Jellyfish
A Jellyfish is a size-4 basic fish for one digit. In one orientation, the digit’s candidates in 4 base rows are confined to 4 cover columns; the transposed orientation swaps rows and columns. The digit can be removed from the cover houses outside the base houses.
What is a Jellyfish?
The size counts 4 required placements, 4 bases, and 4 covers—not a requirement for all 16 intersections to contain candidates. Each base contributes at least one candidate, and every relevant base candidate lies in the selected covers.
How to spot it
Choose one digit and select 4 rows whose candidates are confined to the same 4 columns, or 4 columns confined to the same 4 rows. Sparse intersections are valid. Then inspect the cover houses outside the selected bases.
The logic
Each of the 4 base houses must place the digit, and all 4 placements lie within the 4 cover houses. Those placements occupy the covers, so any other occurrence of the digit in a cover but outside the bases can be removed.