First record of <i>Macrophthalmus japonicus</i> (De Haan, 1835) (Brachyura: Macrophthalmidae) population from the Pacific coast of Iwate Prefecture, northern Japan

Abstract

The tidal flats at Otomo-ura (Iwate Prefecture), which had been converted to farmland through a reclamation project from 1959 to 1969, have been partially restored after the tsunami caused by the Great East Japan Earthquake in 2011. In the course of monitoring the benthic fauna of the tidal flats for 11 years after the tsunami, we found some adult sentinel crabs of the genus Macrophthalmus at a muddy area for the first time on July 28, 2022. During subsequent observations of the mud flat, we noted that the adult male crabs exhibited waving display of vertical non-forward-pointing type. Allocleaning, performed by both sexes, was also observed. Morphological characters include: the carpus and propodus of the ambulatory leg 3 of the adult male are not associated with tuft of setae on their ventral surfaces, and the subdistal tooth of leg 3 is not distinct. Based on these behavioral and morphological characteristics, the macrophthalmid crab was identified as Macrophthalmus japonicus. This is the first record of this species along the coast of Iwate Prefecture. In addition, five of six females captured on July 28 and August 14, 2022, were ovigerous (range in carapace width: 19.1–25.0 mm), which indicates that the crabs are reproducing on the tidal flat.