In cuttlefish, and nautiloids. They are nestled within

In this lesson, we will discuss the structure and function of the cephalopod circulatory system. Cephalopods are marine invertebrate animals including octopuses, squid, cuttlefish, and nautiloids.

What is A Cephalopod?

Go ahead. Press your ear against someone’s chest. You can hear the reassuring lub-DUB of a single heartbeat.Imagine if you could hear three heartbeats at once? That’s exactly what you would find in a cephalopod circulatory system – a fascinatingly complex system that pumps blood through the bodies of these aquatic animals.Cephalopods are incredibly bright invertebrates. Their name means ‘head foot’, emphasizing the simple body plan and the often bulbous heads of these aquatic creatures.

The cephalopod group includes the living octopuses, squid, cuttlefish, and nautiloids. They are nestled within the larger Phylum Mollusca – the mollusks, which also include the cephalopod cousins, snails and slugs.Unlike all other mollusks, cephalopods have a closed circulatory system.

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This means that blood flows through a series of vessels to return to the heart, rather than bathing organs in the blood fluid as in open circulatory systems.

Structure and Function

An overview of a cephalopod circulatory system.
ceph circ system

The core of the cephalopod circulatory system is a series of three beating hearts.

This trio of hearts connects to a high pressure system of veins, arteries, and capillaries – unique among all mollusks. Think about it – no other mollusk has closed-off vessels, almost like a human’s veins and arteries. It’s really unusual.Two of the hearts are branchial hearts, which pump blood through the gills for respiration and gas exchange. These are almost like the parts of the human heart that pump blood to our lungs.

The third heart is a systemic heart, receiving the blood that drains from the gills and pumping that oxygenated blood to the body system. This systemic heart is most like the parts of our human heart that pump blood out to the whole body.Each of the three hearts is innervated by a variety of nerves, though it appears that the cardiac ganglion– a cluster of nerves – acts as the controlling pacemaker of the hearts. We humans have a natural pacemaker, too, and sometimes when it goes awry a surgical pacemaker implant is needed.

The two branchial hearts beat simultaneously, followed by the contraction of the systemic heart to supply the body with blood. In a way, it is a lot like our heart, only it’s split out into three distant orbs.Blood drains to the branchial hearts by the major veins, known as the anterior and lateral vena cavae or vena cava cephalica. Those words are a bit tricky, but just remember they correspond to our own major heart veins, the inferior and superior vena cavae.The vessels within the gills are known as afferent branchial vessels, which drain back to the main ventricle – another name for the systemic heart.

Just like our own ventricles, the systemic heart pushes blood out of the heart.Blood is then pumped from the systemic heart to the body via the main cephalic artery. An easy way to remember this flow is that arteries move away from the heart, and both words begin with the letter ‘a’.

Benefits of a Closed System

Both open and closed circulatory systems have their distinct advantages for an animal’s body plan.

An open system allows for smaller body sizes, without the need for clunky vessels or big, complex heart systems.One advantages of a closed system, which we humans share with cephalopods, is that blood can travel farther through the blood vessels, including veins, arteries, and capillaries. Closed circulatory systems are also more energy efficient, sending larger quantities of oxygenated blood to various tissues.For example, octopuses can maneuver their muscles in tightly coordinated movements to swiftly escape from predators. The closed circulatory system of these cephalopods helps make these fast metabolic activities possible.It may be that the closed circulatory system of cephalopods allows for more intellectually demanding tasks, such as using tools. Octopuses have been observed solving the puzzle of a closed screw-top jar, using the precise and strong movements of its muscular tentacles along with its mental acuity to open the jar.

Perhaps it is, in part, the cephalopod’s advanced circulatory system that contributes to this really remarkable behavior?

Hemocyanin

The oxygen receptor in cephalopod blood cells is a molecule called hemocyanin. Think of ‘cyan’, the turquoise-like color, and ‘hemo-‘, the root word for blood. It literally means ‘blue blood protein’!Because oxygen is picked up by copper-based hemocyanin instead of the iron-based hemoglobin, as in human blood cells, the blood of cephalopods is clear.

That is, until it is exposed to the air – then the clear blood of cephalopods turns greenish blue due to the presence of this copper compound.

Lesson Summary

Cephalopods – the ‘head foot’ invertebrates – are unique among the Phylum Mollusca in that they have a closed circulatory system, complete with three unusual hearts, and with a system of arteries, veins, and capillaries. This distinction helps cephalopods accomplish quick and metabolically demanding tasks. The copper-based molecule hemocyanin is the used (in place of hemoglobin) to cart oxygen around the cephalopod body, giving their blood a strange clear color until it is exposed to air, when the cephalopod blood turns bright blue-green. The blood travels from the branchial hearts to the systemic heart, where it then travels to the entire body system.The closed circulatory system of cephalopods may even provide enough metabolic energy to help this group of animals carry out highly intelligent and energetically demanding tasks.

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