Michael RosenbergDec 2, 2022

Our #fiddlercraboftheweek is the Tetragonal Fiddler Crab, Gelasimus tetragonon. This species has the largest range of any #fiddlercrab, found from southeastern Africa through French Polynesia in the central Pacific.

A large fiddler with a carapace width frequently between 2 and 3 cm, this species is striking colored, usually with a blue and black carapace, red legs, and an orange and white major claw.

https://www.inaturalist.org/taxa/555989-Gelasimus-tetragonon
https://www.fiddlercrab.info/u_tetragonon.html

Source of photo:
https://www.inaturalist.org/observations/65173309

Tetragonal Fiddler Crab (Gelasimus tetragonon)

Gelasimus tetragonon is a species of animals with 187 observations

iNaturalist
Show threadMichael Rosenberg

I strongly suspect "G. tetragonon" is more than one species. In particular there appear to be fairly consistent color differences between individuals found in the central Pacific from Fiji to French Polynesia vs. those in the rest of the range. The little bit of genetic data (16S and COI) available also show divergence between these populations, about as much as between other closely related species of #fiddlercrab

It's up to the on-the-ground taxonomists to do the confirmation work, though.

Dec 2, 2022 at 2:13pmWeb
Show threadMichael RosenbergDec 2, 2022
This pattern would match that of two other #fiddlercrabs where a species with a broad Pacific range was found to contain a cryptic species restricted to the Fiji to French Polynesia region, Gelasimus excisa vs. Gelasimus jocelynae and Austruca citrus vs. Austruca perplexa.
Show threadMichael RosenbergDec 2, 2022

G. tetragonon is the only #fiddlercrab species that stretches from eastern Africa well into the Pacific. There used to be a few others, but all of them have been split and the components species have more reduced ranges.

G. hesperiae goes from E. Africa to the edge of the Pacific, while Austruca annulipes ranges from the western Pacific to Pakistan, but not all the way to SE Africa.

The limited genetic data for G. tetragonon shows little differentiation between Africa and Taiwan.