Roughback Whipray

The roughback whipray has a thin and oval pectoral fin disc slightly longer than wide, with the anterior margins converging at a broad angle to the tip of the snout. The snout terminates in a small, protruding knob, which is relatively larger than in F. signifer. The eyes are small and immediately followed by larger, teardrop-shaped spiracles. There is a curtain of skin between the nostrils with a gently concave or lobed posterior margin. There are 4–5 papillae (nipple-like structures) across the floor of the small mouth. The small teeth are arranged in 4–5 series in the upper jaw and 14–15 series in the lower jaw; in some individuals, the outermost teeth are stained orange-brown. The whip-like tail lacks fin folds and bears one or two serrated, stinging spines on the upper surface, which are seldom found intact. The dermal denticles are better-developed than those of F. signifer, with a central band of denticles covering the dorsal surface of the disc and tail base. There is also an enlarged "pearl organ" in the middle of the back that is present throughout life, and a row of enlarged denticles (some thornlike) running from the pearl to the base of the sting. The disc is light gray or brown to dark orange-brown above, with a white to yellow spot just before the eyes and behind the spiracles, and sometimes a subtle, lighter band running around the margin. …

The original five specimens of the roughback whipray were all caught from the Mae Klong River in Kanchanaburi Province, Thailand, in fresh water above the influence of the tide. Additional records of this ray exist from the Chao Phraya River, and perhaps the Penang River in Peninsular Malaysia. The total range of the roughback whipray is estimated to cover less than 5,000 km2 (1,900 mi2). This bottom-dwelling species inhabits main river channels at a depth of 5–20 m (16–66 ft). In contrast to F. signifer, which favors muddy bottoms, it appears to favor sandy to silty substrates.

All five original roughback whipray specimens bore extensive, healed wounds to the disc and/or pelvic fins, and had much of their tails bitten off such as that the remnant is only 1.3–1.8 times longer than the disc width (the tail of F. signifer measures over three times the disc width). This is in contrast to other freshwater stingrays in Thailand such as F. signifer, Urogymnus chaophraya and F. oxyrhyncha, which are usually found with their tails intact. Kittipong has proposed that these wounds are inflicted by pufferfishes (Monotrete cf. leiurus and M. cf. nigroviridis); though plausible, why these wounds are seldom found on F. signifer remains a mystery. Another possibility is that the truncation of the tail is self-inflicted, which would be consistent with the similar lengths of the tail remnant in all five specimens. With a particularly small mouth, this species likely feeds on small crustaceans and other benthic invertebrates. Like other stingrays, it is aplacental viviparous with females provisioning their developing embryos with histotroph ("uterine milk"). Males reach sexual maturity at around 25 cm (9.8 in) across.