Island Fish Jasconius doesn't untap during your untap step.
At the beginning of your upkeep, you may pay. If you do, untap Island Fish Jasconius.
Island Fish Jasconius can't attack unless defending player controls an Island.
When you control no Islands, sacrifice Island Fish Jasconius.
5/1/2006 Reworded so that it triggers only once at beginning of upkeep, instead of being an activated ability usable any time during your upkeep. You can no longer make infinite combos with this ability.