newman 1997); A, viewed from the left dorso-lateral; B, left lateral; C, ventral sides, respectively (the shard of chitin extending from the aperture in B and C apparently left over from the last molt). It is a first juvenile of a thoracican cirriped and likely that of Imbricaverruca yamaguchii n. gen. and n. sp.; see Fig. 6 and text for explanation. Abbreviations: A1, first antenna; C, carina; “C”, incipient cirri; LS, left scutum; LT, left tergum; RS, right scutum; P, peduncle. Scale bars: 100 µm.

FIG. 6. — Comparison between the primordial plates of the earliest juvenile stages of the principal suborders of thoracic Cirripedia; A, Lepadomorpha, Lepas (from Newman et al. 1979: fig. 87); B, Scalpellomorpha, composite of Neolepas and Pollicipes (respectively from Newman, in prep. and Broch 1922); C, Neoverruca (reconstructed from Newman 1989); D, x-juvenile (Høeg & Newman 1997 & herein); E, F, Verruca and Semibalanus (respectively from Runnström, 1925, 1927). Abbreviations: S, scutum; T, tergum; C, carina).