Cultellus exilis, Yu & Jiao & Zhang, 2024

Yu, Yanan, Jiao, Yingyi & Zhang, Junlong, 2024, Description of a new species of the genus Cultellus Schumacher, 1817 (Bivalvia, Pharidae) from the South China Sea, based on integrative taxonomy, Zoosystematics and Evolution 100 (2), pp. 425-436 : 425

Cultellus exilis sp. nov.

Figs 2 , 3 , 4F

Type specimens.

Holotype: MBM229032, one complete individual, collected on 28 May 2021 by Agassiz trawl on the research vessel "TAN KAH KEE" (muddy bottom, depth 55 m). Paratype: MBM264485, one complete specimen, collected from the Beibu Gulf, China, January 1962 (muddy bottom, depth 55 m); MBM264488, two complete specimens, collected from the Beibu Gulf, China, December 1959 (Habitat unknown); MBM264497, one complete specimen, collected from Haimen, Guangdong Province, China, March 1954 (Habitat unknown); MBM264500, one complete specimen, collected from Shanwei, Guangdong Province, China, January 1995 (Habitat unknown); MBM229040, one complete specimen, collected from Hainan Province, China, January 1959 (muddy bottom, depth 91.5 m).

Type locality.

Neritic zone of the South China Sea (depth 55 m, 20°1'13.44"N, 117°10'45.84"E); Muddy bottom.

Etymology.

The specific epithet “exilis>” is derived from the Latin, referring to its slender shell, which is a remarkable difference from other species in this genus.

Description.

Shell medium in size, flattened, elongate, fragile, glazed, translucent, equivalve, inequilateral. Some specimens are covered with various-sized bubbles on their surface (Figs 2A-D , 3A-D ). Umbo depressed, weakly prosogyrous, situated at anterior 2/9 of shell. Anterior area short, elliptical, posterior area elongated, both ends gaping, anterior end slightly turned-up, posterior end slightly more pointed than anterior end; antero-dorsal margin downward sloping, postero-dorsal margin almost straight, ventral margin rather arcuate, postero-ventral margin more curved than antero-ventral, middle-ventral margin almost straight. Periostracum of valve surface yellowish; shell surface with fine, dense, regular and non-isometric co-marginal growth lines, without radial lines; lunule and escutcheon absent. Ligament short, but strong, dark brown and situated opisthodetic.

Interior shell off-white, with yellowish periostracum in margin. Each valve with one white, strong, thin, straight, internal radial rib extending from umbonal area to anterior end, forming ca. 21° angle with antero-dorsal margin (Fig. 2H-J ). Anterior adductor muscle scar subtrigonal, obvious, near internal rib; posterior adductor scar falciform, almost straight near the dorsal margin; pedal retractor scars rhombic, situated at the corner between umbo and internal rib; pallial sinus shallow, pallial lines connected to adductor scars. Left valve with three strong and projecting cardinal teeth, central tooth bifid at the top and confluent at the base, without lateral tooth (Fig. 2H ); right valve with two long and strong cardinal teeth, anterior protruding downwards, posterior pointing to the back end, without lateral tooth (Fig. 2I ).

Siphons short and bifurcated, situated at posterior area; gills transversely folded; foot strong, depressed, truncated, situated at anterior area.

Measurement.

Holotype: MBM229032: L = 34.66 mm, H = 10.35 mm, W = 2.27 mm. Paratype: MBM264485: L = 45.42 mm, H = 12.65 mm, W = 2.37 mm; MBM264488: L = 61.51 mm, H = 16.26 mm, W = 3.73 mm; MBM264497: L = 77.05 mm, H = 21.14 mm, W = 4.72 mm; MBM264500: L = 73.30 mm, H = 19.74 mm, W = 4.52 mm; MBM264601: L = 44.32 mm, H = 12.42 mm, W = 2.33 mm.

Remarks.

The shells of Cultellus are relatively less elongated compared to those of the other genera of Solenoidea . Typically, the shells of Cultellus have rounded anterior and posterior ends, an anteriorly located umbo, strong internal ribs and three cardinal teeth on the left valve and two on the right ( Thiele 1992). The remarkable morphologic similarity amongst Cultellus members makes species identification challenging. The distinguishing features that separate Cultellus exilis sp. nov. from other species in this genus are its particularly slender, fragile and translucent shells. Additionally, the new species differs from the C. maximus (Fig. 4E ) in that its posterior end is notably narrower than its anterior end ( Gmelin 1791). It closely resembles C. attenuatus (Fig. 4A ) and C. vitreus (Fig. 4D ). However, C. attenuatus (Fig. 4A ) differs from the new species by the anterior area of the shell, which is obviously wider than the posterior part. Additionally, the posterior end of C. vitreus is slightly truncated (Fig. 4D ), whereas Cultellus exilis is more curved. C. subellipticus (Fig. 4C ) differs from the new species by the posterior area of the shell, which is much wider than the anterior part ( Dunker 1862; Clessin 1888). Compared to this new species, the length-to-height ratio of C. hanleyi is 3.3-3.4:1 (Fig. 4B ), whereas the ratio of Cultellus exilis is 3.5-3.8:1.

Geometric morphometrics of shell outlines.

Principal Component Analysis (PCA) was conducted on the Progrustes alignment outline data of 32 samples representing six species. The results revealed that the first three principal components accounted for a cumulative contribution rate of 96.88% (PCA1 80.79%, PCA2 13.97% and PCA3 2.12%), indicating that they can represent the major morphological differences amongst the samples. According to the extreme distortion state of the thin plate spline plots, the main difference of all samples along the PCA1 axis occurs in the relative height of the shell (Fig. 5B, C ). Differences in the PCA2 axis mainly involve the length-to-height ratio of the shells. Shells were increasingly elongated from the PCA2+ to PCA2- direction (Fig. 5A, D ). Considering the distortion observed along both the PCA1 and PCA2 axes, the main variation in the outlines of the six species within the genus Cultellus is the relative length and height of the shell. Discriminant analysis of all data indicated that the first two variance values explain 96.98% of the overall variation. A Discriminant analysis plot clearly demonstrated that Cultellus exilis sp. nov. is distinctly separated from the other species, forming its own cluster (Fig. 6 ). Therefore, the geometric morphometric analysis results, based on outlines, support distinguishing the new species from other species within the genus Cultellus .

Species delimitation analyses.

All species delimitation analyses, namely ABGD, bPTP and GMYC, conducted on the COI sequences, resulted in the delimitation of eleven species in the superfamily Solenoidea . The analysis confirmed that Cultellus exilis sp. nov. is a distinct species from other Cultellus species. Sequences of the genus Cultellus were delimited into four species, i.e. C. attenuatus , C. subellipticus , C. maximus and Cultellus exilis sp. nov. (Fig. 7 ). In addition, based on the available molecular data, the analysis of a 581-bp fragment of the COI gene yielded a 12.2%-13.9% pairwise distance between Cultellus exilis sp. nov. and other congeners, a divergence higher than the intraspecific variation (0-1.7%) of the genus Cultellus (Suppl. material 1: table S2).

Phylogenetic analyses.

The best-fitting evolutionary model of the concatenated dataset (COI, 16S and 28S) was selected as GTR+G+I using the Akaike Information Criterion implemented in jModelTest 2.1.10. The evolutionary relationships amongst the razor clams were depicted on a phylogenetic tree constructed using the ML and BI methods. These trees exhibited highly similar topologies (Fig. 8 ). Phylogenetic trees obtained from the datasets were completely resolved and highly supported, as indicated by posterior probability (PP) or bootstrap scores (BS). Cultellus exilis sp. nov. and the two other Cultellus species formed a clade within the family Pharidae . The sequences of Cultellus exilis sp. nov. recovered a well-supported lineage distinct from the other congeners. Notably, Ensiculus cultellus and Phaxas pellucidus did not cluster together with the genus Cultellus . Instead, they occupied different branch positions in both ML and BI trees. On the ML tree, Ensiculus cultellus clustered with Phaxas pellucidus and formed a sister clade with the genus Siliqua . While on the BI tree, Ensiculus cultellus grouped with the genus Siliqua , constituting a sister lineage to Phaxas pellucidus . All phylogenetic results strongly supported the close relationship between pharids and solenids (PP = 0.99; BS = 92). Furthermore, the family Hiatellidae was found to be closely related as a sister to the superfamily Solenoidea (PP = 1; BS = 100). The best-fitting evolutionary model of the COI sequence was GTR+G. The phylogenetic tree inferred using ML criteria, based on single gene COI (Fig. 8 ), showed a similar overall topology.

Figures

Figure 2. Cultellus exilis sp. nov. (holotype) A, B. MBM 229032, L = 34.66 mm; C, D. Exterior views of left and right valve; E, F. Interior views of left and right valve; H. Hinge of left valve; I. Hinge of right valve; J. Internal radial rib of left valve.

Figure 3. Cultellus exilis sp. nov. (paratypes) A-D. MBM 264485; E-H. MBM 264488; I-L. MBM 264497; M-P. MBM 264500; Q-T. MBM 229040. Scale bars: 10 mm.

Figure 4. A. Cultellus attenuatus Dunker, 1862. Two syntypes, NHMUK 20240145: L = 54.7 mm, H = 13.9 mm; L = 45.2 mm, H = 12.2 mm; B. Cultellus hanleyi Dunker, 1862. One syntype, NHMUK 1986103: L = 54.1 mm, H = 16.1 mm; C. Cultellus subellipticus Dunker, 1862. One syntype, NHMUK 20240146: L = 46.3 mm, H = 16.0 mm; D. Cultellus vitreus Dunker, 1862. One syntype, NHMUK 20240147: L = 39.5 mm, H = 10.9 mm; E. Cultellus maximus (Gmelin, 1791). MBM 264615: L = 33.6 mm, H = 11.3 mm; F. Cultellus exilis sp. nov. MBM 229032: L = 34.66 mm, H = 10.35 mm.

Figure 5. Principal component and thin plate spline analyses, based on outlines of the genus Cultellus. A. The extreme distortion of the outlines in the negative of PC 2; B. The extreme distortion of the outlines in the positive of PC 1; C. The extreme distortion of the outlines in the negative of PC 1; D. The extreme distortion of the outlines in the positive of PC 2. Each colour of the dot in the principal component analysis diagram represents a species.

Figure 6. Scatter plot of discriminant analysis of the genus Cultellus. Each colour of the dot in the principal component analysis diagram represents a species.

Figure 7. Phylogenetic tree obtained by the Maximum Likelihood (ML), based on COI gene sequences. Numbers adjacent to nodes refer to ML bootstrap scores (BS <50 represented by " * "). The results of three species delimitation methods are shown on the right of the figure (Each species is represented by a single colour).

Figure 8. Phylogenetic tree inferred by Bayesian Inference analysis (BI) and Maximum Likelihood (ML), based on concatenated dataset of COI, 16 S and 28 S genes. Bayesian posterior probability and Maximum Likelihood bootstrap scores (left and right, respectively. " - " represents different branch position on ML and BI trees) are shown above the branch.