Feeding is still a very open topic. Roy has (or had) a bimac that lived an astounding 3 years and he feels that keeping them underfed is the key. I am not sold on our vernable professor's diagnosis as the full reason for this amazing longevity though
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As a bit of a compromise to having a constantly hungry octo but taking Roy's observation into account (as well as a long practiced fasting method used with seahorses) we feed our octos once a day, 6 days a week and fast the entire tank 1 day a week - SueNami will occassional coax a small crab from us on the fast day, it just can't be helped
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Experiment with how much but you really can't go by what he eats for the next two weeks as much of it will be nervous eating. Once he knows he will be fed, his eating patterns will change and I am betting he won't eat snails or hermits often.
Do give Fletcher a variety. One of the things to try while he is still nervous eating is a thawed frozen shrimp on a stick. The sooner you can get him/her to accept dead food (I think the hummelincki is more scavenger than some of the others and is easier to train to frozen) the better. If you can find whole saltwater shrimp at the grocery (most are freshwater, tank raised and these will work if you can't get wild caught) these can be a staple once they are accepted. Live (or frozen before there is ANY odor) fiddler crabs, an occassional opened or partially opened clam, piece of raw scallop (they don't seem to like these more than the first couple of offerings for some reason). For a smaller octo, shore shrimp either damaged or recently dead are readily taken. A handfull of these can be left live in the tank and may be snagged as an appetizer on occassion but they are too fast for everyday feeding.
You can put a live clam in the tank, he may or may not decide to open it but it will serve as a clean up critter if he leaves it alone (I have a photo of Octane or OhToo opening one - took him a long time). I put the clams (grocery store on ice - yes they survive) in a bucket of fresh seawater for a few hours and then in a separate tank for a day or two to clean out any pollution. A bucket with an air stone should work but I have a 10 gallon tank for live food that I use. Be sure to eliminate any that die very quickly or it will spoil the water and kill the rest. I find clams to survive better than mussels or oysters and are less damaging to the water quality if they die.
That photo looks like good avitar material!