Pyrgoma cancellatum Leach, 1818

Ross, A. & Newman, William A., 2002, A review of the Pyrgoma cancellatum species complex (Cirripedia: Pyrgomatidae), Journal of Natural History 36 (4), pp. 407-421 : 409-413

Pyrgoma cancellatum Leach, 1818

(®gures 2, 3, 5)

Pyrgoma cancellata Leach, 1818: 171 , pl. 57; Leach, 1824: 171, pl. 57; Lamarck, 1818: 401; Jay, 1839: 7; Delessert, 1841, pl. 1, ®gure 17; Lamy and Andre, 1932: 224; Ross and Newman, 1973: 156, in part, not ®gures 13, 14; Newman and Ross, 1976: 58 (references); Foster, 1982: 225, ®gure 7L, pl. 2f ( Hong Kong); Ren, 1986: 150, in part, ref. to pl. 5, ®gures 11, 14, 15, 18, 19 (South China Sea); Healy and Anderson, 1990: 12, ®gure 8j±m (sperm ultrastructure); Anderson, 1992: 309, ®gures 22, 23, 37e, 38e (cirral activity, Darwin, Australia).

C. [reusia] cancellata: Blainville, 1824: 378 ; Blainville, 1825: 599, pl. 85, ®gure 7±7b; Blainville, 1827, pl. 85, ®gure 7±7b.

Pyrgoma cancellatum: Darwin, 1854: 362 , in part, pl. 12, ®gure 5b±f (`West Indies’); Gruvel, 1905: 303, ®gure 326; Nilsson-Cantell, 1938: 67, ®gure 25, pl. 3, ®gures 2, 3 (Mergui Archipelago); Holthuis, 1982: 316 (nomenclature); Ogawa et al., 1998: 4, ®gure 4 ( Mauritius); Ross and Newman, 2000b: (publication dates).

Creusia cancellata: Des Moulins, 1866: 309 .

Diagnosis. Wall with 28±36 cancellate ribs; articular furrow of scutum deep, narrow, limbus adductorum greater than half overall height; tergum exceptionally narrow, external furrow open, distal end of spur bluntly rounded, lacking depressor muscle crests.

Type locality and host. Shirahama , Honshu Island, Japan, 33ss40¾N, 135ss20¾E, on Turbinaria contorta Bernard ; by subsequent designation (Ross and Newman, 1973: 156) .

Type depository. Lectotype, Natural History Museum, London ( NHM) no. 1998.2836; paralectotypes, NHM no. 1998.2836.1-4.

Remarks. We recognize the original and most subsequent descriptions of this, and the following species described by Weltner (1897), which was not ®gured, were based solely on wall and opercular morphology. Also, recognizing there are several other species herein assigned to this genus, we have limited the two synonymies to those we believe reliably reēct the species in question.

There are diVerences in the external surface of the wall illustrated by Leach (1818; rostro-carinal diameter 8.7 mm), and that by Darwin (1854). Leach’s illustration (see ®gure 2) clearly shows cancellate or chevron-shaped radiating ridges, whereas that of Darwin does not. As it turns out, the specimen Darwin illustrated (pl. 12, ®gure 5a; see ®gure 4B herein) actually represents Pyrgoma lobata here assigned to a new genus. The cancellate ridges are unique to P. cancellatum , despite minor variations in form, but it is noteworthy they are largely produced by the coral, not the barnacle!

FIG. 1. Schematic longitudinal section showing the relationships of two types of pyrgomatids to their coral hosts (left and right halves of the illustration respectively); (A) typical pyrgomatid in which chemical mediation at the suture between the wall and basis (1¾) inhibits adjacent skeletal growth of the coral, thus preventing the suture from becoming sealed. At intervals the calcareous margin of the basis (13) grows upward, more or less in synchrony with growth of the coral. This growth system helps prevent pyrgomatids inhabiting relatively fast-growing corals from becoming entombed. (B) Pyrgoma and Neopyrgoma n. gen., generally found on relatively slow-growing corals or portions thereof develop rows of regularly spaced perforations (2), which are eVectively remnants of the suture (1 ²) between the wall and basis left behind (2) as the basis periodically grows upward. It is by way of these tissue-®lled`perforations’ between calcareous arches of the basis (cf. ®gure 5B and Ross and Newman, 2000 a, ®gure 4) that chemical mediation can continue to occur. Its continuance is apparently necessary to prevent coral tissue from withdrawing from older portions of its skeleton and to stimulate skeletal growth of the coral around the barnacle in otherwise relatively slowgrowing corals. This allows the wall of the barnacle to remain at the general surface level of a slow-growing coral without becoming exposed; note, the general level of the coral is lower on the right than on the left. Key: ®nely stippled=barnacle tissue, black= barnacle shell, cells=coral tissue and coarsely stippled= blocks of coral

Although Darwin (1854: 8, 363) looked at and described the basis of an`. .. authentic specimen bearing the name in question’ he must not have seen the archlike structures forming the major portion of it. This is puzzling because, for example, he did note the presence of a perforated basis in Armatobalanus terebratus (Darwin; cf. Ross and Newman, 2000 a). We have had the opportunity to study several of the specimens Darwin examined and labelled as P. cancellatum , and even with low magni®cation these structures are readily detected.

In comparing the original illustrations of the opercular plates of this species by Leach (1818) with those of Darwin (1854), both of which presumably are based on the same specimens, it is obvious they diVer from those described by Weltner (1897), and subsequently discussed and illustrated by Hiro (1935, ®gure 5; see also Ross and Newman, 1973, ®gure 14). The descriptions and illustrations of specimens from Hong Kong by Foster (1982) compare favourably with certain specimens illustrated by Ren (1986) from the South China Sea, and by Anderson (1992, ®gure 22) from

skeleton with evacuated and tissue-®lled passages between them. (1) Wall/basis suture showing the tissues of the coral and barnacle are separated by a thin chitinous portion of the basis (1¾ and 1 ²) and in the perforations below it (2). (2) Successive series of perforations between calcareous arches formed in the basis as it grew upwards (cf. ®gure 5B and Ross and Newman, 2000 a, ®gure 4), extending from near the basal end to the apical margin of the basis, while the longitudinal calcareous ribs between tiers of arches coalesce with the coral skeleton. (3) Perforations adjacent to older, vacated portions of passageways may become sealed by the barnacle. (4) Simpli®ed cirral net of the barnacle seen from the rear. (5) Distended opercular ¯aps (apertural frill) of the barnacle, inferred to help prevent overgrowth of the ori®ce by the coral (see Anderson, 1992). (6) Opercular plates of right side of barnacle. (7) Wall of barnacle. (8) Coral skeleton growing on wall of barnacle. (9) Coral tissue showing that free passage of coral tissue may be interrupted by blocks of coral skeleton (14) between the suture (1¾) and the basis (13), in contrast to the open passageways in the Pyrgoma growth system (tissue-®lled cavity below 1 ²). (10) Prosoma and thorax of barnacle. (11) Mantle cavity of barnacle; egg lamellae and branchiae omitted. (12) Basal tissue of barnacle, includes ovaries. (13) Calcareous basis of barnacle. (14) Blocks of coral skeleton coalesced to basis.

Darwin , Northern Territory, Australia, and we regard these as likely conspeci®c with those of Leach. The terga of a specimen ®gured by Ogawa et al. (1998, ®gure 4) from Albion, Mauritius diVer somewhat in the height to width ratio (4.1 1) from those of the lectotype (2.76 1) perhaps indicating a distinct species .

One characteristic feature of this species is the porcellaneous appearance of the exterior surface of the scutum, which occludes the entire ori®ce (Anderson, 1992), but not the tergum. We have also examined Anderson’s (1992) specimens from Darwin, Australia as well as others from Indonesia and all have the same polished appearance. How this surface eVect is accomplished remains unknown.

Among the slabs available to us, on the reverse side of one, there is the notation:`This is very similar to a piece in the Royal Scottish Museum, Edinburgh in the Dufresne coll. labelled ``donne fr M. Leach’’ 13.5.59 J.P.H. [= J. P. Harding]’. The reverse side of another slab bears the notation` Pyrgoma sowerbii P. cancellatum’ in Darwin’s cursive handwriting.