These very dark (dark grey to black) banana slugs are an anomaly. I live on the north coast of California and see many slugs every day. These particular slugs I find only on the west-facing headlands at the ocean. There is no cover from the sun since there are no plants taller than about a foot. Also these particular slugs are never longer than 2-1/2" to 3". Are they a sub species, a variety, or something else? Thanks for taking the time to answer.
Banana slugs are comprised of three species:
- Ariolimax californicus
- Ariolimax columbianus
- Ariolimax dolichophallus
Ariolimax columbianus, or the Pacific banana slug is known to have black spots, sometimes so large as to cover their entire body (Source). This is certainly the genus of the slug you've shared.
At times, you could be seeing a Black slug which is well-established in Washington and Oregon. I only include this information as I have noticed this species occasionally in Southern British Columbia and wondered if it was the same species as the Banana slug. It isn't. The picture you've attached is not a Black slug (note the pronounced frills on the back of the Black slug specimen)