Northern Kelp Crab

Northern Kelp Crab

Common name: Northern Kelp Crab, Shield-Backed Kelp Crab

Scientific name: Pugettia producta 

Size range: Carapace 7.8-9.3 cm across, with differing appendage sizes 

Author: Ruby Wallace

Photography: Thank you to Andy Murch for permission to use his photos on our page!

Identifying Features:   Northern Kelp Crab are part of a superfamily of crabs called the Majoidea (spider crabs) and are most easily identified by their shield-shaped carapace and 4 long, hairless walking legs. Telling the difference between males and females is hard, but generally male Northern Kelp Crabs have a bigger carapace, averaging at about 9.3 cm across, and females about 7.8 cm across.  The second pair of their legs is the strongest and longest, being up to 1 ½ times their body length. Adult Northern Kelp Crabs are brown or red in colour, while juveniles are greenish-yellow. Unlike other crabs such as the Graceful Kelp Crab (Pugettia gracilis) who also have a shield-like carapace, the carapace of the Northern Kelp Crab is uniquely smooth and hard. The underside of the Northern Kelp Crab is usually red and/or yellow. You can sometimes tell when a Northern Kelp Crab hasn’t molted in awhile if their body is covered in the white and grey, mold-like appearance of invasive colonial tunicates. A Northern Kelp Crab after its terminal molt will often be covered in barnacles.

Habitat:  Northern Kelp Crabs live primarily along the West Coast of North America, from Southern Alaska to Northern Mexico. They occupy kelp beds and forests and are especially common in swaths of Bull Kelp. Juveniles have been spotted in tide pools or smaller beds of Rockweed or Surf grass. Adults can be found anywhere from intertidal zones, up to 73 meters down. At times, they are seen at wharf pilings. 

Food (Prey):   Generally, the Northern Kelp Crab is a nocturnal feeder. Most of the year, it is a herbivore and feeds on Kelps, seaweeds such as the red algae primarily in the spring and summer. One unique behaviour of the Northern Kelp Crab is that it does not camouflage on purpose like the Graceful Decorator Crab (Oregonia gracilis) or the Graceful Kelp Crab (Pugettia gracilis) does by covering their carapace in kelp. This is probably because the Northern Kelp Crab is faster and larger and has less need for strategic camouflage. The only time the Northern Kelp Crab is observed exhibiting this behaviour is to hold onto their food through the day – similar to a backpack – and only taking bites when needed, while freeing up the pincers for defense and communication. During the Winter months, when the kelp beds and algae have decreased in volume, Northern Kelp Crabs turn to a mainly animal-based diet of barnacles, bivalves, hydroids, and bryozoans.