In Praise of the Freshwater Drum

Freshwater Drum (Aplodinotus grunniens)

Illustration: WPClipart

I wish to say a few words about an odd fish, the Freshwater Drum (Aplodinotus grunniens). Here's a species that I too seldom see alive, but I do find washed up dead, on the shores of Lake Erie with some regularity. Indeed, just this evening I watched a Great Black-backed Gull (Larus marinus) rip apart a smoldering drum carcass, its heavy bill effortlessly shearing the hard overlapping ctenoid scales that smaller scavengers like Ring-billed Gulls (Larus delawarensis) would have had difficulty penetrating. As fascinating as is the process of death and decay, it's the living Freshwater Drum that I'm here to endorse.

A drum's thick scales help protect it from attack by the parasitic native Silver Lamprey (Ichthyomyzon unicuspis) and the highly invasive non-native Sea Lamprey (Petromyzon marinus). Using suction cup-like mouths, lampreys attach themselves to large fish, chewing through their host's scales to the soft tissue below, using a toothed tongue. A lamprey feeds on the blood of its host, sometimes for months at a time. This can be severely detrimental to the host, resulting in reduced reproductive success, or ultimately even death. Freshwater Drum enjoy the advantage of being a among the most well-armored Great Lakes fish, a significant advantage in a world teaming with raspy-tongued parasites.

Besides their boilerplate scales, drum are oddballs among Great Lakes fish for other reasons, and their name, Freshwater Drum, makes it plain. This is the only species of totally freshwater-dwelling drum. All of the other 160 or so drums and croakers (family Sciaenidae) are marine. No other native Great Lakes fish has such a salty pedigree, though the Burbot (Lota lota) comes close, with only one other member of the cod family (Gadidae) found in freshwater - the Atlantic Tomcod (Microgadus tomcod). The name drum, and the specific moniker grunniens, which means "grunting", refers to the sounds that this species makes during mating, and, indeed, when handled by anglers. Drums don't vocalize in the conventional way most mammals or birds do, by issuing vibrations in the throat, instead their sounds are produced by muscular manipulating of the swim bladder. As far as I know, this is the only Great Lakes fish that makes sounds.

Freshwater Drum eggs contain a large oil globule that allows them to float on the water's surface, something totally unique among North American freshwater fish. Most other freshwater fish lay their eggs in nests, like sticklebacks and sunfish, or  stick their eggs to vegetation or other debris, as Yellow Perch (Perca flavescens) do. Some ichthyologists have suggested that planktonic eggs may be particularly good at dispersing long distances, perhaps having played a role in helping Freshwater Drum to attain the greatest natural latitudinal distribution of any fish in North America; they range from the northern reaches of Manitoba's mighty Nelson River to southern Mexico and Guatemala.

From a simple examination of a fish's mouth, it's possible to hypothesize something about its foraging ecology. For example, a Brook Silverside (Labidesthes sicculus) sports an upturned mouth for taking surface-dwelling prey, and don't forget the aforementioned parasitic lampreys with their suctioning and rasping mouthparts. Silversides and lampreys have highly modified external mouthparts, and while Freshwater Drums have extraordinary internal mouth parts. They're highly adapted for crushing the shells of hard-bodied prey. Most perciform fish have two sets of jaws, the external ones which we can plainly see, and a set of internal ones, and it's a drum's internal ones, the pharyngeal jaws, that set them apart. Drum pharyngeal arches are unique among Great Lakes fish in that they are fused together and are covered with large molar-like teeth, adaptations for processing hard foods, namely mollusks and crayfish. No other fish in the Great Lakes has such highly modified arches, though some other species, like Pumpkinseed (Lepomis gibbosus) and Yellow Perch, do feed on mollusks, they don't have the same crushing adaptations. Drums grind native and non-native mollusks, alike, including Zebra (Dreissena polymorpha) and Quagga Mussels (Dreissena bugensis), on those molar-like teeth using powerful muscles which are supported by a series of bone struts on their robust skulls.

Once in a while, while wandering along the beach, I find a polished and intact drum arch washed up on shore, a reminder that there is a thick-scaled, croaking, planktonic egg-laying, mussel-crushing, freshwater version of a marine fish, beneath the Lake Erie waves. Fascinating!