Opossum Shrimp

What are the key characteristics that define opossum shrimp?

Opossum shrimp are small crustaceans belonging to the order Mysidacea. They are defined by several key characteristics:¹²

• Brood Pouch: The name “opossum shrimp” refers to the specialized brood pouch, or marsupium, found in females. This structure, formed by extensions from the thoracic limbs, allows embryos to be carried and protected for several weeks.¹²³
• Size: Most species are relatively small, typically measuring between 1 and 3 cm (approximately 0.4 to 1.2 inches) in length.²
• Habitat: While nearly 1,000 species exist, the majority inhabit marine environments, particularly in cold, deep waters. Some species are found in brackish or fresh water, such as Mysis relicta in North America and Europe.²
• Behavior: Depending on the species, they may burrow into the bottom, creep among vegetation, or swim in open water, sometimes forming large swarms. Certain cave-dwelling species exhibit adaptations like blindness or poorly developed eyes.²

How do opossum shrimp differ from other crustaceans?

Opossum shrimp, members of the order Mysidacea, are distinguished from many other crustaceans primarily by their unique reproductive strategy.

The defining characteristic is the presence of a specialized thoracic brood pouch, or marsupium, in females. Unlike many crustaceans that shed fertilized eggs freely into the water to hatch as free-swimming larvae, opossum shrimp carry their embryos within this pouch for several weeks. Because the embryos develop within this pouch, the typical free-swimming larval stages found in other crustaceans are suppressed, and the young hatch as immature forms of the adult.²¹

While some other groups, such as Isopoda and Amphipoda, also utilize a thoracic brood pouch, opossum shrimp are specifically identified by their shrimp-like appearance, stalked eyes, and the presence of six pairs of leglike appendages that each bear an exopodite. These features, combined with their specific brooding behavior, set them apart within the broader classification of malacostracan crustaceans.³⁵¹

What is the typical habitat and distribution of opossum shrimp?

Opossum shrimp, which belong to the order Mysidacea, exhibit a diverse distribution across global aquatic environments. While there are nearly 1,000 known species, the vast majority are marine organisms that typically inhabit cold, deep waters.²⁵

Their habitat preferences vary significantly by species:

• Marine Environments: Most mysids are found in the sea, ranging from shallow coastal areas to the deep ocean.²⁵
• Fresh and Brackish Water: A smaller number of species have adapted to brackish or freshwater environments. Notably, Mysis relicta is a well-known freshwater species found in cold lakes across North America, Great Britain, and northern Europe.²⁵
• Specialized Habitats: Some species have adapted to unique environments, such as cave-dwelling mysids like Heteromysis cotti in the Canary Islands, which often display reduced or absent vision.²

Within these habitats, they may be found burrowing into the bottom, creeping among vegetation, or swimming in open water, where they occasionally form massive swarms.²

What is the taxonomic classification of opossum shrimp?

The opossum shrimp is classified within the subphylum Crustacea, which encompasses a diverse group of arthropods. Specifically, they belong to the order Mysidacea.²⁶

Within this order, the family Mysidae serves as a primary taxonomic group, consisting of small, shrimp-like crustaceans characterized by stalked eyes and six pairs of leglike appendages, each featuring an exopodite. Historically, the order Mysidacea was sometimes grouped with the Euphausiacea under the name Schizopoda, though they are now recognized as distinct.⁵⁹⁶