Saxicava meriodionalis d’Orbigny, 1846: 521 Saxicava chilensis Hupé, 1854: 379 Saxicava antarctica Martens and Pfeffer 1886: 113–114 Saxicava lebruni Mabille and Rochebrune Saxicava mollis Mabille and Rochebrune Saxicava arctica var. antarctica Melvill and Standen 1907: 151 Saxicava antarctica Philippi, 1845 Saxicava subantarctica Preston, 1913: 223 Saxicava solida Carcelles 1944: 293–294 Carcelles 1950: 82 Hiatella meridionalis Zelaya 2005: 117 Hiatella Decrypting the feared genus Hiatella (Bivalvia): South American species Zelaya, Diego G. Güller, Marina Zoological Journal of the Linnean Society 2023 2023-10-12 199 4 882 905 3LHVD (d'Orbigny, 1846) d'Orbigny 1846 [234,642,1879,1906] Bivalvia Hiatellidae Hiatella Animalia Adapedonta 4 886 Mollusca species meridionalis   ( Figs 3A–L, 4, 5, 6A, B, G, J, 7F, G)    Saxicava meriodionalisd’Orbigny, 1846: 521, pl. 81, figs 21, 22.     Saxicava chilensis Hupé, 1854: 379; Hupé 1858: pl. 8, figs 7, 7a; Rochebrune and Mabille 1889: H101.    Saxicava antarctica:  Martens and Pfeffer 1886: 113–114, pl. 4, fig. 2 (not Philippi, 1845).   Saxicava lebruniMabille and Rochebrunein Rochebrune and Mabille, 1889: H101–H102.   Saxicava mollisMabille and Rochebrunein Rochebrune and Mabille, 1889: H102–H103.    Saxicava arcticavar. antarctica:  Melvill and Standen 1907: 151(in part)(not  Saxicava antarctica Philippi, 1845).     Saxicava subantarctica Preston, 1913: 223, fig. 12.    Saxicava solida:  Carcelles 1944: 293–294, pl. 13, fig. 107;  Carcelles 1950: 82, pl. 5, fig. 91 (not Sowerby I, 1834).    Hiatella meridionalis:  Zelaya 2005: 117; Simone and Penchaszadeh 2008: fig. 18 (only).   HiatellaA: Laakkonen et al.2015: 389.    Typelocalities: Îles Malouines(= Malvinas/ Falkland Islands) (  Saxicava meridionalis); Calbuco, [ 41°46 ʹS], Chile(  Saxicava chilensis); Baie Orange [= Bahia Orange] (  Saxicava lebruni;  Saxicava mollis); [ Malvinas]/ Falkland Islands(  Saxicava subantarctica).   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 4.Shells of  Hiatella meridionalis, external views. A,  Saxicava subantarctica, lectotype, RBINS MT753. B,  Saxicava mollis, syntype, MNHN-IM-2000-31625. C, specimen from Burdwood Bank, MACN-In 44157. D, G–I, DNA vouchers: H37 (D), H39 (G), H40 (H, I). E,  Saxicava lebruni, syntype, MNHN-IM-2000-31626. F,  Saxicava chilensis, original illustration. Scale bars: 10 mm in A–E; 20 mm in F–I.  Material examined: Lectotypeof  Saxicava meridionalis( NHMUK1854.12.4.772); two syntypesof  Saxicava lebruni(MNHN-IM-2000-31626); one syntypeof  Saxicava mollis(MNHN-IM-2000-31625); lectotypeof  Saxicava subantarctica(designated herein: RBINS MT753); and 94 additional lots (Supporting Information, ESM1).   Description:Shell ≤ 61.0 mm L, rectangular to ovate, longer than high, compressed to slightly inflated, with the maximum width at about the posterior third of shell length; thin to moderately solid ( Figs 3A–L, 4). Subequivalve in smaller specimens, slightly to markedly inequivalve in larger specimens, with one of the valves (either the right or the less) wider and larger than the other; larger valve overlapping ventrally over the opposite valve. Posterior end of shell greatly projected; anterior end moderately projected. Posterior area of shell narrow, usually flat, sometimes slightly sunken, ill defined. Posterior and ventral gaps between valves narrow. Anterodorsal margin straight, steeply sloping. Anterior margin extremely short, rounded, sometimes indistinct from anterodorsal margin. Ventral margin straight to slightly sinuous. Posterior margin rounded to obliquely flaưened, usually higher than anterior margin. Posterodorsal margin long, convex to straight. Umbo low, wide, evenly rounded; located at anterior third of shell length. Lunule narrow, depressed in smaller specimens, usually indistinct in larger specimens.   Figure 5.Shells of  Hiatella meridionalis, inner views and hinge teeth. A, specimen from Burdwood Bank, MACN-In 44157. B,  Saxicava subantarctica, lectotype, RBINS MT753. C,  Saxicava chilensis, original illustration. D, H, specimen from Fiordo Ponsonby, MACN-In 44282. E–G, DNA vouchers: H38 (E) and H40 (F, G). Scale bars: 10 mm in A–C, F, G; 2 mm in D; 5 mm in E; 1 mm in H.   Figure 6.Anatomy of  Hiatellaspecies:  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. Outer shell surface white, chalky ( Figs 3C–L, 4). Dissoconch sculptured with low, narrow, commarginal folds near the umbo, gradually increasing in width, forming cords ventrally. In addition, two thin, low and rounded posterior radial ribs, present, similar to each other in solidness. In specimens of ≤ 16 mmL, radial ribs extending all along dissoconch and bearing hollow spines, which increase in size distally. In larger specimens, radial ribs become less evident, lacking spines distally, and early secreted spines, in the vicinity of the umbo, reduced to their bases. Periostracum thick, pale yellowish to yellowish brown; forming thin periostracal folds; usually lost in older parts of the shell.  Hinge ( Fig. 5):With a single, small cardinal tooth at each valve and a socket to accommodate the tooth of the opposite valve. Right valve cardinal high, thin to thick, sharply pointed, ventrally directed. Less valve cardinal elongate, posteroventrally directed, usually narrow, but sometimes strong in larger specimens. Less valve socket anterior to cardinal tooth; right valve socket posterior to cardinal tooth. Teeth present throughout shell ontogeny. Nymph elongate, narrow, delicate. Ligament strong, external, opisthodetic. Pallial sinus deep. Inner shell surface porcellanaceous, white.  Anatomy ( Fig. 6A, B, G, J):Mantle margin fused for most of its length, with a relatively large (anterior) pedal opening and two smaller (posterior) siphonal openings. Suture behind pedal opening extremely short. Anterior and posterior adductor muscles large, the anterior ovoid, the posterior subcircular, somewhat dorsally displaced. Gills low (~40% of shell height), greatly elongate, with the axis almost horizontal; composed of two complete demibranchs at each side; less and right demibranchs posteriorly fused among them and to mantle margin. Outer demibranch uniform in height all along its length. At anterior end, outer demibranch one-third the height of inner demibranch. At anterior half of the gill, ascending lamella of outer demibranch higher than descending lamella, and descending lamella of inner demibranch higher than ascending lamella. At posterior half of the gill inner and outer demibranchs, and ascending and descending lamellae of each demibranch similar in height. Less and right inner demibranchs ventrally connected by ciliary junctions. Foot relatively large, composed of a compressed stalk, a small heel and a markedly projected anterior ‘toe’; base narrow, flat, with byssal groove extending for ~75% of foot sole. Byssus composed of numerous long, narrow, flat byssus threads, arising from a stout byssus seam; each byssus thread bearing a distal adhesive disc. Byssal gland remains functional in adult specimens. Siphons large, fused at the base, distally separated, reddish brown in all their extensions in living specimens. Inhalant siphon slightly wider than the exhalant; both siphons with numerous series of club-shaped papillae surrounding their openings ( Fig. 7G). Inhalant siphon retractor muscle longer and stronger than exhalant siphon retractor muscle. Anterior and posterior labial palps triangular, similar in size, with ≤ 15 sorting ridges in a 57-mm-long specimen.   Habitat:Infaunal on soss sediments ( Fig. 7F), sometimes byssally aưached to gravel, boulders or other rocky substrates.   Distribution:Along the South American coast, from 41°46 ʹSin the Pacific to 54° Sin the Atlantic, including the Magellan Strait, Islade los Estados, Burdwood Bank, Malvinas/ Falkland Islandsand South Georgia( Fig. 1A). From 1.5 to 553 mdepth.   Remarks:d’Orbigny (1846) described  H. meridionalisfrom îles Malouines(= Malvinas/ Falkland Islands) and ‘côte de la Patagonie’. The total number of specimens he had at hand when describing this species was not indicated by the author. Only one of these lots (the former) appears in the NHM database (‘ NHMUK1854.12.4.772’), with no other contemporary lots in the collection ( T. White, personal communication, June 2022). This lot currently contains six valves, although, according to the museum book register, it was originally composed of only two (broken) valves. None of the valves of this lot corresponds to the specimen figured by d’Orbigny (1846: pl. 81, figs 21, 22). Nevertheless, this issue was surpassed with the lectotypedesignation by Simone and Penchaszadeh (2008), and the concomitant (although not formally pointed out by the authors) restriction of the type locality to Malvinas/ Falkland Islands. The lectotype( Fig. 3C) of 4 mmlength closely resembles the specimen figured by d’Orbigny (1846) (here reproduced in Fig. 3A). However, the other specimens studied by Simone and Penchaszadeh (2008), collected as ‘epizoic on …  Zygochlamys patagonica(King & Broderip, 1832)[sic] in grounds 90-130 mdepth’ from ‘ Argentina, Buenos Aires, off Mar del Plata’, and assigned by these authors to  H. meridionalisdo not correspond to this species but to a new species of  Hiatella, which is described below.  Preston (1913)described  S. subantarcticaalso from the Malvinas/ Falkland Islands. The specimen originally figured by the author ( Preston 1913: fig. 12) was located in the collections of the Royal Belgian Institute of Natural Sciences ( RBINS MT753; Figs 4A, 5B). This shell ( 18.5 mmlong) fits within the intraspecific variability here recognized for  H. meridionalisand, consequently, is considered a synonym. Taking into account that Preston (1913)did not indicate the total number of specimens he had at hand when describing this species and the fact that he sold his collections to several museums and private collectors from different parts of the world, the figured specimen is here designated as the lectotype(following ICZNrecommendation 74B), with the express purpose of fixing the species concept. The study of the typematerial of two of the species described by Mabille and Rochebrune (in Rochebrune and Mabille 1889) from Cape Horn, i.e.  S. lebruni(MNHN-IM-2000-31626; Figs 3D, 4E) and  S. mollis(MNHN-IM-2000-31625; Fig. 4B), allows us to confirm that these taxa are also synonyms of  H. meridionalis, as is also the case for the material identified by Carcelles (1944: fig. 107; 1950: fig. 91) as  S. solida(MACN-In 13560) and for some of the specimens reported by Melvill and Standen (1907)as  S. arcticavar. antarctica( NMS.Z.1921.143.728.4). The original illustration of  S. chilensisby Hupé (1858: pl. 8, figs 7, 7a, reproduced herein in Figs 4F, 5C) also fits within our current concept of  H. meridionalis.  Hiatella meridionalisis also the name to apply to the molecular taxon ‘  HiatellaA’ of our Molecular Study section (see above) and of Laakkonen et al.(2015).   Hiatella meridionalisis morphologically similar to the Australian  H. australis(Lamarck, 1818)[=  Saxicava angasiA. Adams, 1865in Angas 1865] (figured by Blainville 1827: pl. 78, fig. 3 and pl. 80 bis, fig. 4; Brunckhhorst 1998: fig. 8.44A; Lamprell and Healy 1998: 199, fig. 578). This is another largesized species (up to 53.5 mmlong), which retains the hinge teeth at a large size (see Brunckhhorst 1998: fig. 8.44B, C). Beu (1971)pointed out that ‘it is possible that South American specimens are conspecific with  Hiatella australis’. However,  H. australishas a proportionally shorter and higher shell than  H. meridionalis, with a more raised umbo, and the cardinal tooth of the less valve directed anteroventrally (see Brunckhhorst 1998: fig. 8.44C). In addition,  H. australishas a different habitat from  H. meridionalis, occurring in the intertidal and shallow subtidal zones ( Middelfart et al.2010). Molecular data provide further evidence to recognize these two entities as distinct ( Laakkonen et al.2015; this study).   Hiatella meridionalisalso resembles the Miocene Antarctic specimens reported by Beu and Taviani (2014: fig. 9a–c) as  Hiatellacf. antarctica. However, the Recent (Magellanic) specimens have a more projected posterior end of the shell, a more differentiated posterior area, and a smaller and lower umbo than the Antarctic fossil material. Furthermore, Beu and Taviani (2014)mentioned that hinge teeth were not seen in the specimens they studied (including specimens from 6 to 28 mmin length), whereas similar-sized specimens of  H. meridionalisshow well-developed teeth. In addition, the authors reported  Hiatellacf. antarcticaaưached to the surface of the pectinid  Austrochlamys forticostaBeu and Taviani, 2013, whereas  H. meridionalislives as infaunal.