For our very first Fiddler Crab of the Week post, our random generator choose the Indus Fiddler Crab, Uca sindensis. Not the easiest species to start with.
The Indus Fiddler crab is a relatively unknown species found on the coasts of Pakistan, Iran, Iraq, and Kuwait. It has also been reported from the United Arab Emirates, near the Oman border (Ismail & Achmed, 1993), so it is feasible that it could be found on the southern shores of the Persian Gulf as well. It is arguably the most abundant species in Pakistan and most of what is known about its biology comes from the recent work and lab of Dr. Noor Us Saher at the Center of Excellence for Marine Biology at the University of Karachi.
It is a smallish fiddler crab (adult male carapace between 9 and 17mm wide, females a little smaller), lacking some of the more conspicuous colors of other species. It is generally grey in color; the carapace has grey and white bands, with the legs a bit darker grey-brown. The large claw of the male is described as generally pink, with a bit darker-purple toward the arm, shifting toward white toward the top and along the dactyl (the upper/movable finger), and more red toward the pollex (the lower/fixed finger). Unfortunately, I cannot find a photo of a living specimen, but some of the basic color can still be seen in this collected specimen.
A specimen of Uca sindensis from Pakistan. Photograph by Sahir Odhano.
This crab is usually found on open mudflats or muddy-sand in the upper intertidal zone, away from the waterline during low tide. In Pakistan it is often found intermixed with another species, Uca iranica; other fiddler crab species found on the same beaches are less likely to directly intermix.
Very little has been reported in the literature about its behavior, although like many species its waving display appears to involve much more than just the large claw. From minimal observations, Jocelyn Crane (1975, p. 108) stated:
“Wave obliquely vertical to lateral straight, the major cheliped not fully unflexed. Minor makes corresponding motion. Several ambulatories on minor side raised in turn during display.”
Phylogenetically, the Indus Fiddler Crab falls into the general category of “broad-front” species from the Indo-West Pacific (IWP) region. It was originally thought to be closely related to (and a subspecies of) Uca inversa, but now not only is recognized as a separate species, but thought to belong to an entirely different group of broad-front crabs (the subgenus Austruca).
So that’s the first Fiddler Crab of the Week entry. It would figure that my first one would involve one of the least-well known species on the planet (although not the very least).