The best replacement for clam is oyster.

The best alternatives to clam are rated as following:
★★★★★  oyster
★★★★★  mussel
★★  frabill
★★  eskimo
★★  beaver
★★  scallop
★  oyster pearl
★  cockle
  quahog
  otter
  oysters
  geoduck
  geoduck clam
  shappell
  brachiopod
  sea clam
  markins
  an oyster
  squid

Your alternative for clam is not listed? Suggest!
Speaking about clam, one probably shouldn't forget about linguine.

Other search terms related to clam and their ratings are:
★★★★★ linguine
★★★ linguine recipe
linguini
You have in mind a companion for clam and it is not yet here? Propose!



Google Trends

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clam
oyster
mussel