Phylogenetic relationships within the Mysidae (Crustacea, Peracarida, Mysida) based on nuclear 18S ribosomal RNA sequences.

Species of the order Mysida (Crustacea, Peracarida) are shrimp-like animals that occur in vast numbers in coastal regions of the world. The order Mysida comprises 1,053 species and 165 genera. The present study covers 25 species of the well-defined Mysidae, the most speciose family within the order Mysida. 18S rRNA sequence analysis confirms that the subfamily Siriellinae is monophyletic. On the other hand the subfamily Gastrosaccinae is paraphyletic and the subfamily Mysinae, represented in this study by the tribes Mysini and Leptomysini, consistently resolves into three independent clades, and hence is clearly not monophyletic. The tribe Mysini is not monophyletic either, and forms two clades of which one appears to be closely related to the Leptomysini. Our results are concordant with a number of morphological differences urging a taxonomic revision of the Mysidae.

A Comparative Study of Static Bodies in Mysid Crustaceans: Evolutionary Implications of Crystallographic Characteristics.

We studied the mineral composition of statoliths in 154 species belonging to 55 genera of Mysidae. Fluorite (CaF2) was found in 86% of Recent species, vaterite (CaCO3) in 9%, and no crystalline component in 5%. Seven samples of fossil statoliths from Upper Miocene deposits were exclusively calcite (CaCO3). Vaterite has the peak of occurrence in fresh water, fluorite in the photic zone of marine waters, and organic statoliths in oceanic deep waters. With respect to population numbers in the different aquatic biota, vaterite prevails in freshwater species and fluorite is dominant among species in all brackish to marine environments. The occurrence of CaCO3 in fresh to brackish waters coincides with fossil records and biogeographical observations. The Ponto-Caspian region is the center of abundance for Recent CaCO3-precipitating species. The rich brackish to freshwater fauna in this region probably has its roots in the brackish Paratethys, where a rich fossil material of calcareous mysid statoliths is known from Upper Miocene sediments. Morphological and scarce palaeontological evidence suggests that the earliest (Carboniferous to Jurassic) Mysidacea were mainly oceanic shrimps without statocysts; these were followed by (bentho) pelagic animals with nonmineralic organic statoliths. With the colonization of coastal to littoral areas by benthopelagic to benthic forms, mineralic statoliths were formed by precipitation of fluorite. Among the modern Mysidae, a special development occurred (in the Miocene) in the Ponto-Caspian region where CaCO3 statoliths appeared in brackish to freshwater forms. As in vertebrates, the patterns of mineral composition of static bodies in the Mysidae reflect both anatomical and ecophysiological differences.

Structure of recent and fossil mysid statoliths (Crustacea, Mysidacea).

Statoliths of 61 Recent species representing all subfamilies of Mysidae were studied with special emphasis on internal structure. In addition 5 samples of fossil statoliths from Miocene deposits were examined. Species of Boreomysinae and Rhopalophthalminae show simple roughly spherical organic statoliths, with setae originating from the sensory cushion and anchored in the statolith with distal branches extending shortly below the surface. All other subfamilies possess mineralized statoliths of greater structural complexity, with differentiation in core and mantle, where each part may consist of up to three layers. Habitus is hemispherical to discoidal. External gross structures are dorsal tegmen, ventral fundus, and the ambitus forming the outer toroidal to semi-toroidal circumference. Setae penetrate the mantle through mineralic canals and insert on the surface of the core. As suggested by congeneric species of Schistomysis, there is no principal structural difference between statoliths mineralized with fluorite compared to vaterite. However, vaterite statoliths tend to be more often of moruloid appearance and are exceptional by showing a central conical hole (the hilum) or a central cavity in certain forms. These structures are typical of fossil calcite statoliths. In vaterite and fluorite statoliths, the mantle shows radially arranged (= spherulitic) crystal aggregates. Such arrangements are badly preserved in fossil calcite statoliths. In large extant statoliths, concentric structures, mainly in the form of superficial striation and/or concentric microstrata, are visible in coexistence with radial aggregates. Stratification is possibly due to stratified deposition of the nonmineralized gland product, while the spherulitic structure is indicative of subsequent radial growth of crystal aggregates. The structure of accessory fluorite statoliths in the statocyst of Mesopodopsis slabberi leads to the hypothesis that mantle material is formed by secretions of the caudal statocyst gland. After demineralization of fluorite, vaterite and calcite statoliths, an organic template remains showing most essential morphological features of the statolith. From this we conclude that the structure of the statolith is (almost) entirely matrix mediated. © 1993 Wiley-Liss, Inc.