Verrucomorpha, Pilsbry, 1916

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 : 81-82

Origin and diversification of the Verrucomorpha

Until recently, the affinities of the verrucomorphs have been puzzling. This was in good part due to the fact that while sessile, they appeared to share as many characters with the scalpellomorphs as with the balanomorphs. Darwin was the first to study their anatomy in detail and work out the homologies of their plates, and in his first volume on the fossil barnacles he notes (1851: 5), Verruca “[…] though hitherto included amongst the Sessile Cirripedes, must, when its whole organization is taken into consideration, be ranked in a distinct family of equal value with the Balanidae [balanomorphs] and Lepadidae [scalpellomorphs], but perhaps more nearly related to the latter than to the Sessile Cirripedes.” In his second volume on the fossil barnacles [1855 (1854): 41], he again notes that “Upon the whole, the Verrucidae are nearly equally related to the Lepadidae and Balanidae ; but certainly nearer to the Lepadidae , than to the sub-family Balaninae or typical sessile cirripedes...” But he then goes on to write, “[…] if compelled to place Verruca in one of these two families, I should place it amongst the Chthamalinae , the other sub-family of the Balaninae ”, and on the same page he notes this was written after his Ray Society monograph on the living sessile barnacles (1854: 495) where he gives a more detailed version of the same thing. So, while the verrucomorphs shared characters more or less equally between the scalpellomorphs and balanomorphs, Darwin considered them not just early sessile barnacles but, if compelled to place them within the existing classification, he would include them with the primitive balanomorphs. However, when it came to his classification, Darwin abandoned the Pedunculata and Sessilia as formal taxa, and subsequent workers ( Pilsbry 1916; Withers 1928; Krüger 1940) followed suit. Consequently the sessile barnacles, Verrucomorpha and Balanomorpha , were considered to have sprung independently from the scalpellomorphs ( Newman et al. 1969: R266, fig. 133; Ghiselin & Jaffe 1973: fig. 1; Newman & Ross 1976: fig. 2).

In the meantime, however, a third group of sessile barnacles, the presumably extinct Brachylepadomorpha, was discovered ( Woodward 1901, 1906), which Woodward considered central to the origin of the sessile barnacles. However, largely due to the authority of Pilsbry (1916: 14), who considered the brachylepadomorphs at least pedunculate barnacles if not simply scalpellomorphs, they came generally accepted as an independent sessile group whereby the sessile barnacles were at least triphyletic (cf. Withers 1928; Krüger 1940; Newman et al. 1969; Newman & Ross 1976; Newman 1982). However, some authors argued that the brachylepadomorphs were better candidates than the scalpellomorphs for the ancestors of the balanomorphs ( Withers 1912; Newman 1987), and if so the sessile barnacles would be diphyletic rather than triphyletic. Furthermore, it was noted the brachylepadomorphs were also better candidates for the origin of the verrucomorphs, and if so, the sessile barnacles would be monophyletic ( Newman 1987: 8, 19 & 33).

Compelling evidence for a brachylepadomorph origin of the verrucomorphs appeared with the astonishing discovery of Neoverruca ( Newman & Hessler 1989) , whereby monophyly for the sessile barnacles became the favored hypothesis ( Newman & Hessler 1989; Yamaguchi & Newman 1990; Buckeridge & Newman 1992; Glenner et al. 1995; Newman 1996). When viewed from one side, this barnacle has the same arrangement of opercular, wall and imbricating plates as a brachylepadomorph. The opposite side was peculiar and if there had been but one specimen available it easily could have been considered an imperfectly developed brachylepadomorph, due to crowding, predation damage or the like. But there were numerous specimens each with an “anomalous” right or left side. The basis for the anomaly was the tergum and scutum of that side being immovably integrated into the wall, as in verrucomorphs. Thus, a better “missing link” between the brachylepadomorphs and verrucomorphs could hardly have been imagined.

Imbricaverruca , while wholly a neoverrucid in organization, differs from Neoverruca in outwardly looking more like a verrucid than a brachylepadomorph ( Fig. 1 ). Thus the question arises as to whether it represents a grade between Neoverruca and the proverrucids. The evidence from both juvenile and adult morphology suggests not. In short, Imbricaverruca has capitalized on the neoverrucid plan rather than making the reductions and acquiring proportions in the direction of those found in modern forms ( Fig. 4 ). Thus the Neoverruca and Imbricaverruca plans are markedly divergent, the former’s being better suited to the derivation of the proverrucid/verrucid plan ( Table 3 ).

In Table 3 , character 1 concerns the existence of fully pedunculate juveniles during the ontogeny of Neoverruca missing in the ontogeny of verrucids, possibly in proverrucids, and in balanomorphs ( Newman 1989). Considering the retention of well-developed imbricating plates and the median latus in Imbricaverruca , it too likely has several pedunculate stages in its ontogeny. If so, like Neoverruca , it is not becoming specialized like verrucids as far as elimination of its pedunculate stages is concerned. Furthermore, it will be noted (character 2), the carina is much wider than high in the presumed juvenile as well as the adult of Imbricaverruca , and together with the reduced rostrum and fixed scutum and tergum, it forms a ring reinforcing the uppermost imbricating plates rather than forming a major part of the wall. A reduction in the extent of the primary wall in Imbricaverruca is an autapomorphy leading away from rather than towards verrucids. Character 3 concerns the median latus, on the verge of being lost in Neoverruca but very well developed in Imbricaverruca . It is apparently completely lost in proverrucids, as in verrucids, so Neoverruca rather than Imbricaverruca is also closer to them in this regard. Character 4 involves the relative importance of the imbricating plates; their reduction and deciduous nature in Neoverruca in contrast to their further development and importance in Imbricaverruca . Character 5 pertains to the relative proportions of the fixed terga and scuta (also note their degree of integration with the rostra and scuta in forming the wall under character 6. In being wider than high, and in their reduced contribution to the wall, those of Imbricaverruca differ conspicuously from the relatively unmodified ones of Neoverruca and the squarely proportioned ones of Verruca . Character 6 pertains to the concomitant changes in the rostrum and carina seen in character 5; e.g. reduction in height and relative importance to the wall which distinguishes Imbricaverruca from Neoverruca and Verruca . It follows, despite Imbricaverruca facies similarity with higher verrucomorphs, that Neoverruca comes much closer to their ancestral organization plan than does 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. 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).