Recently, a newly introduced solitary ascidian, Ascidiella aspersa (Müller, 1776), has occupied rapidly in the aquaculture fields for Patinopecten yessoensis (Jay, 1857) off the Pacific coast of northeastern Japan. In this study, utilization of the invasive ascidian by a native fish as a spawning bed was recorded on the basis of four specimens (41.2-74.3 mm in body length) collected from Ofunato Bay, Pacific coast of northern Hoshu, Japan, on January 22 and February 21, 2018. The egg derived

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... m three of them had embryo with blackish eyes and opened mouth. Compared with the previously known fishes utilizing solitary ascidians as spawning beds, the spawners of the eggs were thought as a native sculpin, Pseudoblennius cottoides (Richardson, 1848), based on the combination of clutch size, egg diameter, and melanophores in embryo. The host selectivity of the native sculpin to the newly introduced A. aspersa shown in the present study is likely due to an adaptation to the newly occupied species in its habitat.