A Finned Jellyfish is an almost-jellyfish with fins. Same fin logic as Finned X-Wing / Finned Swordfish: fish OR fin — only cover-cell eliminations.
Before this: Learn Jellyfish and Finned X-Wing first.
Why it works
Perfect jellyfish story vs fin-true story — only eliminations common to both stories are allowed. Fins shrink the zone.
Demonstration
Find an almost-jellyfish with fin candidates in one box.
The green cells are a Finned Jellyfish on 1. Either the fish holds, or the fin is true.
Only the fin’s cover cells are attacked in both stories.
Eliminate 1 from the fin’s cover cells (red).
Example 1
Look for a jellyfish shape with a fin.
This Finned Jellyfish is on 2.
Remove 2 where the fin covers (red). Fish OR fin — only shared victims.
Example 2
Look for a jellyfish shape with a fin.
This Finned Jellyfish is on 1.
Remove 1 where the fin covers (red). Fish OR fin — only shared victims.