(Pikes and mudminnows)
Class: Actinopterygii
Order: Esociformes
Number of families: 1 or 2
Evolution and systematics
Three key questions to consider regarding the evolution of esociforms are: When did they originate? What other group of fishes is their closest relative? And, what are the evolutionary relationships among esociform species?
As of 2002, the earliest evidence of esociform fishes in the fossil record was in the form of fragmentary fossils found in deposits dating to the Upper Cretaceous from Alberta, Canada. The evidence consists of numerous fossilized fragments of bones that form the mouth skeleton, which displays a distinctive form of tooth attachment that is characteristic of living esociform species. Almost complete fossils of esociform fishes are found in Paleocene deposits from Alberta and Eocene deposits from Wyoming, United States. These fossils are extremely similar to living species of Esox. The fossil evidence suggests that esociform fishes belong to an ancient fish lineage that is more than 70 million years old.
Unfortunately, scientists have a poor understanding of the relationship of esociforms to other fish groups of similar age. The many primitive anatomical features that characterize esociforms hinder the development of a satisfactory theory on the classification of esociforms, despite a long history of phylogenetic studies on this area of fish evolution. Systematic studies based on anatomical comparisons have yielded tentative conclusions based on scant evidence. Studies based on comparisons of genetic sequences have produced the most strongly supported hypothesis of esociform relationships. These studies show unequivocal support for a close evolutionary relationship between the fishes in the order Salmoniformes (i.e., salmons, trouts, chars) and the esociforms. However, a consensus hypothesis is not available.
The order Esociformes includes four genera and 10 to 12 species, depending on the validity of two species described in 1982 from Siberia. The four genera of Esociformes are:
- Dallia (Alaska blackfish or dogfish)
- Esox (Northern pike, muskellunge, and pickerels)
- Novumbra (Olympic mudminnow)
- Umbra (European, central, and eastern mudminnows)
Until the late 1990s, esociform genera were divided into two families: Esocidae, for the genus Esox; and Umbridae, for the remaining three genera, which are collectively termed the mudminnows. This classification conflicts with molecular data first reported in 2000, which showed that these two families do not represent natural evolutionary groups. The molecular evidence suggests that the genera Esox and Novumbra are the closest evolutionary relatives, and that Umbra is the most primitive of the four esociform genera. In light of this evidence, the traditional two-family division of esociforms should be disregarded. A one-family scheme is followed in this chapter but, as of 2002, there was no general agreement on the classification of esociform genera.
Physical characteristics
One striking characteristic of all living esociforms is the posterior placement of the dorsal fin, which is located nearly opposite the anal fin in all species. Esociforms have elongate bodies with round cross sections. Esox species are further characterized by a flattened and elongated snout, which somewhat resembles a duck's bill.
Most species are small or medium sized; but Esox includes very significant exceptions. For example, individuals of E. masquinongy may reach over 5 ft (1.6 m) in length and weigh more than 66 lb (30 kg). At the other end of the scale, individuals of the four species in Novumbra and Umbra rarely reach 4 in (10 cm) and weigh less than 1 oz (28.3 g). In light of their marked contrast in size, it is interesting to note that Esox and Novumbra are thought to be very close relatives.
The coloration of esociforms is varied, but markings or mottled patterns on brown or olive green backgrounds are a common feature. A mottled coloration pattern in the densely vegetated habitats that esociforms favor likely hinders their detection by prey and/or predators. Males of some species show intensified coloration during the breeding season, with attractive iridescent hues on the body and fins.
Distribution
Living species of esociforms are found in all major Northern Hemisphere landmasses, with the exception of Greenland. North America is home to eight species of esociforms, three are found in northern Asia, and two inhabit European waters. The fossil record shows that the diversity of species of esociforms in Europe and Asia has been higher in the past. However, as of 2002, there was no evidence to indicate major changes in the global distribution of the order over its evolutionary history.
There have been changes in the distribution of some esociform species due to human intervention. The ranges of species in Novumbra and Umbra have undergone attrition due to habitat destruction. In contrast, the natural distributions of some Esox species have been expanded by human intervention because of their value as sport fishes.
Habitat
The understanding of the biology of esociforms is much more extensive for members of the genus Esox because these species are important in recreational fishing. However, the few studies conducted on the biology of the other esociform species point to broad similarities among all members of the order. Adults of the larger species of Esox move freely between shore and open water habitat. All esociforms show a similar preference for still or slow-moving water where dense vegetation allows them a place to hide. Other than dense vegetation, mudminnows seek areas with thick and loose muddy substrate, into which they quickly dive when startled. In addition, mudminnows have modified areas of the gut and swim bladder to extract oxygen from ingested atmospheric air. This allows them to withstand the widely fluctuating oxygen levels that may be associated with the heavy vegetation and rich organic matter substrate characteristic of their habitat.
Behavior
Outside the breeding season, esociforms are solitary and sedentary. They are most often found hovering among vegetation using elegant and economical pectoral and medial-fin movements to remain in place, with occasional pelvic-fin motions to correct body orientation. From this stance, the pikes wait for their prey, which they capture with a fast strike. Mudminnows may perch on the vegetation or rest on the substrate, eliminating the need for any fin movement. When dissolved oxygen levels are low, mudminnows occasionally swim up to the water surface and gulp air.
Feeding ecology and diet
Esociformes are carnivorous, predatory fishes. All Esox species prey most commonly on fishes, including smaller individuals of their own species. The larger species may also prey on frogs, waterfowl, and small mammals; their diets seem to be limited only by the size of the potential prey item and the opportunity for its capture. The mudminnows and the Alaska blackfish are also carnivorous, but due to their small size their diet consists almost entirely of aquatic invertebrates and, very rarely, juvenile fish.
Adult northern pike and muskellunge have only a few opportunistic natural predators, including bears, otters, and large birds of prey. Juveniles and fry of species of Esox are commonly preyed upon by larger members of the genus and other predatory fishes that share their habitat (e.g., centrarchid basses). The blackfish and the Olympic mudminnow have few predators because they occupy areas with few other species and are often found among thick vegetation that prevents terrestrial predators from targeting them. Pikes, pickerels, and other piscivorous fishes prey on mudminnows of the genus Umbra.
Reproductive biology
Most esociforms spawn early in the spring when water temperatures begin to rise. Some populations must undertake a migration to reach the spawning grounds. There is no evidence of nest building, but there are reports of territoriality around the spawning site in species of Novumbra and Umbra. Spawning most often involves one female and a few to several males. In some species, the males court the females through swimming displays or aggression. Egg deposition is usually preceded by exaggerated swimming motions and side-to-side contact. Eggs may stick to vegetation or drop to the ground. Fry do not receive parental care.
Conservation status
Habitat destruction is the main threat to populations of esociforms. Mudminnows are particularly vulnerable to this threat because they occur in areas with large human populations, and their habitat preference is not compatible with traditional land-development practices. The European mudminnow (Umbra krameri) is classified as Vulnerable by the IUCN, while the Olympic mudminnow (Novumbra hubbsi) is classified as Lower Risk/Near Threatened. Due to the interest of recreational anglers on all species of Esox, management policies and fishing regulations are in place to ensure stable populations.
Significance to humans
The greatest interest of humans in these fishes in modern times is limited to those species, all of which are members of the genus Esox, targeted by recreational anglers. There has been sporadic interest by aquarium enthusiasts in various mudminnows. Historically, some human populations indigenous to Siberia and Alaska have included blackfish in their diet.
Species accounts
Alaska blackfishNorthern pike
Muskellunge
Olympic mudminnow
European mudminnow
Central mudminnow
Resources
Books:Craig, John F., ed. Pike Biology and Exploitation. New York: Chapman & Hall, 1996.
Morrow, James E. "Mudminnows and Blackfish." In The Freshwater Fishes of Alaska. Anchorage: Alaska Northwest Publishing Co., 1980.
Scott, William B., and E. D. Crossman. "Mudminnows: Umbridae." In Freshwater Fishes of Canada. Oakville, Ontario: Galt House Publishing, 1998. ——. "Pikes: Esocidae." In Freshwater Fishes of Canada. Oakville, Ontario: Galt House Publishing, 1998.
Periodicals:Grande, L. "The First Esox (Esocidae: Teleostei) from the Eocene Green River Formation, and a Brief Review of Esocid Fishes." Journal of Vertebrate Paleontology 19 (1999): 271–292.
López, J., T. Pietsch, and P. Bentzen. "Phylogenetic Relationships of Esocoid Fishes (Teleostei) Based on Partial Cytochrome b and 16S Mitochondrial DNA Sequences." Copeia (2000): 420–431.
Martin-Bergmann, K. A., and J. H. Gee. "The Central Mudminnow, Umbra limi (Kirtland), a Habitat Specialist and Resource Generalist." Canadian Journal of Zoology 63(1985): 1,753–1,764.
Mikschi, E., J. Wanzenböck, compilers. "Proceedings of the First International Workshop on Umbra krameri Walbaum, 1792." Annalen des Naturhistorischen Museums in Wien Serie B Botanik und Zoologie 97B (1995): 437–508.
Organizations:Muskies Canada Sport Fishing and Research, Inc.. P.O. Box 814, Station C, Kitchener, Ontario N2G 4C5 Canada. Web site:
Other:"Alaska Blackfish." Alaska Department of Fish and Game, Wildlife Notebook Series [cited February 6, 2003].
"Blackfish: A Cultural Mini-Unit." Alaska Native Knowledge Network [cited February 6, 2003].
"The International Muskie Homepage." [cited February 6, 2003].
"Understanding Northern Pike and Muskie." The Content Well [cited February 6, 2003].
[Article by: Juan Andrés López, MS]
