Thymus histology was performed as previously described (Tang et al., 2014 (link)). In brief, 3-mo-old fish were sacrificed and fixed in 4% paraformaldehyde, embedded in paraffin, and step sectioned. Slides were stained with hematoxylin and eosin by the Specialized Histopathology Services at Massachusetts General Hospital. Thymus sections were imaged at 200× magnification using an Olympus BX41 compound microscope. Images were imported into Fiji (ImageJ; National Institutes of Health) and the thymus was traced using the Freehand Selection tool and area quantified. The mean area of thymi (n ≥ 5) where graphed and SEM indicated. Significance was calculated by Student’s t test.