Biographies of Southeastern Freshwater Fishes

Northern studfish, Fundulus catenatus, nuptial male.—Northern studfish inhabit shallow, backwater habitats in creeks and rivers draining the Eastern Highlands and Ozarks, regions of remarkable fish diversity in North America.  A member of the topminnow family, Fundulidae, the northern studfish demonstrates interesting examples of sexual dichromatism (differences among sexes in coloration) and functional morphology.  In species that exhibit sexual dichromatism, usually the male displays brilliant, even gaudy, nuptial colors, while female coloration is less intense or even "plain." Differentiation of sexes may also include (sometimes dramatic) variation in size and shapes (sexual dimorphism). These phenomena have evolved in many groups of plants and animals and are often related to an evolutionary process that Charles Darwin termed "sexual selection."

Understanding functional morphology (literally the function of form) enhances our appreciation of the intricate beauty of nature.  In many fishes, the dorsal fin is one of the largest, or at least most prominent, fins. This is usually true for species that live on or near the stream, or up in the water column.  Besides maneuvering, the dorsal fin of male fishes which exhibit sexual dichromatism are also used to convey signals, such as courting females or in aggressive encounters with other males. Accordingly, the dorsal fins are often contrastingly patterned with bands, bars, or spots, and may be multi colored.  However, topminnows, as the common name suggests, usually dwell immediately below the water surface, a location where the dorsal fin is less evident to other topminnows.  Consequently, the anal fin, which is more prominent than the dorsal fin, has functionally replaced the dorsal fin in the communication of behavioral states.

Crescent shiner, Luxilus cerasinus, nuptial male.—Endemic to the Roanoke River drainage on the mid-Atlantic slope, the crescent shiner has been introduced to the adjacent New River and James River drainages. The crescent shiner is one of the minnow species that are spawning associates of the nest-building chubs (several genera of relatively large minnows found in the middle and eastern North America).  These chubs construct stone nests from late April to mid-June. The stone nests have become communal spawning sites by a host of minnow species (i.e., the spawning associates). Sometimes, six or more associate species simultaneously spawn over chub nests, and occasionally the eggs of one species are accidentally fertilized by milt from another, resulting in a hybrid. Hybrids are generally uncommon in nature and usually sterile (e.g., a mule).  However, repeated hybridization in a relatively limited area over extended (evolutionary) time has resulted in the differentiation of new species.  The crescent shiner was a participant in this rare phenomenon.  Geneticists have demonstrated that the crescent shiner introgressed with the related (and widespread) common shiner, Luxilus cornutus, giving rise to the white shiner, Luxilus albeolus. There are several other examples of species of North American minnows that similarly arose via introgressive hybridization. The common denominator of this extraordinary form of speciation is that contact "suddenly" occurred between populations the two parental forms that were previously separated geographically.  The most common explanation this atypical biological event is a natural geological process: stream capture (aka stream piracy or headwater cutting).

Yellowfin madtom, Noturus flavipinnis, female.—The yellowfin madtom is a small catfish that is endemic to the Tennessee River drainage.  Once widespread, this bottom-dwelling denizen of slow pools and runs now persists in fragmented, broadly isolated creek populations.  It is presently listed as threatened under the Endangered Species Act, but some biologist think it may need to be upgraded to endangered status because of the continuing decline throughout its limited range.  Experimental populations of yellowfin madtoms have been established in Citico Creek, in the Great Smoky Mountain National Park, by scientists from Conservation Fisheries, Inc., (a non-profit enterprise that specializes in the propagation of southeastern imperiled fishes). The catfish family Ictaluridae, the only family of fishes endemic to North America, contains many well-known larger species, such as channel catfish and bullheads, but few realize that most native catfishes are small in size, seldom exceeding 150mm (6 inches) in total length.  Called "madtoms," these often-secretive fishes are largely nocturnal and are noted for being one of the few poisonous freshwater fishes.  The leading rays of the dorsal and pectoral fins have become fused into spines, and have a ductless poison gland located at the spine base. In some species, the pectoral spines are edged with formidable appearing saw-like teeth called "serrae.". Indeed, the sting from these spines can be quite painful, but is most often compared to a nasty wasp sting.  The origin of the name "madtom" is unknown, but may allude to the spontaneous reaction upon being stung. As a group, madtoms are disproportionately imperiled (e.g., half of the 16 species in Southern Appalachia). For unknown reasons, these diminutive catfishes are unable to cope with pollution and degradation of creeks and rivers.

Rainbow shiner, Notropis chrosomus, nuptial male.—An aptly named minnow that is native to the upland streams of the Mobile River drainage, the rainbow shiner occurs in small to medium size creeks. This species is one of the many nest associates of chubs (relatively large minnows that build stone nests in the spring as spawning beds). Nest associates spawn over the nest of chubs and derive advantage from the aggressive male chub guarding against predators such as sunfish and bass.  Thus, the eggs of the nest associate minnows also receive protection. For the fish watcher, observing spawning over chub nests is one of the rites of spring, yet has probably been observed less than some of our rarest birds.  However, the chub nests are not difficult to observe; they are usually constructed at the head or the tail of riffles where the water surface rippling has ceased. Nest-guarding chubs are wary.  However, a careful observer, using binoculars or a stealthy approach by snorkeling, may witness dozens–even hundreds—of brilliantly colored minnows engage in frenzied bouts of combat and spawning.  The scene is mesmerizing, even to the most jaded biologist.

Saffron shiner, Notropis rubricroceus, nuptial male.—A relative of the rainbow shiner, but instead native to the Tennessee River drainage, this minnow is an upland inhabitant of small creeks.  Primarily an occupant of pools and slow runs, it too spawns in masse over chub nests, often with other brilliantly colored nest associates.  For the biologist who studies these fishes, lifting a net full of these gaudy minnows can cause the sternest researcher to break out in child-like laughter.  It's like holding writhing, wriggling slippery blobs of color. Suddenly, one can feel absolutely rich in simple, striking beauty. In general, few nest-spawning minnows are imperiled, probably because of the reproductive benefits derived from the communal protection of gametes from predators.

Photo credit:  USGS - Noel Burkhead