The Mysidacea is a group of small, shrimp-like creatures comprising the two related orders Mysida and Lophogastrida. They are sometimes referred to as opossum shrimps though that name is also used for individual species (e.g. Neomysis americana).
Note that despite their name, and their superficial resemblance to shrimp, they are only quite distantly related to the true shrimps, which are classified in the order Decapoda. The characteristics of the Mysidacea include the following:
The name "opossum shrimp" is derived from the females' brood pouch, formed by plates attached to the front legs. The larvae are carried in the pouch until they are able to move by themselves. Although they most commonly occur at sea, they are abundant in certain bodies of fresh water, such as the Great Lakes; some are occasionally found in caves. Wherever mysids occur, whether in salt or fresh water, they are often very abundant and form an important part of the normal diet of many fishes. In many Asian countries, mysids are a common part of local cuisine, and in certain regions, such as the Gulf of Thailand, they support extensive fisheries. Mysids are also used as a bioassay for toxins in seawater.
Although this traditional group has had strong morphological support, recent studies question the monophyly of this grouping. Of particular importance are the molecular studies by Spears et al. (2005) (basal to a Eucarida subtree) and by Meland and Willassen (2007) (who enriched the 26 taxa dataset with an additional 26 taxa from Mysidacea), which remove Mysida from the Peracarida entirely (sister to a krill + mantis shrimp clade). Both nodes however have low bootstrap support (59% and 52% respectively) and do not have much morphological support (Reviewed by Poore, 2005).