Eucalathinae

Lee, Daphne E., Gregory, Murray R., Lüter, Carsten, Zezina, Olga N. & H, Jeffrey, 2008, Melvicalathis, a new brachiopod genus (Terebratulida: Chlidonophoridae) from deep sea volcanic substrates, and the biogeographic significance of the mid­ocean ridge system *, Zootaxa 1866, pp. 136-150 : 139-143

Subfamily Eucalathinae Muir­Wood, 1965

Melvicalathis Lee, Lüter & Zezina , new genus ( Figures 2–6)

Diagnosis. Costae broad, triangular in cross­section; foramen hypothyrid, beak very attrite; transverse band of loop medially directed towards dorsal valve; loop may be incomplete.

Etymology. The generic name is based on the name of the RV Melville which collected most of the specimens on which this investigation is based, and " calathis ", a suffix indicating close relationship with other brachiopod genera in the subfamily Eucalathinae .

Type species. Eucalathis macroctena Zezina, 1981

Type locality. Academik Kurchatov Station #261, southeast Pacific Ocean (30°24'S, 78°59'W) at a depth of 3870 m, collected by Sigsby Trawl, 7 October 1968 ( Zezina 1981: 159, fig. 20).

Material.

Holotype. Accession number XI­52­4 /2, living specimen (adult female, length 3.7 mm; width 3.8 mm).

Paratypes. Paratype 1, accession number XI­52­4 /1, collected with the holotype, and broken during preparation. Paratype 2, accession number XI­52­4 /3, Station #271, 17°42'S, 78°9'W, 2710 m, attached to basalt rock, 20 October 1968. The holotype and 2 paratypes are held in P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia.

Other material examined. Six specimens collected by RV Melville Station #116, Southeast Indian Ridge (48° 52.7' S, 106° 30.7 'E – 48° 52.4'S, 106° 29.6' 'E), 25 February, 1995; one specimen collected from RV Melville Station #138, Southeast Indian Ridge (50° 11.7' S, 112° 52.1'E – 50° 11.2' S, 112° 52.3'E), 4 March, 1995. These specimens are held in the reference collection, Department of Geology, University of Auckland.

One specimen collected live from Eltanin cruise 24, USC1775, 42° 01"S, 130° 02"W – 41°57"S, 130°17"W, southeast Pacific Ocean, at depths of 4831–4851 m, 14 August, 1966, collected by 10' Blake Trawl ( Foster 1989; figs 12, 13). This specimen is held in the United States National Museum, Washington DC.

Two specimens collected live from Sonne SO 144­3 cruise, Galapagos Spreading Centre, east Pacific (2°19.94'S, 84° 25.65W – 2°15.30'S 84° 26.18W), at depths of 2009–2381 m, on 17 November, 1999. These specimens are held in the Museum für Naturkunde der Humboldt­Universität, Berlin, Germany.

Description. ( Figs. 2–6) Very small (maximum length 4.9 mm), punctate, subtrigonal in outline, greatest width anterior to midvalve, gently biconvex, anterior commissure rectimarginate, ornament of broad costae, triangular in cross­section, 7–9 costae on each valve, ridges of costae smooth without tubercles ( Figs. 2 A, 2B; 3B; 6A, 6B). Interior rim of shell anterior slightly scalloped from interlocking costae and slightly tuberculate margin (fig. 2C, 2D). Hinge line narrow, foramen moderately large, hypothyrid, beak very attrite, pedicle collar present, teeth small, triangular; deltidial plates very small, narrow, triangular, disjunct (figs. 2D, 3C–3E). Cardinal process very small; short, low median septum extending ca. 0.25 dorsal valve length; triangular socket ridges narrow, high; crural processes short. Brachidial loop complete or incomplete, descending branches connected by transverse band in holotype and paratype from “Academik Kurchatov” station 261, but not in other specimens; where the transverse band is incomplete, descending branches may have distally serrated edges; unjoined descending branches may represent younger growth stages (figs. 2E; 3A; 4A, 4B). Lophophore tightly coiled, early plectolophous, with large lateral lobes, and a small, possibly underdeveloped central lobe; filaments very long (figs. 2C; 3F, 3G; 5A); highly spiculate, spicules in connective tissue of both lophophore arms; spicules tightly interwoven, forming a dense 3­dimensional structure (figs. 4C, 5B); pedicle short, branched or brush­like, posteriorly directed, attaching specimens closely and perpendicularly to rock substrates (figs. 2C; 5C, 5D). The holotype, an adult female, held 8– 10 eggs in each of 4 gonads (2 in the dorsal, and 2 in the ventral valve); the eggs were at different stages (from underdeveloped to ripe); about 30 eggs were ready for spawning on 7 October 1968. The shell microstructure consists of very flattened lamellar fibres (fig. 6C).

Shell structure. Zezina (2003: 267) noted that in the shell microstructure of deep­sea brachiopods the fibres are composed of very flattened (lamellar) fibres, whereas the fibres in related shallow­water species “have rounded or rhomboidal sections”. She attributed this to “a saving in building material in deepwater forms subject to high calcium carbonate solubility at depths below 2000 m ”. The fibres in Melvicalathis exhibit the lamellar structure described by Zezina. We speculate that there may be some advantage to the brachiopods from living on plagioclase­rich glassy basalts. Dissolution of the unstable glass may leach small quantities of Ca and O which may partly compensate for the difficulties involved in producing a calcite shell at great depths. Note that the Melville brachiopods live at depths between 3820–4900 m at 48–50o S in the Indian Ocean where the carbonate compensation critical depth is at about 3900 m between 50–60o S ( Demopoulos et al. 2003).

Remarks. The new genus can readily be distinguished from the other four genera in the subfamily Eucalathinae ( Lee et al. 2006). Melvicalathis possesses broad smooth costae that are triangular in cross­section (figs. 4D; 6A, 6B). These differ markedly from the costae of species of Eucalathis which possess beaded, tuberculate radial ribs (fig. 4E; 5E). The cardinal margin of the shell in Eucalathis is often extended into “ears”, thereby forming a straight line (fig. 5F). In Melvicalathis the hinge line is narrow, and slightly curved due to the lack of “ears” (fig. 2D). From Bathynanus , Melvicalathis differs in outline, larger size, shape of crura, and well­defined ribs that are triangular in cross section. Nanacalathis differs in its subpentagonal outline, conjunct deltidial plates, and beaded costae, while Notozyga has the typical beaded costae of the subfamily, and thick wide crural bases and crura.