Bristled river shrimp

Macrobrachium olfersii

Family Palaemonidae - grass shrimps

Distinguishing
Characteristics
Body shrimp-like; carapace smooth; color reddish brown to dark brown, sometimes speckled with reddish brown; the rostrum (extension on top of head) is straight or slightly curved downward, 12-15 evenly space teeth on upper surface, with 4-5 behind orbit, lower margin with 3-5 teeth; 2rd abdominal segment overlaps 1st and 3rd segments; 1st pair of legs with small claws; 2nd pair of legs unequal in length and differs between sexes: males with one leg that is extremely long (sometimes as long as body) and inflated with a fat claw at the end, fingers of claw curved and gaping, females with one leg about 1 1/4 as long as the other, leg and claw are slender, claw curved at the tip; both male and females have hairs and small spines covering the legs and claws and 1 small tooth near the joint; the segment of the leg immediately behind the claw (carpus) is about the same length as the next segment behind it (merus); the carpus on the 2nd leg is whole, not subdivided into smaller segments; hepatic spine present, smaller than antennal spine
Similar Species
In the bigclaw river shrimp, M. carcinus, the carpus (segment behind claw) of the second leg is distinctly smaller than the merus (segment behind carpus). In the Ohio shrimp, M. ohione, the tip of the rostrum (extension on top of head) has a dagger-like, toothless point. The cinnamon river shrimp, M. acanthurus, has only 2 teeth behind the orbit on the rostrum and the fingers of its claws are covered with a felt-like growth of fine hairs.
Habitat
Freshwater, bays
Maximum Size
males to 9 cm (3 1/2 in), females to 6 cm (2 1/5 in)
Other Common Names
Previous Scientific Names
Comments
This is a freshwater shrimp, as are all Macrobrachium, that occasionally shows up in the bay. Males of all species are easiest to distinguish because of the differences in the claws between the sexes. Most identification keys only address the males (though they don't always tell you that). Bristled river shrimp have been found in the Guadalupe, San Marcus and Rio Grande Rivers.