The Circulatory System

What is interesting about the octopus's circulatory system is that it is closed, unlike some other molluscs. This is partly because the octopus's respiratory pigment (like many molluscs) is haemocyanin rather than haemoglobin, but, (unlike many molluscs) the octopus requires lots of oxygen for more movement since they are very active creatures. 

Haemocyanin is not bound in cells like haemoglobin is to red blood cells. It is suspended in haemolymph, a fluid similar to our blood without the presence of RBCs. 

Haemocyanin has a lesser affinity to oxygen than haemoglobin because the ion that binds to oxygen is a copper ion (Cu+⇌ Cu 2+ + e-) rather than an iron ion (Fe 2+⇌ Fe 3+ + e-) and copper cations are much less reactive than iron cations.

This means that octopuses had to develop a more efficient circulatory system to meet the demand for oxygen, hence the closed system.

The presence of copper(II) in oxyhaemocyanin means that an octopus's oxygenated 'blood' is blue. Deoxyhaemocyanin contains copper(I) which doesn't absorb visible wavelengths of light. This means the deoxygenated blood is colourless. 

It's also thought that haemocyanin works more efficiently in colder temperatures, which is beneficial for the octopus who averagely lives in <15⁰C temperatures. Some even inhabit sub-zero conditions! 

The octopus has three hearts; one systemic heart and two auxiliary hearts (The doctor has nothing on you, cephalopods). 

The Auxiliary Hearts:

One heart is attached to each of the two gills. These gills are located inside the mantle and have filaments to increase surface area so that more oxygen is exchanged per breath. This is done through counter current flow. 

The auxiliary hearts pump the now oxygenated blood with their single chamber through the branchial veins to the systemic heart.

The Systemic Heart

This is the largest heart which also has a single chamber. It is much more muscular than the auxiliary hearts because it must pump the oxygenated blood all of the way around the body and back to the auxiliary hearts to be oxygenated again. 

When an octopus uses jet propulsion, the systemic heart skips a beat just before the mantle expels the water! Therefore, jetting can be very tiring for octopuses as skipping multiple beats makes it difficult to transport oxygen to meet the increased oxygen demand. 

Questions to be answered: 

Is haemocyanin more susceptible to denaturing in acidic conditions? If so, could this put octopuses at a disadvantage with increasing sea acidity? 

Why is haemocyanin more effective in colder temperatures than haemoglobin? 

What causes the systemic heart to skip beats when jetting? 

REFERENCES 

https://sciencing.com/an-octopus-breathe-4566479.html

'How Does an Octopus Breathe?'

Christy Flora 

2018

https://www.sciencefocus.com/nature/why-does-an-octopus-have-more-than-one-heart

'How Many Hearts Does an Octopus Have?'

Dr. Helen Scales

n.d.

https://chem.libretexts.org/Courses/Saint_Marys_College_Notre_Dame_IN/CHEM_342%3A_Bio-inorganic_Chemistry/Readings/Metals_in_Biological_Systems_(Saint_Mary's_College)/Oxygen_transport_and_Storage/Blue_blood_(Invertebrates_molluscs_and_arthropods)/Haemocyanin

'Haemocyanin' 

Margaret Benjamin 

n.d.

Date published: 14th December 2023