Sex in murky waters: algal-induced turbidity increases sexual selection in pipefish

Abstract

Algal-induced turbidity has been shown to alter several important aspects of reproduction and sexual selection. However, while turbidity has been shown to negatively affect reproduction and sexually selected traits in some species, it may instead enhance reproductive success in others, implying that the impact of eutrophication is far more complex than originally believed. In this study, we aimed to provide more insight into these inconsistent findings. We used molecular tools to investigate the impact of algal turbidity on reproductive success and sexual selection on males in controlled laboratory experiments, allowing mate choice, mating competition, and mate encounter rates to affect reproduction. As study species, we used the broad-nosed pipefish, Syngnathus typhle, a species practicing male pregnancy and where we have previously shown that male mate choice is impaired by turbidity. Here, turbidity instead enhanced sexual selection on male size and mating success as well as reproductive success. Effects from mating competition and mate encounter rates may thus override effects from mate choice based on visual cues, producing an overall stronger sexual selection in turbid waters. Hence, seemingly inconsistent effects of turbidity on sexual selection may depend on which mechanisms of sexual selection that have been under study.Significance statementAlgal blooms are becoming increasingly more common due to eutrophication of freshwater and marine environments. The high density of algae lowers water transparency and reduces the possibility for fish and other aquatic animals to perform behaviors dependent on vision. We have previously shown that pipefish are unable to select the best partner in mate choice trials when water transparency was reduced. However, fish might use other senses than vision to compensate for the reduction in water transparency. In this study, we found that when fish were allowed to freely interact, thereby allowing competition between partners and direct contact between the fish, the best partner was indeed chosen. Hence, the negative effects of reduced water visibility due to algal blooms may be counteracted by the use of other senses in fish.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-017-2310-8) contains supplementary material, which is available to authorized users.

Temperature as a modulator of sexual selection

“…In these fish, water turbidity is known to interfere with mate choice and to relax sexual selection (Seehausen, Alphen, & Witte, 1997; Maan & Seehausen, 2011), and similar findings have been reported in other systems (Engstrom & Candolin, 2007; Candolin, Tukiainen, & Bertell, 2016). By contrast, in the broad‐nosed pipefish ( Syngnathus typhle ; a species with male pregnancy), turbidity strengthens sexual selection (Sundin et al ., 2017). Humidity is an abiotic factor that is also closely linked to temperature, and there is some evidence that it could also affect sexual selection.…”

Trophic Niches of Two Congeneric Pipefishes in the Aegean Sea (Bargilya Wetland, Turkey), Based on Stable Isotope Analyses

“…Thus, under certain environmental conditions, hybridization may foster sympatry, retaining populations in areas where pure species would otherwise be unable to persist. For example, eutrophication (i.e., turbidity or increased algal cover) hamper species-specific sexual signalling, leading to hybridization [69][70].…”

Population decline of Gulf pipefish (Syngnathus scovelli) in a subtropical estuary: ecosystem changes and habitat loss

“…Turbidity due to algae blooms directly impairs pipefishes' choice of larger and possibly more fecund females (Sundin, et al 2016). This finding focused on the reduced ability to function where sexual selection can behaviors rely on vision, however more recent research suggests that negative effects of visibility reduced by algal blooms may be tempered when pipefish can use other senses (Sundin et al 2017). In addition, pipefish and the closely related seahorses are visual feeders that have been found to feed mostly during the crepuscular portions of the day and rarely to never at night (Howard and Koehn 1985;Felício et al 2006).…”