Acartia hudsonica 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 originally described as a subspecies of 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...

but subsequent research concluded it is sufficiently distinct to warrant specific status. It is found in shallow coastal habitats along both the Atlantic and Pacific coasts of northern North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

This species 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).