Question: Did you know that the ascidians exhibit reversal of heartbeats every few seconds or minutes?

Sounds crazy? Let’s take a look at this video that was taken in the laboratory. We were dissecting the ascidians (species Phallusia arabica) to examine if they are reproductively fertile, and while doing so, we paid a closer attention to its heart. At the start of this video, you will see that the pumping motion is from bottom to top, and it pauses at 15-16 seconds mark, and then followed by a reversal in the pumping motion (from top to bottom)!

Wait, what?! Why?

Ascidians, also commonly known as sea squirts or tunicates, are a type of marine invertebrate. While the term ‘invertebrate’ refers to an animal lacking backbone, the ascidians are unique as they possess nerve cords and notochords (similar to our spine but a more primitive version). The heart (white colour in this video) is two gently curved concentric tubes extending across the width of the animal. The heart is peristaltic and periodically reverses direction. This phenomenon is observed widely in all tunicate species, but the reason for it is still relatively understudied.

A recent study using another tunicate species, Corella inflata, reported that not only does the heart pump in reverse direction periodically, the direction of blood flow in all parts of the animal reverses as well! In this study, he proposed that this periodic reversal may have a physiological advantage of helping the ascidian to provide a more uniform delivery of oxygen and nutrients across the various body tissues. But this remains a hypothesis as there is still no generally accepted model for the function of heart reversals.

Imagine that! Even though the ascidians may have a teeny tiny heart compared to ours, their heart remains a mystery waiting to be unravelled! 🙂