Table 1 Phenotypic traits assessed in our present study and (unidirectional) predictions for trait divergence along single components of stream gradients

Body size and condition-related adult traits

Body length

▼Temperature [47, 48], [49, 50]*

Increased body size may translate into an enhanced tolerance to low temperatures.

▲Resources [51, 52]

Increased resource availability may result in larger adult body size, which is often correlated with increased investment into reproduction [53].

▼Oxygen [48]

Oxygen availability is coupled with temperature regimes and probably a major mechanistic determinant of growth and general development.

▲Competition [54]*

Large specimens are more competitive at high conspecific densities.

▼ (Micro)pollution [55]*

Reduced size below sewage treatment works can be a result of the water containing endocrine-disrupting chemicals.

▼Predation [56,57,58]

Relatively larger individuals experience higher predation risk than smaller ones.

▲Sexual selection [59]*, [60]

Male pairing success is positively related to body size.

Body weight

(size-corrected)

▲ Resources [61]*

Higher resource availability (usually after leaf fall in autumn and winter) results in increased body condition.

▼Competition [53]

Intraspecific competition results in fewer resources being available per individual to invest into somatic maintenance and reproduction.

▼ Predation [62]*, [63]

Predator cues can induce behavioural alterations (e.g., reduced foraging), resulting in lower body condition.

Offspring-related phenotypic traits

Fecundity

(number of offspring per brood)

▲ Resources [58]

Higher resource availability allows for more investment into egg production.

▲ Predation [57]*

Predators increase extrinsic mortality, favouring r-selected phenotypes with more, but smaller embryos (see also Embryo size).

▼ Pollution [64]

Pollution (sewage and heavy metals) derived from industrial and domestic sources reduce fecundity.

▼ (Micro)pollution [65]*

Endocrine-disrupting chemicals cause intersexuality in amphipods, leading to a reduced fecundity.

Embryo size

▼Temperature [50, 66, 67]*

Larger embryo size during winter may be driven by a higher tolerance to low water temperatures.

Absence of cold temperatures in thermally-polluted streams reduces selection for large embryo size.

▼ Resources [68]*

Under high resource availability embryo size can be reduced, while embryo size should be increased under resource shortage.

▼ Predation [57]*

The optimal egg size depends on the relationship between juvenile survival and egg size [57]*. The trade-off between offspring size and fecundity [69]* allows females to increase fecundity by producing a few smaller embryos. Smaller size (allowing higher fecundity) is only beneficial if enough offspring survive.

Physiological traits

Gill surface area

▼ Pollution [70]*

Toxic metals are taken up by aquatic crustaceans via the gills. Hence, increased gill area might be disadvantageous under elevated heavy metal concentrations.

▼ Oxygen [46]

High oxygen supply allows species to have smaller gill areas.

Traits used for intrasexual communication and mate defense

Antennae length

▲Sexual selection [60, 71]*

Male antennae are important for locating and evaluating potential mates.

▲ Male biased sex-ratio/intraspecific density

Sex ratios affect male mating behaviour [72] and, therefore, the strength of sexually selected male traits. Male-male competition should increase at high population densities and /or male biased sex-ratios because of the high encounter rate between competitors.

▲ (Micro)pollutants [73]*, but see [74]*

Longer antennae were induced by exposure to non-ionic surfactant 4-nonylphenol [73]; no effect of estrogen 17 α-ethinylestradiol was observed [74].

Gnathopod size

▲ Sexual selection [71]*

Male gnathopods play a central role in holding/securing the female before and during copulation (amplexus).

▲ Male biased sex-ratio [72]*

Under male biased sex-ratios, male-male competition increases and males guard females longer.