Perplexed about the Endemic Chub of the New River? Blame it on the Pleistocene, by Don Orth

The Bigmouth Chub is one of eight endemic fishes in the New River.  It is a special fish in a special place.  Bigmouth Chub is a member of the genus, Nocomis, which was first described in 1856 by Charles Frédéric Girard, a student of Louis Agassiz. However, it wasn’t until 1971 that the Bigmouth Chub was described as Nocomis platyrhynchus.   By that time the distributions of other Nocomis were well described and the Bigmouth Chub had this difficult-to-explain allopatric distribution with its closest relatives, the River Chub Nocomis micropogon and the Bull Chub Nocomis raneyi.   The Bigmouth Chub was all alone in the upper New River, surrounded by other Nocomis. 

Distribution of Nocomis platyrhynchus, Nocomis micropogon and Nocomis raneyi.  Nagle and Simons 2012. 

These and other Nocomis species arose from a common ancestral Nocomis. Let’s call him “Bob.”   Apparently the “Bob” chub moved into the ancient Teays River several million years ago.  (I don’t really know when, no one does for sure.  One day this may be answered by students of Nocomis).    

Populations of the “Bob” chub became vicariant, or isolated from other chubs to an extent that prevents or interferes with genetic interchange.  The Teays River originated in the Tertiary Period and at that time drained much of eastern North American in the pre-glacial period.  However, the original route of the ancient Teays was altered by glacial advances which created a massive ice dam blocking the northward-flowing Teays.   “Bob” experienced dramatic geologic and cycles of glaciation through evolutionary time that permitted the speciation of Nocomis platyrhynchus.  In allopatric speciation there is an extrinsic barrier to gene flow, which for the Bigmouth Chub was the Kanawha Falls at downstream limit and the Atlantic drainage divide at the upstream limits. The upper Teays followed the route of the present-day New River from near Blowing Rock, North Carolina.  Consequently, the descendents of “Bob” found refugia in the upper New River drainage during the Pleistocene and differentiated there. 

Today, the Bluehead Chub Nocomis leptocephalus also occurs in the upper New River. This species is widely distributed in piedmont streams from Mississippi to Virginia and may be five unique species (Nagle and Simons 2012).  Ichthyologists believe that Nocomis leptocephalus entered the New River drainage from the Roanoke drainage via a process called stream capture.  By the time of the stream capture, reproductive isolation mechanisms between the two Nocomis were in place so they would not interbreed.  

Bigmouth Chub.  Photo by Edward Burress.

The species name, platyrhynchus, refers to large mouth gape of the Bigmouth Chub.  The species exhibits sexual dimorphism.  The breeding adults have distinctive pink ventral coloration, olive-orange caudal fin, and yellow pectoral and pelvic fins. For an underwater view, watch the Bigmouth Chub swim in Walker Creek.  Nonbreeding females and juveniles have a dark horizontal lateral body stripe.   Bigmouth Chub inhabit medium- to large-sized tributaries and the mainstem New River.  These streams have a moderate gradient, warm, usually clear water, and a good mix of gravel to boulder substrates.  They may be found in both swift water and pools.  In a late summer investigation by Lobb and Orth (1988), the Bigmouth Chub were found only in riffles and adjacent runs and they avoided the shallowest depths and were always observed near the streambed. 

 

Non-reproductive adult Bigmouth Chub captured in the New River, Eggleston, Virginia

The most distinctive feature of the breeding male Bigmouth Chub is an enlarged nuptial crest and numerous tubercles on the head and snout.  The tubercles are rarely seen in small individuals (< 60mm SL) and the tubercle pattern is not fully developed until individuals exceed 100mm.     

Head of the breeding male Bigmouth Chub.

In the photo, note the pink coloration on ventral side of head and belly region.   Also, the tubercles do not extend to the occipital region in the Bigmouth Chub. The male Bluehead Chub has larger and fewer tubercles and no tubercles on the snout.  Therefore, potential mates can be recognized by presence or absences of tubercles on the snout in addition to coloration differences. 

Tubercle functions are the source of much speculation.  Tubercles possibly serve to warn other individuals who may infringe on the male's territory during nest building.  If the visual warning doesn’t work, head butting might.  Tubercles may assist in attracting a female mate or in maintaining contact during the spawning act.  Tubercles are shed after spawning.

During the spring, the male Bigmouth Chub uses his big mouth to construct spawning mounds in areas of small to large gravel, shallow water (15-75 cm), and moderate water velocity (10-70 cm/sec).   The gravel mound is an impressive alteration of the streambed and is constructed over many days and nights by a single breeding male.  The male may collect pebbles from as far as 10 meters from the nest.  Most mounds measured by Lobb and Orth (1988) were between 50-90 cm wide and 40-70 cm high and consisted of large gravels.  The behavior of mound construction is remarkably similar in the closely related Bigmouth Chub, River Chub, and Bull Chub.  Eugene Maurakis described this characteristic three-stage progression. 

Bigmouth Chub three-stage mound construction (a) concavity; (b) platform; and (c) mound with spawning trough.  Source: Maurakis et al. 1991.

Bigmouth Chub constructs the gravel mound in three distinct stages (a) excavated cavity with channel parallel to current; (b) platform constructed with particles from lateral margins; and (c) mound with a spawning trough in upstream location.    These three behavioral synapomorphies are unique to these three Nocomis and support their derivation from a common ancestor, “Bob.”  The spawning trough is where a single male and female breed.  The mound, modifies the local flow field creating an eddy behind the mound and slow current in the trough where eggs and sperm are deposited.  The large gravel mound enhances survival of embryos and larvae, which would otherwise be eaten by numerous egg predators.   The mound is such a perfect spawning location that other minnows are frequently observed spawning in the vicinity of the Bigmouth Chub gravel mound.   The Central Stoneroller Campostoma anomalum, Striped Shiner Luxilus chrysocephalus, Rosefin Shiner Lythrurus ardens, Rosyface Shiner Notropis rubellus, and Mountain Redbelly Dace Chrosomus oreas are some of the common nest associates with the Bigmouth Chub.

Photos of gravel mound nests of the Bigmouth Chub.

Top view by Brandon Peoples (left) and side view by Del Lobb (right).

The Bigmouth Chub is the result of millions of years of evolution, changing climate, and river erosion.  Recent molecular analyses suggest that it might be subsumed into Nocomis micropogon (Nagle and Simons 2012).  More study is needed of this minnow and the many other minnows that dominate our local streams.  Today we know very little about this endemic minnow and its history and role in its New River home.  It’s not a simple story to untangle.  You can blame it on the Pleistocene! 

References

Lachner, E.A., and R.E. Jenkins. 1971. Systematics, distribution, and evolution of the  chub genus Nocomis Girard (Pisces, Cyprinidae) of eastern United States, with descriptions of new species.   Smithsonian Contributions to Zoology 85:1-97.   

Lobb, M.D., III, and D.J. Orth. 1988. Microhabitat use by the Bigmouth Chub Nocomis platyrhynchus in the New River, West Virginia.  The American Midland Naturalist 120(1):32-40.

Lobb, M.D., III, and D.J. Orth. 1991.  Habitat use by an assemblage of fish in a large warmwater stream.   Transactions of the American Fisheries Society 120(1):65-78.

Maurakis, E.G. 1998.  Breeding behaviors in Nocomis platyrhynchus and Nocomis raneyi (Actinopterygii: Cyprinidae). Virginia Journal of Science 49(4):227-236.

Maurakis, E.G., W.S. Woolcott, and M.H. Sabaj. 1991.  Reproductive-behavioral phylogenetics of Nocomis species-groups.  The American Midland Naturalist 126:103-110.

Nagle, B.C., and A.M. Simons. 2012.  Rapid diversification in the North American minnow genus Nocomis.  Molecular Phylogenetics and Evolution 63(3):639-649.         doi:10.1016/j.ympev.2012.02.013

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