The external gills of captive Mexican axolotl (Ambystoma mexicanum) sometimes develop atypical blood vessels, the cause of which is unknown. We observed the external gill filaments of an individual animal with dilated blood vessels that formed a semicircle within the filament tissue. The positioning of the swollen blood vessels compressed the adjacent capillaries and connective tissues. Normal external gill filaments in urodelans contain a blood-vessel system with afferent and efferent arterioles that connect to circumvent the outer gill periphery. We infer that the dilated blood vessels in the axolotl originated from these arterioles.
