Mutant Fish Sudoku: Mixed Houses in Advanced Fish Logic

Mutant Fish Sudoku expands what can count as a fish house

Mutant Fish Sudoku uses the same counting idea as an X-Wing or Swordfish but relaxes the usual row-versus-column geometry. A basic fish chooses N base rows and N cover columns, or the reverse. A Franken Fish may replace some houses on one side with boxes. Mutant Fish goes further by allowing a mixture of rows, columns, and boxes in both the base and cover sets. For one candidate, every base house must contain that candidate somewhere, and all relevant base candidates must be covered by the selected cover houses. When the number of bases and covers matches and the structure is valid, cover candidates outside the base structure can be eliminated. The pattern is powerful because it can capture candidate distributions that do not line up as a neat rectangle. It is also easy to misread, so Mutant Fish Sudoku should be treated as a set-counting proof, not a visual shape.

Count houses carefully in Mutant Fish Sudoku

To verify Mutant Fish Sudoku, name every base house first. For a size-three fish, you might have two rows and one box as bases. Then name three cover houses, perhaps one row, one column, and one box, that cover every candidate from those bases. The sets must be chosen so the fish counting argument remains sound: each base needs one occurrence of the digit, and the cover structure has only enough capacity for those required placements. A candidate in a cover house that is not protected as a base candidate can then become an elimination. Because boxes overlap rows and columns, some selected houses may intersect in several cells, which makes candidate counting less intuitive than in a normal Swordfish. Use a diagram or highlight the houses rather than trying to infer the pattern from candidate dots alone. If an uncovered base candidate exists, the fish is broken.

Search for Mutant Fish Sudoku only in dense candidate maps

Mutant Fish Sudoku is most useful when simpler fish logic has almost worked but candidate positions keep spilling into boxes. Start with one digit that has several strong restrictions and inspect combinations of rows, columns, and boxes around those restrictions. Before committing, check whether the same elimination can be explained by a simpler Franken Fish, finned fish, or chain. Simpler explanations are easier to trust and easier to learn from. Shapedoku players can translate the method directly to a single symbol: every lotus candidate in a row, column, or box follows the same no-repeat rule as a digit. On an Extreme grid, highlight one shape mentally and think in houses rather than in numbers. Mutant Fish Sudoku is a good reminder that advanced Sudoku often stops being about pictures and becomes about precise sets. If you cannot list the bases, covers, and target in one short sentence, keep searching for a clearer pattern.

A useful way to audit Mutant Fish Sudoku is to temporarily forget the names X-Wing, Swordfish, and Jellyfish and count only selected houses. Ask how many base houses require the digit and whether the chosen cover houses can account for those required occurrences without extra capacity. Then inspect each target. This set-first view helps when a box is used on one side and a row or column on the other, because the familiar rectangular picture no longer guides you. If the same house is being counted twice in a way that inflates the proof, redraw the structure and start again.

1. Choose one candidate value and decide the intended fish size.
2. List the base houses, allowing rows, columns, and boxes where the pattern requires them.
3. List the same number of cover houses and confirm they cover every base candidate.
4. Check that no forgotten candidate escapes the selected cover structure.
5. Eliminate only eligible cover candidates outside the protected base structure.
6. Compare the result with simpler Franken Fish or chain explanations before finalizing it.