Morphological changes to the heart and associated blood vessels during lamprey metamorphosis

Abstract

Representatives of all seven stages in the metamorphosis of the parasitic lamprey Geotria australis and the nonparasitic lamprey Lampetra planeri have been used to describe the way in which the heart and associated blood vessels are modified or lost during metamorphosis from the larval (ammocoete) to adult condition. Attention is focused on discussing the possible functional significance of these alterations and on resolving major anomalies in the literature regarding the adult circulation. The metamorphic changes to the heart and associated blood vessels do not commence until external signs of transformation are apparent and, despite the different modes of adult life of the two species, they subsequently undergo a similar pattern of modification and loss. While the left duct of Cuvier has disappeared by the end of Stage 5, the right duct remains and its walls increase in thickness throughout metamorphosis. The small left dorsal hepatic vein, and also often the right dorsal hepatic vein which in ammocoetes enter the left and right ducts of Cuvier, respectively, are lost during metamorphosis. The median ventral hepatic and prominent inferior jugular veins, which both pass into the ventral region of the sinus venosus at the same level, remain in the same position as in the ammocoete. By Stage 4, the larval hepatic portal vein, which in the ammocoete entered the posterior dorsal tip of the liver, has undergone extensive degeneration and the new adult hepatic portal vein passes into the mid‐dorsal region of the liver. The pericardium develops during metamorphosis through a forward and upward growth of connective tissue on the anterior face of the liver and from connective tissue on the ventrolateral, dorsolateral and dorsal inner surfaces of the body wall. The development of cartilage can be detected in parts of this structure as early as Stage 3. The loss of certain blood vessels, the development of a pericardium and the alterations in the position and structure of the chambers of the heart and associated blood vessels which occur during metamorphosis, are all interpreted as adaptations for accelerating blood flow and increasing the pumping efficiency of the heart. These features would be of advantage to adult lampreys, which are more active than ammocoetes.