Ligodonta obscura Perkins, DeVilbiss & Whelan, 2025

Ligodonta obscura Perkins, DeVilbiss & Whelan, 2025 sp. nov.

Etymology.

obscura from English “obscure” in reference to the species evading discovery until the present study.

Proposed common name.

Solstice creekmussel, in reference to the species’ reproductive period centered around the winter solstice and preferred habitat of small creeks.

Type.

Holotype. • NCSM 221198 View Materials (Fig. 3 A – I View Figure 3 ): Little River adjacent to NC 134 ; coordinates withheld by authors; collectors: BK Jones, KL DeVilbiss, MA Perkins, MA Burchfiel, JK McIver; 24 September 2019. Shell and soft tissues in 95 % non-denatured ethanol, body removed by cut of adductor muscles and separation of mantle from the shell. Description as above in Diagnosis. Shell length = 35.8 mm, shell width = 13.2 mm, shell height = 21.0 mm. GenBank PV 734013 .

Paratypes. • USNM 1607186 About USNM (Fig. 4 A – E View Figure 4 ): Little River adjacent to NC 134 ; coordinates withheld by authors; collectors: KL DeVilbiss, JK McIver, SM Sheats, JW Mays, K Saxton; 18 June 2019. Shell and soft tissues in 95 % non-denatured ethanol, body removed by cut of adductor muscles and separation of mantle from the shell. Description as above in Diagnosis. Shell length = 34.2 mm, shell width = 12.4 mm, shell height = 20.1 mm. GenBank PV 34019 , PV 738973 . • NCSM 47145 View Materials (Fig. 4 F – J View Figure 4 ): Barnes Creek in the vicinity of SR 1303 crossing ; coordinates withheld by authors; 1 whole animal in 95 % ethanol; collectors: CB Eads, RA Harrington; 10 March 2010. Soft tissues detached from left valve by cut of adductor muscles and separation of mantle from the shell. Description as above in Diagnosis. Shell length = 27.3 mm, shell width = 9.6 mm, shell height = 16.2 mm. GenBank PV 738972 .

Other material examined

(Fig. 5 A – J View Figure 5 ). North Carolina: Randolph County. • 1) NCSM 221197 View Materials — Little River adjacent to NC 134 ; coordinates withheld by authors; 3 paired dry bivalves with tags D 116, D 117, D 118; collectors: BK Jones, KL DeVilbiss, MA Perkins, MA Burchfiel, JK McIver; 24 September 2019 . • 2) NCSM 221196 View Materials — Little River in the vicinity of SR 1127 crossing ; coordinates withheld by authors; 1 paired dry bivalve; collectors: KL DeVilbiss, CL Lynch; 18 June 2021 . • 3) NCSM 221195 View Materials — Little River adjacent to NC 134 ; coordinates withheld by authors; 1 paired dry bivalve; collectors: KL DeVilbiss, CL Lynch, RA Hoch; 21 September 2021 . • 4) NCSM 221199 View Materials — Little River in the vicinity of SR 1121 crossing , coordinates withheld by authors; 2 paired dry bivalves; collectors: RJ Heise, MA Perkins, WT Russ, W Xiong, SL Stevens; 2 June 2016 . • 5) NCSM 221200 View Materials — Little River in the vicinity of SR 1135 crossing ; coordinates withheld by authors; 1 paired dry bivalve; collectors: BK Jones, RJ Heise, TR Fox, RA Hoch, JL Griffin; 20 April 2010 . • 6) NCSM 221201 View Materials — Little River , coordinates withheld by authors; 1 whole animal in 95 % ethanol; collectors: KL DeVilbiss, JK McIver, SM Sheats, JW Mays, K Saxton; 18 June 2019 . Montgomery County. • 7) NCSM 47155 View Materials — Barnes Creek in the vicinity of SR 1134 crossing ; coordinates withheld by authors; 1 paired dry bivalve; collectors: CB Eads, RA Harrington; 1 April 2010 . • 8) NCSM 29612 View Materials — Barnes Creek in the vicinity of SR 1303 ; coordinates withheld by authors; 1 whole animal in 95 % ethanol; collectors: CB Eads, P Hubert, E Schubert; 29 March 2004 .

Diagnosis.

Shell morphology (Figs 3 View Figure 3 – 6 View Figure 6 , 10 View Figure 10 ). Shell small, thin, subovate, and moderately inflated; umbo extending slightly beyond the hinge line, with single or weakly double-looped beak sculpture in younger individuals; hinge weak. Anterior laterally compressed; anterior margin strongly rounded towards ventral margin. Ventral margin straight. Posterior margin broadly rounded, higher than umbo distally. Posterior ridge weakly pronounced, rounded, double. Periostracum somewhat thin, papery, appearing dark brown in most individuals; surface with fine, minor growth lines. The following color description was aided by backlight (Fig. 3 D, H View Figure 3 ): periostracum interrupted by dark concentric growth rings, base color yellow or light brown, anterior with variably spaced dark green or brown rays.

Nacre diaphanous, somewhat iridescent with blue or cream tint. Umbo cavity shallow. Anterior adductor scar well defined; posterior adductor scar weak. In the left valve, a single, small, smooth, laterally-compressed pseudocardinal tooth extending to a weak interdental projection, oriented directly above the umbo cavity and parallel to the hinge line. Left lateral tooth absent, appearing as a weakly inflated ridge. In the right valve, a single small, rounded, laterally compressed, triangular pseudocardinal tooth oriented anterior to the umbo cavity and parallel to the anterior margin. Right lateral tooth as described above. No obvious sexual dimorphism in shell morphology.

General external characteristic measurements as follows (n = 14): shell length = 23.2 mm min – 46.2 mm max, x ̄ = 35.9 mm; shell height = 14.6 mm min – 26.3 mm max, x ̄ = 20.7 mm; shell width 7.3 mm min – 17.5 mm max, x ̄ = 13.8 mm; ratios from averages as follows: length 1.73 times height and 2.59 times width, height 1.5 times width.

Soft anatomy (Figs 4 B, F View Figure 4 , 7 View Figure 7 ). Incurrent and excurrent apertures approximately equal in length, separated by thin mantle bridge. Excurrent aperture lacking papillae, smooth or weakly crenulate, dark gray to dark brown, occasionally with irregular cream-colored stripes. Incurrent aperture papillae present, simple or sometimes bifurcate anteriorly, dark gray to dark brown at base and cream-colored to tan distally (Fig. 7 View Figure 7 ). Posterior margin of mantle pigmented, appears as a thin, dark band anterior to apertures. Inner lamellae of gills fully attached to visceral mass; outer gills marsupial. Foot tissue pale pink-orange.

Glochidia (Fig. 8 B – E View Figure 8 ). Larvae subtriangular ( Hoggarth 1999); length approximately 300–350 µm; dorsal margin straight, ventral margin rounded, valves convex; apical styliform hooks present and conspicuous, approximately 50 µm in length and not strongly recurved, mesial surface of hooks with scattered microstylets. Occasionally appearing with a red tint. Charged gills in brooding females pink.

Distribution.

Ligodonta obscura is a rare mussel with an extraordinarily narrow distribution currently known from only two streams in central North Carolina, USA (Fig. 1 View Figure 1 ). A total of 43 live individuals were observed in 2016–2022 from one contiguous 7.15 km reach in Little River, a direct tributary on the eastern edge of the Pee Dee River in Randolph County. Historical collections are represented by only three specimens at NCSM from an approximately 6 km reach of Barnes Creek, a tributary draining to the Uwharrie River (a major tributary to the Pee Dee River) in Montgomery County, NC. Despite intensive surveys, no live specimens have been recovered from Barnes Creek since 2010, but the species may continue to persist. Ligodonta obscura is not currently known from any other locations locally or globally, nor is the species known to be represented in additional museum collections.

Life history.

Occupied habitat. Ligodonta obscura was found in composite sand / gravel substrates among cobble (mid-channel) and in soft silt (side banks). Individuals were observed lying entirely on top of, minorly buried in, or mostly buried in the river-bottom substrate. River runs and shallow pools were the most common habitat, with a few individuals detected in swift moving riffle habitat (Fig. 1 B View Figure 1 ). Occupied habitat also often included a fine layer of benthic sediment.

Reproduction.

Ligodonta obscura is presumably tachytictic, becoming gravid around the autumnal equinox and releasing glochidia in winter, shortly after the solstice. Two individuals in long-term holding at MCAC became gravid in late September; one individual slowly (~ 2 weeks) released approximately 5,500 mature glochidia in late December and early January. Ligodonta obscura does not appear to utilize conspicuous mantle adaptations or conglutinate morphology to attract hosts. Glochidia are released through the excurrent aperture directly into the water column and are attached to a thin mucus thread (Fig. 8 A View Figure 8 ).

Age.

Inspection of growth rings suggests L. obscura is a relatively short-lived species; examined individuals attained a maximum age of 9 or 10 years.

Remarks.

Morphological comparisons.

Ligodonta obscura is morphologically similar to several freshwater mussel species in North Carolina waters, specifically those within the genera Lasmigona and Alasmidonta (Fig. 9 View Figure 9 ). However, this species is distinguishable using a combination of readily identifiable external and internal shell characteristics, life-history traits, and biogeographic information.

The exterior shell morphology of L. obscura is most similar to Lasmigona subviridis , a rare species (recently proposed as federally “ Threatened ”) with a patchy distribution in predominately mid-Atlantic state waters, including the Roanoke, New, Tar, and Neuse River drainages in North Carolina. Unlike L. obscura , L. subviridis has a smooth periostracum, rounded ventral margin, typically straight posterior margin angled ~ 30 ° above the hingeline, conspicuously double-looped beak sculpture, is generally more laterally inflated, and commonly exceeds the maximum recorded size for L. obscura . Internal shell characters are not similar to L. obscura ; L. subviridis possesses strong lateral teeth in both valves, a thickened and weakly serrated pseudocardinal tooth on the right valve, and two conspicuous serrated pseudocardinal teeth on the left valve. These species also differ in life histories; L. subviridis typically prefers larger rivers or streams and glochidia commonly undergo direct transformation without an obligate fish host. Ligodonta obscura is not known to be sympatric with L. subviridis , and L. subviridis has not been recovered in the Yadkin-Pee Dee drainage in at least a century.

Lasmigona decorata (Lea, 1852) is an exceptionally rare protected species (federally listed as “ endangered ”) with a restricted distribution in North and South Carolina with a similar appearance to both L. obscura and L. subviridis . Exterior and internal shell characteristics of L. decorata closely resemble L. subviridis . Therefore, L. decorata can be differentiated from L. obscura by possessing a smooth periostracum, more-pronounced posterior margin, well-developed pseudocardinal and lateral teeth, and attaining a larger size. Distinguishing L. decorata from L. obscura is simplified by the fact that there is no known overlap in their native ranges. Lasmigona decorata is restricted to only a few Yadkin-Pee Dee and Catawba basin streams in North Carolina, which is west of the known range for L. obscura .

While there are exceptions, Alasmidonta is often characterized by poorly developed or absent lateral teeth and weakly developed or smooth pseudocardinal teeth. Several species of Alasmidonta are known from North Carolina, including the recently described A. uwharriensis ( Whelan et al. 2023) . Even though A. uwharriensis is sympatric with L. obscura , they are easily differentiated by external shell morphology. Alasmidonta uwharriensis , and the closely related and comparatively wide-ranging A. varicosa (Lamarck, 1819) , often possess a bright orange foot, conspicuous corrugations along the posterior slope, the umbo sits above the posterior margin, and the shell is more ventrally inflated and attains a larger size than L. obscura . Internal shell characters of A. varicosa are generally similar to L. obscura with rudimentary pseudocardinal teeth and greatly reduced or absent lateral teeth. However, A. uwharriensis is characterized by pronounced, somewhat curved pseudocardinal teeth in both valves and a well-defined interdental projection.

Phylogenetic placement.

Ligodonta obscura was inferred as the sister lineage to Alasmidonta in most phylogenetic analyses (Fig. 2 View Figure 2 ; Suppl. material 1). However, the placement of Ligodonta was not well supported (<95 % UFBoot) in most analysis and only moderately well supported on the COI tree (UFBoot 92 %, see Suppl. material 1). Aside from an absence of morphological features that unite Ligodonta with other genera, the uncertain phylogenetic placement is also justification for erecting a new genus. Moreover, given the long branch that L. obscura is on, additional data is extremely unlikely to suggest that Ligodonta is a synonym of another genus. The long branch may be contributing to the uncertain placement of Ligodonta . We do not anticipate finding additional taxa (i. e. other undescribed species) that could be used to increase taxon sampling and shorten the branch in phylogenetic inference. Therefore, additional data, likely requiring slower evolving genes, may be necessary to determine where Ligodonta resides within the Anodontini phylogeny. This will also be critical for future studies examining biogeography, morphology, and life history evolution of the Anodontini .

Additional comparisons.

Several other mussel species native to the Uwharrie region are superficially similar to L. obscura . Strophitus undulatus Say, 1817 is a wide-ranging, though relatively uncommon species, found throughout Central and Eastern North America that possesses similarly reduced tooth morphology, but it differs from L. obscura by having a smooth peristrocum, single-looped beak structure, low posterior margin, and attaining a much larger maximum size. Young individuals of the common species Uniomerus carolinianus Bosc, 1801 and Elliptio complanata are both characterized by a ventrally compressed shape and thin, papery periostracum and may resemble adult L. obscura , but are easily distinguished by a thickened shell, prominent sculptured pseudocardinal teeth, articulated lateral teeth, and pale white feet. Additionally, U. carolinianus has a single looped umbo structure and Elliptio species have a distinctive barred double looped umbo structure. Toxolasma pullus Conrad, 1838 is a small species with a patchy distribution and is sympatric with L. obscura , but is characterized by well-developed pseudocardinal and lateral teeth, double ridges on the posterior slope, a thickened and more ventrally inflated shell, and a rounded ventral margin.

Ligodonta obscura is known to co-occur with the following mussel species: Alasmidonta uwharriensis , Elliptio complanata , E. icterina Conrad, 1834 , E. producta Conrad, 1836 , Fusconaia masoni Conrad, 1834 , Sagittunio vaughanianus Lea, 1838 , Strophitus undulatus , Toxolasma pullus , Uniomerus carolinianus , Venustaconcha constricta Conrad, 1838 , and Villosa delumbis Conrad, 1834 .