Abstract

Marine mammals are faced with many, potentially stressful, environmental challenges that may influence immune function and thus health. Increasing incidences of marine mammal stranding and emerging disease have highlighted the need for better understanding of marine mammal immunology and adaptation of these animals to these unique environmental challenges. Diving, a natural behavior for most marine mammals, presents numerous physiological challenges. While physiological and behavioral dive adaptations of marine mammals are fairly well known, there are no published reports concerning the effects of diving on marine mammal immune systems. This knowledge is necessary for evaluating the impact of anthropogenic stressors on natural adaptations and implications for health. This study aimed to evaluate neutrophil function in beluga whales (Delphinapterus leucas), harp seals (Phoca groenlandica), grey seals (Halichoerus grypus) and harbor seals (Phoca vitulina) in response to compression and chilling as the first step in assessing the effects of diving and multiple stressors on marine mammal health. Blood samples from beluga whales resident at the Mystic Aquarium, as well as phocids admitted to the Mystic Aquarium marine mammal stranding program, were used to evaluate immune function following in vitro exposures to simulated dive conditions. A stainless steel pressure chamber was used to simulate changes in pressure for two dive profiles; 1) 30 min duration at 2000 psi and 2) two 5 min durations at 2000 psi. Chilling was achieved using water baths (37°C and 34°C) and a 4°C refrigerator (4°C). Neutrophil function was assessed by measuring phagocytosis and respiratory burst, as well as the expression of cd11b (MAC-1). Controls were run without exposure to compression or chilling. Wilcoxon non parametric paired t test and Friedman non parametric ANOVA were used to analyze the effects of compression and chilling (significance p < 0.05). Significant decreases in phagocytosis were measured following dive profile 1 and in both phagocytosis and respiratory burst measurements during chilling in beluga whales. Analysis of cd11b expression indicates no significant changes in beluga neutrophils following either dive profile. All phocid species were grouped together as pinnipeds for analysis and no significant changes in phagocytosis were found following either dive profile. Following chilling, phagocytosis and respiratory burst in pinniped neutrophils showed an initial significant decrease but returned to control levels over time. There was an observed increase in phagocytosis among healthy pinnipeds (immediately prior to release) following both dive profiles, whereas a decrease was seen upon initial stranding. Limited cd11b analysis in pinnipeds suggests there may be an increase in expression following compression. Results suggest that deep (belugas) and shallow diving (phocids) species are differentially adapted to conditions of diving and that characteristics of the dive (e.g., depth, length) can impact the response of peripheral immune cells. Comparing neutrophil function in wild pinnipeds over the course of rehabilitation may provide insight as to the impact of stressors on peripheral leukocyte function during diving. Furthering this work by examining the additional effects of stress hormones is needed in order to better understand dive adaptation in marine mammal immune cells and evaluate the impacts of anthropogenic activity.

Acknowledgements

The authors thank the veterinary staff and arctic coast husbandry staff at the Mystic Aquarium, and Dr. Cara Field at the Audubon Zoo, New Orleans. This work was supported by the Mystic Aquarium and the Department of Marine Sciences, University of Connecticut.