Atetrapachylasma dijonesae Newman & Jones, 2011, sp. nov.
Creators: name: Newman, William A.
name: Jones, William J.
Publisher: name: Zenodo
Year: 2011
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Maxillopoda
Sessilia
Pachylasmatidae
Atetrapachylasma
Atetrapachylasma dijonesae

description: Atetrapachylasma dijonesae sp. nov.
Figures 4, 5, 10–15
Material examined. Vance B Seamount (summit 1932 m), MBARI ROV dive nos T1012 and T1013, 45 ° 30'N– 130° 40'W, 2081 m.
Deposition of types selected from dive no. T1012: Holotype, USNM cat. no. 1145902; first paratype, USNM cat. no. 1145903; second paratype SIO BIC cat. no. 11195; type lot, SIO BIC 11196.
Diagnosis. As for the genus.
Description. Female shells white, wall to 20 mm R-C diameter, 15 mm height. Narrow overlapping margins of R and CL resting on broad, largely exposed alae of CL and C, respectively (Fig. 11), kite-shaped operculum suspended snuggly in orifice with rostral ends of S fitting depression in t rostral sheath (Fig. 12). Occludent margin at carinal end of T with fine teeth formed by alternate growth lines interdigitating when plates closed. Opercular plates individually triangular, simple, without notable articular ridges or furrows, crests (apodemes) for depressor muscles or adductor muscle scars (Fig. 13).
Caudal appendages and penis absent. Labrum thick, crest nearly straight, supporting single row of fine, slightly separated, blunt teeth, flanked by stout mandibular palps (Fig. 15 E); mandible tridentoid, clothed with fine spines, inferior angle armed with rounded comb of short strong spines. First maxillae with stepwise cutting edge armed with strong and relatively weak spines; second maxillae with typical notch (Fig. 15 B, C). Cirri evenly deployed along thorax; first pair modified as maxillipeds, anterior ramus somewhat antenniform, posterior ramus nearly normal in resembling those of posterior pairs; second pair more closely resembling the following than the proceeding pair, rami generally subequal in length; intermediate articles of cirrus VI twice as high as wide, setation ctenopod, each article supporting one pair of strong and one pair of weak setae (Fig. 15 D). Cirral counts for two specimens are given below:
Dwarf males located on external surface of S of female, as many as three per side (Figs 11, 14). Shell, R-C diameter approximately 1 mm, consisting of three parts; 1) opercular plates (S and T) composed of chitinous primordial valves supplemented 2) by largely underlying calcareous plates, and 3) two concentric rings, the juvenile chitinous wall of four plates above concrescent, calcareous, columnar, four-plated wall, leaning to varying degrees toward aperture of female (Fig. 14 A, B).
Etymology. Named for Diana S. Jones, in appreciation of her many works on thoracic Cirripedia, especially Jones (2000).
Affinities. The pachylasmatines constitute a diverse subfamily all members of which, excepting Atetrapachylasma and arguably Tetrapachylasma s.s., display vestiges of an eight-plated ancestor; R-RL-CL1-CL2-C (cf. Jones 2000). Ignoring the exception, the number of wall plates of the 23 species representing five genera are variously reduced; reduction being ontogenetic as well as phylogenetic. Reduction is accomplished by partial to near total concrescence of the rostrolaterals with the rostrum, resulting in six-plated forms (RRL-CL1-CL2-C). Concrescences of the carinolaterals, in some cases simply by elimination of CL2 as suggested by Foster (1988), follows, reducing the number of plates to four. To put it another way, these configurations generally progress from visible vestigial sutures of a tripartite rostrum (RRL) to a compound rostrum (RRL), followed in close order by the carinolaterals (CL1/CL2 to CL1CL2). In such an ontogenetic series, last vestiges of the sutures are best seen on the interior, and to a lesser extent exteriorly, on the older, apical portions of the plates involved.
Five four-plated species were assigned to Tetrapachylasma Foster, 1988 by Jones (2000), but of them only in T. trigonum Foster, 1988 are the compound plates, RLR and CL1CL2, believed, at least by Foster, to be passed through during ontogeny without traces of the fused sutures. However, T. trigonum is known from but three mature specimens, measuring between 8 and 10 mm in R-C, diameter. Jones (2000) argues that, since traces of the sutures are seen in the ontogeny of the other four species and Foster did not have an ontogenetic series to judge by, one cannot be certain they are not also passed through in the ontogeny of T. trigonum. The very small males of T. trigonum, measuring half a millimeter or so in R-C diameter, while perhaps four-plated, do not calcify in the carinolateral region, and so one cannot be sure about them either.
On the other hand, the sample of the new species consists of numerous loose shells and a half dozen or so complete specimens having R-C diameters ranging from ~ 4 to 15 mm, and no traces of vestigial sutures are seen among them. Furthermore, the males, a millimeter or so in R-C diameter, are clearly without vestigial sutures in the early uncalcified as well as the later calcified parts of the wall and, therefore, they too appear to be but four-plated. However, while this indicates that T. trigonum could also have been four-plated at the onset, so different are its opercular plates that it does not seem likely it is allied to the new species. Therefore, the genus may not be a monophyletic taxon. This is heightened by the fact that the same four-plated configuration arose independently in the 1) bathylasmatines (Tetrachaelasma), 2) chthamalines (Tetrachthamalus), 3) elminiines (Elminius, Austrominiu s, etc. (Buckeridge and Newman 2010), 4) most Tetraclitidae (Epopella, Tesseropora, Tetraclitella, Tetraclita, Astroclita, Newmanella and Yamaguchiella), balanoids, including independently the 5) balanids Tetrabalanus and Zulloa, 6) pyrgomatids or coral barnacles (cf. Darwin 1854, Newman 1967, Ross and Newman 1996), and the 7) bryozobiines (Van Syoc and Newman 2010). There are other four-plated configurations, such as R-RLCL-C in Chamaesipho; a configuration arrived at by yet another route. Thus, monophyly of at least Tetrapachylasma if not the pachylasmatines in general is anything but certain.
Four of the five species of Tetrapachylasma are known to have a calcareous basis, albeit it very thin, as it is centrally in at least two, and Jones (2000) infers that the fifth has one too. Unfortunately, its nature (membranous or calcareous) in Atetrapachylasma dijonesae sp. nov. could not be directly ascertained, but the videos taken during capture (cf. Fig. 5) show that, in the unlikely event one is present, it is very thin. Caudal appendages in the genus range from as many as 18 segments in T. arcuatum to as few as one in T. trigonum, but none were found in the new form. The opercular plates of the previously described species of Tetrapachylasma, while relatively generalized as balanomorphs go, have longitudinally striate scuta (a diagnostic feature of the genus), and terga with an incipient spur with an open furrow and strong depressor muscle crests. The operculum of the new form is macroscopically undistinguished, other than for the strong external and internal occludent ridge of the scuta. Furthermore, the tergum resembles that of the brachylepadomorph, Neobrachylepas relica Newman and Yamaguchi, 1995, in its simplicity; that is, in having a straight basal margin (no indication of a spur, incipient or otherwise) and no tergal depressor muscle crests (apodemes), which is also the case in the otherwise more generalized balanomorphs, Chionelasmus and Waikalasma. While the scuta are not crossed by the external longitudinal striae seen in Tetrapachylasma in general, the growth lines alternate to form fine interdigitating teeth along their occludent margins and the external occludent ridge appears to be a unique apomorphy. The simple growth lines of the terga also interdigitate along their occludent margins and this may be a unique apomorphy. Otherwise, the simplicity of the opercular parts of the new form, especially the terga, suggest they have either retrogressed from those of the other species of Tetrapachylasma, its extant allies, or from some extinct pachylasmatine ancestor, or they are simply the least modified of any known pachylasmatoid, or of any balanomorphs for that matter. Whichever, taken a face value, Atetrapachylasma is a unique barnacle, and it is recommended it be placed incertae sedis in the Pachylasmatinae.
description: Published as part of Newman, William A. & Jones, William J., 2011, Two Northeast Pacific deep-water barnacle populations (Cirripedia: Calanticidae and Pachylasmatidae) from seamounts of the Juan de Fuca Ridge; " insular " endemics stemming from Tethys, or by subsequent dispersal from the Western Pacific center of distribution?, pp. 49-68 in Zootaxa 2789 on pages 60-65, DOI: 10.5281/zenodo.202501