1 Abstract
2 The Paddlefish Polyodon spathula is a ram suspension feeding planktivore that resides in fresh
3 water habitats. While there are numerous hypotheses about the external morphology of the Paddlefish,
4 none truly support a definite finding.
5 Context and Content
6 The Paddlefish Polyodon spathula is classified as the following: Kingdom: Animalia, Phylum:
7 Chordata, Class: Actinopterygii, Order: Acipenseriformes, Family: Polyodontidae, Genus: Polyodon,
8 Species: spathula (Ross and Brenneman 2001, Starnes 2016). The order Acipenseriformes includes the
9 sturgeons (family Acipenseridae) and the paddlefishes (family Polyodontidae; Ross and Brenneman
10 2001). The Chinese Paddlefish, Psephurus gladius, is the only other species in the family Polyodontidae
11 (Wilkens and Hofmann 2007) and resides exclusively in China’s Yangtze River (Ross and Brenneman
12 2001). There are a variety of common names associated with the species such as Paddlefish, Spadefish,
13 Spoonbill Catfish, or Duck-Billed Catfish (Ross and Brenneman 2001). Walbaum portrayed the Paddlefish
14 as a shark in 1792; but was corrected by Lacepedѐ in 1797, who classified Paddlefish in the genus
15 Polyodon (McKinley 1984).
16 General Characteristics
17 Paddlefish are an ancient species and exhibit less derived traits. They are typically characterized
18 by the presence of heterocercal caudal fins and the absence of teeth in adults (Ross and Brenneman
19 2001). Polyodon means ‘many teeth’, while Adult Paddlefish do not have teeth, but do have many gill
20 rakers which they use to filter zooplankton from the water column (Ross and Brenneman 2001) and
21 spathula means ‘flat piece’, referring to the long snout, or rostrum, which makes up around one third of
22 the total body length of adults (Wilkens and Hofmann 2007). For the most part Paddlefish have a
23 smooth shark-like exterior appearance and possess a small amount of scales located on the operculum 24 and the bottom lobe of the tail. The Paddlefish skeleton consists mostly of cartilage with the jaws being
25 the only exception (Ross and Brenneman 2001).
26 Distribution
27 Ross and Brenneman (2001) documented Paddlefish in the: Noxubee River system, Pearl
28 drainages of the Gulf of Mexico Basin, and areas of the Mississippi River Basin. Paddlefish have also been
29 documented throughout the Licking, Tennessee, Mississippi, Ohio, Kentucky, and Cumberland rivers and
30 as far north as the Great Lakes (Burr 1980).
31 Form and Function
32 The main theory pertaining to the rostrum of Paddlefish is that it is used as an electrosensory
33 apparatus for locating plankton in aquatic habitats (Wilkens and Hofmann 2007) While earlier writers
34 suggested that the Paddlefish used the elongate rostrum to dig into the benthic layer to locate items for
35 nourishment, later scientific research confirmed that they feed primarily on plankton (Ross and
36 Brenneman 2001).
37 Paddlefish total length is often measured using eye-fork length, taken from the eye to the fork
38 of the tail (Ross and Brenneman 2001). The rostrum of fingerlings and smaller fish account for most of
39 the body length which may increase error if measuring total length from the paddle to the fork of the
40 tail. Eye-fork length (EFL) is the best method for determining an accurate total body length (Ross and
41 Brenneman 2001).
42 Ontogeny and Reproduction
43 Paddlefish journey upstream each spring to spawn. Upon locating areas having an average water
44 temperature of 12ᵒC, Paddlefish spawn over coarse gravel (Ross and Brenneman 2001). The timing for
45 spawning varies with location as more northern populations mature later and have differing thermal 46 regimes. There appears to be a positive relationship between seasonally warming waters and adult
47 Paddlefish abundance (Miller et al 2011), suggesting spawning depends on water temperature.
48 Initially, eggs float, but once the fertilization process is completed the eggs sink, sticking to hard
49 surfaces for up to one week. Eggs hatch when the water temperature ranges from 18-21ᵒC (Jennings
50 and Zigler 2000). Maturity rates seem to vary by location, with males tending to mature at age six (Ross
51 and Brenneman 2001). In most cases females mature later in life around age nine, although a study
52 concluded that half matured by age nine and half matured at age ten (Reed et al 1992).
53 Conservation
54 Paddlefish roe provides a domestic alternative to purchasing expensive imported sturgeon
55 caviar (Clarke-Kolaks et al 2009). However, increased poaching to harvest eggs, driven by the
56 compensation fishers receive for roe, has led to rising concerns Paddlefish conservation. In 2006, fishers
57 could earn as much as $22.50 per pound of roe harvested (Bettoli et al 2009). Although this may be an
58 ongoing problem, management plans have established guidelines for legal harvest regulations, such as
59 regulating areas where harvest is still permitted to prevent over-exploitation. As different habitats can
60 support varying numbers of fish, limits must also vary according to current population status. A
61 commercial harvest of Paddlefish was permitted during the summer of 2006 in Mississippi. Possessing a
62 Paddlefish that was over thirty inches in length was prohibited and the catch limit was two fish per day
63 (Bettoli et al 2009).
64 Paddlefish were plentiful in the Mississippi River drainage before 1900 (Jennings and Zigler 2000
65 However, Paddlefish are listed as vulnerable by the International Union for Conservation of Nature
66 (IUCN) (Grady 2004), largely due to habitat alteration. Paddlefish movement is negatively impacted by
67 the construction of dams as they increase the difficulty of moving upstream to spawn. Increasing river
68 modification, could significantly reduce spawning success and ultimate result in population decline. 69 Coker (1928) participated in a study to determine whether Paddlefish were injured while passing
70 through the Keokuk Dam located on the upper Mississippi River. The results found that Paddlefish were
71 in fact harmed more often than other species of fish, but Paddlefish were able to move through the
72 system at times unharmed (Coker 1928). River modification can result in loss and degradation of
73 spawning habitats due to the destruction of passageways. Jennings and Zigler (2000) cite river
74 modification (along with pollution and over-exploitation) as they discussed environmental issues relating
75 to declines in Paddlefish abundance. Following modification, Paddlefish populations may be confined to
76 an area, resulting in negative impacts on population size, increased competition for resources, and
77 increased rates of disease transmission and mortality. Piers and other channel construction, while not
78 obstructive, may result changes in Paddlefish movement. Wilkens and Hofmann (2007) noted the
79 Paddlefish avoided structures containing steel, perhaps due to abnormal electric signals. Fish in
80 laboratory trials also avoided metal structures. (Wilkens and Hofmann 2007).
81 The Paddlefishes along with sturgeons, are protected by the Convention on International Trade
82 in Endangered Species of Wild Fauna and Flora. This organization was established in order to prevent
83 over-exploitation of any species due to international trade. As extensive concern for Paddlefish
84 populations began to grow people began to get involved with the conservation of the species, as of
85 March 1992 Paddlefish were listed within the Appendix II section titled “Class Actinopterygii” (Jennings
86 and Zigler 2000; CITES 2015). Currently in the state of Mississippi, the Paddlefish is not considered an
87 endangered species, but is listed by Mississippi Natural Heritage Program as a species of special concern
88 (MNHP 2015; Ross and Brenneman 2001).
89
90 Behavior 91 Paddlefish feed on plankton using a method known as ram suspension feeding in which they
92 open their mouths and continuously swim forward, forcing water to flow through their gill rakers, which
93 filter out plankton (Ross and Brenneman 2001). These fish happen to be the only filter feeders in the
94 family Polyodontidae and have adapted specifically for this method of feeding, as their gill arch and jaw
95 configuration produces a wider gape that allows for successful filter feeding (Wilkens and Hofmann
96 2007).
97 Acknowledgements
98 Upon completion of this paper I would like to acknowledge Jason Bies for being a wonderful
99 mentor and teacher throughout the species review. Also, Dr. Mike Colvin who assisted in providing me
100 with a substantial amount of resources and information that led to the successful completion of literary
101 research to make this paper possible.
102 Literature Cited
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