Comparison of the respiratory vasculature of two species of swamp eels, <scp><i>Monopterus albus</i></scp> and <scp><i>Ophisternon bengalense</i></scp> (Synbranchidae)

We compare the cardiovascular anatomy of two synbranchids (Actinopterygii, Synbranchiformes); Ophisternon bengalense, which only infrequently breathes air when in hypoxic water, and Monopterus albus, which is more strongly dependent on air-breathing. Both species use the buccopharyngeal cavity for aerial respiration. The gill vasculature in O. bengalense comprises four pairs of holobranchs. Filaments are lined with secondary lamellae, the blood space of which is studded with the pillar cells, as in most other teleosts. In comparison, M. albus has only three pairs of gill arches exposed to the surrounding water. Filaments are rudimentary, with the afferent and efferent filamental arteries connected by 8-10 (first arch) or fewer (second and third) parallel vessels. There also are shunt vessels directly connecting the afferent and efferent branchial arteries. The fourth arch artery is a large throughfare vessel embedded in tissue with no branchial ramifications. The aerial respiratory capillaries are distributed with no particular pattern in O. bengalense, whereas the capillaries occur in clusters, each composed of repeatedly turning capillaries in M. albus. The arterial architecture of O. bengalense shows no deviation from the typical teleost pattern. The respiratory capillaries over the buccopharyngeal cavity surface are supplied mainly by the branches of the first efferent branchial artery and drained by the anterior cardinal vein. The efferent branchial arteries are connected by the lateral dorsal aorta. In contrast, the arterial system of M. albus shows notable anomalies. These include complete disruption of the lateral dorsal aorta and the presence of pre-gill arteries to the aerial respiratory capillaries (hyoidean artery, ventral esophageal artery and other smaller ramifications of the first to third branchial arches). We discuss the functional implications of these findings and hypothesize a sequence of evolutionary steps from adoption of air-breathing in fish to the development of double circulation as seen in lungfish and tetrapods.