Low-latitude zooplankton pigmentation plasticity in response to multiple threats

Abstract

Crustacean copepods in high-latitude lakes frequently alter their pigmentation facultatively to defend themselves against prevailing threats, such as solar ultraviolet radiation (UVR) and visually oriented predators. Strong seasonality in those environments promotes phenotypic plasticity. To date, no one has investigated whether low-latitude copepods, experiencing continuous stress from UVR and predation threats, exhibit similar inducible defences. We here investigated the pigmentation levels of Bahamian ‘blue hole’ copepods, addressing this deficit. Examining several populations varying in predation risk, we found the lowest levels of pigmentation in the population experiencing the highest predation pressure. In a laboratory experiment, we found that, in contrast with our predictions, copepods from these relatively constant environments did show some changes in pigmentation subsequent to the removal of UVR; however, exposure to water from different predation regimes induced minor and idiosyncratic pigmentation change. Our findings suggest that low-latitude zooplankton in inland environments may exhibit reduced, but non-zero, levels of phenotypic plasticity compared with their high-latitude counterparts.

Knowing the Enemy: Inducible Defences in Freshwater Zooplankton

“…Additionally, young daphnids have been shown to respond more strongly than adults to UV-radiation [62][63][64], which has been reported for predator avoidance by DVM as well [44,61]. Despite this, zooplankton is able to increase pigmentation to protect itself against UV-radiation (see further below) [33,65,66]. Thus, the ultimate factor for DVM in response to both UV-radiation and light intensity is the increase in predation risk due to increased visibility to visually hunting predators.…”

“…On the one hand, food and temperature are crucial for the growth and development of zooplankton. On the other hand, a response to light and UV-radiation may comprise a trade-off between protection against UV-radiation (e.g., by pigmentation) and the increase in vulnerability (due to increased visibility) to visually hunting predators [33,65,66]. Winder et al [60] proposed that temperature and food abundance modulate the depth selection in Daphnia, while predator kairomones and UV-radiation modulate the timing of the DVM (i.e., synchronizing with dusk and dawn), emphasizing the importance of considering these factors in experiments.…”

“…Under favorable conditions, Daphnia typically reproduce asexually, rendering all offspring as clones of their mother, which is a great advantage for ecological and evolutionary experiments. Furthermore, some of its species have fully sequenced genomes [32,33]. Results obtained in laboratory studies on these organisms are crucial for reconstructing and understanding ecological processes in nature [34][35][36].…”

Diel vertical migration of copepods and its environmental drivers in subtropical Bahamian blue holes

“…In addition, copepods can accumulate photoprotective pigments, such as carotenoids, against UVR (Hansson and Hylander 2009a), although this comes at a cost as increased pigmentation makes copepods conspicuous, and thus, increases predation risk (Hylander et al 2009). A recent study demonstrated that copepods exhibited higher levels of pigmentation in fishless and low-predation Bahamian blue holes, but reduced their coloration significantly in blue holes with higher predation pressure (Lee et al 2019). Moreover, copepods may individually adjust their level of pigmentation, as well as other protective, nonpigment compounds such as mycosporine-like amino acids (Hylander 2020), to the prevailing risk from UVR and predation (Brüsin et al 2016;Hansson 2004).…”

Copepods' true colors: astaxanthin pigmentation as an indicator of fitness

“…As in most crustaceans, astaxanthin and canthaxanthin are the principal carotenoid types found in copepods (Czeczuga & Czerpak, 1966; de Carvalho & Caramujo, 2017; Fisher et al, 1964; Matsuno, 2001). Pigmented copepods are found in both marine and freshwater environments, particularly in polar and subpolar regions (Hylander et al, 2015), but also in subtropical systems (Lee et al, 2019). The red copepods from the North Atlantic ( Calanus spp.)…”