The freshwater drum, Aplodinotus grunniens, is easily identified by its distinctive humped back, along with its laterally compressed, deep body shape. Its forehead slopes to a blunt, rounded snout ending in a sub-terminal, or inferior, positioned mouth. The dorsal fin is long and prominently notched, with 10 spines and 29 to 32 rays (Harlan et al. 1987). The anal fin has two distinct spines, the first is short in length and the second much longer, along with seven rays. The scales of the freshwater drum are ctenoid, and there may be between 49 and 53 scales along the lateral line. The lateral line itself is a key identifying characteristic in that it continues into the caudal fin, which is rounded in shape (Page & Bur 2011). Coloration varies, with some individuals appearing silver or gray, and others more bronze. The coloration is almost uniform across the body, with paler scales appearing primarily on the ventral side with some darkening toward the dorsal side (Harlan et al 1987). The common length of A. grunniens is 45cm, but maximum length has been recorded as 95cm, with a maximum recorded weight of 24.7kg (Page & Bur 2011).

Systematics:  

The freshwater drum belongs to the family Sciaenidae, which includes all drums or croakers, in the order Perciformes. The name drum stems from the drumming or croaking sounds this group produces through specialized musculature. The genus Aplodinotus contains the freshwater drum, the only extant member of this genus. Its sister taxon, Pogonias, was thought to also contain only one extant member, the black drum or Pogonias cromis, though in 2019 a new species, Pogonias courbina was described (de las Mercedes Azpelicueta 2019). The relationship to Pogonias is due to many shared morphological characteristics, but the monophyletic clade of which they are a part is largely defined by the specialized muscles used in the drumming that directly attach to the swim bladder (Sasaki 1989). The freshwater drum is the only North American member of Sciaenidae found exclusively in freshwater. The majority of species found in the family Sciaenidae are marine. A fossil specimen of A. grunniens found in Northern Ontario, Canada indicates a former presence in the region and a possible shift in range over time (Harington 1971).

One of the primary food items for the freshwater drum are mussels, though their diet can vary throughout the life cycle (Daibner 1952). The powerful epaxial muscles, which give the drum its distinct humped back, paired with the crushing power provided by the pharyngeal jaw structure allow the drum to engage in durophagy and exploit tough-shelled animals as a food source (Essner et al. 2014). Stomach content analysis has found that their diet varies throughout the life cycle; young-of-the-year individuals have been found to rely on microcrustaceans, such as copepods, for the majority of their diet. As they increase in size, this may shift to larger insect larvae, and later to larger, more diverse prey, including a variety of molluscs, insects, crustaceans, and smaller fish (Daibner 1952). This diversification of prey through the age classes may be due to ontogenetic changes in gape that allows for specificity in prey selection (Schael et al. 1991). They are primarily benthic feeders who hunt on the substrate, and have been observed feeding throughout the night as primarily nocturnal predators (Rypel & Mitchell 2007).

Reproduction: Sexual dimorphism occurs in freshwater drum through variation in growth rate. After about four years of age, the females of the species achieve a faster growth rate, though on average they reach sexual maturity later than the males of the species (Bur 1984). The females have very high fecundity, capable of producing over 85 thousand eggs per ovary (Goeman 1983). This may be an adaptive trait as they engage in pelagic spawning and thus most eggs will be consumed in the water column without any parental care or protection (Hoagstrom & Turner 2015). It is possible that this form of spawning in open water conditions may be a driving factor in the wide latitudinal distribution of this species throughout drainage basins. Spawning occurs on an annual basis in the early summer, driven by changes in the water temperature. Depending on where they are found throughout their wide range, this typically occurs in May or June, when water temperatures reach an average of 20℃. (Swedburg & Walberg 1970). Other Interesting Facts:  

The family Sciaenidae, which includes all drums, is characterized by the unique way in which the species in this group produce sound. The males of the freshwater drum develop specialized musculature, which vibrates against the gas bladder to produce a drumming sound. This sound is used for communication within the species, most likely as a form of sexual selection (source). The males will drum with these specialized muscles, which may play a role in signaling readiness for breeding, although the exact nature of this communication is not well-understood. The size of the fish producing the sound has a correlation with the frequency of the sound, which may communicate essential information about fitness to conspecifics on the receiving end of the sound (Ramcharitar et al. 2006). The mechanisms of how the sound is received and interpreted are not fully understood, though the particularly large otolith of the freshwater drum may be indicative of hearing specializations.

The freshwater drum is abundant throughout its range. They have been considered as a potential biocontrol agent against invasive zebra mussels, but due to their generalist feeding tendencies they have not proven to be useful at controlling zebra mussels (Morrison et al. 1997). Ecologically, small individuals are an important prey item to larger species of game fish that have higher commercial and recreational value. They are harvested and eaten, but are not an important source of recreational or commercial fishing for most of its range compared to other target game fish species. This may be in part due to the fact that they are actively feeding at night, and thus not as active compared to diurnal species in popular fishing areas. Overall it is an abundant species with no specific regulations in place for its management.

Bodensteiner, L.R., Lewis, W.M. 1992. Role of temperature, dissolved oxygen, and backwaters in the winter survival of freshwater drum (Aplodinotus grunniens) in the Mississippi River. Canadian Journal of Fisheries and Aquatic Sciences 49(1): 173-184.

Bur, M.T. 1984. Growth, reproduction, mortality, distribution, and biomass of freshwater drum in Lake Erie. Journal of Great Lakes Research 10(1): 48-58.

Daiber, F.C. 1952. The food and feeding relationships of the freshwater drum, Aplodinotus grunniens Rafinesque in western Lake Erie. The Ohio Journal of Science. 52(1): 35

de las Mercedes Azpelicueta, M. 2019. Morphology and molecular evidence support the validity of Pogonias courbina (Lacepède, 1803)(Teleostei: Sciaenidae), with a redescription and neotype designation. PloS one 14(6): e0216280.

Essner Jr, R. L., Patel, R., Reilly, S.M. 2014. Ontogeny of body shape and diet in Freshwater Drum (Aplodinotus grunniens). Transactions of the Illinois State Academy of Science. 107: 27.

Goeman, T. J. 1983. Freshwater drum spawning and fecundity in the upper Mississippi River. Proceedings of the Iowa Academy of Science. 90(4).

Harlan, J.R., Speaker, E.B., Mayhew, J. 1987. Iowa fish and fishing. Iowa Conservation Commission, Des Moines, Iowa.

Harington, C. R. 1971. A Postglacial Freshwater Drum (Aplodinotus grunniens) from Ontario, and Comments on the Zoogeography of the Species. Canadian Journal of Earth Sciences 8(9): 1137-1144.

Hoagstrom, C. W., Turner, T.F. 2015. Recruitment ecology of pelagic‐broadcast spawning minnows: paradigms from the ocean advance science and conservation of an imperilled freshwater fauna. Fish and Fisheries 16(2): 282-299.

Morrison, T. W., Lynch Jr, W.E., Dabrowski, K. 1997. Predation on zebra mussels by freshwater drum and yellow perch in western Lake Erie. Journal of Great Lakes Research 23(2): 177-189.

Page, L.M., Burr, B.M. 2011. A field guide to freshwater fishes of North America north of Mexico. Houghton Mifflin Harcourt, Boston.

Ramcharitar, J., Gannon, D.P., Popper, A.N. 2006. Bioacoustics of fishes of the family Sciaenidae (croakers and drums). Transactions of the American Fisheries Society 135(5): 1409-1431.

Rypel, A. L., Mitchell, J.B. 2007. Summer nocturnal patterns in Freshwater Drum (Aplodinotus grunniens). The American midland naturalist 157(1): 230-235.

Sasaki, K. 1989. Phylogeny of the family Sciaenidae, with notes on its zoogeography (Teleostei, Perciformes). Memoirs of the Faculty of Fisheries Hokkaido University 36 (1-2): 1-137.

Schael, D. M., Lars G. R., and Post, J. R. 1991. Gape limitation and prey selection in larval yellow perch (Perca flavescens), freshwater drum (Aplodinotus grunniens), and black crappie (Pomoxis nigromaculatus). Canadian Journal of Fisheries and Aquatic Sciences 48(10): 1919-1925.

Swedberg, D. V., Walburg, C.H. 1970. Spawning and early life history of the freshwater drum in Lewis and Clark Lake, Missouri River. Transactions of the American Fisheries Society 99(3): 560-570.