Shallow males, deep females: sex‐biased differences in habitat distribution of the freshwater calanoid copepod Arctodiaptomus alpinus

In a small, circular high‐altitude karst lake situated in the northeastern calcareous Alps of Austria the copepod Arctodiaptomus alpinus (Imhof) develops each spring from resting eggs after the lake basin fills in early spring. After mid‐summer, late developmental stages and adults concentrate in a thin layer near the sediment throughout the lake basin. During daytime an average 86% and 71% of female and male copepods respectively resided near the bottom. Males exhibited stronger diel vertical migrations than females and part of the population concentrated in near‐surface water at night, but the majority of copepods remained at the sediment during darkness. Macro‐photographs and core samples revealed that the proportion of epibenthic male A. alpinus decreased constantly from >70% in 3 m depth to <20% below the 8‐m isobath. The proportion of ovigerous females increased with depth, whereas non‐ovigerous females were comparatively more abundant in shallow water. Concurrently the highest frequencies of copulating copepods were detected in areas of high male density. Above the 3‐m isobath male A. alpinus formed dense swarms in scattered patches of the macrophyte Ranunculus eradicatus (>93% males; maximum abundance: 362 600 individuals m −2 ). Adults and larvae of the alpine newt Triturus alpestris are the top predators in this fishless alpine lake, and A. alpinus constitutes a major component of their diet. However, predator densities and different predation rates on male and female copepods were not sufficient to explain the observed horizontal distribution of A. alpinus . We argue that the optimization of successful sexual encounters in copepods is the ultimate driving force behind this segregation of both sexes of A. alpinus (‘mate encounter hypothesis’).