Bowfin

The ruddy bowfin is a ray-finned fish native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being one of only two surviving species of the Halecomorphi, a group of fish that first appeared during the Early Triassic, around 250 million years ago. The bowfin is often considered a "living fossil" because they have retained some morphological characteristics of their early ancestors. It is one of two species in the genus Amia, along with Amia ocellicauda, the eyespot bowfin. The closest living relatives of bowfins are gars, with the two groups being united in the clade Holostei.
Bowfins are demersal freshwater piscivores, commonly found throughout much of the eastern United States, and in southern Ontario and Quebec. Fossil deposits indicate Amiiformes were once widespread in both freshwater and marine environments across North and South America, Europe, Asia, and Africa. Now, their range is limited to much of the eastern United States and adjacent southern Canada, including the drainage basins of the Mississippi River, Great Lakes, and various rivers exiting in the Eastern Seaboard or Gulf of Mexico. Their preferred habitat includes vegetated sloughs, lowland rivers and lakes, swamps, and backwater areas; they are also occasionally found in brackish water. They are stalking, ambush predators known to move into the shallows at night to prey on fish and aquatic invertebrates such as crawfish, mollusks, and aquatic insects.
Like gars, bowfin are bimodal breathers—they have the capacity to breathe both water and air. Their gills exchange gases in the water allowing them to breathe, but they also have a gas bladder that serves to maintain buoyancy, and also allows them to breathe air by means of a small pneumatic duct connected from the foregut to the gas bladder. They can break the surface to gulp air, which allows them to survive conditions of aquatic hypoxia that would be lethal to most other species. The bowfin is long-lived, with age up to 33 years reported.

Description

The typical length of a bowfin is ; females typically grow to, males to. They can reach in length, and weigh. Young of the year typically grow to by October. Females tend to grow larger than males.
The body of the bowfin is elongated and cylindrical, with the sides and back olive to brown in color, often with vertical bars and dark reticulations or another camouflaged pattern. The dorsal fin has horizontal bars, and the caudal fin has irregular vertical bars. The underside is white or cream, and the paired fins and anal fin are bright green. During larval stage, hatchlings from about total length are black and tadpole-like in appearance. At approximately total length they have been described as looking like miniature placoderms. They grow quickly, and typically leave the nest within 4 to 6 weeks after hatching. Young males have a black eyespot on the base of the tail that is commonly encircled by an orange-yellowish border, while the female's is black, if present at all. It is thought the purpose of the eyespot is 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 that runs from the middle of the back to the base of the tail.
The skull of the bowfin is made of two layers of skull, the dermatocranium and the chondrocranium. The chondrocranium layer cannot be seen because it is located below the dermal bones. The bowfin skull is made up of 28 fused bones, which compose the dermatocranium. The roof of the mouth is made up of three bones, the ectopterygoid, the palantine, and the vomer. They have two sets of teeth, including one set of larger sharp teeth coming out of the mandibular and premaxillary bones to grasp and control the prey. The other set of teeth, located posteriorly and connected to the hyomandibular bone, is made up of pharyngeal tooth patches, which are used for sorting out nutrients and grinding down larger pieces of food. Another three bones make up the lower jaw: the dentary, the angular, and the surangular. The cranial surface of the skull is made up of the nasals, the antorbital, the lacrimal, the parietal, the intertemporal, the post parietal, the supratemporal, the extra scapular, the post temporal, and the opercular. The entirety of the skull is attached to the girdle through another set of bones.
Bowfin are often referred to as "living fossils" or "primitive fish" because they retained some of the primitive characters common to their ancestors, including a modified heterocercal caudal fin, a highly vascularized gas bladder lung, vestiges of a spiral valve, and a bony gular plate. The bony gular plate is located underneath the head on the exterior of the lower jaw between the two sides of the lower jaw bone. Other distinguishing characteristics include long, sharp teeth, and two protruding tube-like nostrils. Unlike all of the most primitive actinopterygians, the scales of bowfin differ in that they are not ganoid scales, rather they are large, single-layered cycloid scales closer in similarity to more derived teleosts.

Fish similar in appearance

s are commonly mistaken for bowfin because of similarities in appearance, most noticeably their elongated, cylindrical shape, and long dorsal fin that runs along their backs. Northern snakeheads are piscivorous fish native to the rivers and estuaries of China, Russia, and Korea that have been introduced and become established in parts of North America. However, unlike bowfin which are native to North America, the northern snakehead is considered an invasive species and environmentally harmful there. Some contrasting differences in bowfin include a black eyespot on their caudal peduncle, a tan and olive coloration, a shorter anal fin, a more rounded head, pelvic fins at a greater distance from the pectoral fins than in the northern snakehead and the presence of the gular plate on the ventral side of the lower jaw. Another noticeable difference is that bowfin scales do not continue uniformly from their body to their head. Bowfin heads are smooth and free of scales, whereas the northern snakehead has scales that uniformly continue from their body through to their head.
The burbot, a predatory 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.

Bowfin body-shape evolution and development

The first fish lacked jaws and used negative pressure to suck their food in through their mouths. The jaw in the bowfin is a result of their evolutionary need to be able to catch and eat bigger and more nutritious prey. As a result of being able to gather more nutrients, Bowfin are able to live a more active lifestyle. The jaw of a bowfin has several adaptations. The maxilla and premaxilla are fused and the posterior chondrocranium articulates with the vertebra which allows the jaw freedom to rotate. The suspensorium includes several bones and articulates with the snout, brain case, and the mandible. When the jaw opens epaxial muscles lift the chondrocranium, which is attached to the upper jaw, while adductor muscles act to close the lower jaw. This ability to open and close the jaw helps the bowfin to be an active predator that can catch bigger prey and digest them.
The vertebral column in bowfin is ossified and in comparison to earlier fish, the centra are the major support for the body, whereas in earlier fish the notochord was the main form of support. In bowfin neural spines and ribs also increase in prominence, an evolutionary aspect that helps provide additional support and stabilize unpaired fins. The evolution of the vertebral column allows the bowfin to withstand lateral bending that puts the column under compression without breaking. This, in turn, allows the bowfin to have more controlled and powerful movements, in comparison to fish that had only a notochord. The bowfin has a rounded heterocercal tail that resembles a homocercal tail. This type of tail gives the body a streamlined shape which allows the bowfin to improve its swimming ability by reducing drag. These types of tails are common in fish with gas bladders, because the bladder supplies the fish with natural buoyancy.
The bowfin is a member of actinopterygii which means that the pectoral girdle is partly endochondral but mostly dermal bone. In this group of fish the fins function to maneuver, brake, and for slight positional adjustments. The pectoral girdle of the bowfin has six parts. The post temporal, supracleithrum, postcleithrum, cleithrum, scapulacoracoid, and the clavicle make up the pectoral girdle. The pectoral girdle is attached to the skull. The paired pectoral and pelvic fins of fish are homologous with the limbs of tetrapods.

Physiology

Bowfin are physostomes, meaning they have a small "pneumatic duct" that connects their swim bladders to their digestive tract. This allows them, like lungfish, to "breathe" in two ways: they can extract oxygen from the water when breathing through their gills, but can also break the water's surface to breathe or gulp air through the pneumatic duct. When performing low-level physical activity, bowfin obtain more than half of their oxygen from breathing air. The fish have two distinct air-breathing mechanisms used to ventilate the gas bladder. Air breathing type I is consistent with the action of exhale / inhale exchange, stimulated by either air or water hypoxia, to regulate gas exchange; type II air breaths are inhalation alone, which is believed to regulate gas bladder volume, to control buoyancy. Bimodal respiration helps bowfin survive and maintain their metabolic rate in hypoxic conditions. Bowfin air breathe more frequently when they are in darkness, and correspondingly more active.
Bowfin blood can adapt to warm, acidic waters. The fish becomes inactive in waters below ; at this temperature they breathe almost no air; however, with increasing temperature their air breathing increases. Their preferred temperature range is between, with the temperature of maximum activity. Air breathing is at a maximum in the range. Bowfin do not use central chemoreceptor regulation for respiration control. Experiments manipulating the oxygen content, carbon dioxide content, and pH of bowfin extradural fluid did not affect breathing rate, heart rate, or blood pressure pointing to a lack of central chemoreceptor regulation. Instead, bowfin respiratory patterns respond to water oxygen content and water temperature, as water temperatures play a role in oxygen content. In the lab, bowfin showed an increase in the breathing rate when the temperatures were raised above 10 °C. Bowfin also showed an increase in breathing rate when exposed to lower oxygen levels in the water.
Herpetologist W. T. Neill reported in 1950 that he unearthed a bowfin aestivating in a chamber below the ground surface, in diameter, 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. Regardless of the lack of evidence confirming the bowfin's ability to aestivate, it has been noted that bowfin can survive prolonged conditions of exposure to air because they have the ability to breathe air. Their gill filaments and lamellae are rigid in structure which helps prevent the lamellae from collapsing and aids gas exchange even during air exposure.