Mirrorshell Crab

• WOTC Official Reminder Textofficial rules
  Ward — Whenever this permanent becomes the target of a spell or ability an opponent controls, counter it unless that player pays [cost].
• WOTC Official Reminder Textofficial rules
  Channel — Discard this card: [effect]. Activate only as a sorcery.

• WWizards of the Coast2022-02-18
  Official Ruling
  If a channel ability requires a target, you may not activate it without a target just to discard the card.
• WWizards of the Coast2022-02-18
  Official Ruling
  Discarding the card is part of the cost to activate a channel ability.
• WWizards of the Coast2022-02-18
  Official Ruling
  If a player casts a spell that targets multiple permanents their opponent controls with ward, each of those ward abilities will trigger. If that player doesn't pay for all of them, the spell will be countered.

Rulings via Scryfall