How many separate hearts does an octopus have in its circulatory system?

Introduction:
Octopuses are fascinating marine animals with many unusual anatomical features compared to vertebrates. One of the most striking facts often quoted in general knowledge is that they have more than one heart. This question tests whether you know the exact number of hearts found in an octopus circulatory system.

Given Data / Assumptions:
- The options are 9, 5, 3 and 2 hearts. - The focus is on true anatomical hearts, not just blood vessels or accessory structures. - We consider a typical octopus species used in biology descriptions. - Knowledge is based on standard zoology textbooks and marine biology references.

Concept / Approach:
An octopus has three hearts. One is the systemic heart, which pumps oxygenated blood to the body. The other two are branchial hearts that sit near the gills and pump blood through the gill capillaries. This arrangement helps circulate blood efficiently in the low oxygen environment of water. Therefore, the correct count of distinct hearts is three, not two or any larger number in the options.

Step-by-Step Solution:
Step 1: Recall that the octopus is often described as having three hearts in biology classes. Step 2: Identify the functions: one systemic heart for the body and two branchial hearts for the gills. Step 3: Compare this known structure with the answer choices 9, 5, 3 and 2. Step 4: Select 3 as the number that matches the standard anatomical description.

Verification / Alternative check:
Marine biology sources and science articles repeatedly mention that when an octopus swims, the systemic heart may slow or stop while the branchial hearts keep working, which helps explain why swimming is tiring for them. These details are built around the accepted count of three hearts, providing strong confirmation of the answer.

Why Other Options Are Wrong:
9 hearts: This number is sometimes associated with earthworms and is not correct for octopuses. 5 hearts: No common description of octopus anatomy uses this number. 2 hearts: This would ignore the presence of two separate branchial hearts plus the systemic heart and so underestimates the actual count.

Common Pitfalls:
Students sometimes confuse facts about octopus hearts with information about other animals, such as earthworms or some fish, and may choose a larger number like 9 or 5. Others remember that octopus has more than one heart but forget the exact count and guess 2. Focusing on the division into one systemic and two branchial hearts is a robust way to remember that the total is three.

Final Answer:
An octopus has 3 hearts in its circulatory system.