Squirmbag

A Squirmbag is the order-5 fish — a digit confined to five columns across five rows, or the other way round. It sits one step beyond the jellyfish and completes the fish sequence (X-Wing, swordfish, jellyfish, squirmbag). In practice it is more a curiosity than a workhorse, and this article explains both how it works and why you will rarely need it.

The idea

The logic is identical to every other basic fish. If a digit in five rows is confined to the same five columns, then those five columns are "full" of that digit from the rows, and the digit can be removed from any other cell in those columns. The catch is a counting fact: on a 9×9 grid, a size-5 fish always has a size-4 complement — the four rows and four columns it does not use form a smaller fish that makes the very same eliminations. So a squirmbag is real, but a jellyfish or smaller fish almost always beats you to the same result.

A worked example

Say the digit 2 across rows 1, 2, 4, 6 and 9 is limited to columns 1, 3, 5, 7 and 8. That is a squirmbag: 2 can be cleared from the rest of those five columns — for instance r3c1. But look at the four rows and four columns left over — rows 3, 5, 7, 8 and columns 2, 4, 6, 9 — and you will usually find a jellyfish doing the same job with less counting. That is why solvers reach for the complement instead.

Digit 2 confined to columns 1/3/5/7/8 across rows 1/2/4/6/9 (blue) — a size-5 squirmbag. It clears 2 from the rest of those columns, e.g. r3c1. The leftover rows 3/5/7/8 and columns 2/4/6/9 usually hold a jellyfish doing the same job.

Step by step

Pick a digit and map its candidates.
Find five base rows in which it is confined to five columns (or the transpose).
Before committing, check the four-row/four-column complement for a simpler fish.
Eliminate the digit from the rest of the five cover columns.

Common mistakes

Missing the complement. A smaller fish nearly always exists — look for it before wading through a size-5 pattern.
Miscounting units. Five base and five cover units must balance exactly.
Treating it as essential. No fair 9×9 needs a squirmbag; it is completeness, not a practical necessity.

Which Sudoku types is this best for?

The same logic applies across variants, but it pays off more in some than others.

Works Classic Sudoku Rounds out the fish family, though a smaller fish nearly always gives the same eliminations. → Limited Killer Sudoku Never realistically needed in Killer. → Limited Sudoku X The diagonals resolve the grid far sooner. → Limited Samurai Sudoku Not a Samurai tool. →