Wednesday, August 16, 2017

Hackelbacks and Old Spade Face, by Don Orth

Scaphirhynchus sturgeons arose in North America but remain among the least studied fishes.  Scaphirhynchus is pronounced Ska-fur-rink-us. Fossils resembling the ancestral Scaphirhynchus were dated from the late Cretaceous (Grande and Hilton 2006).  This fossil (pictured below) was discovered in modern day Montana and the sturgeon swam in rivers at the same time that Tyranosaurus rex, Hadrosaurs, and Ceretopsians roamed the Cretaceous landscapes 70 million years ago.  The sturgeon fishes (Acipenseridae) date back 200 million years and sturgeons exist in all continents of the northern hemisphere. The sturgeons are primarily cartilaginous and lack a backbone with separate vertebrae.  The closest relative of the Scaphirhynchus sturgeons are members of the genus Pseudoscaphirhynchus, which includes three critically endangered species from the Aral Sea basin.  The Scaphirhynchus sturgeons are ideally suited for life in large, fast flowing rivers that formed when the Rocky Mountains arose and the great inland sea receded. Recent molecular genetics methods confirmed the distinctiveness of three species: Shovelnose Sturgeon Scaphirhynchus platorynchus, Pallid Sturgeon Scaphirhynchus albus, and Alabama sturgeon Scaphirhynchus suttkusi (Ray et al. 2007).
Illustration of a specimen named Scaphirhynchus rafinesquii by Jacob Heckel 1836

Fossil sturgeon Priscosturion longipinnis from ~78 MYA. Image from The Field Museum photo archives image A93851c, Chicago, Illinois. photo by Eric Hilton source
The current status of the three Scaphirhynchus sturgeons is precarious due to harvest and dams. Too much harvest means too few breeders.  Too many dams impede the free-flowing river habitat needed for spawning and early development of young shovelnose sturgeons. Pallid Sturgeon is the largest of the three and critically endangered.  The Pallid Sturgeon recovery program  involves three regional teams that must coordinate and implement recovery actions for pallid sturgeon in Recovery Priority Management Areas encompassing the waters of the Missouri and lower Mississippi River basins of the United States.   Watch this video to get a better idea of recovery actions. 
Alabama Sturgeon was feared to be extinct as the last specimen handled by a biologist was in 2007.  Watch this video that describes the recent discovery of environmental DNA of the Alabama Sturgeon. Twenty-five major locks and dams on the rivers of the Mobile Basin fragment the habitat where the Alabama Sturgeon lives.  Shovelnose Sturgeon are listed as threatened because their current range is greatly reduced (Phelps et al. 2017).  

It’s mind boggling to consider the millions of years of survival of the ancestral Scaphirhynchus sturgeon through many major climate disruptions, mass extinction, multiple glacial advances and retreats, and river course changes.  The three species of shovelnose sturgeon are well adapted for life in the murky waters.  Shovelnose Sturgeon complete all aspects of their life cycle in the main channel of rivers. They are often caught by anglers who fish with worms in shifting sands.  Here the anglers call them “Sand Sturgeon” because of their behavior of holding position in sand and associated dune bedforms even at high flows. 

Illustrations of the Pallid Sturgeon (top), Shovenose Sturgeon (middle), and Alabama Sturgeon (bottom).  Sources:    Fishes of Illinois, Flickr,  and Patrick O’Neil  Fishes of Alabama.  Note: the long caudal filaments are typical of only young sturgeon (< 40mm). 
Tens of millions of years living in the large muddy rivers produced a fish with body form and adaptations like no other fish.  The shape of the head earned it the name “old spade face” and “flathead sturgeon” and the bony scutes on the mid-dorsal ridge gave it the name “hackleback.”  The mouth is ventrally located and protrusible and the snout has four long barbels in front of the mouth.
Ventral surface of the head of Shovelnose Sturgeon. Photo by Corey Raimond
Close up of the head of "Old Spade Face" or Shovelnose Sturgeon in Missouri River. Photo by Sam Stukel. 

Sturgeons throughout the world are at risk of overharvest due to the lucrative caviar market.   The Shovelnose Sturgeon is no exception and commercial harvesters in Arkansas, Kentucky, Illinois, Indiana, Iowa, Missouri, Tennessee, and Wisconsin target Shovelnose Sturgeon, many solely for the roe markets (Koch and Quist 2010).  The caviar is marketed as Hackelback caviar and sells for $24 per ounce.  Therefore, an individual gravid female can be worth thousands of dollars.   Harvest pressure to meet the caviar market demands will increase. Therefore, harvest restrictions, which vary among states, are needed to protect populations from overharvest (Koch and Quist 2010). 
Hackelback Sturgeon caviar  Source
The Scaphirhynchus sturgeons have morphological adaptations that are very unique.  The small, adhesive eggs develop quickly into yolk sac larvae which develop as they drift with river currents. Consequently, reservoirs and backwater habitats allow the larvae to sink to the bottom instead of staying in the drift.   The drift hypothesis maintains that the small sturgeon larvae must have sufficient river length in order to drift and develop for many days.  Consequently, Scaphirhynchus sturgeons need spawning grounds and sufficient river length for larval development and drift for populations to persist.  
Yolk sac larva of Shovelnose Sturgeon.  Illustration by Murrie V. Graser
  Juvenile shovelnose sturgeon. USFWS.
The head, bony scutes, barbels with dense arrays of taste buds, and a spiral valve intestine are additional morphological traits unique to the Scaphirhynchus sturgeons.  The bottom of the body is a flat plane,  protected with bony denticles, which allow the fish to swim right at the river bottom and be protected from abrasion.  The body surface is protected by rows of large bony scutes interspersed with smaller denticles.  The mid dorsal, lateral, and ventral ridges form distinct keels. The combination of a flat head, small gas bladder and long caudal filament serves to align the fish in the current but close to the bottom. The whiptail or caudal filament is often lost among older specimens.  They possess numerous electrosensory pit organs on the ventral surface of the rostrum, large nares,  and numerous taste buds on papillose barbels, lips, gill rakers and esophagus (Weisel 1979).   Electrosensory and chemosensory organs all of which serve to facilitate efficient feeding on benthic or drift-feeding invertivores, feeding largely on Trichoptera, Ephemeroptera, Chironomidae, and other small invertebrates (Phelps et al. 2017).   These benthic organisms are consumed via a highly protractile mouth, extending 2/3rds of the head depth.   Digestion is also facilitated by a spiral valve intestine, a highly coiled structure to increase nutrient absorption.  Spiral valve intestines are only present in sharks, skates, rays, and primitive bony fishes.

Scalation pattern from dorsal to ventral for Shovelnose Sturgeon (Weisel 1978)
Head of Shovelnose Sturgeon showing protrusible mouth and barbels (Weisel 1979).
Papillae on barbel of Shovelnose Sturgeon. (Weisel 1979)

Diagram of viscera of the Shovelnose Sturgeon (Weisel 1979).
Like many large riverine fishes, the Shovelnose Sturgeon relies on movement through a mosaic of different habitats as they grow and complete their life cycle (Phelps et al. 2017).   Quinton Phelps, Assistant Professor at West Virginia University, recorded one Shovelnose Sturgeon that moved over 1,000 miles in a single year and others that traveled more than 500 miles in one year.   Therefore, long, undammed and multiple rivers are essential to sustain our remaining Shovelnose Sturgeon populations. These rivers are working rivers that drain industrial and agricultural watersheds and serve as navigation channels for commercial barge traffic.  Habitat degradation, river connectivity, industrial contaminants (PCBs), and entrainment in tow barge prop wash remain contemporary concerns for viability of Shovelnose Sturgeon populations.  Old Spade Face just might live on for many more millions of years.

Koch, J.D., and M.C. Quist. 2010. Current status and trends in shovelnose sturgeon (Scaphirhynchus platorynchus) management and conservation.  Journal of Applied Ichthyology 26:491-498.
Phelps, Q., S.J. Tripp, M.J. Hamel, J. Koch, E.J. Heist, J.E. Garvey, K.M. Kappenman, and M.A.H. Webb.  2017.  Status of knowledge of the Shovelnose Sturgeon (Scaphirhynchus platorynchus, Rafinesque, 1820).  Journal of Applied Ichthyology 32(Suppl. 1):249-260.
Ray, J.M., C.B. Dillman, R.M. Wood, B.R. Kuhajda, and R.L. Mayden. 2007: Microsatellite variation among river sturgeons of the genus Scaphirhynchus (Actinopterygii: Acipenseridae): a preliminary assessment of hybridization. Journal of Applied Ichthyology 23:304– 312.
Weisel, G.F, 1978. The integument and caudal filament of the Shovelnose Sturgeon, Scaphirhynchus platorynchus.  The American Midland Naturalist 100:179-189.
Weisel, G.F. 1979. Histology and the feeding and digestive organs of the Shovelnose Sturgeon.  Copeia 1979:518-525.