The Freshwater Drum, Aplodinotus grunniens, is the only species of the marine drum family, Sciaenidae, to spend its entire life in freshwater. This fish is widely distributed throughout the large lakes and rivers of North and Central America. It is classified as an opportunistic feeder that spends the majority of its time near the bottom of the water column. The Freshwater Drum’s diet changes significantly throughout its life, adapting to its size and the physical capabilities it develops as it matures.
The Adult Diet: Hard-Shelled Prey
Adult Freshwater Drum primarily consume hard-shelled, benthic invertebrates, which are organisms living on or in the bottom substrate. This preference is directed toward freshwater mussels, clams, and various snails. They also prey upon larger crustaceans, such as crayfish, which they encounter while foraging along the bottom.
This heavy reliance on encased food sources can be seen in their impact on invasive species. Where present, adult drum are effective predators of invasive zebra mussels and quagga mussels. Studies show that drum must reach a total length of approximately 25 to 35 centimeters before they can consistently crush and consume the shells of these bivalves.
The diet is not exclusively shell-bearing organisms, as seasonal shifts occur based on prey availability. During the late summer and autumn months, adult drum frequently incorporate small fish, such as young-of-the-year Gizzard Shad, into their feeding routine. This flexibility allows the drum to take advantage of dense seasonal populations, but the bulk of their year-round energy still comes from bottom-dwelling organisms.
Diet Shifts Based on Life Stage
Newly hatched larvae rely on microscopic prey for their first meals. Their initial food source consists of small zooplankton, including rotifers and copepod nauplii, which are abundant in the open water where they hatch.
As the young fish grow, their mouth size, or gape, increases, allowing them to capture progressively larger items. The diet transitions to larger zooplankton, such as copepods and cladocerans, and then gradually incorporates small insect larvae and worms. This shift away from planktonic organisms marks the beginning of their life as benthic foragers.
Juveniles continue to consume soft-bodied insect larvae, such as midges and caddisflies, until they develop sufficient size and strength. The full transition to a diet dominated by hard-shelled prey occurs only when the fish is large enough to crush the shells. This developmental change ensures the fish utilizes the most energetically rewarding prey it is physically capable of consuming at each life stage.
Specialized Feeding Mechanism
The ability of the Freshwater Drum to consume hard-shelled prey is directly linked to a specialized anatomical feature. Located deep in the throat, behind the last set of gill arches, are powerful pharyngeal teeth. These structures are not like the teeth in the jaw but are instead fused bone plates with molar-like cusps designed for grinding.
These crushing plates are progressively developed as the fish matures, transitioning from small, pointed teeth in juveniles to the large, rounded molariform teeth in adults. The drum uses its subterminal mouth and blunt snout to root around in the bottom sediment, often moving rocks to dislodge hidden prey. Its lateral line system, which detects vibrations in the water, helps it locate these buried organisms.
Once the prey is ingested, the pharyngeal teeth crush the shells before the food is passed into the short esophagus and stomach for digestion. This powerful crushing mechanism allows the drum to access a food source unavailable to most other freshwater fish species.