Abstract
The polyclad flatworm Stylostomum ellipse (Dalyell, 1853) has hitherto been recorded from the Antarctic region, Mediterranean Sea, Patagonian region, Scandinavia, South Africa, and South Georgia Island. In this study, we report S. ellipse for the first time from the Pacific Ocean based on specimens collected in Hokkaido, northern Japan. Our specimens are morphologically identifiable as S. ellipse, but may represent a biologically different species from a population of the Mediterranean Sea. This is because, based on the previous genetic data of other cotylean species, the observed uncorrected p-distance 0.02160 between the two distinct populations in terms of a partial 972 bp region of the 28S rDNA sequence may be great enough to separate the species biologically.
Highlights
The euryleptid flatworm genus Stylostomum Lang, 1884 sensu Holleman (2001) is characterized by the oval body shape and the small to moderate size; the mouth and the male gonopore in common opening to the exterior; and the tentacles reduced to small stumps or lacked
We report S. ellipse for the first time from the Pacific Ocean based on specimens collected in Hokkaido, northern Japan
We present the first record of S. ellipse from the Pacific Ocean
Summary
The euryleptid flatworm genus Stylostomum Lang, 1884 sensu Holleman (2001) is characterized by the oval body shape and the small to moderate size; the mouth and the male gonopore in common opening to the exterior; and the tentacles reduced to small stumps or lacked. Stylo stomum consists of nine valid species: S. ellipse (Dalyell, 1853); S. felinum Marcus, 1954; S. frigidum Bock, 1931; S. hozawai Kato, 1939; S. lentum Heath & McGregor, 1912; S. maculatum Kato, 1944; S. mixtomaculatum Pitale & Apte, 2019; S. sanjuania Holleman, 1972; and S. spanis Holleman, 2001. Stylostomum ellipse is known to have the most discontinuous distribution among the polyclads (Prudhoe 1985). This species has been recorded in both the northeastern and southern parts of the Atlantic: Scandinavia to Mediterranean Sea, South Africa, the Patagonian region, South Georgia Island, and the Antarctic region (Fig. 1A). We present the first record of S. ellipse from the Pacific Ocean
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