Idas pacificus, Wu & Lin & Qiu & Xu & Xing, 2025

Wu, Qiong, Lin, Yi-Tao, Qiu, Jian-Wen, Xu, Mei Yu & Xing, Bing Peng, 2025, Two new species of Idas (Bivalvia, Mytilidae) from sunken wood in the East China Sea: description, phylogenetic position and symbionts, Zoosystematics and Evolution 101 (2), pp. 761-778 : 761-778

Idas pacificus sp. nov.

Figs 3 A – G , 4 A – E , 7 , 8 A ( Idas sp. OTU- 51 mentioned earlier; Idas sp. ESU D, as mentioned by Lorion et al. (2010 ))

Materials examined.

All 23 specimens were collected from a piece of wood by trawling at 460–550 m seabed at 30.7°N, 127.8°E in April 2023.

Type material.

Holotype • TIO 2023-1816 B 14 (length: 10.8 mm) ; Paratypes • six specimens, TIO 2023-1816 B 9 , TIO 2023-1816 B 12 , TIO 2023-1816 B 15 , TIO 2023-1816 B 24 , TIO 2023-1816 B 29 , TIO 2023-1816 B 30

Diagnosis.

Adult specimens approximately measure 10 mm in length, resembling the general outline of Idas ghisottii Warén & Carrozza, 1990 from the Mediterranean region. Shells with an H / L ratio of about 0.36 to 0.45, maintaining a consistent overall shape across different sizes, though older individuals are slightly more elongated. Umbo located at approximately the anterior 1 / 7 to 1 / 9 of the shell length. The triangular area, from the posterior margin to the umbo, near the dorsal margin, is covered with bristles that match the external shell surface color. The base of the bristles is triangular in shape.

Shell morphology.

Shells 3.2–10.8 mm in length, 1.4–4.1 mm in height (Fig. 3 A – G ). Shells thin, olive-yellow, semi-transparent, with prominent concentric growth lines. Shells oval, with straight dorsal margin, slightly incurved ventral outline, and rounded posterior margin. Triangular area near dorsal margin, from posterior margin to umbo, sparsely covered with bristles. Umbo located at approximately the anterior 1 / 7 to 1 / 9, shifting forward with increasing shell size and tilting slightly forward. Hinge plate slender and straight (Fig. 4 A – E ). Both inner and outer shell surfaces with pearly luster. Inner shell surface smooth, faint traces of anterior adductor scar and pallial line, difficult to discern under optical microscope. Posterior adductor scar large, nearly circular; anterior adductor scar shallow, curved droplet-shaped (SEM images in Fig. 7 C ). Pallial sinus indentation small. Prodissoconch pink, rounded, smooth, ~ 400 µm in diameter.

Soft tissue anatomy.

Foot well-developed, approximately 40 % of shell length (Fig. 7 ). Like I. macdonaldi , byssal retractors continuous and not divided into posterior and anterior portions ( Gustafson et al. 1998). Byssus retractor muscle and pedal retractor muscle fused into a single strand. Cross-section of anterior adductor muscle teardrop-shaped, posterior adductor muscle round. No byssus observed.

Gill-associated microbes.

The 13 specimens of Idas pacificus sp. nov. share 11 common microbial species (Fig. 6 ), with 7 of these accounting for more than 1 % of the total read abundance. Specifically, three species belong to Gammaproteobacteria, two to Bacteroidia, and two to Campylobacteria. In addition, genetic diversity among symbionts from different Idas pacificus sp. nov. specimens is notable, with 11 different SUP 05 cluster genotypes detected among the 13 specimens (Fig. 5 ). ASV 2 is the most common SUP 05 genotype, dominating in 6 of the specimens where it accounts for over 50 % of all SUP 05 sequences. In the remaining specimens, the predominant SUP 05 genotypes include ASV 4, ASV 10, ASV 96, ASV 144, and ASV 325.

Habitat and distribution.

The Idas pacificus sp. nov. specimens lived inside holes in a piece of sunken wood. These holes are typically round, each containing only a single individual. Unlike those of I. iwaotakii and Idas sinensis sp. nov., the holes occupied by I. pacificus sp. nov. are black (Fig. 8 A ), which may be due to FeS precipitation ( Bienhold et al. 2013). This hypothesis is supported by the functional heatmap of the microbes recovered from the gill tissues (Fig. 6 B ), which showed that I. pacificus sp. nov. had a high abundance of sulfur-oxidizing bacteria. Due to the limited samples of I. iwaotakii and Idas sinensis sp. nov. and the fact that all analyzed Idas samples came from the same piece of wood, we could not determine the generality of this phenomenon.

The type specimens were collected from sunken wood in the East China Sea at depths of approximately 460–550 meters. Idas pacificus sp. nov. was reported on sunken wood in Philippines and Vanuatu as Idas sp. ESU D by Lorion et al. (2009), and experimental bones placed in Japanese waters ( Lorion et al. 2013). Comparing our DNA barcode data for Idas pacificus sp. nov. with those available on GenBank indicates that it is conspecific to Idas sp. ESU D (GenBank Acc. No. HF 545116 , EU 702350 , EU 350071 , EU 702358 ), with K 2 P distances of 0–0.3 %, being smaller than the inter-specific genetic divergence between other species of Idas (e. g. 19.8 % between I. argenteus and I. washingtonius , and 2.8 % between I. macdonaldi and I. modiolaeformis, See Suppl. material 8 for details.). Therefore, Idas pacificus sp. nov. may be widespread in the Western Pacific, including East China Sea (this study), Santo Island in Vanuatu ( Lorion et al. 2009), Bohol Sea ( Duperron et al. 2009), and Japanese waters ( Lorion et al. 2013), with water depth ranging from 460 to 1764 meters ( Duperron et al. 2009; Lorion et al. 2013).

Etymology.

Since our comparison with DNA barcode data obtained from GenBank indicates this species to be widely distributed in the Western Pacific, the name “ pacificus ” was given to the species.

Remarks.

Idas pacificus sp. nov. is notably different from I. iwaotakii , which has a very elongated shape, a sharply pointed posterior margin, and shorter and darker bristles that appear to degrade with increasing size. Bristles are also a key distinguishing feature that separates this species from I. washingtonius , I. coppingeri , and I. indicus . The first two species lack bristles ( Murray et al. 1885; Harbo and Gillespie 2021), while I. indicus has very short bristles and a more pronounced curvature of the ventral outline ( Smith 1899).

Figures

Figure 3. Specimen photographs of Idas pacificus sp. nov. and Idas sinensis sp. nov. A – G. Idas pacificus sp. nov.: A. Sample ID: TIO 2023-1816 B 30, paratype, left valve; B. Sample ID: TIO 2023-1816 B 29, paratype, left valve; C. Sample ID: TIO 2023-1816 B 24, paratype, left valve; C’. Dorsal view; D. Sample ID: TIO 2023-1816 B 9, paratype, left valve; E. Sample ID: TIO 2023-1816 B 12, paratype, left valve; F. Sample ID: TIO 2023-1816 B 15, paratype, left valve; G. Sample ID: TIO 2023-1816 B 14, holotype, left & right valve; G’. Dorsal view; H, I. Idas sinensis sp. nov.: H. TIO 2023-1816 B 21, paratype, left valve; I. TIO 2023-1816 B 6, holotype, left & right valve; I’. Dorsal view. Scale bar: 2 mm.

Figure 4. Photographs of the hinge structures and prodissoconch of the specimens of Idas pacificus sp. nov. and Idas sinensis sp. nov. A – E. Idas pacificus sp. nov. Sample ID: TIO 2023-1816 B 7, length: 8.3 mm. A. Hinge line of specimen; B. Hinge denticles located immediately below the umbo; C. Hinge denticles located posterior of the ligament; D. Prodissoconch; E. Ventral view of the larval shell (left valve); F, G. Idas sinensis sp. nov. Sample ID: TIO 2023-1816 B 6, length: 9.3 mm. hinge line of specimen; F. Prodissoconch; G. Hinge line of specimen (left valve). Scale bars: 200 μm (B – E); 500 μm (F); 1 mm (G).

Figure 5. The ML phylogenetic tree of gill-associated microbiota from 16 Idas samples obtained in this study. Numbers near the nodes represent bootstrap values (only values > 75 % are shown). Branches with a blue background indicate bacterial species shared by 13 Idas pacificus sp. nov. samples. Stacked bar chart adjacent to the phylogenetic tree shows the relative abundance of each ASV in different species (expressed as the percentage of sequences averaged per individual). Venn diagram displays the symbiont species shared among the 13 Idas pacificus sp. nov. individuals.

Figure 6. Gill-associated Bacterial Community Analysis of Idas. A. Relative abundance; B. Functional heatmap generated by FAPROTAX; C. Community heatmap.

Figure 7. Internal View of Idas pacificus sp. nov. A. Sample ID: TIO 2023-1816 B 9, lateral view; B. Schematic drawing of the right shell and internal structures based on TIO 2023-1816 B 9, the positions of the gill and mantle confirmed across multiple samples; C. SEM image showing the anterior adductor scar, pallial line, posterior adductor scar, and pallial sinus, sample ID: TIO 2023-1816 B 7. Note that the posterior margin of the specimen is incomplete. Abbreviations: aa: anterior adductor; ar: anterior byssus retractor; at: anterior hinge teeth; f: foot; g: gill; h: hinge; m: mantle; pa: posterior adductor; ph: pedal heel; ppr: posterior byssus retractor and pedal retractor; pt: posterior hinge teeth. Scale bars: 2 mm.

Figure 8. Situ views of the A. Idas pacificus sp. nov. and B. I. iwaotakii. Both images show the specimens with portions of the wood removed to expose them. Scale bars: 2 mm.