Hiatella barnea ( Ihering, 1907 ) Zelaya and Marina Güller, 2023

Zelaya, Diego G. & Güller, Marina, 2023, Decrypting the feared genus Hiatella (Bivalvia): South American species, Zoological Journal of the Linnean Society 199 (4), pp. 882-905 : 896-900

Hiatella barnea ( Ihering, 1907) comb. nov.

( Figs 3P , 6E, F, I, L , 7H , 11–13 )

Saxicava carnea (sic) Ihering, 1907: 105 (nomen nudum; listed only).

Saxicava barnea Ihering, 1907: 325 , pl. 14, fig. 96; Ihering 1914: 63.

Hiatella solida : Carcelles 1953: 222–223, pl. 5, fig. 111; Aldea and Valdovinos 2005: fig. 11I (not Sowerby I, 1834).

Barnea saxicava Parodiz, 1996: 267.

Type locality: Camarones [44°48 ʹ S, 65°42 ʹ W, Chubut Province, Argentina] .

Material examined: Holotype (MACN-Pi N 510); and 37 additional lots (Supporting Information, ESM1 ).

Other published records: 37°02 ʹ 55″ S, 73°30 ʹ 41″ W, Isla Santa María ( Aldea and Valdovinos 2005: as H. solida ) .

Description: Shell ≤ 37.5 mm long, narrowly elongate, very inflated, with the maximum width at about the anterior third of total shell length; moderately solid ( Fig. 12 ). Right valve somewhat larger and ventrally overlapping over less valve. Anterior and posterior ends projected. Posterior area of shell narrow, flat, ill defined. Anterior area of shell with a wide, deep sulcus, radiating from the umbo to the ventral margin. Ventral gape very wide throughout ontogeny; posterior gape small. Anterodorsal margin short, sloping straight. Anterior margin short and rounded. Ventral margin deeply sinuated anteriorly at the gape, rounded to flat posteriorly. Posterior margin widely arched to flaưened, higher than anterior margin. Posterodorsal margin horizontal, straight to slightly convex, longer than the anterodorsal margin. Umbo low, wide, evenly rounded; located at about the anterior third of total shell length. Lunule short, wide, sunken.

Outer shell surface white, chalky ( Figs 11 , 12 ). Dissoconch sculptured with strong, regularly distributed commarginal elements, separated by wide interspaces. Commarginal sculpture consisting of low, wide cords near the umbo, gradually narrowing and increasing in height, originating lamellae ventrally. Lamellae deeply sinuated at anterior sulcus, where they also bear short but stout, triangular, ventrally projected scales. In specimens ≤ 5 mm L, two extremely thin radial ribs bearing small, hollow spines extend posteriorly: one of them running from the prodissoconch–dissoconch limit to the junction of posterior and ventral margins, the second to the junction of dorsal and posterior margins. In larger specimens, ribs lack spines distally, and early secreted spines (in the vicinity of the umbo) appear reduced to their bases, appearing as tubercles; the radial ribs are recognizable only by an abrupt change in the orientation of commarginal sculpture. Periostracum thick, yellowish straw coloured to brownish, forming thin periostracal folds.

Hinge ( Fig. 13 ): Small-sized specimens (≤ 8 mm long) with a delicate, small cardinal tooth at each valve, and the corresponding depression to accommodate the tooth of the opposite valve. Less valve cardinal triangular; depression anterior to cardinal tooth. Right valve cardinal low, peg-like, ventrally directed; depression behind cardinal tooth. Teeth and depressions lost in specimens> 10 mm long, where valve articulation occurs only by means of the ligament. Nymph short but stout, flaring. Ligament opisthodetic. Pallial sinus deep. Inner shell surface porcellanaceous, white.

Anatomy ( Fig. 6E, F, I, L ): Mantle margin fused for most of its length, with a small (anterior) pedal opening and two smaller (posterior) siphonal openings. Suture behind pedal opening extremely short. Anterior and posterior adductor muscles large, ovoid in section, the posterior somewhat dorsally displaced.Gills low (~40% of shell height), elongated, with almost horizontal axis; composed of two complete demibranchs at each side; less and right demibranchs posteriorly fused to each other and to mantle margin. Outer demibranch gradually increasing in height backwards, reaching the maximum size at about two-thirds of its total length. At anterior end, outer demibranch one-third the height of inner demibranch. Ascending and descending lamellae of inner demibranch similar in height throughout their length. Outer demibranch rectangular, anteriorly with ascending lamella higher than descending lamella; similar in size posteriorly. Less and right inner demibranchs ventrally connected by ciliary junctions. Foot medium sized, composed of a compressed stalk and a markedly projected anterior ‘toe’. A narrow byssal groove, extending for ~60% of foot sole. Byssus composed of a few, short, flat byssal threads arising from a byssus seam; each byssal thread bearing a distal adhesive disc. Byssal gland remaining functional in adult specimens. Siphons large, fused at the base, distally separated, orange towards the tip in living specimens. Inhalant siphon slightly wider than exhalant siphon; both with numerous series of club-shaped papillae surrounding their openings. Inhalant siphon retractor muscle longer and stronger than exhalant siphon retractor muscle. Anterior and posterior labial palps triangular, similar in size, with ≤ 10 sorting ridges in an 11-mm-long specimen.

Distribution: Along the South American coast, from 37°02 ʹ 55″ S in the Pacific to 41°44 ʹ S in the Atlantic and Isla de los Estados ( Fig. 1C ). From 0 to 309 m depth.

Habitat: Associated with hard substrates.

Remarks: Until this study, and for> 100 years, H. barnea had remained known only from its original description, based on a single specimen, inferred to have been collected in Camarones ‘á la base de la formation patagonienne’ (Tertiary) ( Ihering 1907, 1914). The original description is extremely poor in detail and unsatisfactorily figured, and the species was never redescribed. In addition, the type material had not been located subsequently, a fact that has led authors to consider this taxon as a species of questionable status ( Parodiz 1996). To increase the confusion, Ihering (1907: 105) listed the species as Saxicava carnea (indubitably a lapsus calami), and Parodiz (1996: 297), in a revision of the taxa introduced by Ihering, referred to it as Barnea saxicava (instead of Saxicava barnea , the original binomen).

As part of this study, the holotype of Saxicava barnea was relocated in the palaeoinvertebrate collection of the MACN. It consists of an articulated specimen, with the right valve slightly broken dorsally. It is properly figured here, for the first time ( Fig. 11 ). This specimen is morphologically indistinguishable from present-day specimens collected at the Atlantic and Pacific coasts of Patagonia ( Figs 11 , 12 ). Whether H. barnea is a species occurring in Patagonia since the Tertiary or a species from the Present wrongly described from the Tertiary remains uncertain. The exact geological and stratigraphic provenance of the holotype is unknown. Curiously, a well-preserved system of ºuaternary beach, containing several bivalve species, was described from ‘Camarones’ by Zanchetta et al. (2012), although H. barnea was not listed therein. Further palaeontological studies would help to add some precision to the fossil record of this species.

Hiatella barnea is an easily recognizable species owing to its elongated and low shell outline, broad, rounded and depressed umbos, wide byssal embayment and prominent commarginal lamellae, which project ventrally into scales. Another distinctive character is the loss of hinge teeth (and the corresponding hinge teeth depressions) through the ontogeny. Anatomically, a peculiar characteristic of this species is the presence of similarly high ascending and descending lamellae of the inner demibranch. The small foot and size of the pedal aperture of this species resemble the characteristics found in H. umbonata , whereas the shape and position of the adductor muscles resemble the condition found in H. meridionalis .

Based on the material studied herein, H. barnea is not a rare species. In fact, it was collected and figured by Carcelles (1953: fig. 111) and Aldea and Valdovinos (2005: fig. 11I), although, in both cases, identified as ‘ H. solida ’. Examination of the above-mentioned lots (MACN-In 9032-3 and MZUC 29034, respectively) allows us to confirm that these specimens correspond to H. barnea .

Figures

Figure 1. Sites in Southern South America (SSA), Prince Edward Islands (PEI) and South Georgia (SG), from where material was studied. A, Hiatella meridionalis. B, Hiatella umbonata. C, Hiatella barnea. Colours represent geographical areas as follows:blue, South Georgia; green, Atlantic coast; light blue, Prince Edward Islands; lilac, Argentine continental margin; orange, Magellan Strait–Beagle Channel; purple, Burdwood Bank–Malvinas/Falkland Islands; red, Pacific coast; yellow, Isla de los Estados.

Figure 3. Early ontogeny shells of Hiatella meridionalis (A–L), Hiatella umbonata (M–O) and Hiatella barnea (P). A, C, Saxicava meridionalis: original illustrations (A) and photographs of the lectotype, NHMUK 1854.12.4.772 (C). B, F, G, I–K, specimens from Burdwood Bank: MACN-In 44166 (B), MACN-In 44179 (F), MACN-In 44187 (G), MACN-In 44192 (I), MACN-In 44172 (J) and MACN-In 44152 (K). D, Saxicava lebruni, syntype, MNHN-IM-2000-31626. E, specimen from Beagle Channel, MACN-In 10061. L, DNA voucher, H41. H, specimen from Fiordo Ponsonby, MACN-In 44282 (drawn indented lines show the aưachment of the mantle margin to the shell and siphonal muscle scars). M, specimens from Burdwood Bank, MACN-In 44270. N, specimen from Beagle Channel, MACN-In 44235. O, specimen from the Argentine Continental Margin, MACN-In 36596. P, specimens from the Pacific coast, MACN-In 44278. Scale bars: 10 mm in A–L; 2 mm in M–O; 5 mm in P.

Figure 6. Anatomy of Hiatella species:Hiatella meridionalis (A, B, G, J) from the Magellan Strait (MACN-In 12414), Hiatella umbonata (C, D, H, K) from Burdwood Bank (MACN-In 44270) and Hiatella barnea (E, F, I, L) from Chilean ḩords (MACN-In 44278). A, C, E, general view with right valve removed.B, D, F, right view with right mantle fold removed.G–I, right gill. J–L, foot. Abbreviations: aa, anterior adductor muscle; b, byssus; esr, exhalant siphon retractor muscle; f, foot; g, gill; go, gonad; h, heel; id, inner demibranch; isr, inhalant siphon retractor muscle; od, outer demibranch; pa, posterior adductor muscle; t, toe; vm, visceral mass. Scale bars:10 mm in A–F; 2 mm in G–L.

Figure 7. Living specimens of Hiatella: Hiatella umbonata (A–E) from Burdwood Bank (A, C) and the Argentine continental margin (B, D, E); Hiatella meridionalis (F, G) from Puerto Edén (F) and Magellan Strait (G); and Hiatella barnea (H) from ºuellón. Arrowheads (B, E) point to specimens of Hiatella.

Figure 11. Saxicava barnea. A, original illustration.B, original label. C–F, holotype, MACN-Pi N 510, in right (C), less (D), dorsal (E) and ventral (F) views.Scale bar:5 mm.

Figure 12. Shells of Hiatella barnea, external views. A, F, specimens from the Pacific coast, MACN-In 44278. B–E, I–K, specimens from the Atlantic coast: MACN-In 9175-19 (B, I); MACN-In 9156-3 (C); MACN-In 9032-3 (D, K); MACN-In 9237-8 (E), MACN-In 32620 (J). G, H, DNA vouchers:H43 (G) and H42 (H). Scale bars: 10 mm in A–J; 1 mm in K.

Figure 13. Shells of Hiatella barnea, inner and ventral views and hinge teeth. A, specimen from the Pacific coast, MACN-In 44278. B, D, F, G, specimens from the Atlantic coast: MACN-In 9175-19 (B), MACN-In 9156-3 (D), MACN-In 9237-8 (F) and MACN-In 44274 (G). C, DNA voucher, H42 (drawn indented lines show the aưachment of the mantle margin to the shell and siphonal muscle scars). E, specimen from Isla de los Estados, MLP-Ma 4942. Scale bars: 5 mm in A–D, F; 500 µm in E; 2 mm in G.