Creature Shape Feat 17

Your ancestry has grown alongside a favored creature, and you can tap into that bond to transform. When you gain this feat, choose a common polymorph spell with a battle form that matches your ancestry's creature entry; you can't change the form after making your choice. You can cast that spell as an 8th-rank innate spell once per day, except you must transform only into a battle form that matches your ancestral creature. Your form uses your normal AC if it's higher than what is granted by the battle form. If your ancestral creature isn't among the forms listed, you default to the statistics and abilities of the battle form that is closest to your creature at the GM's discretion. This spell has the same tradition as is listed in the feat entry.
If you have the Ancestral Pest feat, you can choose to gain the effects of a 4th-level pest form.

Option(s): primal; crab