The robust redhorse (Moxostoma robustum) is one of the largest species in its genus (Moxostoma) and the sucker family (Catostomidae) in North America. The robust redhorse is a heavy-bodied sucker that can reach 31 inches (78 centimeters) in length and weigh up to 18.6 pounds (8.4 kilograms). It has a bronze-colored back and sides, with scattered, dark, mid-lateral blotches and a faint lateral stripe that can vary in intensity. Juveniles have caudal fin terminal margins that are intensely red and often have red in their other fins. The red coloration in fins fades with age.
The robust redhorse has large molar-like pharyngeal teeth that it uses to crush snails, freshwater mussels and other invertebrates. It was originally described in 1870 by American zoologist Edward Drinker Cope, from specimens collected from the Yadkin River in North Carolina, but wasn’t rediscovered until the 1980s, because it was often confused with other similar species. It occurs in large rivers on the South Atlantic Slope from the Pee Dee River in North Carolina, to the Altamaha River in Georgia. Three genetically distinct populations in the Altamaha, Savannah and Pee Dee rivers are currently managed as separate units.
The robust redhorse is a potamodromous species, meaning that it undertakes long migrations, in some cases up to 62 miles within freshwater rivers, from the low-lying and low-gradient Coastal Plain ecoregion to its spawning grounds on gravel bars near shoals in the upland and higher-gradient Piedmont ecoregion. The species requires clean gravel bars on which to spawn and unimpeded migration routes to complete its life cycle.
Therefore, the primary threats to the species are:
- Development leading to altered flow regimes, erosion and sedimentation that degrade and bury spawning habitat
- Construction of dams for hydropower and reservoir development, which block migration routes to suitable spawning habitat
Non-spawning adult robust redhorse can be found in a wide variety of mainstem river habitats, including riffles, runs and pools, including the upper portion of reservoirs. They can also be found in various substrates, but have been particularly associated with the submerged trees or woody debris in deep water with current over sandy substrates.
Coarse gravel beds in relatively shallow water, which are defined as between 0.95 and 3.6 feet (0.29 to 1.1 meters), with moderate to swift current of between 0.95 to 2.2 feet per second (0.26 to 0.67 meters per second) are required for spawning. Egg and larval survival depend on the aeration and interstitial spaces provided by gravel beds in moderate-to-swift currents.
Very few juveniles have been collected, but they are believed to move downriver into low-flow, pool-like habitats, based on observation of individuals stocked in the Broad River of Georgia that migrated into the Clarks Hill Reservoir upon their release.
A natural body of running water.
Robust redhorse eat a variety of invertebrates. Juveniles, or small size classes, have been found to consume midge larvae (chironomids) and water fleas (Cladocera). Adults have been observed to primarily eat larval midges (chironomids), seed shrimp (ostracods) and invasive clams (Corbicula).
Presumably, robust redhorse would have historically also consumed small or young native mussels (Unionids) and snails (gastropods), based on their large, molar-like pharyngeal teeth, which are used to crush shelled prey.
Robust redhorse exhibit site fidelity to both their spawning and non-spawning habitats. In some rivers, presumably where suitable habitat is present, there are two behavioral subgroups: one remains in the Piedmont ecoregion and members make only localized migration to spawning grounds; members of the second migrate to the spawning grounds in the Piedmont ecoregion to spawn and then return down river to the Coastal Plain for the rest of the year.
Outside of their potentially lengthy migrations, robust redhorse appear to have relatively small home ranges, although they may abandon these home ranges periodically and establish new ones. During high flows, robust redhorse may abandon the deep and swift main channel in favor of submerged floodplains.
?Robust redhorse have a maximum weight of 18.6 pounds (8.4 kilograms).
Robust redhorse have a maximum total length of 31 inches (78 centimeters). The robust redhorse is a stout fish for its length, in addition to being somewhat cylindrical and fusiform-shaped. Adults become deeper-bodied, exhibited by a relatively high-arching back. Like most suckers, robust redhorse have inferior mouths and fleshy lips; their lips are divided into longitudinal sections, known as plicae, with the rear margin of the lower lip relatively straight, except for some central plicae that extend beyond the margin in adults. Spawning adult males develop large prominent bumps, known as tubercles, on their anal and caudal fins, as well as their heads and snouts.
Robust redhorse have bronze backs and sides, with scattered, dark, mid-lateral blotches and faint lateral stripes that can vary in intensity. Adult breeding males have darker and more-intense stripes that extend to the snout tips. In juveniles, the caudal fin terminal margin is intensely red, and they often have red in their other fins; red coloration fades with age.
The robust redhorse is potamodromous, migrating long distances within river systems to spawn.
Robust redhorse can live 27 or more years.
Annual migrations of robust redhorse to spawning grounds typically begin between February and mid-March, as water temperatures increase and the photoperiod lengthens. Spawning occurs from late April to late May, when water temperatures range from 60.8 to 75.2 °F (16 to 24 °C). Adults may migrate to spawn shoals up to a month before spawning.
Males hold and defend territories on gravel beds and wait for females to enter their territory. Females typically use flow refugia, including backwaters, woody debris, banks and point bars, upstream of the spawning shoal until they’re ready to approach males on the gravel bed.
Spawning consists of a trio of robust redhorse, with a male on each side of the female, with other males trying to disrupt or displace the spawning males. Fertilized eggs are deposited into the gravel substrate after vigorous quivering of the spawning trio and the disturbance of a large amount of gravel. Females can produce between 1,485 to 86,295 eggs.
Notchlip redhorse (Moxostoma collapsum) is distinguished from the robust redhorse by a less-stout shape and semipapillose lips, with a V-shaped rear margin on the lower lip. Striped jumprock (Moxostoma rupiscartes) is distinguished from the robust redhorse by its more-slender shape and circumpeduncle scale row counts of 16 (12 in Moxostoma robustum). Brassy jumprock (Moxostoma sp. cf. lachneri) is distinguished from the robust redhorse by its more-slender shape and circumpeduncle scale row counts of 16 (12 in Moxostoma robustum). Spotted sucker (Minytrema melanops) is distinguished from the robust redhorse by rows of dark spots on the sides (most individuals) and a slender caudal peduncle with 16 circumpeduncle scales (12 in Moxostoma robustum).
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