Size-assortative pairing in the lotic amphipod Gammarus zaddachi, an examination of hypotheses and the influence of current speed

Abstract

Three main hypotheses have been put forward to explain size-assortative pairing in gammarid amphipods: microhabitat separation, sexual selection and loading constraint. In order to determine which hypothesis best explains this phenomenon in the estuarine species Gammarus zaddachi, I first measured the body lengths and dry weights of precopula pairs collected from two field sites with substantially different current speeds. Second, I performed three laboratory experiments in order to estimate the importance of the following processes: (1) male choice; (2) male-male competition and (3) male-female acceptability. The loading constraint hypothesis seemed best supported by the data in that fieldcollected male G. zaddachi size correlated well with female size in precopula pairing in both fast and slow flowing water. In the laboratory, males preferred females of their same size group (large versus small), and 'won' them in the male-male competition experiments. Size-assortative pairing is thus likely a consequence of the loading constraints imposed upon these males by virtue of them having to carry and manoeuvre their partners through flowing water, while attempting to maintain station in an optimal microhabitat.Males may therefore forego the largest, most fecund females, in favour of a practicable payload (small male-large female pairings were rare). However, there seems to be a lower limit to this selection, indicated by the high degree of cannibalism on small females by large males.

Mating patterns ofGammarus lacustris(Amphipoda: Gammaridae) from three populations across a gradient on the Tibetan Plateau

“…The guarding for females is more costly in habitats with fast than with slow currents, therefore the size-assortative patterns potentially increased their fitness. Williams (2007) recently found similar results in G. zaddachi, suggesting that the loading constraint hypothesis seemed to explain that size-assortative pairing. In summary, we propose that pairing patterns of G. lacustris be regarded as population-level consequences of the sexual behaviour of individuals (Andersson 1994;Arnquist 1997), and thus may reflect the outcome of sexual selection on behavioural and morphological characters.…”

“…The mating patterns of freshwater amphipods are often non-random with respect to body size (Birkhead & Clarkson 1980;Ward 1983;Crespi 1989;Hume et al 2002;Bollache & Cézilly 2004;Williams 2007;Franceschi et al 2010;Cornet et al 2012). By investigating mating patterns in G. lacustris under natural conditions from three sites, we demonstrated that the species shows a size-related non-random mating pattern.…”

“…We also found size-assortative mating for G. lacustris at the three sites (Figures 1 and 2). There have been many previous studies on size-assortative mating of Gammarus species (Birkhead & Clarkson 1980;Ward 1983;Crespi 1989;Hume et al 2002;Bollache & Cézilly 2004;Cornet et al 2012), and it has been suggested to be due, in part, to the outcome of male-male competition and male mate choice (Ridley 1983;Ward 1983;Elwood et al 1987;Dick & Elwood 1996;Bollache et al 2000;Williams 2007;Franceschi et al 2010). There is a strong sexual size dimorphism (SSD) in G. lacustris, which does not vary along the altitudinal gradient.…”

“…Pairs may remain in precopula for up to two weeks (Hartnoll & Smith 1978), but males are unable to guard females during their own moult. Although males may be larger than females, precopula pairs tend to show positive size-assortative pairing with large males pairing with large females and smaller males pairing with smaller females (Williams 2007), as in other Gammarus species (e.g. Birkhead & Clarkson 1980;Ward 1983;Crespi 1989;Hume et al 2002;Bollache & Cézilly 2004;Cornet et al 2012).…”

Sexual and natural selection on morphological traits in a marine amphipod,Pontogammarus maeoticus(Sowinsky, 1894)

“…Megalorchestia californiana Brandt, 1851, Gammarus duebeni Liljeborg, 1852 and Gammarus pulex (Linnaeus, 1758)) are indeed larger than females (Hatcher & Dunn 1997;Hume et al 2005;Iyengar & Starks 2008), especially in species where males carry females (pre-copula mate guarding; Adams & Greenwood 1983;Dick & Elwood 1996;Tsoi & Chu 2005;Williams 2007). The importance of male body size is illustrated by studies showing that pair formation appears to be size-sorted, and males in amplexus pairs are, on average, larger than females (Birkhead & Clarkson 1980;Hatcher & Dunn 1997;Williams 2007). In mate-guarding amphipods of the 'carrier type', several anterior body appendages are enlarged in males at the age of maturity, which may be an adaptation to carry the female (Conlan 1991).…”