The curse of being single: both male and female Gammarus pulex benefit energetically from precopulatory mate guarding

Complexity of trophic relationships between Gammarus aequicauda (Amphipoda) and Eucypris mareotica (Ostracoda) in a hypersaline lake

In the hypersaline lakes of Crimea, only 5-7 species of animals are common including Gammarus aequicauda (Amphipoda) and Eucypris mareotica (Ostracoda). Two experiments were carried out to characterize the trophic interactions between these species. In one, G. aequicauda were used freshly caught from the lake, and in the other, after they were starved for 2.5 days in the laboratory. The experimental options were one male or female G. aequicauda and either 10, 20, or 30 ostracods. The gammarids actively fed on the ostracods, and significant differences were between females and males. The feeding intensity of males was higher than that of females. However, ostracods are not passive prey, they can actively attack their predator and are capable of causing them harm. Ostracods attacked a Gammarus only if it had captured one of the ostracods. Sex differences in G. aequicauda were also evident in the reaction to the group's aggressive behavior of ostracods. Interactions between Ostracoda and Gammarids cannot be considered unidirectional; they both may be prey or predators depending on the situation.

Carry-over effect of artificial light at night on daytime mating activity in an ecologically important detritivore, the amphipod Gammarus pulex

Artificial light at night (ALAN) is a growing environmental problem influencing the fitness of individuals through effects on their physiology and behaviour. Research on animals has primarily focused on effects on behaviour during the night, whereas less is known about effects transferred to daytime. Here, we investigated in the lab the impact of ALAN on the mating behaviour of an ecologically important freshwater amphipod, Gammarus pulex, during both daytime and nighttime. We manipulated the presence of ALAN and the intensity of male-male competition for access to females, and found the impact of ALAN on mating activity to be stronger during daytime than during nighttime, independent of male-male competition. At night, ALAN only reduced the probability of precopula pair formation, while during the daytime, it both decreased general activity and increased the probability of pair separation after pair formation. Thus, ALAN reduced mating success in G. pulex not only directly, through effects on mating behaviour at night, but also indirectly through a carry-over effect on daytime activity and the ability to remain in precopula. These results emphasise the importance of considering delayed effects of ALAN on organisms, including daytime activities that can be more important fitness determinants than nighttime activities.

The behavior of Gammarus aequicauda (crustacea, amphipoda) during predation on chironomid larvae: Sex differences and changes in precopulatory mate-guarding state

The foraging behavior determines the diet size influencing the growth and reproduction of a predator and its prey populations. Amphipods play a key role in various aquatic ecosystems. Gammarus aequicauda is an abundant widespread omnivorous species and the only known amphipod species of hypersaline waters. Its predation can suppress the populations of planktonic and benthic invertebrates. Males of G. aequicauda actively eat chironomid larvae but there is no data on predatory females. Experiments were conducted to study sex-related differences in feeding behavior including during precopulatory mate guarding (PCMG). There were significant sex differences in feeding characteristics. For females, calculations showed that maximum consumption could be 3-4 chironomid larvae per day and for males about 9-10 larvae per day. During PCMG, males did not feed and females fed as efficiently as single females. Males transported females and alone carrying the energetic costs of swimming during PCMG. Less expenditure of energy on swimming means more energy for reproduction for females. This allows more energy-efficient use of food resources and increases the reproductive success of the population. There are currently two alternative views on PCMG among crustaceans. 1. It is a sexual conflict between males and females serving as a male adaptive strategy to achieve a female during strong male competition, while females bear many costs during the long guarding period. 2. There is also the opposite view that pairs get energetic benefits from long PCMG. Obtained data suggest that PCMG is likely to be intersexual cooperation and not intersexual conflict.

Natural variation in social conditions affects male mate choosiness in the amphipod Gammarus roeselii

The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.

Feeding Behavioural Studies with Freshwater Gammarus spp.: The Importance of a Standardised Methodology

Freshwater Gammarids are common leaf-shredding detritivores, and they usually feed on naturally conditioned organic material, in other words leaf litter that is characterised by an increased palatability, due to the action and presence of microorganisms (Chaumot et al. 2015; Cummins 1974: Maltby et al. 2002). Gammarus spp. are biologically omnivorous organisms, so they are involved in shredding leaf litter and are also prone to cannibalism, predation behaviour (Kelly et al. 2002) and coprophagy when juveniles (McCahon and Pascoe 1988). Gammarus spp. is a keystone species (Woodward et al. 2008), and it plays an important role in the decomposition of organic matter (Alonso et al. 2009; Bundschuh et al. 2013) and is also a noteworthy prey for fish and birds (Andrén and Eriksson Wiklund 2013; Blarer and Burkhardt-Holm 2016). Gammarids are considered to be fairly sensitive to different contaminants (Ashauer et al. 2010; Bloor et al. 2005; Felten et al. 2008a; Lahive et al. 2015; Kunz et al. 2010); in fact Amphipods have been reported to be one of the most sensitive orders to metals and organic compounds (Wogram and Liess 2001), which makes them representative test organisms for ecotoxicological studies and valid sentinel species for assessing water quality status (Garcia-Galan et al. 2017).

First come, first served: precopulatory mate-guarding behavior and male-male contests by a hymenopteran saproxylic parasitoid

Precopulatory mate-guarding behavior is a common strategy that maximizes male reproductive success when female receptivity to copulation is low. This behavior has been demonstrated in vertebrates, aquatic crustaceans, terrestrial isopods, and some species of insects, but there is very little available information about hymenopteran insects. A few studies have clarified the factor that determines the outcome of a contest between a guarding male and an invader male. We investigated the male-male contest and mating behavior of a saproxylic parasitoid wasp, Ibalia japonica (Hymenoptera: Cynipoidea: Ibaliidae) using field observations in Japan. These observations indicated that I. japonica males show precopulatory mate-guarding behavior and that four types of male-male contests occur on the Magnolia liliiflora (Magnoliales: Magnoliaceae) tree that virgin females emerge from. We show that the arrival order of I. japonica males that found the future emergence point of a female was key factor that allowed males to secure virgin females.