Finned Mutant Jellyfish: row 6, row 8, col C, col E and box 8, row 5, row 9, col G on 6 (A9-6)
All the possible positions of the value 6 in four base row/col are in the same four cover box/row/col. The number of row/cols matches the number of box/row/cols, and each row/col must contain the value 6; so each box/row/col will have a 6 in one of the intersecting cells.
Hence the other possible positions of the value 6 that are in the box/row/col but not in one of the row/col may be removed.
Therefore we can remove A9-6.
Explanation
Mutant Fish use all three types of regions (box, row, and col) as both base AND cover regions. Note that MOST mutant eliminations will have already been found by simpler Fish types, so they never show-up, and with all three region types in play the number of combinations of regions EXPLODES, so searching them is MUCH slower; but we still find a few, so we do it anyway; mainly just because it was there.
Note that HoDoKu incorrectly reports Mutants as Frankens. I'm not going to fix it, because the code is MENTAL.