Explanations for answers to quiz about why cockles in comparison with other bivalves are preferred prey for octopuses:

They are easy to catch.  Yes.  Cockles have short siphons and live right at the sediment/water interface.  No digging or chasing (as for scallops) are required by the octopus.

They taste good and are large and meaty.  These are good attributes for a prey.  However, other bivalves are large, just as meaty, and may taste just as good to an octopus.

They are easier to open than other bivalves.  No.  Although no data are available, it seems unlikely that bivalves will differ markedly in the effort required by the octopus to open their shell valves.

They are easier for the octopus to see.  No.  Enteroctopus dofleini mostly hunts at night, so vision is unlikely to be as important as, say, chemotactile perception by the suckers.

NOTE  lit. “chemistry touch” (G. & L.)  An octopus has remarkable chemical sensitivity in its suckers.  Scientists believe that at night, especially, octopuses navigate by tasting their way around their habitats.  The closest thing that humans have to this type of sense organ is a tongue

 
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