Changes in water chemistry can disable plankton prey defenses

Abstract

The effectiveness of antipredator defenses is greatly influenced by the environment in which an organism lives. In aquatic ecosystems, the chemical composition of the water itself may play an important role in the outcome of predator–prey interactions by altering the ability of prey to detect predators or to implement defensive responses once the predator’s presence is perceived. Here, we demonstrate that low calcium concentrations (<1.5 mg/L) that are found in many softwater lakes and ponds disable the ability of the water flea, Daphnia pulex to respond effectively to its predator, larvae of the phantom midge, Chaoborus americanus. This low-calcium environment prevents development of the prey’s normal array of induced defenses, which include an increase in body size, formation of neck spines, and strengthening of the carapace. We estimate that this inability to access these otherwise effective defenses results in a 50–186% increase in the vulnerability of the smaller juvenile instars of Daphnia, the stages most susceptible to Chaoborus predation. Such a change likely contributes to the observed lack of success of daphniids in most low-calcium freshwater environments, and will speed the loss of these important zooplankton in lakes where calcium levels are in decline.

Synergistic interactions of biotic and abiotic environmental stressors on gene expression

“…We employed a clonal isolate whose lower calcium thresholds for survival (0.1-0.5 mg/L Ca +2 ) and reduced reproduction (1.5 mg/L Ca +2 ) are known (Ashforth and Yan 2008). This obligate parthenogenetic isolate undergoes inducible phenoplastic changes in the presence of Chaoborus, but low calcium reduces its ability to form defensive structures that otherwise are normally induced by Chaoborus (Riessen et al 2012). The particular Daphnia lineage used in our study was isolated from Sherborne Road Pond, a shallow (less than 1 m maximum depth), ephemeral, forest pond situated near Dorset, Ontario, Canada.…”

“…We predict that a joint exposure of kairomones and low calcium would increase the number of genes differentially transcribed in comparison with either low calcium stressor or kairomone exposure on their own. As kairomones increase Daphnia's calcium requirements (Riessen et al 2012), we predict that Calb, a gene involved in calcium absorption, and Serca, a gene involved in calcium storage, will be up-regulated at lower calcium concentration in the presence of predator kairomones.…”

“…Because of their great calcium need, declining calcium concentrations reduce daphniid growth, reproduction, and survival. For example, calcium concentrations under 1.5 mg/L lower the growth of Daphnia (Riessen et al 2012), thus delaying reproduction (Ashforth and Yan 2008), while the animals die when calcium falls below 0.5 mg/L. Since daphniids are keystone organisms in a wide variety of freshwater habitats, their loss as a major grazer of algae and food source in freshwater ecosystems is expected to have strong repercussions up the food chain (Lampert 2006).…”

“…The phenoplastic changes induced by the predator greatly alter the structure, rigidity, ornaments, and size of the carapace in daphniids (Riessen and Trevett-Smith 2009), thus, likely increasing the animals already great calcium demand. Low environmental calcium interferes with the production of these defensive traits (carapace size, neck spines, and rigidity), rendering the animals more vulnerable to predation (Riessen et al 2012). A significant percentage of Canadian Shield lakes currently have calcium levels of <1.5 mg/L, levels low enough to impede the production of these defensive traits (Riessen et al 2012), but we do not know if the transcriptional patterns of genes linked to caparace growth and metabolism are influenced by the interactions of low calcium stress in the presence of Chaoborus kairomones.…”

Phenology of Daphnia in a Northern Italy pond during the weather anomalous 2014

“…Zooplankton is a key component of aquatic food webs and the climate change may cause different adverse effects on species diversity by means of elimination of sensitive species, replacement of sensitive species/strains by less sensitive ones, shifts in food-web interactions; acclimation of species/strains to stress, selection of tolerant genotypes, outcome of predator-prey interactions by altering the ability of prey to detect predators or to implement defensive responses both in marine and freshwater habitats (Riessen et al, 2012;Vadadi-Fülöp et al, 2012;VadadiFülöp and Hufnagel, 2014). Shallow water bodies are highly sensitive to climate warming due to substantial changes in thermal and hydrological regimes; changes in phenology may be important to ecosystem function because the level of response may vary across functional groups and multiple trophic levels (Scheffer et al, 2001;Meerhoff et al, 2007;Feuchtmayr et al, 2009;Landkildehus et al, 2014).…”