Chaoborus flavicans Meigen (Diptera, Chaoboridae) is a complex of lake and pond dwelling species: a revision

Chaoborus flavicans (Meigen) is a widespread and much studied lacustrine phantom midge. As larvae, these insects are important aquatic predators. Based on the available type material, morphology of immature stages and adults, their aquatic habitat, and DNA barcodes, C. flavicans is shown to be a composite of at least four species, with three of these named here. Chaoborus flavicans is primarily a lake-dwelling species with a Holarctic range. Chaoborus albipes (Johannsen, 1903 stat. rev.) and C. posio Salmela sp. n. are pond-dwelling Holarctic and north European species, respectively. The position of the larval subordinate mandibular tooth at the vertex of the second and fourth teeth is a synapomorphy of the Chaoborus flavicans species complex. We present an identification key to fourth instar larvae, pupae, and adult males. We also designate the lectotype and paralectotypes of Sayomyia rotundifolia Felt, 1904 (syn. nov. of C. albipes). We hypothesize that a fourth species of the species complex is present in Japan. Our revision indicates that Holarctic shallow ponds contain a hidden diversity of predators (C. albipes and C. posio sp. n.).

Vertical migration patterns of the different larval instars of Chaoborus flavicans and the influence of dissolved oxygen concentrations

Here we investigate the diel vertical migration (DVM) of the different larval stages of Chaoborus flavicans between spring and summer in two different lakes and three different years. Specific attention is given to the influence of the vertical distribution of dissolved oxygen (DO) on the DVM of the different larval instars. To our knowledge, this study is the first that combines continuous observations of DVM of C. flavicans with continuous measurements of DO distributions over several months, allowing the assessment of changes in DVM due to the development of hyperoxic conditions in the deep water of lakes. With ontogenetic development, C. flavicans larvae increase their sensitivity to changes in light intensity and their tolerance to low oxygen conditions. Our results suggest that the physiological changes of C. flavicans larvae are adaptations to seasonal changes in DO, improving migration abilities to enable utilization of hypoxic and anoxic waters to avoid predation. Interannual change in the abundance and vertical distribution of phytoplankton affecting DO concentrations was sufficient to alter DVM patterns of C. flavicans larvae between years.

Predation by a visual planktivore perch (Perca fluviatilis) in a turbulent and turbid environment

Turbulence and turbidity are thought to independently affect the foraging success of fish, but little is known about their interactive effects on the feeding of fish larger than a few centimetres. We experimentally tested for this interaction on the feeding of planktivorous perch (Perca fluviatilis). There was an interactive effect of root mean square (RMS) velocity (0, 1.3, 2.7, 5.5, and 18.3 cm·s−1) and turbidity (0, 30, and 60 nephelometric turbidity units; NTU) on perch feeding on phantom midge larvae (Chaoborus flavicans). In the 0 and 60 NTU conditions, there was no significant change in the feeding efficiency of perch. However, at 30 NTU, increasing turbulence enhanced perch feeding by increasing encounter rates and disabling the prey escape response. The proportion of encountered Chaoborus larvae that were consumed showed a linear decline with increasing turbulence under clear and 30 NTU conditions and a dome-shaped response under 60 NTU. The results indicate that turbulence has a strong effect on the post-encounter stages of the foraging cycle.