Atlas of Crustacean Larvae

38. Lophogastrida and Mysida

38 general: Lophogastrids and mysids are shrimp-like crustaceans that are known as opossum shrimps, due to the presence of a ventral brood pouch (marsupium) in mature females. They are readily distinguished from other shrimplike crustaceans, such as euphausiids and carideans, by the presence of a statocyst in the proximal part of the endopod of the uropods. The statocyst is missing in the order Lophogastrida (the families Lophogastridae, Gnathophausiidae, and Eucopidae) and in the family Petalophthalmidae of the order Mysida. The marsupium is composed of either seven pairs of oostegites on the thoracopods (the Lophogastrida, Petalophthalmidae, and Boreomysinae) or two or three pairs (the Gastrosaccinae, Siriellinae, Ropalophthalminae, Mysidellinae , and Mysinae). The Lophogastrida and Mysida currently include 58 and 1,106 species, respectively (G. Anderson 2010a, 2010b). Lophogastrids are marine pelagic swimmers and have a cosmopolitan oceanic distribution. The majority of mysid species are marine (mainly coastal) and live in a suprabenthic (hyperbenthic) habitat. Although there are only a few freshwater species, they are widespread and often numerous; thus they are of ecological importance. The life cycle consists of an embryonic stage, a nauplioid stage, a post-nauplioid stage (intramarsupial stages), a juvenile, and an adult, all of which are free living. Developmental stages are all lecithotrophic. larval types: Breeding females of both the Lophogastrida and Mysida carry their embryos and larvae in a marsupium , within which their entire embryonic and larval development takes place. Young individuals emerge from the marsupium as early juveniles; only in Boreomysis arctica have larval stages occasionally been found in the plankton (Jepsen 1965). In non-ovigerous females, the mature ovary fills the posterior dorsolateral regions of the thorax, and the eggs present in the ovary can be easily observed through the carapace. Within the marsupium, the eggs emerge from the external genital openings of the oviducts, which are located on the body near the bases of the sixth pair of thoracopods. Eggs sensu stricto do not occur in the marsupium, since they are fertilized immediately once they are extruded from the oviducts (Mauchline 1980). As described by Mauchline (1980), there are three main phases of marsupial development: the egg, an eyeless larva, and an eyed larva. Confusion has persisted in the literature, however, over the use of such terms as egg, embryo, and larva (Mauchline 1980; Cuzin-Roudy and Tchernigovteff 1985). The terminology used in this chapter follows Wittmann (1981a) in defining embryos as the developmental stage inside the egg membrane, and larvae (nauplioid and post-nauplioid) as the stages occurring after hatching. Embryos: After fertilization, the development of the spherical embryo inside the egg membrane is termed the embryonic phase, which ends with the shedding of the egg membrane. The early embryos are spherical or subspherical, and their size is closely similar to that of ripe eggs within the oviducts. The term embryonic stage (Wittmann 1981a) refers to this egg-like embryo, which was called the “egg” by Mauchline (1972, 1973) or “early embryo” (stage 1 larva) by Mauchline (1980). Nauplioid, Post-Nauplioid: Two successive main phases of post-embryonic development, differentiated by hatching (ecdysis ) of the larvae, occur within the marsupium of lophogastrid and mysid females (K. B. Nair 1939; Matsudaira et al. 1952; Jepsen 1965; Davis 1966, 1968; Childress and Price 1978). These phases correspond to the stages described by Mauchline (1972, 1973) as eyeless larvae and eyed larvae (Mauchline 1980), and to the stages described as nauplioid and post-nauplioid stages by Wittmann (1981a). The emerging nauplioid form is immobile , yolk-filled, and encased in an unsegmented cuticle. It has the same appendages as a classic nauplius, but it lacks the nauplius eye and swimming setae. Later in this phase, all body segments and appendages of the adult are formed. The first larval molt results in the non-motile post-nauplioid form, which continues to rely on yolk reserves and has all its appendages, its carapace, and its eyes free. A second ecdysis, Carlos San Vicente Guillermo Guerao Jørgen Olesen Carlos San Vicente, Guillermo Guerao, and Jørgen Olesen Lophogastrida and Mysida 200 Carlos San Vicente, Guillermo Guerao, and Jørgen Olesen just before or after liberation from the marsupium, yields a free-living juvenile. Depending on the species, the three intramarsupial phases (embryonic, nauplioid, and post-nauplioid) can be divided further into substages, based on successive morphological changes (Wittmann 1981a; Cuzin-Roudy and Tchernigovtzeff 1985; Greenwood et al. 1989; Wortham-Neal and Price 2002; Fockedey et al. 2006; Ghekiere et al. 2007). Development within a brood is usually synchronous...