The Bowfin, Amia calva, is a stout, elongate-bodied freshwater fish characterized by a rounded, abbreviated heterocercal caudal fin (a caudal fin where the upper lobe is longer than the bottom, and the vertebrae extend into the upper lobe of the caudal peduncle) (Becker, 1983). Bowfin possess a long soft dorsal fin, which is rapidly undulated for both forward and reverse locomotion (Becker, 1983). Coloration is mottled dark green or brown, with a cream or white belly (Becker, 1983). Males possess sexually dimorphic coloration, with bright green fins, mouth, and tongue, and and a yellow-ringed eyespot on the upper caudal peduncle, a feature retained from juvenile coloration (Burr and Bennett, 2014).The Bowfin’s lateral-line cycloid scales number 63-70 (Becker, 1983).Adults have a maximum weight of 9.8 kg, with lengths up to 109 cm TL recorded (Burr and Bennett, 2014). In captivity, Bowfin have been kept alive as long as 20-30 years, suggesting that bowfin are relatively long-lived fish (Becker, 1983); wild Bowfin have significantly shorter lifespans (Burr and Bennett, 2014).
Bowfin possess terminal mouths lined with canine-like teeth, and are relatively unique in possessing a bony gular plate underneath the lower jaw (Becker, 1983). Other identifying skeletal features include a quadrate and symplectic double articulation of the lower jaw, loss of suborbitals, and ossified vertebral centra (Schultze and Wiley, 1984). Holostei such as Bowfin also possess a unique trait among neopterygians— the presence of open kidney tubules connecting directly to the coelomic cavity— a character common in embryonic teleosts, but not adults (Becker, 1983). Overall, bowfins possess many unique, primitive characters owing to the retention of primitive characteristics not seen in more modern fish clades.
Systematics:
The ancestors of modern gar and Bowfin appeared during the late Paleozoic, and their relatives formed a highly successful sister clade, the teleosts (Becker, 1983). The only species in its genus, family, and order, the Bowfin is the only extant member of the subdivision Halecomorphi, which includes many extinct taxa related to the modern-day bowfin in the family Amiidae (Schultze and Wiley, 1984). Among the extinct amiids, the closest phylogenetically is the genus Kindleia, which shares its long dorsal fin with Amia (Schultze and Wiley, 1984).
Conflicting phylogenies exist for the placement of the halecomorph fishes within the Neopterygii (the clade comprised of teleosts, gars, and Bowfin). The traditional phylogenetic tree has the Holostei as a monophyletic clade including the amiids (Bowfin) and the lepisosteids (gars) as a sister group (Holostei) to the teleosts, while an alternative phylogeny places the amiids and teleosts as sister clades, with lepisosteids as an outgroup (Grande and Bemis, 1998). More recent studies suggest a monophyletic Holostei, with gars and Bowfin as the closest relatives to each other (Grande, 2010).
The extant, or living, bowfin species is a relic of the large species diversity once present in its lineage millions of years ago. About nine genera of fossil amiids exist, with the earliest records found in the Cretaceous (Burr and Bennett, 2014). During the Mesozoic Era, amiiform fishes were highly diverse and made up a significant amount of fish fauna abundance and biodiversity; however, the amiiforms were still less morphologically diverse than other contemporaneous fish groups, such as the highly diverse and more adaptable fishes known as pycnodontiforms, restricting the spread of amiiforms compared to more ecologically and morphologically plastic related groups (Martin-Abad and Poyato-Ariza, 2013).
The oldest fossil record of the bowfin family comes from limestones in Solnhofen, Germany, with some primitive amiids even occupying marine environments before making a transition to freshwater (Martin-Abad and Poyato-Ariza, 2013). Other fossil records of fishes closely related to A. calva include a piscivorous Cretaceous amiiform, Calamopleurus, from Brazil (Mulder, 2013) and Sinamia kukurihime, a Cretaceous amiiform found in Japan (Yabumoto, 2014).
Habitat & Range: Bowfin prefer heavily vegetated and low-lying lakes, marshes, and slow-flowing rivers, often with higher water clarity (Becker, 1983). In the Midwest, bowfin occur in the Mississippi, Lake Superior, and Lake Michigan watersheds in connected tributaries and glacial lakes (Becker, 1983), with most of its distribution in North America restricted to the central and south-eastern U.S. and Canada (Schultze and Wiley, 1984). The populations in Minnesota are at the far-western edge of its geographic range, with few records farther west of Minnesota (Becker, 1983, p. 252). Within the state, the species occurs mainly in the southern half of the state, not extending farther north than Lake Superior (Becker, 1983, p. 252). Amia species throughout the Cenozoic occupied a much broader range across North America than the currently eastern-limited distribution of A. calva seen today (Schultze and Wiley, 1984).Food:
A piscivorous diet in the bowfin order Amiiformes dates back to at least the Cretaceous (Mulder, 2013). Bowfin are opportunistic apex predators that utilize a variety of food sources, including aquatic invertebrates, fishes, amphibians, birds, and reptiles (Burr and Bennett, 2014). Young Bowfin prey mainly on aquatic invertebrates such as chironomids, odonates, and zooplankton (Burr and Bennett, 2014). Bowfin use suction feeding to capture their prey, with capture events taking as little as 0.075 seconds (Burr and Bennett, 2014). Feeding in Bowfin most often occurs at dawn, dusk, and night (Burr and Bennett, 2014). Despite its varied diet and high level of feeding behavior, Bowfin are capable of surviving prolonged periods of starvation, with one study finding a living Bowfin after a year of food deprivation (Becker, 1983).
The piscivorous diet of Bowfin has led to their consideration as controllers of prey fish populations. One study conducted in Lake Winona, Minnesota found that while Bowfin predated bluegills, they were not an effective manager of bluegill populations due to a combination of low reproductive rates in the lake, low predation rates in extremely vegetated areas, and low feeding preference for sunfish (Mundahl, 1998). The same study found that sunfish were never the preferred prey of Bowfin over cyprinids or crayfish (Mundahl, 1998).
Reproduction:
Bowfin become sexually mature between 2-4 years old for males, and 2-5 years for females (Burr and Bennett, 2014). Bowfin spawn nocturnally in early spring and summer, when water temperatures are 16-19 degrees Celsius, with southern populations beginning spawning as early as February (Burr and Bennett, 2014). Males clear vegetation with their mouths, excavating a deep trough with a sandy or gravelly bottom (Burr and Bennett, 2014). Male Bowfin perform complex courtship behaviors that can take several hours involving biting, touching, and circling the female, and often taking her snout in his mouth before continuing (Burr and Bennett, 2014). Both sexes can spawn with multiple partners during the breeding season, with males often fighting each other over female mating rights (Burr and Bennett, 2014).
Bowfin eggs are yellowish and color, oval-shaped, and adhesive, clinging to the vegetation around the nest area (Burr and Bennett, 2014). Once hatched, the young stay adhered to vegetation until they reach a length of around 12 mm (Becker, 1983). Bowfin are the only North American primitive fishes to provide care to young, the males displaying territoriality and defending free-swimming young (Burr and Bennett, 2014). The young form a polarized swarm that the male defends until the offspring are approximately 10 cm TL, with guarding behavior decreasing once the young are 3.5 cm TL (Burr and Bennett, 2014). Male Bowfin have even been known to lunge at larger animals, including humans, when protecting their young (Becker, 1983). This protective behavior represents a single evolutionary origin for male parental care within the amiiform lineage (Ah-King et al., 2005). The origin of male parental care in the amiiform lineage represents one of only 22 independent evolutions of male care within fishes, in which territorial behavior like the Bowfin’s is common (Ah-King et al., 2005).
Other Interesting Facts:
Air-breathing is present in Bowfin, with the optimum temperature for this behavior occurring at 18.4-29.6 degrees C, and very little or no air breathing at 4.4-10 C (Becker, 1983). There have been several unconfirmed reports in the literature of Bowfin estivating in underground chambers similar to the air-breathing lungfishes (McKenzie and Randall, 1990). Estivation is a process similar to hibernation seen in lungfishes, where the fish shuts off most of its metabolic functioning and sits in suspended animation in a muddy burrow until hydrated again. Successful estivation requires a reduction in oxygen consumption and metabolism paired with the ability to tolerate or breakdown harmful waste product buildup that can’t be excreted.
More recent studies have called anecdotal observations of Bowfin estivation into question. One study found that oxygen consumption and metabolism in the Bowfin remained unchanged after gradual exposure to air, and that toxic ammonia built up in the fish’s system (McKenzie and Randall, 1990). Based on this study, which found an inability to detoxify nitrogenous compounds and no slowing of oxygen consumption or aerobic metabolism, there is no evidence to suggest that Bowfin are capable of successful estivation for any prolonged period (McKenzie and Randall, 1990).
Conservation & Economic Importance:
Bowfin are common throughout their natural range, and currently show no need for conservation measures, likely due to the limited commercial value they possess (Burr and Bennett, 2014).
They are rarely used as a food fish, with most anglers considering their flesh poor eating (Burr and Bennett, 2014). Bowfin eggs are occasionally sold as a cheaper alternative to sturgeon caviar, with a price of $80/kg of eggs in 2008 (Burr and Bennett, 2014). Bowfin are taken in large numbers by commercial fishers on the Mississippi, with over 2,900 kg of fish harvested over several decades in Wisconsin and a 1975 value of $0.04 per kilogram (Burr and Bennett, 2014). Bowfin are also widely used as lab dissections, and can be kept in home aquaria (Burr and Bennett, 2014). Although considered for many years an undesirable pest by many anglers, in recent years some fisheries managers have considered the possibility of Bowfin control of gamefish populations (Burr and Bennett, 2014). However, the ability of Bowfin to effectively manage and control centrarchid populations is limited in heavily vegetated areas and areas and/or in waters with a diversity of other prey choices (Mundahl et al., 1998).
Resources:
Ah-King M., Kvarnemo C., and Tullberg B.S. The influence of territoriality and mating system on the evolution of male care: a phylogenetic study on fish. 2005. Journal of Evolutionary Biology 18: 371-382.
Becker, G. C. 1983. “Bowfin Family— Amiidae.” In: Fishes of Wisconsin. 249-254. University of Wisconsin Press, Madison.
Burr, B.M.. and Bennett M.G. 2014. “Chapter 8: Amiidae: Bowfins.” In: Freshwater Fishes of North America Volume 1: Petromyzontidae to Catostomidae, edited by, Burr, B. M. and Warren Jr., M.L. 279-298. Johns Hopkins University Press.
Grande, L. and Bemis, W. E. 1998. A comprehensive phylogenetic study of amiid fishes (Amiidae) based on comparative skeletal anatomy: an empirical search for interconnected patterns of natural history. Journal of Vertebrate Paleontology 18(1): 1-696.
Grande, L. 2010. An empirical synthetic pattern study of gars (Lepisosteiformes) and closely related species, based mostly on skeletal anatomy: the resurrection of Holostei. American Society of Ichthyologists.
Martin-Abad, H. and Poyato-Ariza, F. J. Historical patterns of distribution in Pycnodontiform and Amiiform fishes in the context of moving plates. 2013. Geologica Belgica 16(4): 217-226.
McKenzie, D. J. and Randall, D. J. Does Amia calva aestivate? 1990. Fish Physiology and Biochemistry 8(2): 147-158.
Mulder, E. On the piscivorous behaviour of the Early Cretaceous amiiform neopterygian fish Calamopleurus cylindricus from the Santana Formation, northeast Brazil. 2013. Netherlands Journal of Geosciences 92: 119-122.
Mundahl, N. D. et al. Effectiveness of bowfin as a predator on bluegill in a vegetated lake. 1998. North American Journal of Fisheries Management 18: 286-294.
Schultze, H. P. and Wiley, E. O. 1984. “The Neopterygian Amia as a Living Fossil.” In: Living Fossils, edited by Eldredge, N. and Steven, M. S., 153-154. Springer, New York.
Yabumoto, Y. Sinamia kukurihime, a new Early Cretaceous amiiform fish from Ishikawa, Japan. 2014. Paleontological Research, 18(4) : 211-223