Tunicates comprise an invertebrate, chordate subphylum which has been shown to be the closest group to vertebrates. Colonial tunicates are clusters of genetic clones generated asexually from a single free swimming larval “tadpole”. Each individual, or zooid, of the colony has a peristaltic heart which circulates blood through that individual. In addition, each zooid is connected to a common, external vascular network. This vascular network has radial extensions that end at the colony periphery in bulbs, or ampullae, which contract and expand to generate reciprocating flow between ampullae and zooids. Surgically detached ampullae continue to beat.
Quantitative scans of videos of individual ampullae in a young Botrylloides viocella colony demonstrate ampullae contractions are often in phase, with occasional abrupt phase shifts out of and back to synchrony. The vessels connecting the ampullae to the zooid also contract, mostly in phase with the ampullae. Total volumes pumped by this colonial system are a significant fraction of the zooid volume, since it contracts 180 degrees out of phase and at the same frequency as the ampullae. Reversals of the peristaltic heart are at least partially synchronized with ampullae contractions. Ampullae that have been surgically detached from the colony contract at a more uniform rate with more symmetrical profiles than when part of the colony.
Contractions of the ampullae and associated vessels pump sufficient blood in and out of the zooid that they should be considered functional hearts, and the partial synchrony of ampullae contractions results in a larger blood flow compared to an alternative asynchronous contraction pattern. The manner in which the ampullae abruptly fall out of and back to synchrony indicates synchrony is due to entrainment while the out of phase contractions of the zooid may be a direct result of pumping. The shape of contraction curves of detached ampullae pairs is almost indistinguishable from a pure sine wave, indicating that the more complex original pattern was due to interactions between out of phase ampullae. Ampullae and associated vessels might be analogous with the system of lymphatic vessels in vertebrates.