If you're familiar with local crabs, you'll probably recognize that this is not a species that we usually find in the Bodega Bay/Tomales Bay area.
Keep in mind that this summer follows two warm-water years ("The Blob" in 2014-2015 and El Niño in 2015-2016).
Did you notice the rounded, flattened, paddle-like legs in the photo above? The paddles are used for both swimming and burying in the sand (and are a characteristic of the swimming crabs, Family Portunidae).
Here's another photo of this striking crab, this time from below:
This image emphasizes the impressive length of the claws. And isn't that purple color amazing?!
From the front you can see the spines on the claws and on the carapace. (For those of you who have spent time on the East Coast, you might have noticed a similarity to a blue crab.)
Meet Xantus' Swimming Crab (Portunus xantusii)! This species has a more southern distribution, with most records occurring south of Morro Bay. They were recorded in Monterey Bay in June 2016 (see story here), and there are a few iNaturalist records from San Francisco Bay in March 2016 and June 2016.
Since these crabs have planktonic larvae, it's likely that larvae reached this area in either 2014 or 2015 (based on their relatively large size, I'm leaning towards 2014). The crabs survived and are now being discovered in sandy/muddy habitats far to the north of their normal range.
Jason and Ted found four Xantus' Swimming Crabs near Sacramento Landing in Tomales Bay. It would be great to hear about any other sightings, so if you see one of these crabs, take pictures, measure the width of the carapace, and let us know. This is a rare opportunity to document Xantus' Swimming Crab in northern California.
Here's one more picture — Jason smiling about his exciting discovery!