Jellyfish Sudoku: The Advanced Fish Pattern Beyond Swordfish

A Jellyfish is a single-digit fish with four base rows and four cover columns, or the transposed column version. It extends the same occupancy argument used by X-Wing and Swordfish.

Check the complete candidate pattern

Choose a digit not yet placed in four rows. All its remaining candidates in those rows must lie in exactly four columns. Check every candidate in each base row; do not ignore an extra position just to make the pattern fit.

Each base row needs one copy of the digit. Those four placements must use four different columns, because a column cannot repeat the digit. The four cover columns are therefore occupied by the base rows: remove that digit from other cells in those columns, outside the base rows.

A worked candidate example

In the reviewed interactive Jellyfish lesson, candidate 7 occurs in R1 at C2/C7, R3 at C3/C6, R4 at C6/C7, and R8 at C2/C3. Together these four rows use C2, C3, C6 and C7. Candidate 7 can be removed from R7C3, which is in a cover column but outside the four base rows.

Four rows restricted to only three columns would be inconsistent, not a Swordfish: four required placements cannot fit into three columns without a repeated digit. Recheck the givens and candidate notes if you encounter that situation.

Compare the fish family

X-Wing uses two base lines and two cover lines; Swordfish uses three of each; Jellyfish uses four. The number of eliminations depends on candidates outside the base lines, so a larger fish does not necessarily remove more candidates.

When there is a fin

An extra candidate outside the cover columns prevents the ordinary Jellyfish elimination. A finned Jellyfish requires a separate proof: the target must be eliminated both when the fin is false and when it is true. The target must remain outside the base rows and see every relevant fin; merely being in a cover column is insufficient.

Practise the deduction

Use the lesson’s candidate grid to check the four rows, four columns and target before removing anything. Return to singles after an elimination. Difficulty labels and daily puzzles do not guarantee a Jellyfish; study the fixed example when you want to practise this exact pattern.