Zoomorphology(1992) 112:67 79 Zoomorphology 9 Springer-Verlag1992 Comparative morphology and actuopalaeontology of mysid statoliths (Crustacea, Mysidacea) Thomas A. Schlacher 1, Karl J. Wittmann 1, and Antonio P. Ariani 2 1 Institut ffir AilgemeineBiologie,Universit/itWien, Schwarzspanierstrasse 17, A-1090 Vienna, Austria 2 Dipartimentodi Zoologia,Universitfi di Napoli, Via Mezzocannone8, 1-80134 Napoli, Italy Received November26, 1991 Summary. A comprehensive and comparative study of the external statolith morphology of the family Mysidae is presented. The study covers 48 species from major systematic groups occupying a large number of habitats in different biogeographical zones of the globe. Stato- liths generally show high morphological diversity. The traditional classification scheme of subtaxa and the cor- relation of statolith characters with segmentation pat- terns of body appendages suggest that the organic com- position and the nearly spherical structure of the stato- liths of Boreomysinae and Rhopalophthalminae are ple- siomorphic compared with the more complex mineral- ized statoliths found in all other subfamilies. During on- togenetic development the number of sensorial setae and associated pores and pore groups on the statolith in- crease with body size and statolith diameter. Although patterns of caudal pores are highly specific for some genera, the high intraspecific variance of pore numbers strongly reduces the diagnostic value of this feature in most species. Statolith characters can be successfully used for identification of subfamilies, tribes, and especially genera. For future palaeontological applica- tions a proper diagnosis of fossil mysid statoliths is es- sential. Therefore, we provide a key to subfamilies and tribes based exclusively on statolith characters. A. Introduction It hds been stated that mysid statoliths provide impor- tant information for stratigraphy, pelaeogeography, tax- onomy, and phylogenesis (Voicu 1974, 1981; Fuchs 1979; Maissuradze and Popescu 1987). On the basis of his findings, Voicu (1974, 1981) even proposed a new zoological discipline, for which he coined the name "sta- tolithology". Although it may sound somewhat prema- ture, we expect that the importance of statolith studies will increase, especially in palaeontology. Fossil stato- Correspondence to: K.J. Wittmann liths have been found in Miocenic to Pleistocenic layers in Europe, California, and Japan. Voicu (1981) and Ma- issuradze and Popescu (1987) based their descriptions of new mysid taxa exclusively on fossil statoliths. However, our current knowledge, hypotheses, and as- sertions are still based on very weak ground. Hitherto, the statoliths of only six extant and two fossil species have been described in detail (Bethe 1895; Voicu 1981; Kharkeevich and Lebsky 1984; Espeel 1987; Maissur- adze and Popescu 1987). All eight species so far studied belong to the same tribe, Mysini, representing only a small spectrum of mysid morphology, physiology, and ecology. Despite this narrow range there are two current concepts of statolith morphology which are incompati- ble in several aspects: Bethe (1895) described the statolith consisting of two parts, an organic core and an inorganic mantle, the latter with canals for penetrating sensorial hairs. Espeel (1987) has shown that both mantle and core are mineralized, for Neomysis integer, with the organic matrix mainly present in the core. In accordance with this concept, Kharkeevich and Lebsky (1984) distinguished a central and a cortical part in the statolith of Mysis oculata. In contrast, Voicu (1974, 1981) described a central tubular depression called the hilum, surrounded by the sensorial zone and a peripheral gravitational zone. He also distinguished groups of pores on the statolith. He introduced the resulting grouping as the statolith formu- la and claimed it to be species specific. His views were largely adopted by Fuchs (1979) and Maissuradze and Popescu (1987) on fossil statoliths. As shown by Franco et al. (1989) Recent mysid stato- liths can be completely organic or mineralized with ei- ther fluorite (CaF2) or vaterite (CaCO3). Calcite (CaCO3) was only found in fossil statoliths. The occur- rence of these constituents is correlated with ecological, taxonomical and biogeographical factors (Ariani et al. 1983; Franco et al. 1989). Earlier works on statolith morphology are largely based on typological concepts. The aims of this paper are: (1) to examine statolith form and structure over