Amazonia versus Pontocaspis: a key to understanding the mineral composition of mysid statoliths (Crustacea: Mysida)

Karl J Wittmann,Antonio P Ariani

doi:10.21426/b634142438

Abstract

We have determined the mineral composition of statoliths in 169 species or subspecies (256 populations) of the family Mysidae on a worldwide scale. Including previously published data, the crystallographic characteristics are now known for 296 extant species or subspecies: fluorite (CaF2) in 79%, vaterite (a metastable form of crystalline CaCO3) in 16%, and non-crystalline (organic) components in 5%, the latter exclusively and throughout in the subfamilies Boreomysinae and Rhopalophthalminae. Within the subfamily Mysinae vaterite or fluorite were found in three tribes, whereas other three tribes have fluorite only. The exclusive presence of fluorite was confirmed for the remaining seven subfamilies. Hotspots of vaterite were found in Amazonia and the Pontocaspis, in each case with reduced frequencies in main and tributary basins of the Atlantic and N-Indian Ocean. Vaterite is completely absent in the remaining aquatic regions of the world. In accordance with previous findings, fluorite occurred mainly in seawater, vaterite mostly in brackish to freshwater. Only vaterite was found in electrolyte-poor Black Water of Amazonia, which clearly cannot support the high fluorine demand for renewal of otherwise large fluorite statoliths upon each moult. Vaterite prevails in Diamysini, distributed over most of the area once occupied by the Tethyan Sea. It also prevails in Paramysini with main occurrence in the Pontocaspis, where fossil calcareous statoliths in the stable form of calcite are known from Miocene sediments of the brackish Paratethys. Four Recent genera from three tribes are heterogeneous in that they comprise both vaterite- and fluorite-precipitating species. Previous hypotheses are expanded to cover greater geographic and time scales, proposing that fluorite-bearing marine ancestors penetrated freshwaters in Tethyan and Paratethyan basins, where they developed precipitation of vaterite. This gave their successors predispositions for shifting into separate evolutionary lines from fluorite to vaterite precipitation and vice versa.

Highlights

Mysid shrimps of the family Mysidae are exceptional among invertebrates by having endogenous static bodies, analogous to otoliths of most vertebrates
The first identification of vaterite statoliths in a mysid species was made by Ariani et al (1981) for Diamysis bahirensis
The peculiar crystal habit of vaterite is recognizable in scanning electron micrographs of statoliths published by Almeida Prado-Por (1981), who, did not identify the mineral upon first descriptions of D. bahirensis hebraica Almeida Prado-Por 1981, and D. bahirensis sirbonica Almeida Prado-Por 1981 - acknowledged (Ariani and Wittmann 2000) at species level as D. hebraica and D. sirbonica

Summary

Introduction

Mysid shrimps of the family Mysidae are exceptional among invertebrates by having endogenous static bodies (statoliths), analogous to otoliths of most vertebrates. The first identification of vaterite (a metastable crystal phase of CaCO3) statoliths in a mysid species was made by Ariani et al (1981) for Diamysis bahirensis (G.O. Sars 1877), later partly transferred to D. mesohalobia Ariani & Wittmann 2000, from certain brackish water populations in the Mediterranean. The peculiar crystal habit of vaterite is recognizable in scanning electron micrographs of statoliths published by Almeida Prado-Por (1981), who, did not identify the mineral upon first descriptions of D. bahirensis hebraica Almeida Prado-Por 1981, and D. bahirensis sirbonica Almeida Prado-Por 1981 - acknowledged (Ariani and Wittmann 2000) at species level as D. hebraica and D. sirbonica. Franco et al (1989) reported that certain mysid species have non-crystalline (organic) statoliths

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