Acartia omorii is a species of marine copepod

Copepod

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

 belonging to the family Acartiidae

Acartiidae

Acartiidae is a family of calanoid copepods distinguishable by the rostral margin not being extended. They are epipelagic, planktonic animals, not being found below a depth of . There are over 100 described species distributed throughout the world's oceans, mainly in temperate areas....

. This species was discovered when specimens previously identified as Acartia clausi

Acartia clausi

Acartia clausi is a species of marine copepod belonging to the family Acartiidae. This species was previously thought to have a worldwide distribution but recent research has restricted its range to coastal regions of the north-east Atlantic Ocean as far north as Iceland, the Mediterranean Sea and...

were examined and found to belong to a separate species. This species is found around the coast of Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. It is similar to A. clausi but lacks the prominent spines on the dorsal

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 part of the posterior body segment (metasome).