Salinity-driven size variability inCyprideis torosa(Ostracoda, Crustacea)

Ian Boomer,Peter Frenzel,Martin Feike

doi:10.1144/jmpaleo2015-043

Abstract

The living ostracod Cyprideis torosa (Jones, 1850) is geographically widespread, often abundant, occurring in modern and late Quaternary marginal-marine and athalassic environments world-wide. The species is capable of withstanding varying salinity over short (diurnal) timescales as well as adjusting to longer-term changes. Much attention has been paid in the past to the development of eco-phenotypic nodes and the shape of sieve-type pores on the external, lateral surfaces as indicators of particular salinity levels. In this paper we demonstrate a bimodal distribution between shell size (which can be determined directly from optical microscopy) and the salinity of the water in which the carapace formed. Between almost ‘freshwater’ salinity of about 1‰ up to about 8‰ the length of C. torosa increases linearly by about 10%, after this point there is a sharp break in the size–salinity relationship with carapace length reverting to values at or below those of freshwater and gradually declining in size by about 5% through the observed range (a maximum salinity of almost 40‰ in this study). This switch in size–salinity relationship coincides with a physiologically important switch between hypo- and hyper-osmotic regulation at about 8‰ known for C. torosa .

Highlights

Quaternary ostracods have been shown to have great utility as archives of changing environmental conditions (De Deckker et al 1988; Boomer et al 2003; Horne et al 2012) and these approaches generally fall into one of three categories: (1) indicator taxa/assemblages, (2) trace element and isotope chemistry and (3) intra-specific morphological variability
At salinities above that boundary there appears to be little or no relationship between salinity and shell size, if anything there is a slight decrease in size; there are too few data points to be certain of the physical response in this part of the range
From 1 to about 8‰ there is an increase in valve size, from about 8 to 38‰ there is a slight decrease in size but the data are too scattered to have confidence in this relationship

Summary

Introduction

Quaternary ostracods have been shown to have great utility as archives of changing environmental conditions (De Deckker et al 1988; Boomer et al 2003; Horne et al 2012) and these approaches generally fall into one of three categories: (1) indicator taxa/assemblages, (2) trace element and isotope chemistry and (3) intra-specific morphological variability. Majoran et al (2000) cultured the marine genus Krithe at different temperatures and noted that the average size of individual adult and juvenile stages was greater in cooler waters This broadly reflects the Temperature Size Rule (TSR) or Bergmann’s rule (Bergmann 1847) sensu lato whereby individuals of a species grow more slowly in colder environments but result in larger adults; see Angilletta &. It has been noted that certain groups of brackish-water organisms show a direct relationship between size and salinity with decreasing size and thinner carbonate shells in salinity ranges that diverge from the ecological optima of those species (Remane 1958) Both temperature and salinity are primary controls on the distribution of all aquatic organisms, but it is more common for organisms to accommodate temperature variability more than salinity changes, true euryhaline taxa being relatively rare. Reconstructing past salinity levels (and variability) plays a fundamental role when studying the evolution of marginal-marine and estuarine ecosystems, where isolated water bodies such as lakes and lagoons are subjected to climatically driven salinity change. Barker (1963) discussed the possible relationship between ostracod carapace size and salinity based on a study from the Tamar Estuary in SW England; subsequent analyses of those data suggest that the failure to identify adults and juveniles correctly had largely led to this erroneous conclusion, with the smaller juveniles having been transported post mortem

Results

Discussion

Conclusion