Acartia hudsonica is a species of marine copepod belonging to the family Acartiidae. Acartia hudsonica is a coastal, cold water species that can be found along the northwest Atlantic coast. Acartia hudsonica was originally described as a subspecies of Acartia clausi, 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 Anatomy Acartia hudsonica anatomy is different for the nauplius (larval) stage than the copepodite (juvenile) and adult stages. A nauplius has a head and a tail, but no defined abdominal region. After six stages of molting, a nauplius develops into a copepodite, which now has a distinct abdomen. After molting six more times, a copepodite will have grown enough to be considered an adult copepod. An adult copepod is usually under 1 millimeter long. Their bodies are split up into three sections: 1. the head (cephalosome); 2. the abdomen (metasome); and 3. the tail (urosome). The head has a single eye in the center with two pairs of antenna, one long and one short. Copepods also have five pairs of swimming legs that are located on the underside of the abdomen. An anatomical characteristic that distinguish A. hudsonica from other Acartia species is blue lines on the anterior of their abdomen. Distribution Geographic Like all Acartia species, A. hudsonica is found primarily in estuaries. They can be found in open coastal waters, as well, but they are less abundant in those regions. A. hudsonica is not found farther south than Chesapeake Bay, and is not found farther north than Labrador/Newfoundland. Temporal A trait specific to A. hudsonica is that they are only found in cool water. North of Cape Cod, the water stays cold enough throughout the year for A. hudsonica to be abundant year-round. However, south of Cape Cod A. hudsonica is only found in the winter and spring months. This is because the summer and fall months are too warm for the A. hudsonica population to thrive. In response to this temperature increase A. hudsonica has developed a genetic mutation that allows it to lay two different types of eggs: subitaneous and diapause eggs. Subitaneous eggs hatch immediately. Diapause eggs are laid when water temperatures rise above 16˚C and then hatch when exposed to temperatures more typical of winter and spring. Climate change is affecting the temporal