An experimental heat wave changes immune defense and life history traits in a freshwater snail

Abstract

The predicted increase in frequency and severity of heat waves due to climate change is expected to alter disease dynamics by reducing hosts' ability to resist infections. This could take place via two different mechanisms: (1) through general reduction in hosts' performance under harsh environmental conditions and/or (2) through altered resource allocation that reduces expression of defense traits in order to maintain other traits. We tested these alternative hypotheses by measuring the effect of an experimental heat wave (25 vs. 15°C) on the constitutive level of immune defense (hemocyte concentration, phenoloxidase [PO]-like activity, antibacterial activity of hemolymph), and life history traits (growth and number of oviposited eggs) of the great pond snail Lymnaea stagnalis. We also manipulated the exposure time to high temperature (1, 3, 5, 7, 9, or 11 days). We found that if the exposure to high temperature lasted <1 week, immune function was not affected. However, when the exposure lasted longer than that, the level of snails' immune function (hemocyte concentration and PO-like activity) was reduced. Snails' growth and reproduction increased within the first week of exposure to high temperature. However, longer exposures did not lead to a further increase in cumulative reproductive output. Our results show that short experimental heat waves do not alter immune function but lead to plastic responses that increase snails' growth and reproduction. Thus, although the relative expression of traits changes, short experimental heat waves do not impair snails' defenses. Negative effects on performance get pronounced when the heat waves are prolonged suggesting that high performance cannot be maintained over long time periods. This ultimately reduces the levels of defense traits.

Heat stress enhances LTM formation inLymnaea: role of HSPs and DNA methylation

“…In addition, the higher temperatures increase parasite virulence in snails (Seppala, and Leicht, 2013;Leicht et al, 2013). Lymnaea that have experienced an experimental 'heat wave' (i.e.…”

“…Lymnaea that have experienced an experimental 'heat wave' (i.e. maintained at 25°C for up to 11 days) show reduced immune function after 7 days and within the first 7 days of the 'heat wave' their growth and reproductive functions increase (i. e. number of snails that laid eggs and the total number of eggs laid; Leicht et al, 2013). Snails also reduce their innate response to predation when reared at higher environmental temperatures (Dalesman and Rundle, 2010).…”

“…It has been demonstrated that laboratory-reared Lymnaea prefer temperatures of around 20°C for optimal growth and mortality (Vaughn, 1953;McDonald, 1969). At higher temperatures, Lymnaea have reduced immune function and parasitic infestation increases (Seppala and Jokela, 2011;Hermann et al, 2013;Seppala and Leicht, 2013;Leicht et al, 2013). Thus, it is safe to assume that higher temperatures are an ecologically relevant stressor for Lymnaea.…”

Direct and transgenerational effects of an experimental heatwave on early life stages in a freshwater snail

“…This procedure ensured, first, that the snails were exposed to their respective temperature treatments long enough to induce strong effects on their performance before the clutches were collected (see Leicht et al, 2013) and, second, that the collected clutches were exposed to the maternal temperature treatments only briefly. During the following 10 days, we checked the cups twice a day for new clutches to be collected for the next step of the experiment (see the next section).…”

“…In this species, exposure of adult individuals to high temperature (≥25°C) initially increases growth and reproduction, but prolonged exposure (1 week or longer) ceases reproductive rate and reduces immune function (Leicht, Jokela, & Seppälä, 2013;Seppälä & Jokela, 2011). We tested which juvenile traits are affected by maternal effects and how strong these effects are compared with the effects of direct exposure of offspring to high temperature.…”