Ylistrum japonicum japonicum (Gmelin, 1791)
GMELIN, J. F. 1791. Caroli a Linné, Systema Naturae per Regna Tria Naturae, secundum classes, ordines, genera, species, cum characteribus differentis, synonymis, locis. Tomus I. Pars VI. Classis VI. Vermes Species, cum characteribus, differentiis, synonymis, locis. 13th edition, Aucta, Reformata, 3021-3910 p. Impensis Georg Emanuel Beer, Lipsiae [Leipzig], 1791. [p. 3317, sp. 8]
1791 Ostrea japonica Gmelin, 1791
"Amusium Japonicum maximum"; J. H. Chemnitz, 1784, Neues Systematisches Conchylien Cabinet, plate 62, figure 596.
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«O. testa aequivalvi convexiuscula flavo marginata: valvae superioris lineis obsoletis fascias transversas concentricas decussantibus: striis intus elevatis quadraginta octo.
Chemn. Conch. 7. t. 62. f. 596. Habitat rarior ad Guineae et Japoniae littora, pleuronecti affinis, at testa crassiore, auriculis subtiliter rugosis, et infra nodo auctis, valva inferiore ochroleuca glaberrima, testa ad 5½ pollices longa et lata.» JOHANN FRIEDERICH GMELIN, 1791
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«Remarks: Valve colour and the pattern of concentric lines and spots differ between the two Ylistrum species. In Y. japonicum, auricles generally are darker than the base colour of the left valve and only Y. balloti has spots in a concentric pattern on both left and right valves. Internal rib count could not be used to differentiate between the two Ylistrum species as counts had completely overlapping ranges. Their geographical distributions are not known to overlap. Additional molecular sampling will be needed to address the validity of species within Ylistrum.
Comparison of genera: Ylistrum can be separated from Amusium based on several morphological features, including size, number of ribs and coloration (Table 4). Ylistrum species are typically larger, reaching 120 mm in height as compared to 100 mm in Amusium; however, number of ribs and coloration are better characteristics to separate the two genera as they do not appear to be age dependent.
Ribbing data have been used previously to define the three different Amusium s. l. species (Fig. 4; Bernardi, 1861; Habe, 1964; Dijkstra, 1988), but their reliability has not been rigorously tested. Our statistical analysis suggests that internal ribbing is a reliable character in distinguishing between the two genera. Ylistrum has a significantly greater number of internal ribs on both valves (Table 1; Supplementary Material, Appendix SB), with 30–45 and 36–51 ribs on left and right valves, respectively. On average, the number of ribs in Ylistrum differs between left and right valves, the right valve always having a greater number of ribs than the left. In contrast, Amusium bears between 19–27 internal ribs on the left valve and 20–30 ribs on the right and, on a single individual, the number of ribs tends to be similar between valves. Coloration of the left valve is variable in both genera, but general patterns can be observed. Ylistrum typically has a reddish hue and lacks conspicuous radial lines on the exterior left valve (Fig. 1D, G, J), while Amusium appears cream, pinkish or light brown and bears obvious radial lines that are bluish or purple (Fig. 1A). It is important to note that colour may be variable when examining specimens from different localities, in which case ribbing may be more reliable in separating the two genera (Fig. 1B, C cf. E, F, H, I, K, L).» MYNHARDT, G., A. ALEJANDRINO, L. PUSLEDNIK, J. CORRALES & J. M. SERB. 2014. Shell shape convergence masks biological diversity in gliding scallops: description of Ylistrum n. gen. (Pectinidae) from the Indo-Pacific Ocean. Journal of Molluscan Studies, 80 (4): 400-411, figs. 1-5. [p. 408, 409]
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Ylistrum japonicum (Gmelin, 1791), new combination; G. Mynhardt, A. Alejandrino, L. Puslednik, J. Corrales & J. M. Serb, 2014, Shell shape convergence masks biological diversity in gliding scallops: description of Ylistrum n. gen. (Pectinidae) from the Indo-Pacific Ocean, figures 1D-1F.
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«A broken left valve and several fragments of young indiviuals. The surface is smooth with radiating ribs inside arranged two and two.
Fossil occurrence.— Upper Byoritz Beds: (1) Tenshi, Shiko Shō (四湖庄店仔) and (2) Enri (苑複) Shinchik (新竹州首栗郡); (3) Tanbunko, Shinchik (同州竹南郡淡文湖). Living.— Central and Western Japan. China.» YOKOYAMA, M. 1928. Mollusca from the Oil-Field of the Island of Taiwan. Imperial Geological Survey of Japan, Report, 101: 1-112, pls. 1-18. [p. 95]
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Pecten (Amusium) japonicus (Gmelin): M. Yokoyama, 1928, Mollusca from the Oil-Field of the Island of Taiwan, plate 13, figure 12.
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