Swampfish: A Lesson in Preadaptation, by Don Orth

When I think of dark stained waters, my mind always brings up an image of the Creature from the Black Lagoon, a 1954 movie.  But the star of that movie, Gill-Man, was humanoid with gills and webbed hands. The fishes of the dark stained waters of the coastal plain are Swampfish Chologaster cornuta.   A movie about the Swampfish would have to take us into the subterranean world of the cavefishes. Let's learn more about the world of the Swampfish. 

Gill-Man, the main character from Creature from the Black Lagoon 

Swampfish are small fish (1- 2 ½ inches) that live only two years.  They are easily distinguished from other local fishes by the brown coloration dorsally and creamy-yellow belly with three dark longitudinal stripes on each side.   The body shape is distinctive with a combination of flattened head, small eyes, upturned mouth, no pelvic fin, and a rounded caudal fin.  

Swampfish  Photo by Scott Smith NCFishes.com 

The Swampfish is found in acidic blackwater swamps, sloughs, and streams of the coastal plain from southeast Virginia to east-central Georgia. These waters are stained from high levels of organic matter and often have dense aquatic vegetation and coarse woody debris.  The best way to sample Swampfish is with a dipnet because they are so closely associated with cover, a reaction known as thigmotaxis.   The largest series of Swampfish collected from Virginia were taken during application of the ichthyocide, rotenone (Jenkins and Burkhead 1994).

The family Amblyopsidae is most closetly related to the Pirate Perch.  The Swampfish ancestor was likely adapted for life in the changing coastal plain habitats  and changing sea levels after the Cretaceous-Palogene extinction event. There are currently between 7 and 9 species of Amblyopsidae, all of which are geographically isolated from the Swampfish and at least partially cave-adapted. The most recent cavefish, the Hoosier Cavefish, was discovered in Indiana by Chakrabarty et al. (2014). Now there's a movie plot.  Cue the music!  Millions of years before emergence of cavefish, there were ancestral swampfish that give rise to the diversity of amblyopsids we see today.   Rise of the Cavefish -- that's a good movie title. 

 

 Range map of the Swampfish (Niemiller and Poulson 2010). 

Many of the weird characteristics of the Swampfish seem be preadaptions for life in caves. Feeding may be nocturnal or crepuscular.  Amphipods, chironomid larvae, and cladocerans were the most frequent diet items reported by Ross and Rohde (2003). Swampfish have tiny black spots for eyes and are negatively phototactic. They possess numerous rows of neuromasts, or sensory papillae, on their head, body, and caudal fin. Nostrils are tubular. The vent (anus) is located in the throat position, similar to the Pirate Perch, its sister group. Why?  Keep reading!  The mature male possesses a strange appendage on the snout; its function is still unknown. It may be revealed in the movie.  The small size, small eyes, nocturnal behavior, and enhanced sensory receptor for feeding and orientation in a dark environment are preadaptations for cave-dwelling descendent species (Poulson 1963). All species of Amblyopsidae occur in regions that were not glaciated.  The cave-dwellers in the family occur in regions of karst where the limestone and dolomites have dissolved to create caves with sufficient water to support a simple food web (Noltie and Wicks 2001). 

What about the vent location? Young are born with the vent (i.e., anal–genital pore) positioned just anterior to the anal fin and it migrates forward as the Swampfish matures (Ross and Rohde 2003). You read that correctly.  All excreta, egesta, and gametes are released near the head region. This vent location facilitates transfer of eggs directly to the gill chamber cave-dwelling Northern Cavefish Amblyopsis spelaea (Eigenmann 1909).  However, the Swampfish with a similar vent location never carried eggs or yolk-sac fry in its gill cavity (Ross and Rohde 2003).  Interesting plot twist for the movie.  

View of the dorsal surface of the snout in male and female Swampfish from April sample.  Ross and Rohde (2003).

Unlike the small, isolated populations of cavefishes, the Swampfish populations appear to be more secure.  Channelization and removal of streamside forest and riparian vegetation have altered the lowland swamps and streams, but the populations are resilient.  Native fish enthusiasts can easily collect and keep Swampfish, which adapt well in dimly lit aquaria with peat moss to increase acidity (Goldstein 2000).  They may keep you entertained until the release of Rise of the Cavefish. 

References

Agassiz, J.L.R. 1853. Recent researches of Prof. Agassiz. American Journal of Science and Arts 16: 134.

Eigenmann, C.H. 1909. Cave vertebrates of America, a study in degenerative evolution. Carnegie Institute of Washington Publication 104:1-241.

Goldstein, R. J. 2000.  American aquarium fishes.  Texas A&M University Press. College Station, Texas. 

Jenkins, R.E. and N.M. Burkhead. 1994. Freshwater Fishes of Virginia. American Fisheries Society, Bethesda, Maryland, 1079 pp.

Niemiller, M.L., and T.L. Poulson. 2010. Subterranean fishes of North America: Amblyopsidae.  Pages 169-280 in E. Trajano, M.E. Bichuette, and B.G. Kapoor, editors.  Biology of Subterranean Fishes.  CRC Press, Science Publishers.

Noltie, D.B., and C.M. Wicks. 2001. How hydrogeology has shaped the ecology of Missouri’s Ozark cavefish, Amblyopsis rosae, and southern cavefish, Typhlichthys subterraneus: insights on the sightless from understanding the underground. Environmental Biology of Fishes 62:171-194.

Poulson, T.L. 1963. Cave adaptation in amblyopsid fishes.  American Midland Naturalist 70:257-290.

Chakrabarty P., J.A. Prejean, and M.L. Niemiller. 2014. The Hoosier cavefish, a new and endangered species (Amblyopsidae, Amblyopsis) from the caves of southern Indiana. ZooKeys 412: 41–57. doi: 10.3897/zookeys.412.7245

Ross, S.W. and F.C. Rohde. 2003. Life history of the swampfish from a North Carolina stream. Southeastern Naturalist 2: 105-120.