Netastoma darwinii (G. B. Sowerby II, 1849)

Delfino, Marina & Signorelli, Javier H., 2021, Fig. 13 in Fig. 12. Majella pristis n in Periclimenaeus djiboutensis Bruce 1970, Zoological Studies 60 (28), pp. 1-20 : 11-12

Netastoma darwinii (G. B. Sowerby II, 1849) ( Fig. 6 )

Pholas darwinii G.B. Sowerby II, 1849: 490 , pl. 107, figs. 76–77.

Pholas macrostoma Philippi, 1858: 23 .

Netastoma darwinii – Bernard 1983: 61; Pastorino 1995: 17, pl. 8, fig. 36; Scarabino 2003: 242; Rios 2009: 594, fig. 1637; Huber 2010: 479; Dias Passos and Magalhães 2011: 148; Scarabino et al. 2016: 7.

Pholadidea (Nettastomella) darwini – Dall 1909: 276; Carcelles 1950: 82, pl. 5, fig. 93; Carcelles and Williamson 1951: 348.

Pholadidea darwini – Lamy 1926: 155; Castellanos 1970: 259, pl. 25, fig. 5.

Nettastomella darwinii – Turner 1955: 142, pl. 85, figs. 1–3, pl. 86, figs. 1–4; Rios 1966: 37; Klappenbach 1967: 167; Scarabino 1977: 215; Aldea and Valdovinos 2005: 379, 396, fig. 11J.

Ty p e m a t e r i a l: P h o l a s d a r w i n i i: NHMUK unnumbered, holotype, articulated specimen without soft parts; Pholas macrostoma : not found.

Type locality: Chiloé Island, Chile, collected by Charles Darwin.

Description: Shell: white fragile, equivalve, inequilateral and triangular; anterior end open and back closed; shell length up to 38 mm; umbo prosogyrate, in some specimens the umbos are found displaced from each other; umbonal reflection located in the anterior third of the shell ( Fig. 6A–D ); dorsal and ventral margin straight; posterior margin extended outward, forming a calcareous siphonoplax ( Fig. 6G ); external surface white with differentiated concentric ornamentation in three zones ( Fig. 6E–F ): zone 1 with continuous marked growth lines without crests or radial ribs (z1), zone 2 with growth lines also marked, but with weak radial ribs formed by small crests (z2), zone 3 with marked growth lines without crests or radial ribs (z3); mesoplax small and rudimentary; protoplax (pt) not calcareous, present on the dorso-anterior margin covering the adductor muscle anterior ( Fig. 6G–H ); periostracum not observed; inner surface white; pallial sinus wide, although not very visible; adductor muscle scar poorly visible; hinge plate rudimentary with a single cardinal tooth in each valve ( Fig. 6C–D ).

Mantle cavity organs: mantle only exposed at the pedal gape ( Fig. 6I ); siphons of equal size, totally fused, with a row of simple tentacles surrounding both apertures; incurrent siphon with a single row of tentacles; excurrent siphon without tentacle ( Fig. 6JL ); two pairs of gills arranged along the antero-posterior axis; inner demibranchs wider than outer ( Fig. 6M ); food groove observed over the distal margin of both demibranchs; two pairs of labial palps longer than wide, triangular in shape, with external surface smooth and internal surfaces with plicate folds ( Fig. 6M ); trimyarian with a ventral adductor muscle ( Fig. 6O ).

Material examined: Argentina ─ Mar del Plata ( MACN 11974), Bahía Blanca ( MACN 24482), Punta Norte ( MACN 11509), San José gulf ( MACN 9175-25), Puerto Madryn ( MACN 9172-30), Punta Este ( CNP-INV 2901), Punta Ninfas ( CNP-INV 2898, 2899, 2900, 2903), Puerto Deseado ( MACN 9237), Puerto San Julián, Playa La Mina ( CNP-INV 2902, 2904, 2905, 2906, 2907).

Distribution: From southern Chile to Rio Grande do Sul, Brazil ( Kennedy 1974; Scarabino 2003; Aldea and Valdovinos 2005; Ríos 2009; Scarabino et al. 2016). Specimens from Mar del Plata, Buenos Aires province, to Puerto San Julián, Santa Cruz province were examined in this work.

Remarks: The specimens of N. darwinii were mostly sampled on the vertical walls of soft rocks at low tide. The species was widely reported along the western Atlantic ocean ( Rios 2009; Scarabino et al. 2016; Zelaya 2016, and literature therein). The type material of Pholas macrostoma described by Philippi (1858) from Chonos Archipelago was not found. Nevertheless, the author reported characters compatible with N. darwinii such as an oval to cuneiform shell, with two sculptured areas, the anterior one with radial ribs and concentric lamella, and the posterior one with only concentric ornamentation. This synonymy was also suggested by Coan and Valentich-Scott (2012). Netastoma darwinii differs from the Eastern Pacific Netastoma rostratum and Japanese Netastoma japonicum in having a larger shell, with a relatively smoother posterior end and callum, and with a more diverging siphonoplax.

Figures

Fig. 6. Netastoma darwinii. A−D, NHMUK without numeration, holotype. E−F, external sculpture showing the three differentiated zones. G, detail of calcareous siphonoplax. H, detail of protoplax. I, pedal gape. J−L, general view of the siphons. M−O, general aspect of visceral mass. Scale bars: A−F, J = 5 mm; G−I = 2.5 mm; K−L = 1 µm; MN = 2 mm; O = 4 mm.