Octopus

By any measure used for invertebrates, very. Octopuses solve puzzles, open child-proof jars and screw-top containers, navigate mazes, use tools, recognise and respond differently to individual human keepers, and show what looks like play and curiosity. They have the largest brain of any invertebrate relative to body size, plus the extra processing spread through the arms, and they can learn quickly and remember for months. What makes this striking is that octopus intelligence evolved entirely separately from that of vertebrates, in an animal with a completely different body plan and a lifespan of only a year or two, and with almost no parental care, so each octopus works most of it out alone.

The blue colour comes from the oxygen-carrying molecule in the blood. Instead of the iron-based haemoglobin that makes vertebrate blood red, octopus blood uses a copper-based molecule, haemocyanin, which turns blue when it binds oxygen. Haemocyanin is less efficient than haemoglobin in warm, oxygen-rich conditions but performs better in the cold, low-oxygen water many octopuses live in. The three hearts support this system, with two branchial hearts driving blood through the gills to pick up oxygen and one larger systemic heart pumping the oxygenated blood to the rest of the body. The systemic heart actually stops beating when the octopus swims by jet propulsion, which is one reason octopuses tire quickly and prefer to crawl.

Yes, as long as the gap is bigger than its beak. The beak, a hard structure a bit like a parrot's, is the only truly rigid part of an octopus, so it sets the limit. Everything else, muscle and skin, can be squeezed, folded, and stretched, and an octopus will actively probe an opening with an arm, then work the rest of its body through, often changing shape dramatically in the process. A large octopus can pass through a hole a couple of centimetres across, which is how aquarium octopuses escape through pipework and drains and why keeping one contained is a real engineering challenge.

Almost all of them do, and it is not simply old age. After mating, an octopus enters a programmed decline controlled by a gland behind the eyes, the optic gland. In the female, this switches her into full-time egg care, so she stops feeding, ventilates and cleans the eggs constantly, and by the time they hatch she is starving and her body is failing, and she dies soon after. Males also decline and die within months of mating. Removing the optic glands in experiments stops the female eating her way to death and greatly extends her life, which shows the process is a built-in self-destruct rather than an inevitable wearing out. It means most octopuses live one to two years and reproduce only once.