Some interesting facts about cephalopods from the FAQs on the Cephalopod Page. The Cephalopod Page (TCP) was created and is maintained by Dr. James B. Wood and is hosted by Dalhousie University and the University of Texas Medical Branch.
How can you tell octopus and squid apart?
Octopuses, squid and cuttlefish all have 8 arms. In addition, cuttlefish and squid have 2 tentacles. Tentacles are longer then arms and they usually only have suckers at their tips.
The public often interchanges the two terms, as they are not aware that they have different meanings. The presence/absence of tentacles is one way to distinguish an octopus from a squid. In addition, cuttlefish have a shell (the cuttlebone) on their back.
Are cephalopods dangerous?
Much of the "dangerous" image of cephalopods is Hollywood fantasy where you can have tales of cephalopods attacking ships and devouring sailors. 20,000 Leagues Under the Sea and Toilers of the Sea by Victor Hugo are good examples. Since cephalopods can be large, strong, and intelligent and because most people don't know much about them, they make great monsters of the deep. Yes, people have died from octopuses. However, the 'killer octopuses' are very small (smaller than your hand) and rather beautiful. These octopuses, called blue ringed octopuses, are only found in Australia. To get bit, you have to pick one up, or pick up its home (it lives in empty shells).
There are giant octopuses (O. dofleini) - they live in the Northeast and Northwest Pacific. An average giant pacific octopus weights around 15 kg or so and is regularly encountered by divers off the West coast of North America. They can weigh up to 50 kg.
The giant squid, however, is much more impressive. They can weigh up to a ton and can attain 20m long - they are the largest invertebrates on Earth.
Octopus are very short-lived - why?
To me this is a very interesting question and something that I've though a lot about. It is hard for us, as mammals to understand why a fairly large and intelligent creature such as an octopus lives for only a year. I think the answer is easier to understand if the question is altered slightly to 'Why would evolution favour cephalopods to have large brains and well developed senses'. A large brain could help any animal regardless of its life span. Cephalopods were once one of the dominant groups in the worlds oceans. Back in evolutionary history, cephalopods faced competition from a relatively intelligent and fast moving competitor - fish. When fish arrived in the world oceans - many species of cephalopods went extinct but some, those that were able to adapt by losing their heavy external shell and becoming faster as well as developing better senses and larger brains to process the increase in information, survived. A large brain seems worth the cost in cephalopods, whereas in say a clam, diverting resources to a large brain would not be selected for as that energy could be put to better use elsewhere. Evolution looks at brain size just as any other adaptation. If having that bit become larger, be it muscle, brain or gonads helps the animal survive and produce fit offspring then natural selection will favour it.
We tend to link intelligence with life cycle. We are most familiar with the life cycles of cats, dogs, birds, rodents, elephants, fish and other vertebrates, many of which have relatively long life cycles and are iteroparous (reproduce more then once). However, cephalopods are not the only creatures that reproduce once and die, salmon are a vertebrate example of an animal that only reproduces once. And as far as life cycles go, there are far weirder ones out there. Many jellyfish and hydrozoans have alternation of life style and many species of algae experience alternation of ploidy in addition to alternation of life style. The life cycle of many parasites is truly bazaar and interesting.
Cephalopods, except Nautilus, are though to live fast and die young. They do not store lipid reserves like fish. Therefore, it might be better for them to get to maturity as fast as possible and get out a second generation instead of surviving a lean period. Keep in mind that a lean period for an adult may be a good period to be a hatchling that is several orders of magnitude smaller and feeding on a completely different food source. Also, jet power isn't as efficient as fin undulation except at very small sizes. Having short life cycles may help cephalopods spend a large part of their life in a stage where they have the advantage.
Can cephalopods regenerate themselves?
A starfish can regenerate a complete clone from a lost arm but an octopus can not. An octopus can regenerate a lost arm though, and octopuses missing arms are fairly common in nature.
You probably know that octopuses have very advanced brains for invertebrates - but did you know that 2/3 of the nerves of an octopus are in their body and arms, not the brain! This gives the arms a lot of local control. It also the allows the detached arm to continue to crawl, suckers to suck, and chromatophores to change colour.What is the difference between squid and cuttlefish?
In general terms, squid tend to be more streamlined as most of them live well away from the shore, while most cuttlefish live near the shore and in association with the bottom. Cuttlefish tend to be broader and less streamlined.
Squid have a pen (very thin shell remnant) for support while most cuttlefish have a thicker internal "shell" - the cuttlebone used by many to supply calcium to birds.
Where does the name "cuttlefish" come from?
"The name Cuttlefish originally came about as the best guess of how to spell or pronounce the Dutch or perhaps Norwegian name for these beasts. It is derived from something like 'codele-fische' or 'kodle-fische'. In German today, cuttlefish and squids are called tintenfische, meaning ink-fish. I've been told that the term fische actually refers to any creature that lives in the sea or are caught in nets when fishing, not just fishes. Anyway, that's what I understand the derivation of name to be.