Semi-aquatic warm-temperate annual fern, Ceratopteris gaudichaudii Brongn. var. vulgaris Masuyama et Watano, was collected on October 4, 2019 in a rice paddy where traditional farming practices were used in Matsushima, Miyagi Prefecture, Japan. The collection site (38.4°N) is north of the previous northernmost recorded location in Yamagata (38.1°N) on the Sea of Japan side and in Fukushima (37°N) on the Pacific side. Furthermore, the location in Matsushima is the northernmost range of the genus Ceratopteris in the world. Warmth index near the collection site (87.9) is close to the value in Yamagata (88.1) but considerably lower than in Fukushima (103.1). Characteristics such as the ferns' small size and some sporophytes without fertile leaves, even in early October, indicate the severe conditions of the collecting site for this species. The climatic condition at the collection site may be close to the limit of compensatory progenetic adaptation to a shorter growth period and subsequent dwarfism in the cold climates.
