Sturgeons (Acipenseridae) are some of the largest fish in lake environments and brackish waters. Given their peculiar appearance, seemingly-armored bodies, and their crepuscular nature, they are still fairly mysterious. Some species have extremely limited distributions, with strains that exist in just one or two freshwater systems on earth. Most favor cool, dark depths where they can go about their daily activities undisturbed.
Unlike the scary, giant creatures of open seas, sturgeons are far from threatening. If anything, they are more like the freshwater equivalents of whale sharks. ‘Gentle giant’ is a better term for them as they usually keep to themselves on the lowermost reaches of the water column. Humans and most pelagic fish need not worry about being attacked, but other bottom feeders do. The challenge in their feeding strategy lies in their being toothless, yet this doesn’t wholly delimit prey choices!
The stark similarity of the sturgeon morphology to those of primitive Acipenseriformes highlights their capacity to survive under natural pressures. For these living fossils, maybe being toothless isn’t so bad after all! Long-lived, they have evolved to persist on most types of prey that they can lodge in between their jaws. Despite looking a bit like sharks due to their cartilaginous profiles, they don’t feed like them at all.
Mouth Morphology of Sturgeons
Regardless of size or species, the mouth morphology of sturgeons is largely conserved. As they are all toothless at maturity, they must rely on the expansion of their jaws, which determines their maximum mouth gape. The size of their gape influences what types of prey they can feed on. This means relatively small sturgeons, with consequently smaller mouths, can be grown alongside large pond fish (e.g. jumbo koi, smallmouth bass).
Sturgeons are distinguished by their hard, pointy snouts. Their mouths are found on the ventral region of their heads. When closed, the “lip” forms an inverted U-shape or semicircle, much like a frown. The cranial morphology of Acipenseriformes allows their jaws to move independently of their neurocranium. This has resulted in specialized, protrusible mouths.
The oral organs of adults are significantly more differentiated compared to those of juvenile sturgeons. Note that, as bibliographic data is fairly scarce for most sturgeon species, there may be unreported discrepancies. In general, the region above the upper lip in adults is protected by a hardened and lubricated tissue layer. This protects the fish from getting wounded while feeding. Ampullae, which contain some taste buds, are located in this region. In the oral cavity, more taste buds are found alongside mucus cells on the tongue.
Apart from the tongue, gustatory papillae (likewise equipped with tastebuds) are located throughout the oral surfaces. Taste buds tend to be more widespread in juveniles compared to adults. One important thing to note – there is an exception to sturgeon “toothlessness”! Before the mouth has fully differentiated, those of the larvae and juveniles of some species may have tiny, canine-like teeth on the oral floor. These disappear upon maturity.
Feeding Mechanism of Acipenseridae
A combination of complex sense-organs and evolutionary adaptations has allowed sturgeons to efficiently detect, capture, and process live-caught food. The presence of barbels and mechanoreceptors on their snouts compensate for reduced visual acuity. As benthophagous animals that tend to feed in the evening, they rely on these organs to detect external stimuli. These allow them to perceive subtle vibrations of potential prey.
Sturgeons tend to move toward zones with the highest concentration of recognizable vibrations. Their search is narrowed down with the aid of electroreceptors in their ampullae. These can supposedly detect short-range and low-frequency electrical signals. The barbels and external taste buds then evaluate the suitability of prey. Suction feeding, which involves a strong influx of water into the mouth, is used to capture desired prey.
Protrusion of the upper jaw of sturgeons increases their mouth gape and allows their lips to quickly come into contact with a target. Depression of the lower jaw, which is mostly composed of cartilage, likewise increases peak gape. There is evidence that water flow is generated into the mouth while feeding, causing the prey to be “sucked” towards it even before the jaws have fully opened. Once the prey is drawn into the oral cavity, the jaws quickly snap together to secure it.
A Gizzard-Like Organ in Sturgeon Stomachs
You might find yourself wondering how a sturgeon can properly digest its food without teeth. It turns out the wall of its stomach is hypertrophied to a gizzard-like organ that helps break down organic matter. This organ has a velvety texture and is made of thick muscles. Situated in the second region of a U-shaped stomach, it mechanically processes prey after they are transported through a relatively short digestive tube.
The Sturgeon Diet
The predominant components of the sturgeon diet are largely influenced by prey availability in their habitats. As they are benthic feeders, they usually favor macroinvertebrates. Prey need to be adequately sized relative to their mouths so that they can be consumed whole. Crustaceans, shellfish, and small fish along bottom substrates are highly desirable. Sturgeons require high-protein food sources to mature normally and live out their remarkably lengthy lifespans.
Larger sturgeon species may occasionally feed on protein-rich pelagic fish, such as salmonids. Smaller species tend to search for insect larvae, aquatic insects, snails, leeches, and sludge worms. They’ll also eat fish fry and eggs if these are accessible. A study on the diet of Acipenser sturio (the European sturgeon) collected from an estuary, found that polychaetes comprised the highest proportion of their stomach contents. This was followed by crustaceans.
In contrast, the stomach contents of Acipenser transmontanus (white sturgeon) collected from a river showed that Amphipoda was the most dominant food group consumed. The second most dominant prey type was fish. Preferred prey can be highly variable depending on the species and its environment. Studies on diet are quite difficult to carry out due to the dwindling number of wild sturgeons and their protected status.
Feeding Sturgeon in Ponds
The sturgeon’s calm nature has permitted smaller species to be reared without aggression-related issues in ornamental ponds or large aquariums. They can be trained to feed on pellets that float to the pond or tank floor. These need to have a high-protein component that makes up at least 40% of the formulation.
Pond sturgeons should not be relied upon to feed on naturally occurring prey items as these may be present in such small quantities. This applies to those in any artificially developed habitat. Moreover, their food consumption has to be monitored, especially in the presence of large fish that may compete with them for food. For detailed information, consult our feed guide on sturgeon feeding frequency and food types.
