Bowfins (Amia calva) are basal bony fishes related to gars in the Superorder, Holostei. Common names include mudfish, mud pike, dogfish, griddle, grinnel, cypress trout and choupique. They are regarded as taxonomic relicts, being the last surviving members of both the order Amiiformes (which includes three additional, now-extinct families dating from the Jurassic to the Eocene), and of the family Amiidae which contains numerous species in about four subfamilies, only one of which, Amiinae, is extant. They are often improperly referred to as "primitive fishes" because they have retained some morphological characters common with ancestors of modern fish.
Bowfins are demersal freshwater piscivores commonly found in much of the eastern United States, and in southern Ontario and Quebec. They inhabit the drainage basins of the Mississippi River, Lake Superior and Lake Michigan. Their preferred habitat includes vegetated sloughs, lowland rivers and lakes, swamps, backwater areas, and they are also occasionally found in brackish water. They are stalking, ambush predators that will move into the shallows at night to prey on fish, and aquatic invertebrates such as crawfish, mollusks, and aquatic insects.
Like the distantly related gar fishes, bowfins are bimodal breathers which means they have the capacity to breathe both water and air. Their gills exchange gases in the water allowing them to exploit oxygen for breathing, but they also have a vascularized swim bladder lung that not only serves to maintain buoyancy, it allows them to breathe air through a small duct connected from the esophagus to the gastrointestinal tract. They break the surface to gulp air, thus surviving conditions of aquatic hypoxia where most other species cannot survive.
Description and biology
The average length of a bowfin is 50 cm (20 in); females typically grow to 65–70 cm (26–28 in), males to 50–65 cm (20–26 in). Records indicate bowfins can reach 109 cm (43 in) in length, and weigh 9.75 kg (21.5 lb). Young of the year typically grow to 13–23 cm (5.1–9.1 in) by October. Females tend to grow larger than males.
The body of the bowfin is elongated and cylindrical, with the sides and back olive in color, often with dark reticulations. The underside is white or cream. Bowfins have large scales that are technically ganoid, but actually very similar to cycloid scales of Teleostei. The paired fins and anal fin are bright green. Males have a black "eye spot" on the base of the tail (caudal peduncle) that is commonly encircled by an orange-yellowish border. It is thought to confuse predators, deflecting attacks away from the head of the fish to its tail, which affords the bowfin an opportunity to escape predation. The bowfin is so named for its long, undulating dorsal fin consisting of 145 to 250 rays, and running from the middle of the back to the base of the tail.
Bowfins are often referred to as "living fossils", and sometimes inaccurately described as "primitive" due to the retention of some primitive features of ancestral predecessors, including a modified (rounded externally) heterocercal caudal fin, a swim bladder lung, vestiges of a spiral valve, and a bony gular plate underneath the head. A distinctive feature of bowfin is the gular plate, a bony plate located on the exterior of the lower jaw between the two sides of the lower jaw bone. Other characteristics include long, sharp teeth, and two protruding tube-like nostrils.
Northern snakeheads (Channa argus) are invasive piscivores fishes that are native to the rivers and estuaries of China, Russia, and Korea. They are commonly mistaken for bowfin because of their similarities in appearance. Bowfins and snakeheads have elongated, cylindrical bodies, a long dorsal fin that runs along their backs, round tails, and sharp teeth. Both species are bimodal breathers, (can breathe both air and water), are capable of aestivation, and are found in similar habitats. Some contrasting differences in bowfins include a black "eye spot" on their caudal peduncle, a tan and olive coloration, a shorter anal fin, a more rounded head, and an upper jaw that is longer than its lower jaw. The burbot, a predacious fish native to streams and lakes of North America and Eurasia, is also commonly mistaken for bowfin. Burbots can be distinguished by their flat head and chin barbel, long anal fin, and pelvic fins situated beneath the pectoral fins.
Bowfins are capable of bimodal respiration, being able to breathe air by utilizing their gas bladder as a lung. When performing low-level physical activity, they obtain more than half of their oxygen consumption from breathing air. Bowfins have two distinct air-breathing mechanisms used to ventilate the gas bladder. Type I air breaths are consistent with the action of exhale-inhale stimulated by aerial or aquatic hypoxia to regulate O2 gas exchange; type II air breaths are by inhalation alone which is believed to regulate gas bladder volume for buoyancy control. Bimodal respiration helps bowfin survive and maintain their metabolic rate in hypoxic conditions. The rate of air breating is higher in darkness, coincidental with the higher activity of the fish.
Bowfin blood can adapt to warm, acidic waters. The fish becomes inactive in waters below 10 °C (50 °F); at this temperature they practically do not breathe air. With increasing temperature the air breathing increases. Their preferred temperature range is between 12–26 °C (54–79 °F), with 18 °C (64 °F) the temperature of maximum activity. Air breathing is maximum in the 18.4–29.6 °C (65.1–85.3 °F) range.
Herpetologist Wilfred T. Neill, reported in 1950 that he unearthed a bowfin aestivating in a chamber 4 inches (10 cm) below the ground surface, 8 inches (20 cm) in diameter, .25 miles (0.4 km) from a river. It was further noted that flood levels had previously reached the area, and receded. It is not unusual for riverine species like bowfin to move into backwaters with flood currents, and become trapped when water levels recede. While aestivation is anecdotally documented by multiple researchers, laboratory experiments have suggested instead that bowfin are physiologically incapable of surviving more than three to five days of air exposure. However, no field manipulation has been performed.
Habitat and feeding
Bowfins are usually not easy to spot, living in clear, slow water with abundant vegetation, preferring places with roots, logs and other types of shelter. Oxygen-poor environments can be tolerated because of its ability to breathe air.
Bowfins are indiscriminate predators that readily prey on a broad variety of arthropod and vertebrate prey, from insects and crawfish to other fish and frogs.They ambush and stalk their prey, and are remarkably agile. Young fish feed mostly on young crustaceans, while older fish (longer than 10 cm or 3.9 in) are mostly piscivorous. The long dorsal fin is used to propel silently through water up to the last inches, when the prey is then quickly engulfed by the mouth, with a movement that lasts only 0.075 seconds - this last part of the attack is similar to that of gars, but it attacks on the front rather than on the side. Its feeding habits are preferentially nocturnal.
Bowfins spawn in the spring or early summer, typically between April and June, more commonly at night in abundantly vegetated, clear shallow water in weed beds over sand bars, and also under stumps, logs, and bushes. Optimum temperatures for nesting and spawning range between 16–19 °C (61–66 °F). The males construct circular nests in fibrous root mats, clearing away leaves and stems. Depending on the density of surrounding vegetation there may be a tunnel-like entrance at one side. The diameter of the nests commonly range between 39–91 cm (15–36 in), at a water depth of 61–92 cm (24–36 in).
During spawning season, the fins and underside of male bowfins often change in color to a bright lime green. The courtship/spawning sequence lasts one to three hours, and can repeat up to five times. Courtship begins when a female approaches the nest. The ritual consists of intermittent nose bites, nudges, and chasing behavior by the male until the female becomes receptive, at which time the pair lie side by side in the nest. She deposits her eggs while he shakes his fins in a vibratory movement, and releases his milt for fertilization to occur. A male often has eggs from more than one female in his nest, and a single female often spawns in several nests.
Females vacate the nest after spawning, leaving the male behind to protect the eggs during the eight to ten days of incubation. A nest may contain 2,000 to 5,000 eggs, possibly more. Fecundity is usually related to size of the fish, so it isn't unusual for the roe of a large gravid female to contain over 55,000 eggs. Bowfin eggs are adhesive, and will attach to aquatic vegetation, roots, gravel, and sand. After hatching, larval bowfin do not swim actively in search of food. During the seven to nine days required for yolk-sac absorption, they attach to vegetation by means of an adhesive organ on their snout, and remain protected by the parent male bowfin. Bowfins aggressively protect their spawn from the first day of incubation to a month or so after the eggs have hatched. When the fry are able to swim and forage on their own, they will form a school and leave the nest accompanied by the parent male bowfin who slowly circles them to prevent separation.
A common parasite of bowfin is the anchor worm (Lernaea). These small crustaceans infest the skin and bases of fins, with consequences ranging from slowed growth to death. The mollusk Megalonaias gigantea lays eggs in the bowfin gills, that are then externally fertilized by sperm passing in the water flow. The small glochidia larvae then hatch and develop in the gill tubes.
The bowfin is considered an evolutionary indicator, a window to the fossil past of bony fishes. Based on a mitogenetic perspective on the phylogeny of "ancient fish", the basal actinopterygians comprise four major lineages, including Polypteriformes, Acipenseriformes, Lepisosteidae, and Amia calva. Following the radiation of basal actinopterygians, fossil records indicate the evolution of a new lineage of ray-finned fishes in the Late Permian Epoch which grew to prominence in the Mesozoic and Cenozoic Eras.
Neopterygians are the second major occurrence in the evolution of ray-finned fishes, and are distinguished from their earlier ancestors by major changes to the jaws, shape of the skull, and tail. Neopterygians include four main groups of fishes:
- the semionotids (now extinct) appeared in the Permian Period, and were small, streamlined swimmers that occupied freshwater and marine habitats;
- the lepisosteids which include extant species of garfishes that first appeared in the Cretaceous,
- the bowfins (halecomorphids) the only extant species in the order Amiiformes which date back to the Triassic Period, and
- the stem group of Teleostei from which modern fish arose, including most of the bony fish we are familiar with today.
As a sport fish, bowfin are not considered desirable to many anglers. They were once considered a nuisance fish by anglers and early biologists who believed the bowfin's predatory nature was harmful to sport fish populations. As a result, efforts were taken to reduce their numbers. Research has since proven otherwise, and that knowledge together with a better understanding of maintaining overall balance of ecosystems, regulations were introduced to help protect and maintain viable populations of bowfins. Bowfin are strong fighters, a prized trait in game fish. However, they do have a jaw full of sharp teeth which requires careful handling. The current tackle record is 21.5 lb (9.8 kg)
Bowfin were once considered to have little commercial value because of its poor tasting meat which has been referred to as "soft, bland-tasting and of poor texture". However, it is considered quite palatable if cleaned properly and smoked, or prepared fried, blackened, used in courtbouillion, or in fishballs or fishcakes. Over the years, global efforts have imposed strict regulations on the international trade of caviar, particularly on the harvest of sturgeons from the Caspian Sea where the highly prized caviar from the beluga sturgeon originates. The bans imposed on Caspian sturgeons have created lucrative markets for affordable substitutes in the United States including paddlefish, bowfin, and various species of sturgeon. In Louisiana, bowfin are harvested in the wild, and cultured commercially in hatcheries for their meat and roe. The roe is processed into caviar, and sold as "Cajun caviar", or marketed under the trade name "Choupiquet Royale".
Accumulation of toxic substances
In some areas of the United States where aquatic environments have tested positive for elevated levels of toxins, such as mercury, arsenic, chromium, and copper, there are posted signs with warnings about the consumption of fish caught in those areas. Concentration of mercury biomagnifies as it passes up the food chain from organisms on lower trophic levels to apex predators. It bioaccumulates in the tissues of larger, long-lived predatory fishes. When compared to smaller, short-lived fishes, bowfin tend to concentrate mercury at higher levels thereby making them less safe for human consumption.
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