When feeding the whelk crawls onto the barnacle, tubeworm or shellfish and drills a hole in the calcium carbonate covering of its prey. In the case of barnacles, whelks usually attack the doors that open to allow the animal to feed. The whelk releases an acid from a gland in the front part of its foot. This softens the calcium carbonate which is then licked away by the rasp-like tongue (radula) of the whelk. When a hole has been made in the prey the whelk inserts its tube-like mouthpart into the victim and, with its radula, tears off and eats the soft tissues.
... It takes 30-40 minutes for each application of the acid then about a minute of rasping before the process is repeated. The whelk takes about 8 hours to penetrate a shell 2mm thick and can take up to 4 days to get into a larger barnacle.
This one may have been a young Amphissa. It's just over 4mm. long. It would have been eating dead algae and other detritus.