Looks like a ciliate! Euplotes species are quite common in marine waters. However, image is not good enough to identify any further.

Thank you for your helpful answers. Yes, it looks like genus Euplotes too. Unfortunately I don't have any other photo of this specimen. I will examine about Euplotes.

In my opinion, the ciliate looks like genus Diophrys, similar with Euplotes. Because a conspicuous depression in the end of the cell (left side of the cell from the image) and the position of caudal cirri. But sorry I'm not quite sure. No more details in the photo.

In my view, it is more likely to be Diophrys than Euplotes.

Thank you for your nice answer. Indeed, the posterior shape and caudal cirri are very similar to Diophrys. But, unfortunately, I don't have any knowledge about this taxon.They are common in coastal water??

Yes, Diophrys and Euplotes are all very common in coastal water.

Today I found line figures of Euplotes spp. and Diophrys spp.in the book "The ciliated protozoa" (Corliss, JO). I agree with Zhuo's opinion; as far as I can see, this specimen is more similar to Diophrys than Euplotes. Apparently some studies reported Diophrys can be found in coastal water in Japan. Thank you very much again.

My pleasure~