Polyodon spathula (American paddlefish) is currently found in 22 states that are part of the Mississippi River drainage (Mims, 2001). American paddlefish distribution is now restricted to this system of large, slow-moving rivers. In the past, P. spathula was located in 4 more states in this drainage, the Great Lakes, and Canada (Graham, 1997). The many dams that have been added throughout the Mississippi River drainage have limited the ranges of many populations of these fish (Wills, 1993). Due to these unnatural blockages, migratory breeding behavior has been disrupted, and many areas previously sustaining paddlefish must be annually stocked by local conservation agencies (Graham, 1997). (Graham, 1997; Mims, 2001; Wills, 1993)
American paddlefish have also been introduced into the lower Danube River, in the Balkans region of Europe, as sportfish. (Vassilev, 2006)
American paddlefish are mainly freshwater fish but can survive in brackish water (Billard, 2001). They generally reside in large rivers with deep water (greater than 6 meters) and slow moving currents (less than 5 cm/s) (Zigler, 2003; Southall, 1984). Turbid (muddy) water is preferred by P. spathula. During migratory breeding events, P. spathula needs access to areas with sand or gravel bars (Wills, 1993). (Billard and Lecointre, 2001; Southall and Hubert, 1984; Wills, 1993; Zigler, et al., 2003)
American paddlefish are clearly distinguishable from other North American fish by the presence of an extended snout, or rostrum. This rostrum is covered with electroreceptors to locate zooplankton and facilitate migratory behavior. American paddlefish are large, reaching maximum lengths of up to 2.5 meters and weighing from 18 to 70 kg. Males are generally larger than females (Wilkens, 2002). Large gill rakers are present in these fishes for zooplankton consumption from turbid waters (Russell, 2002). They also have a cartilaginous skeleton, heterocercal tail, and lack scales (Wills, 1993). (Russell and Neiman, 2002; Wilkens, et al., 2002; Wills, 1993)
After fertilization, developing American paddlefish may be seen through their transparent roe, or eggs. In 24 hours the notochord develops, and a heartbeat is apparent by day four. By the fifth day, young P. spathula hatch and begin their larval stage of life. They are then carried somewhere downstream by slow moving river currents (Wills, 1993). Once gill rakers fully form, juvenile paddlefish can effectively filter feed (Mims, 2001). Female American paddlefish are not fully mature until they reach 12 to 14 years of age and males are mature at ages of 6 to 7 years (Wills, 1993). (Mims, 2001; Zigler, et al., 2003)
Paddlefish are broadcast spawners whereby multiple males swim near and release milt onto the eggs liberated by a female. The eggs are very sticky and thus adhere to a substrate such as gravel or sand (Wills, 1993). A spawning migration occurs within the Mississippi River system making paddlefish a potamodromous species. During the peak of the breeding season in the spring, large shoals of male and female fish form in specific breeding areas to spawn (Billard, 2001). (Billard and Lecointre, 2001; Wills, 1993)
Female American paddlefish are very particular about when they will release their eggs for reproduction. It has been noted that spawning generally occurs only every 2 to 3 years based upon certain environmental stimuli. At a specific spring photoperiod, there must be a rise in the water level of the river accompanied by a water temperature of approximately 55 to 60 degrees Fahrenheit (Wills, 1993). (Wills, 1993)
Beyond the production of roe and milt, female and male American paddlefish provide nothing in the form of parental investment. The yolk sack of larval padlefish is consumed after hatching which may be considered a form of pre-fertilization provisioning by the female (Wills, 1993). These fishes are an excellent example of a species with a life history strategy designed to maximize success by producing many more eggs than will survive, rather than providing parental care. (Wills, 1993)
American paddlefish are relatively long-lived, they may live up to 55 years. The average lifespan as estimated in dentary studies seems to be about 20 to 30 years (Wills, 1993). (Wills, 1993)
Relatively little is known of P. spathula behavior towards conspecifics or members of different species. More is known about American paddlefishes physiological behavior in terms of feeding and respiration. American paddlefish can often be seen swimming around with their very large mouths wide open. This is a behavior that allows the fish to filter feed and ventilate their gills (ram ventilation) at the same time (Burggren, 2003). It is also known that American paddlefish swim in a fairly primitive manner involving undulation of nearly the entire body (Wills, 1993). (Burggren and Bemis, 1992; Wills, 1993)
Polyodon spathula ranging behavior has been inferred from radio tagging experiments. These show that paddlefish generally restrict their movement to a certain home range, but that they frequently stray from this area, especially during spring breeding seasons (Zigler, 2003; Jennings, 1993). During the breeding season, American paddlefish migrate upstream to gravel or sand bars, but they generally never leave the freshwater of the Mississippi River basin (Wills, 1993) (Jennings and Wilson, 1993; Wills, 1993; Zigler, et al., 2003)
The large rostrum (paddle) of P. spathula is covered with electroreceptors (Russell, 2002). This paddle is essentially a highly sensitive antenna used to gather information about the surrounding environment through changing electrical fields. This electric sense is used by American paddlefish to locate prey and successfully migrate during spawning seasons (Wilkins, 2002). This system is so sensitive that juveniles can locate single zooplankton from up to 9 cm away by sensing the miniscule electrical pulses given off by the animal’s muscle contractions (Wilkins, 2002; Wills, 1993). It is unclear how P. spathula individuals communicate during spawning, but they may use a combination of visual and tactile cues. (Russell and Neiman, 2002; Wilkens, et al., 2002; Wills, 1993)
Polyodon spathula is a faunivore specialized for filter feeding. As described above, American paddlefish use electroreceptors to locate zooplankton in turbid water (Wilkins, 2002). Examples of animals in the P. spathula diet are copepods, cladocerans such Daphnia pulex, and ephemeropteran nymphs (Hoxmeier, 1997). Polyodon spathula possesses huge gill rakers along with jaws that are anatomically independent from the neurocranium (Carroll, 2003). These features allow for a larger surface area to be filtered by American paddlefish. (Carroll and Wainwright, 2003; Hoxmeier and DeVries, 1997; Wilkens, et al., 2002)
It is speculated that the large size and extended life span of P. spathula is an adaptation to avoid predation. Producing high concentrations of paddlefish offspring in a single season may be a predator satiating mechanism used to maximize the chances of survival for a smaller, but significant proportion of P. spathula. When American paddlefish are in their larval stage they are easy prey for many different birds and fishes, but at maturity their only real predators are humans (Wills, 1993). (Wills, 1993)
American paddlefish are predators of zooplankton and prey to other fishes, birds, and humans. Also, silver lampreys have been found to use American paddlefish as hosts (Cochran, 2004). Beyond these relationships, relatively little is known of the role of American paddlefish in their ecosystem. (Cochran and Lyons, 2004)
Perhaps to the benefit of humans and the detriment of American paddlefish, these fish are highly valuable in many ways. Due to their large size and succulent flesh, P. spathula has long been utilized as a source of meat. Also, being closely related to sturgeons (family Acipenseridae), American paddlefish produce highly coveted roe, or caviar. In the 1980’s a trade embargo of Iranian imports severely limited the amount of caviar that was imported to the United States from the Caspian Sea. As the demand for caviar increased, American paddlefish suffered heavy population losses as the illegal acquisition of their roe became a highly profitable endeavor (Wills, 1993). (Wills, 1993)
Beyond P. spathula being a source of caviar, their skin is thick, scaleless, strong, and tans well, making it a marketable product. There is also a growing interest in the immune system of American paddlefish. Despite their extremely simple immune system, they rarely, if ever, get cancer due to their cartilaginous skeleton. Cartilage prohibits blood flow to cancerous cells that is necessary for their growth, thereby stopping the spread of cancer. Therefore, American paddlefish have been a useful and promising test subject in the field of aquatic pharmacology (Wills, 1993). (Wills, 1993)
Since American paddlefish are extremely beneficial to humans in many ways, they are currently being farm raised in the U.S. (Mims, 1999), and as far away as Russia, Romania, and Moldavia (Vedrasco, 2001). Most of the fish at these farms are produced for their valuable meat and caviar (Vedrasco, 2001). (Mims and Shelton, 1999; Vedrasco, et al., 2001)
There are no known adverse affects of P. spathula on humans.
Polyodon spathula populations are most threatened by dams throughout the Mississippi River basin. These cause a separation of American paddlefish populations which limits gene flow and thus genetic variability. Dams also prevent the natural migratory spawning behavior of American paddlefish (Wills, 1993). Due to their highly valuable meat and roe, P. spathula has been over-harvested in the past. This has lead to more regulations on paddlefish harvesting, and many states now actively stock areas of river with American paddlefish (Graham, 1997). This species is presumed to be extirpated from the state of Michigan. (Graham, 1997; Wills, 1993)
Polyodon spathula was first described by J. Walbaum in 1792 (Walbaum, 1792). (Walbaum, 1792)
Tanya Dewey (editor), Animal Diversity Web.
John Jerome (author), University of Michigan-Ann Arbor, William Fink (editor, instructor), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
areas with salty water, usually in coastal marshes and estuaries.
uses smells or other chemicals to communicate
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
uses electric signals to communicate
fertilization takes place outside the female's body
union of egg and spermatozoan
a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.
A substance that provides both nutrients and energy to a living thing.
mainly lives in water that is not salty.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
Animals with indeterminate growth continue to grow throughout their lives.
referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
makes seasonal movements between breeding and wintering grounds
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an animal that mainly eats plankton
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
uses sight to communicate
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
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