Final answer:
The heaviest invertebrate is the colossal squid. Among the given options, the blue whale and African elephant are vertebrates, not invertebrates.
Step-by-step explanation:
The question asks about the heaviest invertebrate. The options given include the giant squid, blue whale, African elephant, and colossal squid. It is important to note that a blue whale and an African elephant are not invertebrates; they are vertebrates.
The blue whale is a mammal, and the African elephant is a land mammal, making both of them unsuitable as answers. Among the options, the correct answer is the colossal squid. The colossal squid (Mesonychoteuthis hamiltoni) is indeed the heaviest invertebrate known, with some specimens weighing in at an estimated 495 to 990 pounds (225 to 450 kilograms) or more.
In comparison, the giant squid (Architeuthis dux) is also a large cephalopod but does not typically reach the same mass as the colossal squid. Cephalopods like the nautilus, giant cuttlefish, reef squid, and blue-ring octopus are much smaller by comparison, according to the given references.