Atetrapachylasma dijonesae, Newman, William A. & Jones, William J., 2011

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?, Zootaxa 2789, pp. 49-68 : 60-65

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 .

Figures

FIGURE 4. Numerous females of Atetrapachylasma dijonesae gen. et sp. nov. in situ on pillow basalt from 2080 m of depth on Vance B Seamount, with nozzle of suction sampler on ROV Tiburon to their right (distance between red laser dots 30 cm). The pristine white appearance of the dead shells accumulating at the base of the slope, as well as of the living individuals, suggests that there is little if any hydrothermal activity nearby to stain them (rather than the barnacle just being short lived, which they are). Thus, as might be expected, dissection revealed feeding appendages (Fig. 13) of the deep-sea rather than hydrothermal type. N. B.: Inspection of MPEGs taken by ROV Tiburon during collection of most of the barnacles seen in this frame (also, Fig. 5) detected no loss of material in the process. Therefore, it can be said with confidence that the shell fragments at the base of the basalt below the barnacles represent natural, postmortem accumulations.

FIGURE 5. Atetrapachylasma dijonesae gen. et sp. nov. in situ on pillow basalt from 2080 m of depth Vance B Seamount, with nozzle of suction sampler of ROV Tiburon largely to their right. A, the remains of several walls broken away by the nozzle can be seen to the right of “ 1 ” leaving membranous bases exposed. B, the barnacles seen between “ 2 ” and “ 3 ” and to the right of “ 4 ” in A have been collected. N. B.: Inspection of MPEGs taken by ROV Tiburon during collection of the barnacles seen removed in these frames detected no loss of material in the process. Therefore, it can be said with confidence that the shell fragments at the base of the basalt below the barnacles (cf. Fig. 4) represent natural postmortem accumulations.

FIGURE 10. Calantica moskalevi Zevina and Galkin, 1989; appendages, first topotype: A, portion of crest of labrum with left palp attached; B, mandible; C, first maxillae; D, second maxilla; E, intermediate articles of cirrus VI; and F, penis still attached to the coxal portion of pedicle of the left sixth cirrus.

FIGURE 11. Atetrapachylasma dijonesae gen. et sp. nov. holotype: A, exterior view of female with males; note largely exposed alae and the pair of males on each side of the tight fitting occludent ridges of the scuta. B, interior view of the same female, note the tight fit of the bearing surfaces of the alae on the adjacent plates, and the close fit of the sutures between plates, especially between the opercular plates and the interior of the orifice.

FIGURE 12. Atetrapachylasma dijonesae gen. et sp. nov., exploded four-plated wall arranged from loose plates, type lot: A, exterior of a rostrum; B and D, left and right carinolaterals; C, a carina; and E, the interior of a portion of a rostrum. Note the variably pronounced depression in the sheath of rostrum (E), which, together with the edges of the carinolateral alae, receives the rostral ends of the occludent ridges of the scuta (Figs 9 A; 11 C, D).

FIGURE 13. Atetrapachylasma dijonesae gen. et sp. nov., opercular plates of female: A and B, interior and exterior of terga, and C and D, interior and exterior of scuta. Both the terga and scuta are isosceles triangles, the bases of which are formed by their occludent margins. Note that the weakly developed articular ridges and furrows, as well as the lack of adductor and depressor muscle crests (apodemes), pits or scars, are remarkable. Comparably simple opercular plates are seen in no other balanomorph.

FIGURE 14. Atetrapachylasma dijonesae gen. et sp. nov., dwarf males: A, viewed obliquely (R-C axis running between 10: 30 and 4: 30 o’clock); B, another individual, viewed from the carinal end, leaning markedly to its right, toward the aperture of the female (see Fig. 9 for four males in situ). Note that the chitinous primary wall is made up of four plates, with the rostrum overlapping the carinolaterals. There is no indication of sutures in the rostrum or the carinolaterals of the male or the female.

FIGURE 15. Atetrapachylasma dijonesae gen. et sp. nov., appendages: A, mandible; B, first maxillae; C, second maxilla; D, intermediate articles of cirrus VI; and E, crest of labrum flanked by left palp.