The effect "When this card is added from your Deck or Graveyard to your hand by a card effect: You can Special Summon this card from your hand." is a Trigger Effect that is activated from the hand. (This effect does not target. It can be activated during the Damage Step.)[1]
This effect cannot be activated if "Mermail Abyssdine" is added from your Deck or Graveyard to your hand by a card effect activated as Chain Link 2 or higher.[1]
The effect "When this card is Special Summoned by the effect of a "Mermail" monster: You can target 1 Level 3 or lower "Mermail" monster in your Graveyard; Special Summon that target." is a Trigger Effect that is activated from the field. (This effect targets 1 Level 3 or lower "Mermail" monster in your Graveyard. It can be activated during the Damage Step.).[1]
This effect cannot be activated if "Mermail Abyssdine" is Special Summoned by the effect of a "Mermail" monster activated as Chain Link 2 or higher..[1]
"Mermail Abyssdine" is a monster with 2 effects and since the effect of "Mermail Abyssdine" can only be used once per turn, if either of those effects is activated, the other effect cannot be activated during the same turn.[2]