Abstract
We developed an automated, non-invasive method to detect real-time cardiac contraction in post-larval (1.1–1.7 mm length), juvenile oysters (i.e., oyster spat) via a fiber-optic trans-illumination system. The system is housed within a temperature-controlled chamber and video microscopy imaging of the heart was coupled with video edge-detection to measure cardiac contraction, inter-beat interval, and heart rate (HR). We used the method to address the hypothesis that cool acclimation (10°C vs. 22°C—Ta10 or Ta22, respectively; each n = 8) would preserve cardiac phenotype (assessed via HR variability, HRV analysis and maintained cardiac activity) during acute temperature changes. The temperature ramp (TR) protocol comprised 2°C steps (10 min/experimental temperature, Texp) from 22°C to 10°C to 22°C. HR was related to Texp in both acclimation groups. Spat became asystolic at low temperatures, particularly Ta22 spat (Ta22: 8/8 vs. Ta10: 3/8 asystolic at Texp = 10°C). The rate of HR decrease during cooling was less in Ta10 vs. Ta22 spat when asystole was included in analysis (P = 0.026). Time-domain HRV was inversely related to temperature and elevated in Ta10 vs. Ta22 spat (P < 0.001), whereas a lack of defined peaks in spectral density precluded frequency-domain analysis. Application of the method during an acute cooling challenge revealed that cool temperature acclimation preserved active cardiac contraction in oyster spat and increased time-domain HRV responses, whereas warm acclimation enhanced asystole. These physiologic changes highlight the need for studies of mechanisms, and have translational potential for oyster aquaculture practices.
Highlights
Cardiac control is relatively conserved between phyla, despite inter- and intra-species differences; for example, myogenic propagation of cardiac electrical impulses and the excitatory and inhibitory regulation of heart rate (HR) retain common design principles across evolutionarily distant organisms (Koester et al, 1979)
The HR of temperatures of 10◦C (Ta10) spat at the initial 22◦ acute temperature was not different from the Ta22 spat, they did exhibit a lower rate of decrease during cooling
We provide the first measurements of the prevalence of asystole, HR response, and time-domain heart rate variability (HRV) profile of spat exposed to an acute temperature ramp (TR) protocol from 22◦C to 10◦C to 22◦C
Summary
Cardiac control is relatively conserved between phyla, despite inter- and intra-species differences; for example, myogenic propagation of cardiac electrical impulses and the excitatory and inhibitory regulation of heart rate (HR) retain common design principles across evolutionarily distant organisms (Koester et al, 1979). Cardiac control is altered by changes in temperature, which are a routine challenge for the inter-tidal bivalve mollusc, the Pacific oyster (Crassostrea gigas; Thunberg, 17933; Quayle, 1969; Newell and Branch, 1980; Bernard, 1983) Factors such as temperature, salinity, oxygenation, and seston/nutrient loads alter growth rates, influence survival, and affect physiology and health in open aquaculture environments (Newell and Branch, 1980; Bernard, 1983; Helm, 2005)
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