Key to Australian Freshwater and Terrestrial Invertebrates
Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Order Mysidacea
Common names: mysids, opossum shrimps
Overview
Mysidacea are small, shrimp-like aquatic crustaceans found in marine and freshwater environments. Mysids have stalked eyes on a head that is fused with up to four of the thoracic segments and covered by a well-developed carapace, which may also be fused to the anterior abdominal segments. Mysids can most easily be separated from decapod prawns and shrimps by the number of thoracic appendages: six or seven in mysids; five in decapods. Mysids may also possess a distinctive pair of statocysts (sac-like organs used for balance) in the tail fan (uropods), whereas in decapod prawns these are typically associated with the antennule and not obviously visible. Their pereopods (thoracic legs) are biramous (separated into two branches), except sometimes the last pair, which may be reduced. Their pleopods (swimming legs) are reduced and may be modified in males. Mysids are often referred to as �opossum shrimps� due to the peracaridian oostegites (plates attached to the front legs) that form a ventral brood pouch (or marsupium) in mature females. Mysidaceans are typically transparent and range in size from about 6-350 mm in length, with the majority of species being >25 mm.
Distribution and diversity
Mysids are found globally with the majority of species inhabiting coastal and open ocean waters, with a handful of species recorded continental freshwater and groundwater habitats. Worldwide there over 1,000 described species in around 160 genera. Around 146 species from 45 genera are described from Australian waters, mostly from marine habitats with a small number from surface and groundwaters.
Life cycle
Receptive females of some species are known to release a pheromone into the water that attracts males. Mating occurs at night and during copulation, males deposit sperm into the brood pouch of the female. Eggs are deposited into the brood pouch soon after mating where they are fertilised. Females brood the eggs, which take weeks to several months to hatch, depending on species and water temperature. Well-developed juveniles hatch from the eggs and are released as soon as they hatch. Females can produce several broods during their adult lives.
Feeding
The majority of Mysidacea are filter feeders and generally regarded as omnivores, feeding on algae, detritus, and zooplankton. Mysids typically feed in one of two ways: large food particles are taken with the thoracic legs and the food is consumed while being held by the legs or by grooming tiny food particles that settle on the body and appendages, and/or get trapped in setae. Some species are also known to be active filter feeders, generating a water current from which food particles are filtered. A few species are predators on zooplankton and some are scavengers, feeding on carcasses of copepods, amphipods and even other mysids.
Ecology
Mysids species occur in marine, brackish and occasionally fresh water. Several troglobitic (cave-dwelling) species are also known. In both marine and freshwater habitats, mysids can often be very abundant, forming large shoals, making them important in the diet of many fish.
