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

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Crappie and mastodon

Crappies and mastodons shared the same habitat.  Crappies are primarily a lake fish, preferring clear still water where they prey on small minnows and insects.  Mastodons often waded into lakes to feed upon submerged aquatic plants.  Perhaps they even aided crappies by forcing minnows away from the cover of underwater vegetation into the open to be picked off by schools of crappie.  Of course, mastodons weren’t as tied to the water as crappies and could travel overland whenever they desired.

Black crappie (Pomoxis nigromaculatus).  A beautiful fish and tasty too.  I once had a fish dinner of bass and crappie.  The bass were meatier, but the crappie were sweeter and better tasting.

Kurt Hamlin, a curator at the Milwaukee Museum, was lucky enough to find fur on a mastodon specimen.  The fur resembled that found on river otters and beavers, so we know mastodons spent a lot of time in the water.

River otter (Lutra canadensis) pelt.  River otter fur is water proof and dries quickly.  Mastodon fur was similar.

Although these two different species shared the same habitat, the fossil remains of mastodon and crappie have been found together at only 1 locality–The Charles Adams Mastodon Site in Livingston County, Michigan.  It was a lake deposit also containing fossils of meadow vole, 1300 snail shells, 500 freshwater clam shells, and bones of white sucker fish.  In Georgia mastodon fossils have also been found associated with fish remains, but not crappie.  An alluvial deposit in Little Kettle Creek contained bones of channel catfish along with mastodon.  The  deposit in Kingston Saltpeter Cave in Bartow County contained over 500 fish bones in addition to a mastodon fossil. Here, predatory birds brought  fish from the nearby Ashpole Creek, a tributary of the Etowah River.  They included gar, pickerel, channel catfish, bullheads, sucker fish, chubsuckers, largemouth bass, and unidentified sunfish.  Coastal fossil sites often have marine species of fish as well as mastodon.

Overall, fish populations during the Pleistocene were much higher than those of today’s waters.  Then the rivers were undammed, unpolluted, and unfished by men.  But in what’s now Georgia crappie populations were probably an exception.  They thrive in modern artificial reservoirs formed behind the numerous hydroelectric dams.  Suitable habitat during the Pleistocene was temporary and sporadic.  Oxbow lakes were plentiful during interstaidals and interglacials.  Cut off channels were common during stadials.  But large lakes akin to modern day reservoirs were nonexistent.  Instead, crappies relied on favorable habitats that constantly changed with the evershifting river patterns.  They can live in rivers where water flow is obstructed and forms pools.  Lower water levels with still channels, and higher water levels with wide bends that form slow moving water both provide favorable habitat.  Crappies could also inhabit large beaver ponds.  But the constantly changing conditions probably made crappie numbers fluctuate much more than they do in today’s reservoirs.

Centrarchid Evolution

An evolutionary tree of the centrarchid family of fishes as proposed by some scientists.

 The centrarchid family of fish includes sunfish, crappie, and bass (though not white bass, striped bass, and sea bass which are in completely different families).  The opportunity for speciation occurs often among this family because populations of fish get isolated when connecting streams run dry or become obstructed.  This makes the centrarchid family an excellent one for evolutionary scientists to study.  Scientists can look at rates of genetic divergence between closely related species and estimate the length of time it has taken since speciation occurred from a mutually ancestral species.  They call this a speciation clock. Many species of centrarchids hybridize but genetic compatibility decreases with time since speciation occurred.  One study found that hybrid embryo viablity declines 3% per million years of separation.

Here’s a list of centrarchid species which are strictly an American family of fish.  Note:  Some scientists recognize 33 species and 7 genera; others recognize 31 species and 9 genera.

Mud sunfish–Acantharchus pomotis

Shadow bass–Amploplites aribmarus

Roanoke bass–A. cavifrons

Ozark Bass–A. constellatus

Rock Bass–A.  rupestris

Sacramento perch–A. choplites

Blackbanded sunfish–Ennearanthus gloriousus

Flier–Centrarchus macropterus

Redbreast Sunfish–Lepomis auridus

Green sunfish–L. cyanellos

Pumpkinseed sunfish–L. gibbosus

Orange spotted sunfish–L. humili

Warmouth–L. gulobus

Bluegill–L. marochirus

Dollar sunfish–L. marginatus

Longear sunfish–L. megalatis

Redear sunfish–L. microlophus

Red spotted sunfish–L. miniatus

Spotted sunfish–L. punctatus

Bantam sunfish–L. symmetricus

Redeye or Coosa Bass–Micropterus coosae

Spotted Bass–Micropterus punctatus

Largemouth Bass–Micropterus salmonoides

Shoal Bass–M. cataractus

Guadalupe bass–M. treculi

White crappie–Pomoxis annularis

Black crappie–P. nigromaculutus

Redeye bass

A redeye bass.

In my blog entry of a few weeks ago about the food I would eat, if I could live in Georgia 36,000 BP, I mentioned smallmouth bass as a fish I might find in my fish traps on the Broad River.  At the time of European colonization of North America smallmouth (aka spotted) bass only occurred in extreme northern Georgia.  It did not naturally occur in the Broad River, though it has since been introduced.  During the Pleistocene, smallmouth bass may have ranged further south, but maybe not.  Instead, redeye bass, a closely related species, did occur on the Broad River.  Redeye bass and smallmouth bass probably evolved from a common ancestor that diverged due to the geographical separation of the watersheds where this common ancestor lived.  So, I believe I would find redeye bass in my Pleistocene fish traps in the Broad River, and probably not smallmouth.

Reference:

Bolnick, Daniel; and Thomas Neal

“Tempo of Hybrid Inviability in Centrachid Fish (Telestei: Centrarchae)”

Evolution 59 (8) Augusts 2005