Beaks in octopuses are large, sharp, and formidable-looking.  A bite is invariably accompanied by injection of toxins from the posterior salivary glands that act quickly to paralize and/or kill prey such as crabs.  Although used to kill and dismantle prey during hunting and feeding, beaks are not as commonly used in defense as their appearance might suggest.  An exception is the feisty Indo-Pacific blue-ringed octopus Hapalochlaena lunulata, whose toxin has been known to kill humans.  On our coast, there are several references to divers, snorklers, and aquarium staff being bitten while handling the small west-coast red octopus Octopus rubescens including, in a few instances, injection of toxin.  Similarly, at least one report notes that giant octopuses Enteroctopus dofleini will also bite in defense of (human) attack, mostly on hand and wrist areas.  The account describes a lot of bleeding and prolonged healing.  The fact that most close contact by humans with both of these cold water-inhabiting species is done in protective suits, including gloves, may explain why reports of biting are relatively few.  High 1976 Mar Fish Rev 38: 17.

NOTE  a description of a bite on the back of the hand from a specimen of Octopus rubescens caught in Puget Sound, Washington includes the following: localised pain commencing 10min after the bite, accompanied by swelling at the wound site and a “fiery” pain extending up the forearm.  The victim bathed the hand in water as hot as could be tolerated and this may have helped detoxify the venom. The following day the bite could hardly be seen or felt. Another week was required for the hot-water blisters to heal. Anderson 1999 The Festivus 30: 45.

NOTE  in the cases described the toxin appears to have only minor topical effects, such as localised necrosis and prolonged healing.  As the main or perhaps only component of tetrodotoxin is a protein, then any treatment that destabilises protein is likely to be a useful prophylactic for an octopus bite.  This would include immersion of the affected part in hot water (as hot as possible, but not so hot that scalding occurs) and treatment with mild acid such as vinegar

Beak of an octopus showing radula. The 2 halves of the beak are arranged
opposite to that of a bird's beak. In an octopus the dorsal part (on the
Left in the photo) is overlapped by the ventral part

The beak and radula are really the only hard parts of an octopus, and the 2 parts of the beak may be all that remain of of an octopus consumed by a predator.  A researcher interested in feeding energetics of a lingcod with octopus beaks in its stomach will need to know the original size of the prey.  Scientists at Simon Fraser University, British Columbia have worked out the relationship between beak size and live mass in Enteroctopus dofleini for just this purpose.  Their first question was to determine which measurement on the beak would give the best fit.  The data in the graph are for a measurement of height of upper beak as shown on the drawings on the Left (see orange line).  To determine live mass of the octopus from its beak measurement, simply substitute in the regression equation:  lnX = (lnY-a)/b where a = 2.674 and b = 0.274.  For example, an upper beak measuring 24.5mm in height would have come from an octopus weighing 6.8kg.  The authors find no significant difference in beak size between sexes.  Robinson & Hartwick 1983 Veliger 26: 26.

NOTE the otoliths, or balance-receptor stones, are quite soft

NOTE  73 octopuses are collected, weighed, and their beaks dissected out.  Twelve measurements on the beak are selected that are convenient and easy to take. Five are on the upper beak and 7 on the lower beak.  Each of the 12 measurements is then plotted against live mass (using log-transformed values to produce linear relationships), and the one that gives the best fit as measured by correlation coefficient r is selected (the parameter chosen produces an r value of 0.97)

Analyses of stomach contents of 250 Steller sea lions in the Gulf of Alaska indicate the presence of 4% by volume cephalopod prey (mainly octopuses and squids). The author uses the presence of beaks to estimate numbers of cephalopod prey and the volume of identifiable cephalopod matter in relation to volume of all stomach contents to obtain the % volume represented. However, owing to the ease of digestibility of cephalopods (absence of hard parts), this latter is likely to be an underestimate at best. In the case of cephalopods a better substitute for such visual estimations would have been to employ a similar technique to that used in Research Study 2 above.  Pitcher 1981 Fish Bull 79: 467.