The mooneye is a freshwater fish with very large eyes. Usually measuring between 16-21 in (40-51 cm) long, it has a deep, laterally thin body. Because it generally resembles a shad or herring, the mooneye has acquired such common names as toothed herring, big-eyed shad, or white shad. Mooneyes can be distinguished from shads and herrings by the presence of well-developed teeth on their jaws and tongues and by the absence of an adipose fin, which is a small, extra dorsal fin located well back on the fish's spine in front of its tail. The anal fin of a mooneye is moderately long with 23–33 rays, and its caudal, or tail, fin is well-developed. Its pelvic fins have 7-10 rays. Laterally, this fish has between 54 and 61 scales.
The mooneye belongs to the Order Osteroglossids, generally referred to as bonytongues. Within the Order, there are two Suborders: (1) bonytongues (Osteoglossoidei) and (2) featherbacks (Notopteroidea). The mooneye is classified in the latter suborder and in the family Hiodontidae. There are two species commonly accepted to be in this family, the Hiodon tergisus (mooneye) and the Hiodon alasoides (goldeye). A third species, the southern mooneye (Hiodon selenops), used to be considered a separate species, but scientists now think that it is identical to the common mooneye.
The two species can be differentiated by their eye coloring, dorsal fins, and the position of their ventral keels. As its name infers, the goldeye has gold-colored eyes. Furthermore, the mooneye has 11–12 rays comprising its dorsal fin. The same fin on the goldeye has only 9 or 10 rays. Finally, while both the mooneye and the goldeye have a fleshy keel on their belly, known as a ventral keel, its position on each fish is different. On the mooneye, the ventral keel does not extend in front of its pelvic fins; on the goldeye, it does.
The northernmost range of the mooneye extends northwest of the St. Lawrence River to the Hudson Bay and Manitoba. In the United States, these fish live in Lake Champlain west to the Great Lakes, mostly in Lake Ontario and Lake Erie. In the Mississippi River, these fish occur as far south as Arkansas. The mooneye can also be found in the Mackenzie River system.
Mooneyes generally eat insects, but larger fish will eat mollusks and minnows if given the opportunity. They are abundant in their natural ranges, however, they are little known by fishermen. Although mooneyes are not considered to be gamefish, they will fight actively if hooked. Commercially, it is of negligible importance, its flesh being edible but not tasteful.