BIOL630 TERM PROJECT
CIRCULATORY SYSTEM
The circulatory system is essential for transporting oxygen and nutrients around the body. For octopuses, this has evolved to be more advanced than other invertebrates. Their circulatory system is closed, meaning that the blood remains inside of the blood vessels as it travels through the body.
Octopuses also are known for having three hearts! The main heart, called the systemic heart, has a powerful muscular chamber, while the two other hearts, called branchial hearts, pump blood to the gills (Hill, 2018). The breakdown of this circulatory plan is seen in figure 1. When blood enters the systemic heart, it gets pumped into the aortae to the systemic tissues. Blood returns in the major veins, where it is split into two paths to the gills. The branchial hearts reside at the base of the gills, so as the blood passes through them, it then returns to the systemic heart via the branchial vessels. A study of octopus embryo development found that the branchial hearts develop before the systemic heart, and the branchial heart sets the pace for the heartbeat rhythm, because they beat at a faster rate during this time (Maldonado et al, 2019).
Figure 1. Diagram of the octopus circulatory system. (Courtesy of Thoracic Key)
Figure 2. Structure of hemocyanin with and without the presence of oxygen. (Courtesy of ResearchGate)
Within the octopus’s blood is a respiratory pigment called hemocyanin. It has a high molecular weight, contains copper, and carries oxygen while being dissolved in the blood plasma (Wells and Smith, 1987). However, the oxygen carrying-capacity for an octopus is lower than for an individual that uses hemoglobin as their respiratory pigment instead. The carrying-capacity is just 2 to 5 ml O2 per 100 ml of blood. Because of this low carrying-capacity, octopuses are intolerant to waters where they don’t have access to oxygen freely (Hill, 2018). However, it does provide some advantages in colder climates. When the temperature is low, oxygen binds even tighter to hemocyanin, making oxygen transport and release to the tissues even more of a challenge. Octopuses that live in freezing waters around -2°C have evolved to produce even more hemocyanin than an octopus in warm waters, to increase the release of oxygen in tissues even further away from the body of the octopus to the tips of the arms (Oellermann et al, 2015).
