Integrated taxonomy reveals new threatened freshwater mussels (Bivalvia: Hyriidae: Westralunio) from southwestern Australia

Genetic variation

The best fitting substitution models for COI codons 1–3 were identified as TN + F + G4, F81 + F + I, and TN + F, respectively. The maximum likelihood (ML) and Bayesian inference (BI) trees showed similar topologies of the main nodes, although the BI tree displayed greater resolution of the ingroup branches (Fig. 1). Furthermore, the BI tree revealed three monophyletic clades, while two of those clades were merged in the ML tree. Two of the three molecular species delimitation methods (ASAP and TCS) recovered three groups in the BI tree as distinct taxa (Fig. 1), corresponding to the three previously described ESUs27,28. The third method (bPTP) recovered between 8 and 43 groups (mean = 28.03) suggesting that there is evidence of additional genetic differentiation within the three groups identified by ASAP and TCS. The outputs of the three methods are provided in the Supplementary information. The molecular diagnosis uncovered several fixed nucleotide differences COI characters for each taxon (Table 1: “W. carteri” I = 10; “W. carteri” II = 3; “W. carteri” III = 5). There were also 13 fixed nucleotide differences in W. carteri for the 16S gene. The remaining two taxa had no fixed nucleotide differences for the 16S gene.

Figure 1
figure 1

Phylogenetic trees obtained by maximum likelihood (left) and Bayesian inference (right) analysis of “Westralunio carteri” mtDNA COI sequences, including support values for the major genetic clades [ultrafast bootstrap values (left) and Bayesian posterior probabilities (right)]. Colour coded bars show support for the three major clades by the species delimitation methods (ASAP = dark shade; TCS = lighter shade). Green = WcI = “W. carteri” I; blue = WcIII = “W. carteri” III; red = WcII = “W. carteri” II. Results of bPTP analysis not shown (see supplementary data). Haplotype names correspond to Benson et al.28. Outgroup taxa are Velesunio ambiguus (Philippi, 1847) (Hyriidae: Velesunioninae) and Cucumerunio novaehollandiae (Gray, 1834) (Hyriidae: Hyriinae: Hyridellini).

Table 1 Molecular diagnoses of “Westralunio carteri” Evolutionarily Significant Units (ESUs) from southwestern Australia (after Bolotov et al.122 with reanalysis of data from Klunzinger et al.27 and Benson et al.28).

Variation in shell morphology

Based on results from analyses of variances (ANOVAs), shells of “W. carteri” I were significantly larger (for size metrics total length (TL), maximum height (MH), beak height (BH) and beak length (BL)) and more elongated (i.e., had a lower maximum height index (MHI)) than shells of “W. carteri” II and “W. carteri” II + III combined (Table 2). However, there was no difference in size or shape metrics between “W. carteri” I and “W. carteri” III (Table 2). The lack of significant differences in beak height index (BHI) and beak length index (BLI) among any of the taxa (Table 2) indicates that wing and anterior shell development was not discernibly different between any of the ESUs.

Table 2 Shell size metrics [mm], shape indices [%] and scores for the first two principal components (PC) obtained by Principal Component Analysis of shape indices and 18 Fourier coefficients generated by Fourier Shape Analysis for each “Westralunio carteri” species and subspecies-level Evolutionarily Significant Units (ESUs): n, number of specimens measured; minimum (min) to maximum (max) and mean (± standard error (SE)).

This pattern was partly confirmed in the principal component analysis (PCA) of these three shell shape indices, where PC1, largely explained by variation in BLI (Fig. 2A), did not differ between the two species (i.e., “W. carteri” I vs. “W. carteri” II + III) or among the three taxa (Table 2). The PC2, largely explained by variation in MHI and BHI (Fig. 2A), differed significantly between “W. carteri” I and “W. carteri” II (Table 2). Accordingly, 70% (70% jack-knifed) of specimens were assigned to the correct species in the corresponding discriminant analysis (DA), whilst this was true for only 55% (54%) at the MOTU-level.

Figure 2
figure 2

Scatterplots of the first two PC axes obtained by PCA on (A) calculated shape indices based on shell measurements, and (B) 18 Fourier coefficients for “Westralunio carteri” I, “W. carteri” II and “W. carteri” III. 95% Confidence Intervals are displayed at the species level, i.e., for “W. carteri” I (full line) and “W. carteri” II + III (dashed line). Extreme shell outlines in (B) are depicted to visualise trends in sagittal shell shape, along PC axes.

The difference in shell elongation between “W. carteri” I and “W. carteri” II was confirmed by Fourier shape analysis. As visualised by synthetic outlines in Fig. 2B, shell elongation is expressed along the PC1 (explaining 15% of total variation in Fourier coefficients). The PC1 as well as PC2 scores differed significantly between the two species (i.e., “W. carteri” I vs. “W. carteri” II + III) as well as between “W. carteri” I and “W. carteri” II, respectively (Table 2). Combined with synthetic outlines, this indicated a tendency towards a more elongated, somewhat wedge-shaped shell in “W. carteri” I, whilst “W. carteri” II shells tended to be relatively high with a stout anterior margin (Fig. 2B). An analysis of similarities (ANOSIM) analysis on all Fourier coefficients revealed no significant difference between the two species (i.e., “W. carteri” I vs. “W. carteri” II + III; ANOSIM: R = − 0.018, p = 0.097), but did indicate a significant difference between the three ESUs (ANOSIM: R = 0.0625, p = 0.0051). Specifically, “W. carteri” I differed significantly from “W. carteri” II (Bonferroni-corrected p = 0.0009). Only 66% and 65% (62% and 62% jack-knifed) of specimens were assigned to the correct species and taxon in DAs on that dataset, respectively.

Taxonomic accounts

Class: Bivalvia Linnaeus, 175831.

Subclass: Autobranchia Grobben, 189432.

Infraclass: Heteroconchia Gray, 185433.

Cohort: Palaeoheterodonta Newell, 196534.

Order: Unionida Gray, 185433 in Bouchet & Rocroi, 201035.

Superfamily: Unionoidea Rafinesque, 182036.

Family: Hyriidae Parodiz & Bonetto 196337.

Genus: Westralunio Iredale, 19349.

Type species: Westralunio ambiguus carteri Iredale, 19349 (by original designation).

Redescription: Westralunio carteri (Iredale, 1934)

Synonymy

Unio australis Lamarck38: Menke39, p. 38, specimen 219. (Non Unio australis Lamarck, 181938).

Unio moretonicus Reeve40: Smith41, p. 3, pl. iv, Fig. 2. (misidentified reference to Unio moretonicus Reeve, 186540).

Hyridella australis (Lam.38): Cotton & Gabriel42 (in part), p. 156. (misidentified reference to Unio australis Lamarck, 181938).

Hyridella ambigua (Philippi26): Cotton & Gabriel42 (in part), p. 157. (misidentified reference to Unio ambiguus Philippi, 184726).

Westralunio ambiguus carteri: Iredale, 19349, p. 62.

Westralunio ambiguus (Philippi26): Iredale9, p. 62, pl. iii, Fig. 8, pl. iv, Fig. 8. (Non Unio ambiguus Phil. 184726), Iredale43, p. 190.

Centralhyria angasi subjecta Iredale, 19349, p. 67 (in part), Iredale43, p. 190.

Westralunio carteri Iredale9: McMichael & Hiscock10pl. viii, Figs. 1, 2, 3, 4, 5, 6 and 7, pl. xvii, Figs. 4, 5.

Type material

Lectotype: AMS C.61724 (Fig. 3A) Westralunio ambiguus carteri Iredale, 19349.

Figure 3
figure 3

(A) Westralunio ambiguus carteri Iredale, 1934, Lectotype: Victoria Reservoir, Darling Range, 12 mi E of Perth, AMS C.061724. Detail of fusion in anterior muscle scars from either valve represented by dashed lines and black polygons. Bottom image showing detail of hinge teeth. Photos provided with permission by Dr Mandy Reid, AMS. (B) Valves and detail of sculptured umbo of a juvenile W. carteri from Yule Brook, Western Australia, UMZC 2013.2.9. Photo by Dr Michael W. Klunzinger. (C) Glochidia of W. carteri from Canning River, Western Australia. Photo by Dr Michael W. Klunzinger.

Paralectotypes: AMS C.170635 Westralunio ambiguus carteri Iredale, 19349 (n = 4).

Type locality: Victoria Reservoir, Darling Range, 12 miles east of Perth, Western Australia (Fig. 4A).

Figure 4
figure 4

(A) Victoria Reservoir, Canning River, near Perth, Western Australia, type locality for W. carteri. Photo by Corey Whisson. (B) Goodga River, Western Australia, type locality for W. inbisi inbisi, at vertical slot fishway where holotype of W. inbisi inbisi was collected from. Photo provided with permission by Dr Stephen J. Beatty. (C) Margaret River, Western Australia, type locality for W. inbisi meridiemus, at Canebreak Pool. Photo by Dr Michael W. Klunzinger.

Lectotype: BMNH 1840–10-21–29 Centralhyria angasi subjecta Iredale (selected by McMichael & Hiscock10).

Type locality: Avon River, Western Australia.

Material examined for redescription: For W. carteri (= “W. carteri” I), molecular data examined included 52 and 61 individual COI mtDNA and 16S rDNA sequences, respectively, for species delimitation. Additionally, Fourier shell shape outline analysis and traditional shell morphometric measurements were examined from 238 and 290 individuals, respectively. Complete details on all specimens examined are provided in Supplementary Table S1.

ZooBank registration: urn:lsid:zoobank.org:act:6B740F4D-40C3-4D6A-8938-B0FD7FD1F6D7.

Etymology: The species name carteri is most likely named after the surname of the collector who provided original type specimens to the Australian Museum, although Iredale9 did not specify this as the case. We have applied ICZN Articles 46.1 and 47.144, designating W. carteri as the nominotypical species.

Revised diagnosis: Specimens of W. carteri are distinguished from other Australian Westralunio taxa by having shell series that are significantly larger and more elongated than Westralunio inbisi inbisi subsp. nov., but not different from Westralunio inbisi meridiemus subsp. nov. The species has 10 diagnostic nucleotides at COI (57 G, 117 T, 210 G, 249 T, 255 C, 345 G, 423 T, 447 T, 465 A, 499 T) and 13 at 16S (137 T, 155 C, 228 C, 229 T, 260 G, 290 A, 305 G, 307 T, 310 A, 311 C, 321 T, 330 A, 460 A), which differentiate it from its sister taxa, W. inbisi inbisi and W. inbisi meridiemus (each described below) using ASAP and TCS species delimitation models.

Redescription

This species is of the ESU “W. carteri” I27,28.

Shell morphology: Shells of relatively small to medium size, generally less than 70 mm in length, but to a maximum length of approximately 100 mm10,45, MHI 46–89%; anterior portion of shell with moderate development, BLI 22–49%; larger shells with abraded umbos scarcely winged; wing development variable, generally decreasing with size, BHI 76–104% (Table 2). Shell outline oblong-ovate to rounded; posterior end obliquely to squarely truncate, anterior end round; ventral edge slightly curved, nearly straight in larger specimens; hinge line curved, hinge strong. Umbos usually abraded in specimens > 20 mm in length; unabraded umbos with distinctive v- or w-shaped plicated sculpturing (Fig. 3B and Zieritz et al.46). Shell substance typically thick; shells of medium width with pronounced posterior ridge; periostracum smooth, dark brown to reddish, with fine growth lines. Pallial line less developed in smaller specimens and prominent only in large specimens (e.g., > 60 mm TL). Lateral teeth longer and blade-like, slightly serrated to smooth and singular in left valve, fitting into deep groove in right valve; pseudocardinal tooth in right valve coarsely serrated, thick, and erect, fitting into deeply grooved socket in left valve. Anterior muscle scars well impressed and anchored deeply in larger specimens; anterior retractor pedis and protractor pedis scars both small and fused with adductor muscle scar; posterior muscle scars lightly impressed; dorsal muscle scars usually with two or three deep pits anchored to internal umbo region.

Anatomy: Supra-anal opening absent, siphons of moderate size, not prominent but protrude beyond shell margin in actively filtering live specimens, pigmented dark brown with mottled lighter brown to orange splotches; inhalant siphon aperture about 1.5 times size of exhalant and bearing 2–4 rows of internal papillae (Fig. 5A); ctenidial diaphragm relatively long and perforated. Outer lamellae of outer ctenidia completely fused to mantle, inner lamellae of inner ctenidia fused to visceral mass then united to form diaphragm; palps relatively small, usually semilunar in shape; marsupium well developed as a distinctive swollen interlamellar space in the middle third of the inner ctenidium of females. Outer ctenidia in both sexes thin, with numerous, short intrafilamentary junctions and few, irregular interlamellar junctions; in females similar, but marsupium has numerous, tightly packed, well-developed interlamellar junctions. Thus, brooding in females is endobranchous.

Figure 5
figure 5

Live specimens of actively filtering freshwater mussels in the burrowed position. (A) Westralunio carteri (Iredale, 1934), Canning River at Kelmscott, Western Australia, inhalant siphon with 2–4 rows of papillae oriented toward substrate. Photo by Dr Michael W. Klunzinger. (B) Westralunio inbisi meridiemus subsp. nov., Canebreak Pool, Margaret River, Western Australia; inhalant siphon edges lined with protruding papillae facing towards water surface, away from substrate. Photo by Dr Michael W. Klunzinger.

Life history: Sexes are separate in W. carteri, and hermaphroditism appears to be rare47,48,49. Males and females both produce gametes year-round but brooding of glochidia appears to be seasonal and ‘tachyticitc’ (i.e., as defined by Bauer & Wächtler19, fertilisation and embryonic development occurring in late winter/early spring and glochidia release in early summer)50. Glochidia are released within vitelline membranes, embedded in mucus which extrude from exhalant siphons of females (i.e., ‘amorphous mucus conglutinates’) during spring/summer. Glochidia attach to host fishes and live parasitically on fins, gills or body surfaces for 3–4 weeks while undergoing metamorphosis to the juvenile stage. Host fishes which have been shown to support glochidia metamorphosis to the juvenile stage in the laboratory include Afurcagobius suppositus (Sauvage, 188051), Gambusia holbrooki (Girard, 185952), Nannoperca vitttata (Castelnau, 187353), Pseudogobius olorum (Sauvage, 188051) and Tandanus bostocki Whitley, 194454 but not Carassisus auratus Linnaeus, 175831 or Geophagus brasiliensis (Quoy & Gaimard, 182455)47. Wild-caught fishes observed to be carrying W. carteri glochidia have included A. suppositus, Bostockia porosa Castelnau, 187353, G. holbrooki, Galaxias occidentalis Ogilby, 189956, N. vittata, P. olorum, T. bostocki, Leptatherina wallacei (Prince, Ivantsoff & Potter, 198257), and Phalloceros caudimaculatus (Hensel, 186858)47. Juveniles which have detached from host fishes have a characteristic ciliated foot and two distinct adductor muscles47. Probable age at maturity is 4–6 years old and estimated longevity is at least 36 to 52 years59. Inheritance of mitochondria is doubly uniparental60.

Glochidium: Following release, glochidia hatch from vitelline membranes but remain tethered by a larval thread and characteristically ‘wink’; valves with single adductor muscle; shells subtriangular and scalene in shape with smooth surface which lack surface spikes and dotted with pores, 305–310 μm long, 249–253 μm high and have a hinge length of 210–214 μm; apex of the ventral edge protrudes and is off-centre and closest to the posterior region of the glochidial shell, giving a sub-triangular scalene shape; larval teeth slightly curved towards adductor muscle with concave protuberance on base of the right valve tooth and convex protuberance on base of the left valve tooth; larval tooth of the right valve lanceolate, terminating with three sharp cusps; tooth of left valve blunt with two rounded cusps and groove at the midpoint to accommodate the middle cusp of the right valve; larval teeth lack microstylets (Fig. 3C and Klunzinger et al.48).

Distribution: Found in freshwater catchments from Gingin Brook, north of Perth to westerly flowing drainages north and west of the Blackwood River, within 150 km of the coast28,61 (Fig. 6).

Figure 6
figure 6

Distribution of the Westralunio specimens used for analyses in this study. Stars indicate type localities, labelled in bold, with colours corresponding to taxa (red—W. carteri, blue—W. inbisi inbisi, yellow—W. inbisi meridiemus). Waterbodies: South Coast: 1—Waychinicup R, 2—Goodga R, 3—King George Sound (N.B. museum records provided locality which we presume include freshwater streams or rivers draining to King George Sound rather than being collected from the marine environment), 4—Marbellup Bk, 5—Kent R, 6—Bow R, 7—Frankland R, 8—Walpole R, 9—Deep R, 10—Inlet R, 11—Weld R, 12—Shannon R, 13—Gardner R, 14—Warren R, 15—Lk Yeagarup, 16—Lefroy Bk, 17—Donnelly R, 18—Scott R, 19—Chapman Bk, 20—St. John Bk; Capes: A—Boodjidup Bk, B—Ellens Bk, C—Margaret R, D—Wilyabrup Bk; West Coast: W1—Carbunup R, W2—Vasse R, W3—Abba R, W4—Ludlow R, W5—Capel R, W6—Preston R, W7—Collie R, W8—Serpentine Res/R/Birrega Drain, W9—Wungong Bk, W10—Neerigen Bk, W11—Canning Res, W12—Canning R, W13—Southern R, W14—Yule Bk, W15—Swan R, W16—Victoria Res, W17—Helena R, W18—Lk Leschenaultia, W19—Gingin Bk, W20—Marbling Bk. Mapping methods provided in text. River basins within the South West Coast Drainage Division of Australia as defined under AWRC102. Spatial data were mapped as vector data in QGIS Desktop 3.24.3 (qgis.org/en/site/) using the GCS_GDA_1994 coordinate system103. The country outline for Australia was drawn from the GADM database (www.gadm.org), version 2.0, December 2011 under license. The rivers were mapped from the Linear (Hierarchy) Hydrography of Western Australia dataset (catalogue.data.wa.gov.au/dataset/hydrography-linear-hierarchy/resource/9908c7d1-7160-4cfa-884d-c5f631185859), under license.

Habitat: Found in freshwater streams, rivers and sometimes lakes or wetlands with permanent water, salinities less than about 3.0 mg/L, pH ranging from about 4.5 to 10 and more common in habitats not prone to nutrient pollution61.

Comments: McMichael & Hiscock10 suggested that the species aligns with other Velesunioninae in having smooth umbos, later refuted by Zieritz et al.46, as illustrated in Fig. 3B. Additionally, Iredale9 separated the genus Westralunio from Velesunio based on adult hinge tooth morphology, such that pseudocardinal hinge teeth are erect, serrated and strongly grooved in Westralunio as opposed to Velesunio which are suggested as not serrated and not strongly grooved. We contend that while W. carteri typically does have serrated pseudocardinal teeth that are usually erect/conspicuous and strongly grooved, so too are some Velesunio specimens (M. Klunzinger, unpublished data). In terms of distribution, there is one record of a specimen from the Gascoyne River collected ca. 1891 (BMNH-MP-110 listed as Diplodon ambiguus Parreyss in Philippi26 = Unio philippianus Küster, 186162; from Graf & Cummings63) which is well north of the species currently known range boundary. It is unclear whether the species occurs in that river as it has not been collected from north of the Moore-Hill Basin apart from this individual record.

Westralunio flyensis (Tapparone Canefri, 1883)

Synonymy

Unio (Bariosta) flyensis Tapparone Canefri, 188324, pp. 293–294, text Fig. 1.

Diplodon (Hyridella) flyensis (Tapp. Can.24), Simpson64, p. 1295.

Hyridella flyensis (Tapp. Can.24), Haas65, pp. 74–75, pl. ii, Figs. 4 and 5.

Westralunio flyensis (Tapp. Can.24), McMichael66, p. 41.

Type material

Holotype: The Holotype is held at Museo Civico di Storia Naturale, Genoa, Italy.

Paratypes: Two Paratypes are held at Museo Civico di Storia Naturale, Genoa, Italy.

Type Locality: Fly River, Papua New Guinea.

Description: As described by McMichael & Hiscock10.

Distribution: Southern rivers of New Guinea.

Westralunio albertisi Clench, 1957

Type material

Holotype: MCZ 212908.

Paratype: AMS C.62268.

Type Locality: inland from Daru, Papua.

Paratype: MCZ 191391, Lake Murray, Fly River, Papua New Guinea.

Description: As described by McMichael & Hiscock10.

Distribution: Lakes of the Fly River district, Papua New Guinea.

Westralunio inbisi sp. nov.

Westralunio inbisi inbisi subsp. nov.

Type material

Holotype: WAM S82756 (Fig. 7A–C), collected by M.W. Klunzinger.

Figure 7
figure 7

Westralunio inbisi inbisi subsp. nov., (A) Paratype: Goodga River, Western Australia, WAM S5620. Detail of fusion in anterior muscle scars from either valve represented by dashed lines and black polygons. Bottom image showing detail of hinge teeth. Photos by Corey Whisson. (B) Holotype: Goodga River, Western Australia, WAM S82756. Photo by Corey Whisson. (C) Valves and detail of sculptured umbo of a juvenile, Lake Yeagarup, Western Australia, WAM S82697. Photo by Dr Michael W. Klunzinger. (D) Westralunio inbisi meridiemus subsp. nov. Holotype: Margaret River, Western Australia, WAM S56235. Photo by Corey Whisson.

Type locality: Goodga River at vertical slot fishway, Western Australia (34.9485°S, 118.0799°E, GDA94) (see Fig. 4B).

Paratypes: WAM S56200, WAM S56201, WAM S56202, WAM S56203, collected by M. W. Klunzinger.

Type locality: Goodga River, Western Australia (34.9597°S, 118.0981°E, GDA94).

Material examined: For W. inbisi inbisi (= “W. carteri” II), molecular data examined included 82 and 93 individual 16S rDNA and COI mtDNA sequences, respectively, for species delimitation. Additionally, Fourier shell shape outline analysis and traditional shell morphometric measurements were examined from 127 and 139 individuals, respectively. Complete details on all specimens examined are provided in Supplementary Table S2.

Etymology: The specific epithet, inbisi, is derived from the Nyoongar word ‘inbi’, translating to ‘mussel, fresh water’ in English67.

Diagnosis: Specimens of W. inbisi inbisi are distinguished from other Australian Westralunio taxa by having shell series that are significantly smaller and less elongated than W. carteri, but not W. inbisi meridiemus. The subspecies has three diagnostic nucleotides at COI (75 A, 87 T, 318 T) and none at 16S, which differentiate it from its sister taxa, W. carteri and W. inbisi meridiemus using ASAP and TCS species delimitation models.

Description: This subspecies is of the ESU “W. carteri” II27,28. Shell morphology in juveniles and adults same as W. carteri as described above (see Fig. 7A–C). Total adult shell length generally < 80 mm but known to reach in excess of 90 mm68,69. MHI 55–74%; anterior portion of shell with moderate development, BLI 23–51%; larger shells with abraded umbos scarcely winged; wing development variable, generally decreasing with size, BHI 79–99% (Table 2); anatomy same as W. carteri; life history: sexes appear to be separate based on macroscopic examinations of marsupia in non-gravid and gravid females, examined in the field in September and March 2011, respectively; wild-caught fishes from Fly Brook, Lefroy Brook and Shannon River, observed to be carrying what we presume to be W. inbisi inbisi glochidia have included B. porosa, N. vittata and T. bostocki47. Mature glochidia of W. inbisi inbisi have not been formally described. Reproductive phenology, age and growth have not been elucidated in W. inbisi inbisi.

Distribution: Found in southerly to south-westerly flowing freshwater streams, rivers and sometimes lakes or wetlands with water salinities less than approximately 3.0 mg/L from Boodjidup Brook in the Capes region to the west of the Blackwood River catchment to Waychinicup River within 150 km of the coast, primarily along the South Coast of Western Australia27,28,29,61,69 (Fig. 6).

Habitat: Similar to W. carteri although habitats can also include perched dune lakes. Waters that W. inbisi inbisi inhabit are often more tannin stained due to their occurrence in more heavily forested catchments with greater densities of native riparian vegetation than for W. carteri.

Westralunio inbisi meridiemus subsp. nov

Type material

Holotype: WAM S56235 (Fig. 7D), collected by M.W. Klunzinger.

Paratypes: WAM S56236, WAM S56237, WAM S56238, WAM S56239, collected by M. W. Klunzinger.

Type locality: Canebreak Pool, Margaret River, Western Australia (33.9460°S, 115.0707°E, GDA94) (see Fig. 4C).

Material examined: For W. inbisi meridiemus (= “W. carteri” III), molecular data examined included 9 and 12 individual 16S rDNA and COI mtDNA sequences, respectively, for species delimitation. Additionally, Fourier shell shape outline analysis and traditional shell morphometric measurements were examined from 12 individuals each. Complete details on all specimens examined are provided in Supplementary Table S3.

Etymology: The subspecific epithet, meridiemus, is derived from Latin ‘meridiem’, translating to ‘southwest’ in English in reference to the location of its type locality, which sits in the southwestern region of Western Australia.

Diagnosis: Specimens of W. inbisi meridiemus have five diagnostic nucleotides at COI (69 C, 123 C, 126 T, 483 A, 526 A) and none at 16S, which differentiate it from its sister taxa, W. carteri and W. inbisi inbisi using ASAP and TCS species delimitation models.

Description: This subspecies is of the ESU “W. carteri” III. Shell morphology in juveniles unknown, but adults same as W. inbisi inbisi and W. carteri as described above; total adult shell length generally < 80 mm. MHI 60–71%; anterior portion of shell with moderate development, BLI 28–35%; larger shells with abraded umbos scarcely winged; wing development variable, generally decreasing with size, BHI 84–93% (Table 2); anatomy same as W. carteri and W. inbisi inbisi, including siphon pigmentation and morphology (illustrated in Fig. 5B). Life history observations for this ESU cannot be derived from existing field observations as all known populations overlap the distribution of either W. carteri (in Margaret River) or W. inbisi inbisi (in the lower Blackwood Basin); however, mussels from those locations appear to have separate sexes based on macroscopic examinations of marsupia in both gravid and non-gravid females. Similarly, gravid female mussels have been observed from Margaret River during late spring to summer (November to December). Species of wild-caught fishes from Canebreak Pool in Margaret River that have been observed carrying glochidia include G. occidentalis and N. vittata47. Reproductive phenology, age and growth have not been elucidated in W. inbisi meridiemus.

Distribution: Found in the neighbouring catchments of Margaret River and the Blackwood River of Western Australia, where it is sympatric with W. carteri and W. inbisi inbisi, respectively27,28 (Fig. 6).

Habitat: Similar to W. carteri and W. inbisi inbisi in either lotic or lentic freshwater rivers, streams, and pools with varying degrees of riparian vegetation.

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