Question

I noticed a green-colored marine arthropod in harbor in the US San Juan Islands (early July 2022, around noon), specifically Haro Strait, and took several pictures from the dock.

The bug was approximately 1 centimeter in length. I am guessing that it is algivorous (or algivore-vorous) based on the green color. It was in constant motion in sunlight, possibly hunting though it wasn't clear what it was going after (though the area was very algae-rich).

Apologies for the low quality images, some of these are stills from a movie (I had issues focusing because it was underwater and it was swimming around quite quickly).

Answer 1

The green-colored marine arthropod observed in the harbor in the US San Juan Islands is likely a crab that feeds on algae.

Step-by-step explanation:

The green-colored marine arthropod you observed in the harbor in the US San Juan Islands is likely a species of crab. Crabs are commonly found in marine environments and can vary in size and color. The green color you observed could be a result of the crab's diet or camouflage adaptation.

Crabs are known to be opportunistic feeders and can consume a variety of food sources, including algae. It is possible that the crab you saw was feeding on the algae-rich environment in the area.

Without having specific details or clear images of the crab, it is difficult to determine the exact species. However, based on your description, it is likely an algivorous marine arthropod, potentially a crab.