Mirrorshell Crab — ruling 1

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.

Mirrorshell Crab — ruling 2

Discarding the card is part of the cost to activate a channel ability.

Mirrorshell Crab — ruling 3

If a channel ability requires a target, you may not activate it without a target just to discard the card.

Mirrorshell Crab : règles, fonctionnement et interactions (MTG)

Mirrorshell Crab is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Ward (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays .) Channel — , Discard this card: Counter target spell or ability unless its controller pays .

How it works : Mirrorshell Crab

Original English text — Wizards of the Coast / Scryfall

#1February 18, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#2February 18, 2022· Wizards of the Coast
Discarding the card is part of the cost to activate a channel ability.
No other card is named in this ruling.
#3February 18, 2022· Wizards of the Coast
If a channel ability requires a target, you may not activate it without a target just to discard the card.
No other card is named in this ruling.