One of the top 20 most observed crabs on the iNaturalist is the Speckled Swimming Crabs (Arenaeus cribrarius).
Swimming crabs (Family Portunidae), have paddles on the tips of their back legs and many of the larger species are often eaten, such as the familiar Atlantic Blue Crab (Callinectes sapidus)
But the genus Arenaeus has two species in it. Speckled Swimming Crabs on the Atlantic coast of the Americas and the lesser known but very similar Pacific Speckled Crab (Arenaeus mexicanus) on the Pacific side.
When I looked yesterday there were only 13 observations of Pacifici Speckled Crab. I wanted to see if I could find some more among the other swimming crab observations but I’m still not super well versed in the swimming crabs.
In my experience, once you’re familiar with identifying a group of organisms, you go by gestalt. By that I mean what goes on in your brain as an identifier is more like recognizing people you know. When we look at a picture of a friend, we just say “thats Jack” we don’t count the number of freckles. The same goes with species. I’m pretty familiar with lots of crabs and its much easier for me to just say what crab it is than to pick out individual characters and explain why I think that (e.g. “uh, its shell is bumpier and the way the spines are is different”). Getting a feel for gestalt just comes from experience and is hard to explain or teach.
In contrast learning how to identify a species and explaining how to identify a species is often much easier to do when there’s a specific character to get a handle on. I had trouble wading into the genus Arenaeus because there are a lot of other swimming crabs and I didn’t have a good feel for the gestalt of the group. I usually turn to the museum literature for characters but this can be extremely frustrating because often the literature focuses on characters that might be good for a preserved specimen but rarely show up in photos (e.g. hence so much focus on things like the shape of sexual organs). Also the jargon and unfamiliar anatomical terms in the museum literature can be really hard to translate.
Thats why I was so excited to find this great character for identifying Arenaeus. Its described in the literature as “Superior fissures of orbit open , V-shaped” but it wasn’t until I found this great FAO guide in Spanish for identifying Pacific fisheries species that I understood what that means. The character shows up amazingly well iNat photos as shown below:
The most similar crabs to Arenaeus in the range are swimming crabs in the genera Callinectes and Portunus. In these species the notch is closed and much less well marked as shown below:
Once I had this character to hook on to, after flipping through some North American Swimming Crab observations in the identify tool I was quickly able to develop a sense of gestalt for the players in this group beyond this one character. I was able to more than double the number of observations of Pacific Speckled Crab pretty quickly.
I wish we could figure out a way to encourage the sharing of this type of information more on iNaturalist. My sense is its so much easier for identifiers to just identify other people’s observations than it is to put things in terms like this that help people learn how to identify a species and develop a sense of gestalt and build bridges with the often inaccessible information in the museum literature. Its kind of like the whole “give a man fish vs give a man a fishing pole” and I think teaching one another how to identify is going to be critical if we’re going to continue to scale the number of identifier on iNaturalist alongside the rapidly growing number of observers.
I think the reality is authoring this kind of content takes a lot of work. But there must be something we could do to make it more painless. What would that be? And what kind of format/distribution is best for creating and sharing kind of content and what kind of technology tools could accelerate it? Looking forward to hearing ideas from others. And if possible, I’d love to see other attempts at explaining how to ID a species even if its just focused on a single obscure genus with two species as in this case here…