Cephalopods have quite a neat circulatory system (file that away under “dorkiest things to say at a party”.) I’m not joking, though; they do! They have a closed circulatory system, meaning that their blood is contained within blood vessels, instead of just filling their body cavity. All other molluscs have an open circulatory system, where blood is still pumped by one or more hearts, but it flows around the body cavity bathing the organs rather than moving through a set of vessels like it does in cephalopods (and, incidentally, mammals like us!) Powering the closed circulatory system of cephalopods are three hearts, two brachial hearts that pump blood through the vessels of the gills and one systemic heart that pumps blood from the gills through the rest of the body. This figure (from Wells, 1980,) illustrates the circulatory system of (who else?) the octopus: To get oriented, imagine an octopus laying on a table with its arms pointed away from you (towards the top of the image.) This is what you’d see (in beautiful color, with lots of other gunk in there) if you dissected the animal’s mantle. The brachial heart is only shown on one side, and is labeled “Gill heart”. M. J. Wells studied the octopus hearts rather extensively, describing their basic anatomy and physiology. By recording the pressure inside the aorta of the systemic heart (a method described in his 1979 paper, cited below) he found that the octopus’s heart behaves much like most of the other hearts that you may familiar with. Although the heart normally beats at a steady rate, when an octopus is startled or excited (such as by Wells’ elegant procedure of sticking a hand in to its tank,) its heart rate and blood pressure increase. He also found that something very curious happens when an octopus is excited: it undergoes temporary cardiac arrest. When you get an octopus really riled up, its heart misses a beat or two. (Keep in mind that any single octopus heart can stop for some time, and the remaining two will provide more than enough circulation to keep the animal alive. Otherwise, the octopus would die when its heart stopped.) In the way of theorizing why this might be, Wells wrote:
It is true that the animal often tenses its muscles when startled. It may flatten out into the ‘dymantic’ response (see Wells, 1978) or grip tightly to the bottom of its tank. It often ceases to make respiratory movements. Any of these responses might restrict the venous return. But there would appeartbeats, and the effects observed (both the cessation of the beat and its return) often seem to be far too abrupt and complete to permit an explanation based on strangulation of the venous return… It is far more likely that sudden to be plenty of blood in the large vena cavae to supply at least two or three systemic hear cardiac arrest is a direct nervous effect, ordered by the CNS through the visceral nerves.
Basically, the fact that the octopus systemic heart stops very abruptly after the animal is startled suggests that this effect is not due to the surrounding muscle putting too much pressure on incoming veins and physically blocking the blood flow to the heart. Rather, Wells speculates, it is controlled by the central nervous system of the octopus. Why this could be adaptive, I have no idea; in the acute stress responses I am more familiar with (those of mammals), the heart usually speeds up when one is scared. If anybody has a thought on why the octopus would have evolved a heart that quits when the animal is surprised, please post it in the comments. Anyways, I’ve covered all the background we need to get to the point of this post. Wells did what any curious person would do, given a laboratory full of octopuses; he put males and females together and observed them mating. Unusually, though, he observed them mating while he was recording their heart activity. Specifically, he found that when a male octopus is copulating, his heart often skips a few beats; when he initiates the act, and usually once each time he deposits a spermatophore (these are packets of sperm that the male octopus leaves inside the body of the female.) In his 1979 paper, Wells presents his recording from the systemic heart of a copulating male octopus:
The waves in this recording show the pressure in the heart as it changes over time. Each peak-and-valley pair is one heartbeat. If you follow it from the first line, you can see the heart skip a beat when the female is first introduced to the tank, then twice more as the couple come closer to each other and make contact, and then three more times when the male deposits his spermatophores. Crazy, right?
Before the cephalopod fanboys and fangirls among you run off talking about “octopuses in love” and the like, I should remind you that octopus sex is not generally a very romantic thing, regardless of how romantic it seems that his heart skips a beat when he touches her. In case you’re unfamiliar with the mechanics of octopus sex, it goes like this: a male approaches a female and attempts to slide one of his arms inside of her mantle. At the end of this arm is an organ called a hectocotylus. He uses this organ to deposit a packet of sperm (a spermatophore) inside of her. Then he tries to get away without being eaten. It probably skips the beat not because of love, but because he’s scared of being strangled during the act! If you don’t believe me, just take a look at this video of two octopuses “in love”:
Of course, the magic moment can occur in a less violent fashion, with the whole ritual taking a more calm attitude. This occurs more readily when you play Neil Diamond to the octopuses while they are mating. The biological basis of this so-called “Neil-facilitated mating behavior” is still under investigation, though we owe its recent documentation to rman751 :
Watch for the actual copulation at around 2:15, when the male unrolls his arm and inserts it into the female’s mantle. Ok, maybe it is kind of cute.
Thanks for reading, and I’ll see you next time!
Wells MJ (1980). Nervous control of the heartbeat in octopus. The Journal of experimental biology, 85, 111-28 PMID: 7373208