Cardiac responses in Crassostrea gasar: An experimental approach of how the tidal cycle influences the heart function of the mangrove oyster

Abstract

Cardiac physiological studies in oysters are scarce and these kinds of responses can be key issues for understanding behavioral and baseline adaptive responses. In this study we characterized the electrocardiogram (ECG) tracing patterns, wave intervals (RR; QT), and complex duration (QRS) of Crassostrea gasar during immersion followed by air exposure, simulating what occurs in a tide cycle. Initially, the ECG was analyzed in the oysters under immersion for 30 min to evaluate the basal recordings (immersion exposure condition). Then, the same animals were removed from the water and the ECG was analyzed for another 30 min to assess cardiac activity under air exposure (air exposure condition). For this, a technique of cardiac recordings was developed, allowing us to analyze, beyond ECG, other cardiac parameters such as the heart rate (HR) in beats per minute, the spectral power of HR, and the amplitude. The basal cardiac parameters analyzed in oysters in the immerse condition clearly show the waves and intervals (R-R: 11.03 ± 0.63 s, P-Q: 2.09 ± 0.06 s), with a normal and regular electrocardiographic tracing and sinus rhythm without alterations. When in the air exposure condition, oysters maintained the ECG tracing of sinus rhythm, but with changes in intervals and a prolonged isoelectric period. Moreover, in this condition, oysters presented a biphasic response: initially (phase I), heart rate increased (9.83 ± 0.98 BPM), and consequently the R-R and P-Q intervals decreased (5.86 ± 2.01 s e 1.91 ± 0.13 s, respectively); in phase II, heart rate (3.68 ± 0.82 BPM) and spectral power (21.26 ± 5.44 mV2/Hz x 10^-3) decreased and consequently, the R-R interval increased (14.83 ± 2.92 s). But, the P-Q interval remained (2.45 ± 0.65 s) in phase II. The QRS complex of oysters in both phases decreased (Phase I: 0.57 ± 0.13 s; Phase II: 0.62 ± 0.05 s) compared to the immersion exposure condition (0.79 ± 0.09 s). We conclude that air exposure affected cardiac function in C. gasar leading to arrhythmia in response to the beginning of air exposure, as a means of maintaining oxygen supply, followed by bradycardia to decrease metabolism as a survival strategy. The basal responses of the mangrove oyster in the physiological modulation against the environmental factors of the tidal regime provide information about the species for possible application as model organisms in studies of toxicological evaluation of chemical products and in conservation and sustainability studies.

Keywords: Aerial exposure; Bivalve; Electrocardiogram; Physiology.