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Occurrence of the Estuarine Mud Crab in Texas Reservoirs

Adult Mud Crab

The "white-fingered", or "Harris" mud crab, Rhithropanopeus harrisii, is a common inhabitant of Texas and Florida estuaries and has been introduced into estuarine systems all over the world. Adult crabs are known to migrate upstream in estuaries and can tolerate freshwater, but because their larvae are not normally able to reproduce at extremely low salinities they have not established permanent populations in typical freshwater habitats. The lowest salinity that has been demonstrated in the literature that crab larvae can survive to the juvenile stage has been 2.5 ppt, and only a few out of hundreds survived at this salinity.

Adults and small juveniles were first identified in Possum Kingdom Reservoir, Palo Pinto County, Texas in the summer of 1998. Since that time, we have confirmed the presence of this species in eight additional Texas impoundments including: Trading House Creek Reservoir, Lake Colorado City, Lake E.V. Spence, Lake Granbury, Lake Balmorhea, Lake Whitney, Squaw Creek Reservoir in Glen Rose and Lake Texoma. Mud crabs have also been reported from Braunig Lake in San Antonio but we have not conformed this finding.

The salinities at Possum Kingdom and Lake Granbury usually range between 1 and 2 ppt, and Trading House Creek Reservoir generally ranges between 0.4 and 0.5 ppt. These salinities are below the minimum that has been demonstrated for larval survival yet there is strong evidence that these crabs have established permanent breeding populations.

These are the first known occurrences of this species in inland freshwater lakes. Exactly how this crab was introduced into the reservoirs is questionable though fish-stocking programs are suspected. Evidence that the crab has established breeding populations at all seven impoundments is demonstrated by the extreme abundance of crabs with size variation from 1.5 mm to 23 mm, the occurrence of gravid females, and the presence of zoea larvae in the plankton. Eggs from gravid females have been hatched in our lab at salinities from 0.5 to 15 ppt with no significant difference in the percent of eggs that hatched (average about 85%)

The crabs have caused fouling problems at Possum Kingdom by clogging PVC inlets to lakeshore homes, and have been found on the intake screens at the power generating plant at Glen Rose. There is also some evidence that they may be disrupting natural community structure by replacing the crayfish that naturally occur in these freshwater habitats. Because of these concerns, it is important to understand how the crab has managed to adapt and reproduce in its new habitat. Our research is attempting to establish the potential for this species to reproduce and spread to other freshwater bodies, and determine the degree of genetic alteration from their coastal relatives. This is being accomplished by attempting to establish the lowest salinities that larvae can survive to the juvenile crab stage, and making DNA comparisons between the lake and estuarine populations. DNA comparisons are also being conducted to determine variations that occur in populations of different lakes.