Anodonta exulcerata, Porro, 1838

Anodonta exulcerata View in CoL ‘Villa’ Porro, 1838

Anodonta piscinalis exulcerata – Drouët, 1883 Anodonta exulcerata – C. B. Adams, 1847

Margaron (Anodonta) cygnea (Drap.) [in part] – Lea, 1852

Anodon exulceratus – Sowerby, 1870

Margaron (Anodona) cygnea (Linn) .[in part] – Lea, 1870

Anodonta (Acalliana) exulcerata – Bourguignat, 1881 Anodonta (Acalliana) exulcerata – Bourguignat, 1882 Anodonta exulcerata – Bourguignat, 1883

Anodonta exulcerata – Catlow & Reeve, 1845; Clessin, 1874

Anodonta (Groupe de l’A. acallia) exulcerata – Locard, 1890

Anodonta (Euanodonta) exulcerata – Westerlund, 1890 Anodonta (Groupe de l’A. acallia) exulcerata – Locard, 1893

Anodonta cygnea (Linnaeus, 1758) View in CoL [in part] – Simpson, 1900; Simpson, 1914

Anodonta anatina (Linnaeus, 1758) View in CoL [in part] – Germain, 1931

Anodonta palustris exulcerata – Modell, 1945 Anodonta (Anodonta) cygnea (Linnaeus, 1758) [in part] – Haas, 1969

Anodonta exulcerata – Froufe et al., 2017

Comments: We present only a chresonymy for A. exulcerata and determine the earliest described Anodonta from northern Italy. We have included Anodonta idrina Spinelli, 1851 as the next available taxon for this species. However, due to the confusion of shell forms of A. anatina , A. cygnea and A. exulcerata , we have not attempted a complete review of all Anodonta taxa described from Italy in the later part of the 19 th and early 20 th centuries. This list of taxa includes at least 56 taxa described from Italy (e.g. Alzona, 1971).

Based on the similarity of the shell and on the coincidence of the sampling spots (including one of the type localities, i.e. Lake Oggiono), the rediscovered species was recognized as Anodonta exulcerata , (‘Villa’) Porro, 1838, using the oldest available name for the Anodonta taxa in the studied region ( Haas 1969; Graf & Cummings, 2019). The shells of the lectotype specimen of A. exulcerata deposited in the Natural History Museum, London (NHMUK1841.5.6.127; Supporting Information, Fig. S1 View Figure 1 ) and of the paratype specimens from the ‘original series’ ( Zilch, 1967: 111; Senckenberg Museum, N°5166) were analysed in detail before attributing this name to the erroneously synonymized species. Johnson (1971), in reviewing the unionid types in NHMUK, found a specimen labelled Anodon exulceratus and listed it as the specimen from Ziegler figured in Sowerby (1870). Ziegler is listed in the Malacology ledger as the donor of Anodon exulceratus (Dr T. White, pers. comm. 2/4/2019). Sowerby credited the name to a ‘Villa’ manuscript in the British Museum, indicating that it was Sowerby’s figured type. Johnson credited the species description to Sowerby (1870). Sowerby (1870: species 131 page [48], pl. 33 species 131, page 48, plate 33) listed ‘Villa. MS in Mus. Brit’. Johnson (1971) cited Anodon exulceratus ‘Porro’ Sowerby, 1870. Thus, Johnson was aware of the citation of the Villa manuscript by Sowerby, but chose to ignore it and claim it was a Porro manuscript name, ignoring Porro’s (1838) description of Anodonta exulcerata . Listing of that specimen figured by Sowerby as the figured holotype represents an inadvertent lectotype fixation under Art. 74.6 of the Code ( ICZN, 1999). However, Porro (1838) mentioned in his description ‘the plurality of individuals’ observed. He also listed three lakes in his distribution. This documents that the description of A. exulcerata by Porro was based on multiple individuals. Thus, the inadvertent lectotype designation by Johnson, (1971) for A. exulcerata ‘Porro’ Sowerby, 1870 may be valid, but the application of the lectotype to A. exulcerata Porro, 1838 by assumption of holotype is invalid as Porro mentions multiple specimens in his description. This NHMUK specimen, NHMUK 1841.5.6.127 is here designated as the lectotype for Anodonta exulcerata ‘Villa’ Porro, 1838.

Shell description: Shell generally thin, equivalve and inequilateral, large (max. length 103 mm, N = 109) elliptical to suboval, moderately inflated. Angle between dorsal margin and posterior margin 124° to 147° (mean = 135°). Anterior margin broadly rounded, posterior margin narrowly rounded to bluntly pointed; ventral margin convex, occasionally flat straight in the middle nearer to the posterior edge; dorsal margin straight to slightly convex in passing from the posterior margin, occasionally extending into a low dorsal wing; posterior ridge rounded, occasionally weakly biangulated distally; posterior slope moderately steep, flat to slightly convex; umbo broad, moderately inflated, elevated slightly above hinge line; umbo sculpture with thin wavy rugae; umbo cavity wide, shallow. Pseudocardinal and lateral teeth absent. Adductor muscle scars rather light shallow (not deep). Nacre is white to bluish white, usually iridescent. Periostracum tawny to olive or brown; small individuals yellowish brown to dark olive, large individuals brownish black with dark green rays of varying width and intensity. Morphological shell features correspond well to the first description of the species ( Porro, 1838) and to the lectotype made available from the Natural History Museum, London (Supporting Information, Fig. S1 View Figure 1 ). One discrepancy lies in the fact that, contrary to what is indicated by Porro, we cannot argue that ‘in the majority of individuals the upper and lower margins are parallel, and only in a few individuals are distant posteriorly’. On the contrary, the shape of the shell is so variable that it appears haphazard to draw any generalization ( Fig 4 View Figure 4 ; Supporting Information, Fig. S8).

Umbo sculpture also appears to be highly variable, ranging from a clearly double-looped to a finely concentric lines arrangement (Supporting Information, Figs S7S, S9).

Soft anatomy description: In life the mantle is creamy white to yellowish or light-brownish (respectively, 79 and 21% of individuals examined), brownish or tan at the openings of the apertures, mantle outside of apertures transparent white to grey; visceral mass creamy white to pink powder, may be pale-orange adjacent to foot; foot pale orange to creamy-white.

Gills creamy to gold; dorsal margin sinuous to concave, ventral margin convex; anterior margin of inner gills slightly longer and wider than outer gills. Outer gills marsupial; glochidia held across gill length; well-padded when gravid; light brownish to brownish orange.

Labial palps creamy white; straight to concave dorsally, convex ventrally, pointed distally; with a smooth external surface and a finely canaliculated internal surface.

Incurrent aperture longer than excurrent and supra-anal apertures; supra-anal and incurrent apertures occasionally of similar length. Incurrent aperture creamy white to grey within; greyish or brownish basal to papillae; papillae in two to three rows, inner row usually larger, longer, thick; papillae white-creamy to light tan; whitish in living animals. Excurrent aperture smooth, whitish at the external margin, with darkly coloured irregular band at the base. Supra-anal aperture smooth, creamy white within, without marginal coloration.

Voucher specimens: Six voucher specimens of this species were deposited: two at the Museo de La Specola-Florence (catalogue numbers: MZUF BC/ 51405 and MZUF BC/51406), two at the Naturhistorisches Museum der Burgergemeinde Bern (NMBE 549733 and NMBE 549734) and two at the North Carolina Museum of Natural Sciences (NCSM 102851 and NCSM 102852) ( Table 5; Froufe et al., 2017). Since Anodonta exulcerata Porro, 1838 is the oldest available name for the Anodonta taxa in the studied region ( Haas, 1969; Graf & Cummings 2019), A. exulcerata is used herein for this newly detected Anodonta species. The shell morphology of A. exulcerata specimens sampled in this study (Supporting Information, Fig. S3 View Figure 3 ) is consistent with the lectotype of A. exulcerata (Natural History Museum, UK: Lectotype NMNHUK 1841.5.6.127) and with the paratype specimens of the Senckenberg Museum, Frankfurt am Main ( Zilch, 1967). Furthermore, in one of its type localities (Lake Annone), it was the only Anodonta species found ( Froufe et al., 2017).

Distribution: Anodonta exulcerata is found from the Italian Peninsula to Croatia west of the Dinaric Alps ( Froufe et al., 2017), which confirms the distribution reported by Clessin (1876). In northern Italy it appears to be the most common Anodonta species.

Habitat and biology: Anodonta exulcerata occur in waters with little or no current and substrates typically composed of mud or muddy sand, often with detritus. Due to misidentification with other Anodonta species, information on biology is scarce. Gravid individuals brooding glochidia at different stages of development have been observed from early September to late December in Lake Maggiore and Lake Varese (N. Riccardi, pers. obs.). Glochidial host fish species are unknown.

Conservation status: The fact that A. exulcerata has not been previously recognized has precluded any assessment of its conservation status. However, it is widely distributed in the region and locally abundant, which might suggest that currently the species is not at risk.

Comparison with similar species: Close conchological similarity and wide shell plasticity make the use of shell shape for the discrimination of A. exulcerata from coexisting congeneric species (i.e. A. anatina and A. cygnea ) unreliable. Like A. anatina , A. exulcerata tends to be more swollen than A. cygnea slightly posterior to the umbo. However, the difference, whenever it exists, may be masked by the broad shell plasticity. Indeed, except for the index of convexity standardized over length, the mean values of the shell measurement ratios were not significantly different ( Table 2). To the extent that reliable external features could be identified to distinguish the two central, northern and eastern European Anodonta species ( Gallenstein 1895, Möller 1933, Bloomer 1937, Franz 1939), it also became apparent that the Italian forms could not be clearly identified ( Gallenstein 1894, Falkner 1994). Rather, a mixture of the otherwise species-specific characteristics was often found. Only the analysis of further characters (allozymes, DNA) contributed a new view on this problem providing an objective basis to older assumptions about the peculiarities of the Italian unionid fauna.

Clessin (1874) already stressed the close similarity of A. exulcerata and A. anatina (‘belongs to the Formenkreis of Anodonta anatina Rossm’) and attributed A. exulcerata , as well as the similar A. idrina ( Spinelli, 1851) , to the A. anatina ‘group’. Kobelt (1876) reiterated that A. idrina , A. exulcerata and A. gibba (a nomen nudum) should not be separated, and emphasizes the enormous difficulties and uncertainties in distinguishing the species of Anodonta . This is the only final message to be drawn after getting lost in the enormous variety of conflicting opinions among malacologists of the time.

For the determination of live animals or shells in the field, a diagnosis based on external characters is highly desirable. For this purpose, a larger number of molecularly determined forms must be examined anatomically and conchologically. This step is reserved for future investigation.