Mysida

Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in this pouch and are not free-swimming characterises the order. The mysid's head bears a pair of stalked eyes and two pairs of antennae. The thorax consists of eight segments each bearing branching limbs, the whole concealed beneath a protective carapace and the abdomen has six segments and usually further small limbs.

Mysids are found throughout the world in both shallow and deep marine waters where they can be benthic or pelagic, but they are also important in some fresh water and brackish ecosystems. Many benthic species make daily vertical migrations into higher parts of the water column. Mysids are filter feeders, omnivores that feed on algae, detritus and zooplankton. Some mysids are cultured in laboratories for experimental purposes and are used as a food source for other cultured marine organisms. They are sensitive to water pollution, so are sometimes used as bioindicators to monitor water quality.

Description

The head of a mysid bears two pairs of antennae and a pair of large, stalked eyes. The head and first segment (or sometimes the first three segments) of the thorax are fused to form the cephalothorax. The eight thoracic segments are covered by the carapace which is attached only to the first three. The first two thoracic segments bear maxillipeds which are used to filter plankton and organic particulate from the water. The other six pairs of thoracic appendages are biramous (branching) limbs known as pereopods, and are used for swimming, as well as for wafting water towards the maxillipeds for feeding. Unlike true shrimps (Caridea), females have a marsupium beneath the thorax. This brood pouch is enclosed by the large, flexible oostegites, bristly flaps which extend from the basal segments of the pereopods and which form the floor of a chamber roofed by the animal's sternum. This chamber is where the eggs are brooded, development being direct in most cases.

The abdomen has six segments, the first five of which bear pleopods, although these may be absent or vestigial in females. The fourth pleopod is longer than the others in males and has a specialized reproductory function.

Behavior

thumb|left|Mysis relicta

Some species are benthic (living on the seabed) and others pelagic (living in mid-water), but most are found close to, crawling on or burrowing into the mud or sand. Most marine species are benthic by day but leave the seabed at night to become planktonic. Locomotion is mostly by swimming, the pleopods being used for this purpose. Some mysids live among algae and seagrasses, some are solitary while many form dense swarms. Mysids form an important part of the diet of such fish as shad and flounder. The species Mysidium integrum has a mutualistic relationship with longfin damselfish, the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators.

The majority of Mysida are omnivores, feeding on algae, detritus, and zooplankton. Scavenging and cannibalism are also common, with the adults sometimes preying on their young once they emerge from the marsupium. The age at which mysids reach sexual maturity depends on water temperature and food availability. The young are released soon afterwards, and although their numbers are usually low, the short reproductive cycle of mysid adults means a new brood can be produced every four to seven days.

Uses

Some species of mysids are easy to culture on a large scale in the laboratory as they are highly adaptive, and can tolerate a wide range of conditions. Despite low fecundity, these species have a short reproductive cycle which means they can quickly reproduce in vast numbers. In flow-through systems, juvenile mysids are continuously separated from the adult brood stock in order to reduce mortality due to cannibalism. Their high protein and fat content also makes them a good alternative to live enriched Artemia when feeding juveniles (especially those that are difficult to maintain such as young seahorses) and other small fauna.

Systematics

The Mysida belong to the superorder Peracarida, which means "near to shrimps". Although in many respects mysids appear similar to some shrimps, the main characteristic separating them from the superorder Eucarida is their lack of free-swimming larvae. The order Mysida is extensive and currently includes approximately 160 genera, containing more than 1000 species.

Traditionally, Mysida were united with another, externally similar group of pelagic crustaceans, the Lophogastrida, into a broader order Mysidacea, but that classification is generally abandoned at present.

While the previous grouping had good morphological support, molecular studies do not corroborate the monophyly of this group. Previously Mysida included two other families, Lepidomysidae and Stygiomysidae, but these have now been placed in a separate order, Stygiomysida.

Subfamily Boreomysinae Holt & Tattersall, 1905
2 genera
Subfamily Erythropinae Hansen, 1910
54 genera
Subfamily Gastrosaccinae Norman, 1892
10 genera
Subfamily Heteromysinae Norman, 1892
14 genera
Subfamily Leptomysinae Hansen, 1910
30 genera
Subfamily Mysidellinae Czerniavsky, 1882
2 genera
Subfamily Mysinae Haworth, 1825
55 genera
Subfamily Palaumysinae Wittmann, 2013
1 genus
Subfamily Rhopalophthalminae Hansen, 1910
1 genus
Subfamily Siriellinae Norman, 1892
3 genera

Family Petalophthalmidae Czerniavsky, 1882
Genus Bacescomysis Murano & Krygier, 1985
Genus Ceratomysis Faxon, 1893
Genus Hansenomysis Stebbing, 1893
Genus Parapetalophthalmus Murano & Bravo, 1998
Genus Petalophthalmus Willemoes-Suhm, 1875
Genus Pseudopetalophthalmus Bravo & Murano, 1997

Description

Behavior

Uses

Systematics

External links

References