Mysidacea
Introduction

The order Mysidacea belongs to the phylum Arthropoda, subphylum Crustacea, class Malacostraca, superorder Peracarida (Bowman and Abele, 1982). The superorder Peracarida consists of 6 orders, including Spelaeogriphacea, Amphipoda, Isopoda, Tanaidacea and Cumacea, in addition to Mysidacea. With these groups, Mysidacea have in common that adult females are provided with a marsupium between pairs of pereopods and keep their embryos in it until they grow to juveniles. Hence, Mysidacea are also known as "opossum shrimps", as well as "mysids". The first mysid species was described by O. F. Müller in 1776 as Cancer flexuosus, now known as Praunus flexuosus, which occurs abundantly along coasts and estuaries of northern Europe (see Tattersall and Tattersall, 1951). Many discussions on the taxonomic position of mysids were made by carcinologists during the 19th century. Comprehensive classification systems currently used for this order were established by Boas, Calman, Hansen and several others. Historical development of the taxonomic status of Mysidacea was described in detail in "British Mysidacea" by Tattersall and Tattersall (1951). Recently, it was proposed that the suborder Lophogastrida should be elevated to the status of order (e.g. Schram, 1986). In the present chapter, however, the older classification is followed.

Mature females spawn their eggs inside their own marsupium. The brood size varies according to species, and to season and body size within the same species. In most cases, the number of eggs spawned is fewer than 100, but Mysis stenolepis breeds as many as 140-245 eggs (Amaratunga and Corey, 1975). Brood size in mysids was summarized by Mauchline (1980). At the time of spawning, a female copulates with a male, so that "eggs" produced into the marsupium are actually embryos. Hatching to larvae and subsequent growth occur in the marsupium.

Morphological changes of embryos and larvae in Neomysis intermedia are shown in Growth of embryos. The breeding period in Neomysis intermedia, which is a temperate-subarctic species, varies from 7 to15 days depending on water temperature (Murano, 1964). On the other hand, a deep-sea gigantic mysid, Neognathophausia ingens was estimated to have a period of larval development in the marsupium of 530 days (Childress and Price, 1978). In N. intermedia, release from the marsupium starts about one hour after sunset. One or two individuals at a time are released at intervals of 20-30 seconds. Liberated juveniles begin to swim and feed immediately after release. The female mysid, after releasing all the juveniles, undergoes a sequence of molting, copulation, and subsequent spawning within a night. Spawning is carried out even in the absence of males, but these non-fertilized eggs are abandoned after one or two days. Released mysids need about a month to reach the adult stage at about 20ºC (growth of N. interm. at 20 degrees C). The life-span of N. intermedia is estimated to be five months for a winter generation, two months for spring and autumn generations, and one and a half months for a summer generation. On the other hand, in N. ingens the life-span of females was estimated to be 2950 days, with the onset of reproduction at 2400 days (Childress and Price, 1978).

Mysids seem to prefer animal-matter as food, but stomach contents of those collected in coastal environments are in most cases dominated by detrital, unidentifiable materials with small amounts of diatom shells, tintinnid loricae, bodies and appendages of small crustaceans, etc. In general terms, mysids are sea-bottom scavengers. Many reports on the relationships between mysids and fish indicate that mysids can be one of the most important food organisms for the latter, especially in coastal regions where they occur in abundance. Besides fish, birds (Prince, 1980, Schneider, 1981) and whales (Hudnall, 1983) have been reported as predators of these crustaceans.

In East Asian countries, mysids are frequently caught commercially for human consumption and as food for cultured fishes. In Japan Neomysis intermedia and N. japonica are used for a popular dish known as "tsukudani", a food boiled in soy sauce, and in southeastern Asia some species (name unknown) are used in the preparation of a particular type of sauce.