There May Be 5 Species of Shoal Bass Previously Described as 1

New discoveries await those entering the field of biological research.  Scientists have uncovered volumes of knowledge, yet we still know so little.  Nevertheless, it’s surprising that in Georgia, in the midst of what most would consider an advanced civilization,  there may be as many as 5 distinct species of bass left undescribed by science.  Byron Freeman and other scientists are using anatomical and DNA comparisons to remedy this unlikely gap in ichthylogical classification.

 

Redeye bass.  There may be 5 different species of shoal bass wrongly considered the same species.

Shoal basses; along with largemouth bass, crappies, and sunfish; belong to the Centrarchidae family.  Shoal basses favor rocky stretches of river too warm for trout, yet too cool for largemouth bass, though there is some overlap on both ends.  Dr. Freeman discovered that various specimens of a species of shoal bass, known as the redeye bass (Micropterus coosae), differed strikingly, depending upon which river they came from.  By comparing 20 anatomical characteristics, Dr. Freeman was able to determine that shoal basses from the Coosa, Apalachicola, Chattahoochee, Savannah, and Altamaha rivers all differed from each other enough to be considered separate species.  Differences in shoal bass DNA sequencing supports his discovery. The scientific name, Micropterus bartrami, or Bartram’s bass, has been proposed for a species living in the Savannah River.

All of these species share a close evolutionary relationship.  It’s likely the founding population of shoal bass became geographically isolated due to stream capture events when the headwaters of 1 stream eroded backwards and captured the flow of another. For example a tributary of the Savannah River captured a tributary of the Chattahoochee, explaining how speciation occurred between shoal basses from these 2 rivers.  (See:https://markgelbart.wordpress.com/2014/02/06/tallulah-gorge/) Similar geological events probably explain the evolutionary divergence of other shoal basses.

All of these species hybridize with each other, showing the difference between species is a gray area.  Hybridization via backcrossing is another way new species can originate.  By introducing a species of shoal bass into another species of shoal bass’s range, man may inadvertently create a new species.  However, scientists are concerned that introductions of smallmouth bass into shoal bass habitat will result in the latter’s extinction.

See also: https://markgelbart.wordpress.com/2013/04/26/the-evolutionary-history-of-the-black-bass-genus-micropterus-sp/

Reference:

Freeman, Byron; et. al.

“Shoal Basses, a Clade of Cryptic Identity”

Black Bass Symposium 2013

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The Evolutionary History of the Black Bass Genus (Micropterus sp.)

Scientists used a molecular clock to determine all 8 species of black bass share a common ancestor that lived 11 million years ago.  The molecular clock is a method scientists have of estimating the age of a species.  It’s especially helpful when studying the evolutionary history of organisms that are absent or rare in the fossil record.  It is based on the assumption that DNA  protein sequences evolve at a relatively constant rate.  Scientists assume a substitution rate of appearance of new mutations in each member of the population.  Scientists eventually learned the substitution rate varies among different kinds of organisms, so they calibrate the molecular clock with the fossil record and known geological events that may have caused species divergence.  This involves a lot of computer modeling and statistics, but they are confident that using a molecular clock is a reliable method of estimating evolutionary ages.

The oldest bass fossil ever found was unearthed in Texas and dated to 23 million BP.  Scientists estimate the Micropterus (black bass) genus originated about 26 million years ago, based on molecular clock data.  This suggests their findings are consistent with the fossil record.  Today, there are 8 species of bass that began to diverge from a common ancestor 11 million years ago when a marine transgression flooded most of the coastal plain in the southeast.  This dramatic rise in sea level isolated many populations of bass, resulting in allopatric speciation.  Scientists believe this is the most common type of speciation, and it occurs when founding populations become geographically isolated.

Range map of 8 species of black bass.  The dots represent collecting localities for specimens used in study.  From “Speciation in Micropterus” referenced below. Click to enlarge.

Time calibrated phylogram showing evolutionary history of black bass.  This is also from “Speciation in Micropterus.”  The scientific names translated to common names are: Micropterus punctulatus–spotted bass, M. dolomieu–smallmouth bass, M. coosae–redeye bass, M. cataractae–shoal bass, M. notius–Suwannee bass, M. treculi–Guadulupe bass, M. salmoides–largemouth bass, M. floridanus–Florida largemouth bass.

The common ancestor of smallmouth and spotted bass diverged from the ancestor of the other 6 species of bass during the marine transgression of 11 million BP.  Later rises in sea level between 8 million BP-2.5 million BP further resulted in the evolution of redeye, shoal, Suwannee, Guadalupe, largemouth, and Florida largemouth basses.  Before this study scientists thought speciation of bass occurred due to climate perturbations of the Pleistocene, but now they realize most Micropterus species evolved before Ice Ages began.  Only the divergence of spotted bass and smallmouth bass occurred during the Pleistocene about 1 million years BP. These sister species may have been isolated from each other by a glacier, but the other species of bass originated in warmer climates that remained glacier-free.

Most black bass species readily hybridize, showing just how closely related they are.  This can prove a problem when a common species is introduced to waters occupied by a rare species.  Shoal bass were formerly abundant in the rocky cataracts found in the Flint, Chattahoochee, and Appalachiacola Rivers, but most of those areas were inundated with reservoirs that make the habitat unsuitable.  Shoal bass became rare, and to make matters worse spotted bass were introduced.  Spotted bass compete for the same foods and hybridize with the remaining shoal bass.  

Comparison photos and illustrations between 4 species of bass.  The bottom photo is of a a redeye bass.  It has a curious distribution.  It’s found in 2 different river drainages–the Coosa and the upper Savannah.  At one time these 2 drainages must have been connected.

The below link has some nice photos comparing the shoal bass with other species of bass.

http://www.flmnh.ufl.edu/fish/sharks/shoalbass/ShoalAbstract.htm

White bass (Morone chrysops), striped bass, (Morone saxatilis), and black sea bass (Centrophis stiata) are not closely related to the Micropterus genus, despite their common names.  Instead, the Micropterus genus is part of the Centrarchid family that includes sunfish and crappies.  Surprisingly, largemouth bass can hybridize with some of their distant sunfish and crappie relatives.

Adult largemouth bass prey on smaller fish, frogs, crayfish, and occasionally birds and mice.  Most of the largemouth bass’s sister species grow smaller and feed more on insects and small crayfish.  Smallmouth and spotted bass feed on prey intermediate in size between that taken by the largemouth and its 6 sister species.  The largemouth bass’s range overlaps with those of its sister species, but they occupy different ecological niches.  The introduction of intermediate feeders, like the smallmouth or spotted, may disrupt the ecosystems in southern rivers.

Largemouth bass are some times found in brackish waters located within salt marshes.  Anglers catching them here still call them by their archaic common name of “green trout.”

References:

Near, T.J.; et. al.

“Speciation in North American black bass, Micropterus (Achioptergii: Centrarchidae)”

Evolution 57 (7) 2003

http://www.yale.edu/eeb/near/14.pdf

Simon, Ho

“The Molecular Clock and Estimating Species Divergence”

Nature Education 1 (1) 2008