Black Widows Eat...Crabs?! - Observation of the Week, 11/2/23

Our Observation of the Week is this Black Widow (genus Latrodectus) and its American Broad-front Fiddler Crab (tribe Minucini) prey! Seen in the United States by @evan_tree.

“As part of Texas Master Naturalists, I was fortunate enough to visit the East Foundation's El Sauz Ranch in deep south Texas,” says Evan Trees. 

While most of the group was looking up at birds, I was looking down in a very old (and dry) concrete cattle trough and in an enclosed corner I spotted the black widow's web, which I was very surprised to see had trapped a crab. I saw that the spider was still feeding on this particular crab (a type of American Broad-front Fiddler Crab) and there was another wrapped and discarded below in the leaf litter. I later told Texas Parks and Wildlife Dept urban wildlife biologist Sam Kieschnick (@sambiology), who was nearby, and he immediately gave it a precise identification as a Northern Black Widow (Latrodectus variolus).

I believe the crabs must scuttle up from a nearby brackish water pond and get trapped in the enclosure, eventually stumbling into the web to be consumed.

The genus Latrodectus - known as widow spiders, redbacks, and button spiders, among other English names, is one of the world’s most notorious spider groups, as their venom is medically significant to humans. A widow bite causes latrodectism, the symptoms for which include fever chills, and intense muscle pain. It sounds like a truly miserable illness but is rarely fatal, and widow spiders do not seek out humans to bite, they must be provoked and generally given no other option. 

It’s decapodian prey is an unknown species of fiddler crab (family Ocypodidae), and fiddler crabs show up in tropical and temperate areas of the world. They’re fairly terrestrial and can be seen on beaches and sandy areas where the males court females by waving their one large “fiddling” claw. Latrotoxin, as Evan pointed out to me, contains at least one chemical that specifically affects crustaceans, so if the spider is able to get a bite in then the crab can be subdued.

Originally from the US state of Pennsylvania, Evan (above) credits his interest in nature to his family, his rural upbringing and the scouts. He moved to Texas about a decade ago and tells me

I've completely fallen for the incredibly unique and fragile ecosystems here -- karst springs, chalk prairies, vernal pools. I spend a lot of time in the greenbelts, urban creeks and overgrown lots around Austin observing the effects of development on wildlife, what manages to endure in hidden places and how life rebounds in the aftermath of disturbance.

For me, iNaturalist is a lens through which to see all the hidden, incredible things around me. I like to go into “Explore” and find plants or trees nearby that I'm learning about to go out and see in person. And when I see something I don't recognize, after identifying it I like to look through observations by other users to see what the plant looks like in different seasons, under different conditions and at different phases of development. And I'm just now learning more about connecting with and communicating with other users since some more training given by Texas Master Naturalists.

(Photo by Pearl Chen. Some quotes have been lightly edited for clarity.)

- check out this interview with @sambiology from 2017! 

- a birder’s fiddler crab observation in California extended the species’ range by hundreds of kilometers.

- Deep Look has some phenomenal footage of Latrodectus spiders here, as well as some surprising facts about their reproductive behaviors.

Posted on November 2, 2023 07:13 PM by tiwane