Caudal Circulation in the Short-finned Eel Anguilla australis schmidtii (Phillips). P.S. Davie. Department of Zoology, University of Canterbury, Christchurch, New Zealand. The anatomy, neuroanatomy and neural control of the caudal lymph heart of the short-finned eel are described. The evolution of accessory caudal vascular pumps is discussed.
An isolated saline perfused eel tail preparation is described which is suitable for investigation of vascular and interstitial fluid circulation. Dose versus vascular resistance response curves for adrenaline, noradrenaline and isoprenaline are presented. The eel tail vascular bed has alpha constrictory and beta two dilatory adrenergic receptors. Beta two receptors may be stimulated in vivo by sympathetic nervous activity. Vascular volume changes in the eel tail during constriction and dilation were small; less than three per cent of the tail vascular volume. Significant amounts of perfusate were extravasated when resistance increased more than ~ 100% of baseline resistance. Frequency and amplitude of the lymph heart of the perfused tail increased when resistance increased by more than ~ 100% of baseline resistance. Frequency increases were probably caused by interstitial fluid build up rather than direct actions of vasoconstrictory drugs. One important function of the lymph heart is recovery of interstitial fluid from the tail. Cardiac output, and ventral aortic, dorsal aortic and caudal venous blood pressures were recorded from unrestrained eels at rest, while swimming at 15 cm.s-1 and 22-25 cm.s-1, and after exercise. During swimming cardiac output fell slightly and transbranchial pressure differential increased. Elevated ventral aortic pressure was the principal cause of the increase in transbranchial pressure differential. Plasma catecholamine concentrations probably do not increase during swimming in the short-finned eel. Spontaneous cessation of swimming often occurred at very high branchial resistances. It is suggested that this response has a protective function for the gill tissues.