Each Nibbler has the indicated abilities.

Bitelings
Match five fruits in an "L" or "T" pattern. When released, they will clear out rows and columns in each of the four directions.

Bouncer
Cascade multiple combinations. He will clear almost an entire board with one drop. Connect to Facebook and invite friends to play to summon him faster.

Coral the lavender anglerfish
Match four of the same fruit. You can launch her in any direction to clear out that row or column.

Octo
Match five fruits in a line. He will break through three rows or columns when released.