Studying the cardiovascular system of a marine crustacean with magnetic resonance imaging at 9.4 T

Non-invasive MRI Studies of Ventilatory and Cardiovascular Performance in Edible Crabs Cancer pagurus During Warming Under Elevated CO2 Levels

The thermal tolerance of marine decapod crustacea is defined through their capacities for oxygen uptake and distribution. High ambient CO₂ levels were previously shown to reduce hemolymph oxygen levels at enhanced cardiac performance during warming. This study investigated the impacts of warming under two CO₂ levels on ventilation and hemolymph circulation in edible crabs Cancer pagurus. It also highlights changes in the ventilatory and cardiac pauses displayed by Decapoda under routine metabolism. Animals were exposed to step-wise, sub-critical warming (12–20°C over 5 days) under control (470 μatm) and high (1,350 μatm) water PCO₂. Flow-through respirometry was combined with magnetic resonance imaging and infra-red photoplethysmography to allow for simultaneous, non-invasive measurements of metabolic rates (M˙O2), ventilation and cardiovascular performance. Crabs spent significantly more time in a low M˙O2 state (metabolic pause), when experiencing high CO₂ conditions above 16°C, compared to normocapnic warming. Heart rates leveled off beyond 18°C at any CO₂ level. Cardiac output continued to increase with high-CO₂-warming, due to elevated cardiac stroke volumes. Consequently, temperature-dependent branchial hemolymph flow remained unaffected by CO₂. Instead, a suppressing effect of CO₂ on ventilation was found beyond 16°C. These results indicate constrained oxygen uptake at stable cardiovascular performance in a decapod crustacean.

Cancer pagurus: urn:lsid:zoobank.org:act:B750F89A-84B5-448B-8D80-EBD724A1C9D4

Non-invasive quantification of cardiac stroke volume in the edible crab Cancer pagurus

Background

Brachyuran crabs can effectively modulate cardiac stroke volume independently of heart rate in response to abiotic drivers. Non-invasive techniques can help to improve the understanding of cardiac performance parameters of these animals. This study demonstrates the in vivo quantification of cardiac performance parameters through magnetic resonance imaging (MRI) on the edible crab Cancer pagurus. Furthermore, the suitability of signal integrals of infra-red photoplethysmographs as a qualitative tool is assessed under severe hypoxia.

Results

Multi-slice self-gated cardiac cinematic (CINE) MRI revealed the structure and motion of the ventricle to quantify heart rates, end-diastolic volume, end-systolic volume, stroke volume and ejection fraction. CINE MRI showed that stroke volumes increased under hypoxia because of a reduction of end-systolic volumes at constant end-diastolic volumes. Plethysmograph recordings allowed for automated heart rate measurements but determination of a qualitative stroke volume proxy strongly depended on the position of the sensor on the animal. Both techniques revealed a doubling in stroke volumes after 6 h under severe hypoxia (water PO₂ = 15% air saturation).

Conclusions

MRI has allowed for detailed descriptions of cardiac performance in intact animals under hypoxia. The temporal resolution of quantitative non-invasive CINE MRI is limited but should encourage further refining. The stroke volume proxy based on plethysmograph recordings is feasible to complement other cardiac measurements over time. The presented methods allow for non-destructive in vivo determinations of multiple cardiac performance parameters, with the possibility to study neuro-hormonal or environmental effects on decapod cardio physiology.