Bridges as optical barriers and population disruptors for the mayfly Palingenia longicauda: an overlooked threat to freshwater biodiversity?

Abstract

Freshwater biodiversity is declining faster than marine or terrestrial diversity, yet its drivers are much less known. Although dams were shown to negatively affect river habitats, fragmentation by bridges has received less attention and is not as well understood. We tested whether and how bridges present barriers to aquatic insects by studying mass swarmings of Palingenia longicauda mayflies on river Tisza (NE-Hungary). Behavioural observations showed that upon approaching the bridge, upstream-flying mayflies typically turned back and 86% of them never crossed the bridge. Lack of physical contact showed that the bridge was an optical, rather than a mechanical barrier for the polarotactic mayflies. Imaging polarimetry showed that the bridge disrupted the horizontally polarizing channel guiding the flight of mayflies above the river. Energy loss, demonstrated by calorimetry, and time constraints forced females to lay eggs only downstream from the bridge. Counts of larval skins shed by swarming individuals showed nearly 2 to 1 female per male downstream from the bridge, while sex ratio above the bridge was slightly male-biased. We suggest that the surplus of parthenogenetic females, that produce only female larvae, downstream from the bridge may have led to the observed sex-ratio bias since the construction of the bridge (1942). Our results demonstrate that bridges can be optical barriers for aquatic insects and can cause population-level impacts, such as biased sex ratios, in natural populations. Sex ratio biases due to bridges may decrease effective population size and genetic variability, which may have contributed to the recent extinction of this species from most of Europe.

Mayflies are least attracted to vertical polarization: A polarotactic reaction helping to avoid unsuitable habitats

“…Therefore, the continuous highly and horizontally polarized signal of the river surface, guiding the flight of mayflies above water, was broken up by the vertically polarized mirror image and shadow of the bridge crossing the river. Imaging polarimetric measurements of Horváth and Varjú [11], Bernáth et al [3,4] and Málnás et al [21] and the reflection-polarization patterns presented here show that a weakly and non-horizontally (mainly vertically) polarized area is also formed along the riverside where the mirror image and shadow of the riparian vegetation are observable on the water surface. May this weakly and non-horizontally polarized signal keep flying mayflies away from the edge regions of water bodies and keep them above the open water surface?…”

“…A special consequence of this behaviour is that mayflies turn back from a bridge, from the mirror image and shadow of which vertically polarized light is reflected. This latter behaviour can be disadvantageous for the mayfly population as showed by Málnás et al [21]). …”

“…In the case of mayfly species swarming immediately above the water surface, such as Ephoron virgo [Olivier 1791] ( Fig. 1) and P. longicauda [Olivier 1791], their positive polarotaxis is partly responsible for keeping them above water during their whole flying activity [9,21,26], while other mayflies may leave the water bodies up to a distance of 1 km [5]. In the latter case, positive polarotaxis guides the females back to water to oviposit.…”

“…After copulation, the females increase their altitude and begin their upstream-directed compensatory flight, which ends in oviposition onto the water surface [14]. The swarming behaviour of E. virgo is also typical for P. longicauda inhabiting rivers [18,21]. Caenis robusta mayflies swarm above the water surface, where they form groups including several hundred individuals comprising both males and females.…”

“…Málnás et al [21] showed an example where mayflies were influenced by an artificial object: the upstream-directed compensatory flight of P. longicauda females was interrupted by a bridge and its mirror image and shadow on the river surface. The latter formed an optical barrier displaying a weakly and vertically polarized reflectionpolarization signal.…”