How does salinity influence the swimming speed of the estuarine calanoid copepod Eurytemora affinis?: Reply

Abstract

The effects of salinity on Eurytemora affinis small-scaleswimming behavior have been investigated in twostudies, leading to conflicting results (Seuront, 2006;Michalec et al., 2010). In an attempt to ensure the rel-evance of his earlier work (Seuront, 2006), Seuront(Seuront, 2010) presented results demonstrating that thebehavioral response of E. affinis to salinity changes wasnot affected by the origin (i.e. continuous culture versusfield) of the organisms tested or by the presence/absence of food, two major differences between the twostudies. These results might lead to the conclusion thatexperimental conditions have little effect on copepodbehavior. However, they should be carefully taken intoconsideration and attention must be paid to prevent anysynergistic effects. We discuss hereafter some of theimportant differences between the two studies whichcould have accounted for these discrepancies.(1) The sampling dates were not indicated either inSeuront (Seuront, 2006)orinSeuront (Seuront, 2010),despite the evidence for seasonal and temporal variationin copepod swimming behavior (Sullivan et al.,1983;Gill and Crisp, 1985;Hough and Naylor, 1992;Moisson,2009). Moreover, at the individual scale, seasonal meta-bolic variations have been demonstrated in severalcopepod species (Anraku, 1964; Conover and Corner,1968), depending on the physiological state of individuals(Hirche, 1983; our personal observations). Fieldobservations conducted in the Seine Estuary (during themultidisciplinary national cruises: EURYPROD 2005;ZOOTRANZ 2008 and 2009) have revealed apparentdifferences in the in situ optimal salinity of E. affinis,depending on the season and the estuarine hydrology(Souissi et al., unpublished results). Finally, seasonal andtidal variations in hydrophobic organic contaminant con-centrations have been observed in the water column ofthe Seine Estuary (Cailleaud et al., 2007a,b;2009).Thesehave been shown to induce modifications in acetylcholin-esterase and glutathione-S-transferase in E. affinis(Cailleaud et al.,2009) and to profoundly affect its swim-ming behavior (Cailleaud, 2006).(2) Seuront (Seuront, 2010) presents new results on: (i)the possible effect of food on the physiological state ofindividuals originating from culture, (ii) the possiblesynergetic effect of food and salinity during his previousexperiments. However, differences in copepod swimmingbehavior depending on the presence, quality and quan-tity of food have been widely reported (e.g. Buskey, 1984;Tiselius, 1992;Saiz, 1994;Tiselius et al.995,1 ;van Durenand Videler, 1995; Seuront and Vincent, 2008).Moreover, while several species of marine microalgae areacknowledged to be tolerant to salinity variations, sali-nities as low as 0 or 5 might however have a lethal effect,affecting the algal distribution within the experimentalvessel. This can, in turn, affect the distribution of