The physiological state of crustaceans varies over time and is strongly linked to their moult stage (Chang, 1995; Carvalho & Phan, 1998). For example, levels of ecdysone and vitellogenin fluctuate during the moult cycle and are involved in pairing decision (Dunham, 1978; Ducruet, 1982; Subramoniam, 2000). However, females do not reproduce at each successive moult as noticed for many species (Jormalainen & Merilaita, 1995; Souty-Grosset

Tanespimycin mouse et al., 1998) and even during the breeding period, egg-depositing moults (with reproduction) may alternate with growth moults (without reproduction). The types of moult are expected to affect differentially the pairing outcomes (Sparkes, Keogh & Haskins,

2000), as well as size-assortative mating. This latter point has not specifically been investigated in crustaceans. Predictions can be made about the role of the moulting cycle in size-assortative mating. If size-assortative pairing results from the constraints of carrying a female for males or from the spatial heterogeneity in the local environment, then the stage of moulting cycle should not have an effect. In this case, the variability in size-assortative pairing should be low and independent of where males and females are in their moulting cycle. If sexual selection indeed determines male choice (‘timing and takeover Lorlatinib molecular weight hypotheses’), then the level of size-assortative pairing should vary according to female moult stage and be more pronounced late in the female moult cycle. In the freshwater gammaridean amphipod Gammarus pulex, a pattern of positive size-assortative pairing during PCMG has been well described. The objective of the present work was to study the influence of female moult stage on the variation and intensity of size-assortative pairing in MCE G. pulex. Both paired and unpaired individuals were collected in the field, sized and their position in the moult cycle was characterized by observing the new cuticle formation. The data allowed us to determine whether female moulting status influenced the individual’s

decision and therefore the outcome of pairing. Males and females of G. pulex were collected from the Suzon River at Val Suzon (in the north of Dijon, Burgundy, France) using a hand net and brought back to the laboratory. Individual females were maintained in the laboratory (temperature 15 ± 1°C, light : dark cycle 12:12 h) in dishes (10 cm diameter, 8 cm height) filled with aerated dechlorinated ultraviolet-treated tap water and fed ad libitum with elm leaves. At the end of the assays, gammarids were anesthetized with CO2 gas and then killed in 70% ethanol. Body size was measured by linear dimensions (height of the fourth coxal plate) using a Nikon SMZ 1500 stereoscopic microscope and Lucia G 4.91 software (Nikon, Tokyo, Japan).