Seasonality of immunological and health-state parameters of wild broadnose sevengill shark, Notorynchus cepedianus

Seasonal fluctuation in environmental parameters can influence immune responses of vertebrates and consequently influence their health and disease resistance. Although seasonality of immune function is well documented in a broad range of vertebrate taxa, this information remains virtually unexplored in cartilaginous fish. Here we examine seasonal variation in immune and general-health parameters of free living adult broadnose sevengill sharks, Notorynchus cepedianus, along an annual cycle. We sampled sharks during autumn/winter (i.e., coolest temperatures and nonreproductive period) and spring/summer (i.e., warmest temperatures and active reproductive period) and assessed aspects of immunity, general condition, and reproductive hormone levels. A seasonal influence was observed in some, but not all, parameters evaluated. Lower lymphocyte counts and higher heterophil counts and granulocyte to lymphocyte (G∶L) ratios were observed in sharks sampled during autumn/winter than in those sampled during spring/summer. On the other hand, total leukocyte counts, eosinophil counts, bacterial agglutination mediated by natural antibodies, and hematocrit did not vary seasonally. The observed seasonal patterns could be explained as (1) greater levels of stress based on the G∶L ratio, (2) a sign of immunosuppression or depressed immune investment based on the low lymphocyte counts, and/or (3) a sign of ongoing infection based on the higher heterophil counts in the colder seasons with respect to the warmer ones. In addition, the pattern is in line with the notion that while acquired components are usually depressed by lower temperatures, some innate components might increase to offset that reduction. Immune and health-state parameters were mostly independent of reproductive hormone levels, providing little support for a trade-off with reproduction. Overall, the observed seasonal pattern in immunity of broadnose sevengill sharks could be related to changes in abiotic environmental condition, such as water temperature and photoperiod, although other factors such as availability of high-quality food may play a part.