Red Blood Cell Swelling Prevalence in Elasmobranch Fishes
Fishes exposed to stress such as extreme hypoxia and capture events exhibit elevated plasma pCO2, potassium, and lactate; decreased plasma pH; and a suite of other physiological responses including increased cardiac output (Pickering, 1981; Wood, 1980; Korsmeyer et al., 1997; Mandelman and Skomal, 2009; Marshall et al., 2012; Skomal and Bernal, 2010; Skomal and Mandelman, 2012). These physiological disturbances can persist for hours, causing cellular damage and other detriments. These disturbances may then contribute to post-release mortality, negating any beneficial management strategies that mandate release of undersized, prohibited, or otherwise non-targeted species (Skomal, 2007, Moyes et al., 2006, Campana et al., 2009, Heberer et al., 2010, Whitney et al., 2016).
The mechanisms for maintaining blood oxygen delivery following exposure to hypoxia or exhaustive exercise are generally understood for teleost fishes: As blood pCO2 levels increase, plasma pH decreases, which initially decreases red blood cell (RBC) intracellular pH. This reduces hemoglobin (Hb) oxygen (O2) affinity, a phenomenon known as the Bohr effect (Berenbrink, 2011). In many teleost fishes, however, the reduction in blood pH is accompanied by a decrease in Hb-O2 carrying capacity, known as the Root effect (Waser, 2011). Because the influence of intracellular pH on Hb-O2 affinity is so profound, catecholamine-driven activation of (β-adrenergic) sodium (Na+) proton (H+) exchange (βNHE) on the RBC membrane is in place to protect intracellular pH by pumping H+ out of the cell. This results in an increase in Hb-O2 affinity and carrying capacity. Thus, as the blood returns to the gill, Hb can still effectively bind O2, and subsequent O2 delivery to the tissues is safeguarded (Nikinmaa, 1983, Hladky and Rink, 1977). This ion transfer results in an increase in intracellular [Na+], drawing water moves into the cell and causing measurable RBC swelling (Farrell, 2011).
It has long been thought that elasmobranch fishes do not exhibit RBC swelling in response to stress because their Hb does not exhibit a Root Effect, which makes reductions in Hb-O2 carrying capacity less of a liability. Recent work on both juvenile sandbar sharks (Carcharhinus plumbeus) and epaulette sharks (Hemiscyllium ocellatum) has, however, demonstrated these species do exhibit RBC swelling following exhaustive exercise and exposure anoxia, respectively (Brill et al., 2008, Chapman and Renshaw, 2009). Brill et al. (2008) also exposed blood from unstressed individuals to relevant catecholamine concentrations in an attempt to induce swelling in vitro, but did not observe any change in mean RBC size. This is not surprising, as to-date there is no published evidence that elasmobranch RBCs possess adrenergic Na+/H+ exchangers (Lowe et al., 1995, Berenbrink et al., 2005). Additionally, Graham et al. (1990) found little skates (Leucoraja ocellata) do not exhibit any signs of RBC pH regulation or swelling. Together, these studies suggest the mechanism driving swelling (when present), and indeed the function of swelling, may differ between teleost and elasmobranch fishes.