The jellyfish completes the fish family: an X-Wing spans two lines, a swordfish three, a jellyfish four. Take one digit: if four columns each confine their candidates for it to the same four rows, the digit’s four placements in those columns must occupy four of the crossings — one per column, one per row — and the four rows are entirely claimed.
The exchange argument is the same as its smaller siblings’. Each of the four columns places one copy of the digit, no two of them can share a row, so the four rows’ quota for the digit is spent inside the pattern — and it is struck from every other cell of those rows. (Row-based jellyfish claim four columns the same way.)
To spot one, you need bookkeeping: a candidate grid for the digit, read column by column, looking for four columns whose entries stay inside four shared rows. Genuine jellyfish are rare and late — the reward for the housekeeping is usually a burst of eliminations at once, as here.
On the board
Follow the digit 7. Column 1 can hold it only in rows 3 and 4; column 2 only in rows 3, 7, and 9; column 6 only in rows 4 and 7; column 7 only in rows 4 and 9. Four columns, and every candidate lies in the same four rows — 3, 4, 7, and 9. Those rows must draw their 7s from these columns, so every other 7 in them is struck: R3C3, R3C4, R3C5, and R3C8 across row 3; R4C8 in row 4; and R9C8 with R9C9 along the bottom. Seven eliminations in one stroke.
This is the exact board the app's interactive Jellyfish demo uses — step 24 of its 71-step walkthrough, drawn here with its live pencil marks.
