Octopuses belong to the phylum Mollusca which includes snails, clams and chitons. Their closest relatives are the chambered nautilus, cuttlefish and squids. The largest and smallest octopuses are found off the United States. The largest is the North Pacific Octopus (Octopus dofleini) that may grow to over 30 ft. And weighs more than 100 lbs. The smallest is the Californian (Octopus micropyrsus) which only reaches 3/8″ to 1″ in length.
Octopuses have the most complex brain of the invertebrates (animals with out backbones). They have long term and short-term memories as do vertebrates. Octopuses learn to solve problems by trial-and-error and experience. Once the problem is solved, octopuses remember and are able to solve it and similar problems repeatedly.
Octopuses sense of touch is acute in it’s suckers. The rim of the cups are particularly sensitive. A blindfolded octopus can differentiate between objects of various shapes and sizes as well as a sighted octopus.
Octopuses have highly complex eyes which compare to human visual acuity. Focusing is done by moving the lens in and out rather than by changing its shape as the human eye does.
When threatened, octopuses will often try to escape by releasing a cloud of purple-black ink to confuse the enemy. Its body will change color, release an ink cloud and jet away to safety. Several blotches of ink can be released before the ink sac is empty. The ink is toxic to an octopus in a confined space such as in a cave with little water current or in captivity. If the the octopus can not escape the ink (or water is not changed quickly when held in an aquarium), the octopus will become ill or perhaps die.
Color change in octopuses is initiated by the eyes. If an octopus is disturbed, special pigment cells (chromatophores) in the skin will be activated in an attempt to blend in with the surroundings. The chromatophores consist of three bags containing different colors which are adjusted individually until the back ground is matched. Coloration reflects mood, white for fear, red for anger, brown is the usual color.
Many octopuses produce venomous secretions. This venom is fatal to their favorite prey – crabs and lobsters. The tiny Blue Ringed Octopus in Australia is deadly to humans. Its tiny beak can even penetrate a SCUBA diver’s wet suit!
Octopuses have separate sexes (male and female) and fertilization is internal. In some species, the male can be distinguished by modified sucker discs at the tip of one of its arms. This modified arm is used to remove a packet of sperm from within his mantle cavity and insert it into the mantle cavity of the female. Within two months after mating the female attaches strands of clustered eggs to the ceiling of her lair. The number of eggs laid by a female varies greatly depending on the species laying the eggs. The Common Octopus may lay 200,000 – 400,000 tiny eggs. The Pygmy Octopus lays about 150 large eggs. Once the eggs have been laid, the female octopus will gently caress the eggs with her suckers to keep algae and bacteria from growing on them. She keeps the eggs oxygenates by gently squirting them with streams of water from her syphon. After the developing octopus turns in the egg so the tip of its mantle is at the unattached end of the egg and is ready to hatch, the female’s gentle caresses become more violent to help the baby octopus escape from the egg case. Most females will not eat after laying eggs and die soon after her eggs have hatched. Some baby octopuses, like Octopus vulgaris, are carried about in water currents for about a month before they settle to the bottom. Other baby octopuses, like the large egged Octopus joubini, look like miniature adults and immediately start living their life on the bottom. Only one or two out of 200,000 eggs will survive to become adult.
Cephalopods (for example squids) are found in all of the world’s oceans, from the warm water of the tropics to the near freezing water at the poles. They are found from the wave swept intertidal region to the dark, cold abyss. All species are marine, and with a few exceptions, they do not tolerate brackish water.
Cephalopods are an ancient group that appeared some time in the late Cambrian several million years before the first primitive fish began swimming in the ocean. Scientists believe that the ancestors of modern cephalopods (Subclass Coleoidea: octopus, squid, and cuttlefish) diverged from the primitive externally shelled Nautiloidea (Nautilus) very early – perhaps in the Ordovician, some 438 million years ago. How long ago was this? To put this into perspective, this is before the first mammals appeared, before vertebrates invaded land and even before there were fish in the ocean and upright plants on land! Thus, nautilus is very different from modern cephalopods in terms of morphology and life history.