When performing a Special Summon, you do not need to Tribute monsters as you normally would for a Tribute Summon. However, you have to fulfill the requirements to make the Special Summon. For example, Megarock Dragon requires that Rock monsters be removed from the Graveyard first.
If a card attempts to Special Summon a monster with a "can only be Special Summoned by" effect, the Special Summon is only allowed if the monster was already Special Summoned by its own effect during the Duel, and has not been returned to the Hand, Deck, or Fusion Deck since that original Special Summon.
If a card attempts to Special Summon a monster with a "cannot be Special Summoned except by" effect, this is not allowed.
However, if the card performing the Special Summon tells you to ignore Special Summon restrictions or requirements (like Underworld Circle), then you can as long as you fulfilled the requirements to activate the card.
If a card attempts to Special Summon a monster with a "can only be Special Summoned by" effect, the Special Summon is only allowed if the monster was already Special Summoned by its own effect during the Duel, and has not been returned to the Hand, Deck, or Fusion Deck since that original Special Summon.
If a card attempts to Special Summon a monster with a "cannot be Special Summoned except by" effect, this is not allowed.
However, if the card performing the Special Summon tells you to ignore Special Summon restrictions or requirements (like Underworld Circle), then you can as long as you fulfilled the requirements to activate the card.