The Gulf sturgeon, Acipenser oxyrinchus desotoi, is one of seven species of sturgeon found in North America, and the only one to appear off the Texas Gulf Coast. It is a subspecies of the Atlantic sturgeon, Acipenser oxyrinchus. As you might guess, Gulf sturgeon and Atlantic sturgeon are very similar in appearance. In fact, the only significant morphological difference is the length of their spleen (Gulf sturgeons have a mean spleen length versus fork length measurement of 12.3 percent while Atlantic sturgeon’s is only 5.7 percent). Both species are part of an ancient lineage dating back to the Triassic period, some 245 to 208 million years ago. Sturgeon are one of the oldest lines of actinopterygian fishes, a class of bony fishes known as the ray-finned fishes because their fins are webs of skin supported by bony or horny spines (‘rays’). Actinopterygii are the largest and most successful group of fishes and make up half of all living vertebrates. Sturgeon have evolved very little since the Late Cretaceous period, 100.5 to 66 million years ago, earning them the informal status of living fossils. They are unique among bony fishes, along with other members of their subclass Chondrostei, because their skeleton is almost entirely cartilaginous. This might lead you to believe they should be classified with sharks and rays, but the cartilaginous skeleton is a derived feature, not a primitive one. Sturgeon ancestors had bony skeletons.
Gulf sturgeon are anadromous, meaning they are born in freshwater rivers, migrate to marine environments, and return to those freshwater rivers to spawn. They are generally found from the Mississippi River in Louisiana, east to the Suwannee River in Florida, but sporadic occurrences have been recorded as far west as the Rio Grande River in Texas and Mexico. Typically they spend late fall through early spring in bays, estuaries, and the nearshore Gulf. During that time, they hang out in shallow shoals (5 to 7 feet), deep holes near passes, unvegetated sand habitats such as sandbars, and intertidal and subtidal energy zones. The warm summer months are spent in freshwater. Gulf sturgeon are truly gargantuan fish, growing up to eight feet long and weighing up to 200 pounds. Females grow larger than males. Rows of bony plates, called scutes, armor their back and head. The snout is V-shaped and blade-like with four fleshy barbels. The vacuum-like mouth lacks teeth and is positioned on the lower surface of the snout. Similar to sharks, Gulf sturgeon have heterocercal tails, meaning one lobe is larger than the other (the upper lobe, in the sturgeon’s case). Adults are usually dark brown on top, fading to a creamy white underneath.
Sturgeon are opportunistic and indiscriminate benthivores (bottom-feeders) that change their diets and foraging areas during different life stages. They feed like a vacuum – sucking up amphipods, isopods, crabs, grass shrimp, lancets, brachiopods, mollusks, small fish, and polychaete worms – and have a special spiral valve digestive system to help them absorb food and other nutrients. (The spiral valve is a screw-like, symmetrical shape within a portion of the small intestine. It adds surface area for digestion and absorption to an otherwise relatively short intestine.) Sturgeons feed non-visually. They detect prey with their barbels – which have olfactory, tactile, and chemosensory sensors – and with their ampullae of Lorenzini, a type of electroreceptor located in the head that are sensitive to weak electric fields generated by other animals. Adults only feed from fall to spring when they reside in the Gulf; they appear to fast during the summer when they are in freshwater. In freshwater habitats, Gulf sturgeon have very few predators due to their size and body armor. Alligators may feed on them occasionally. In the Gulf of Mexico, sharks may be potential predators.
Although not intentionally dangerous to humans, there are incidents of Gulf sturgeon leaping out of the water and injuring boaters. Due to its large size and heavy body armor, some injuries have been significant. All sturgeon species will jump at times. Gulf sturgeon can jump six feet out of the water, but why they leap is still a mystery. Many speedy, near-surface predators (such as billfish, tuna, and tarpon) jump, but sturgeon are bottom dwellers. There are several theories as to why they leap, including group communication, escape from predators, shedding parasites, shedding eggs (during spawning), gulping or expelling air, courtship displays, or simply that it “feels good.”
Hatched in freshwater rivers, Gulf sturgeon head out to sea as juveniles, and return to the rivers of their birth to spawn when they reach adulthood. Individuals don’t necessarily spawn every spring. Some may only spawn once every five years. Temperature, flow, and pH must be optimum to induce spawning. If conditions aren’t quite right for a successful spawn, sturgeons will skip that spawning season, often waiting years before spawning again. When they do spawn, sturgeon make a noise similar to a creaky door hinge. Spawning occurs over bedrock, boulders, or gravel bottoms. Fertilization is external. Eggs are demersal and adhesive. Eggs are about 2 millimeters in diameter and vary in color from gray to brown to black. After hatching, juvenile Gulf sturgeon generally disperse, most heading downstream, some traveling upstream. They stay in the river for the first two or three years and then move to the estuary where they forage until they reach sub-adult size, three to four feet. Average growth is just under ten inches per year for the first two to five years, and about three inches per year to the age of eight. It takes seven to twelve years for males to sexually mature, and eight to sixteen for females. They can live for as long as sixty years, but the average lifespan is twenty to twenty-five. Not only do the sturgeon’s spawning habits make them susceptible to fishing pressure, but the long generation lifespan means that it takes time for populations to recover. Gulf sturgeon also appear to be river-specific spawners, meaning once a specific river’s spawning population dies out, there’s almost no chance of it being naturally repopulated by sturgeon from another river system. Additionally, they are unable to climb fish ladders like salmon can, so dams, weirs, reduced water flow, and other forms of habitat loss all threaten the survival of the species. With a relatively small and widely scattered population, the Gulf sturgeon is classified as Vulnerable on the IUCN Red List.
Historically, sturgeon populations were greatly reduced by overfishing because of the high value of their roe used to produce caviar; their flesh for smoking; and as a source of isinglass, a semi-transparent gelatin prepared from the swim bladder, used in jellies, special cements, glues, parchment preservation, and clarification of wine and beer. Now that rampant overharvest has been stopped, the main threats to survival are habitat degradation, pollution, dredging, and climate change. Contamination from industrial, agricultural, and municipal activities is believed to cause a variety of impacts, including muscle atrophy; abnormal gonad, sperm, egg, and larval development; organ mutations; tumors; and disruption of hormone production. The dredging of river channels can destroy or suffocate sturgeon eggs and can also affect the quality, quantity, and availability of prey, since they’re bottom feeders. Global climate change may cause changes in habitat, such as saltwater intrusion, water temperature fluctuations, exacerbation of dead zones, and extreme weather periods that increase both the frequency and intensity of droughts and floods. As one of the oldest lineages of living fish, perhaps the sturgeon will adapt to some of these challenges. Or perhaps they’ll go the way of the dinosaurs they lived alongside so long ago. But I hope not.
**Fun Fact: In the UK, sturgeon are classified as a "royal" fish - a status granted by King Edward II. The law decrees that every sturgeon caught belongs to the Treasury and has to be offered to the monarch. This means that the Queen has to be consulted before anything is done with one.**
Where I learned about sturgeon, and you can too!
US Fish & Wildlife Service
IUCN Red List
Gulf States Marine Fisheries Commission
Florida Fish & Wildlife Conservation Commission
Florida Natural Areas Inventory
University of Southern Mississippi: Gulf Coast Research Laboratory
Alabama Dept of Conservation & Natural Resources
Convention on International Trade in Endangered Species of Wild Fauna & Flora
The Diversity of Fishes: Biology, Evolution, and Ecology
By Gene Helfman, Bruce B. Collette, Douglas E. Facey, Brian W. Bowen