1.
crab
The katakana spelling of 蟹 (crab), the same word written with kanji. Katakana is the usual spelling on restaurant menus, product packaging, and store signs.
カニ is simply 蟹 written in katakana. Animal and food names with complex or rare kanji are often written in katakana instead, especially on menus, packaging, and signage, where it reads faster than the kanji 蟹.
Writing
All three spellings are seen: 蟹 (kanji), かに (hiragana), and カニ (katakana). Katakana is the most common on restaurant menus and seafood packaging; kanji appears more in formal or literary writing.