Mirrored from Sudopedia, the Free Sudoku Reference Guide
In a real Jellyfish, you will rarely see all 4 rows have 4 candidates for the selected digit. Each row can contain between 2 and 4 candidates.
Jellyfish for digit 8 in rows 3,4,6,7 and columns 2,5,8,9.
Because a Jellyfish in the columns often has a complementary Jellyfish in the rows, many computer solvers only report the Jellyfish in the rows. This example also has such a complement:
Jellyfish for digit 3 in columns 1,2,6,7 and rows 1,3,5,9
Complementary Jellyfish in rows 2,4,6,7 and columns 3,4,5,9
Each of the defining lines can have 2, 3 or 4 candidates, but together they cover 4 of the crossing lines. Because there are many possible configurations for 2, 3 or 4 candidates in 4 lines, a Jellyfish is much harder to find than an X-Wing or Swordfish. See Trawling for Fish Using Locked Sets for a technique that makes Jellyfish easier to find.
Because a Jellyfish has 4 defining lines, there will always be 2 lines in a single chute. However, when the defining lines of a Jellyfish occupy only 2 chutes, the eliminations it causes immediately trigger a subsequent Locked Candidates move, like column 8 in this diagram.