A CLADISTIC ANALYSIS OF THE ARTHROPODS: CHELICERATES, CRUSTACEANS, AND UNIRAMIDS, IN ORDER TO DETERMINE THEIR PHYLETIC CLASSIFICATION.

Kristi Koch, Laura Lancieri, Julie Merkle, Sarah Wise.  EVERYBODY LOVES FUNGI

ABSTRACT

 We performed a cladistic analysis of 25 characters to determine how the chelicerates, crustaceans, and uniramids should be classified, possibly under 1 phylum, Arthropoda, or in 2 or more phyla.  We chose 7 genera to analyze from each of the groups chelicerates, crustaceans, and uniramids.  We selected Cephalofovea, Peripatus, Macrobiotus, and Milnesium as out-groups.  We determined that the three groups did not share enough similar characteristics to classify them together under Phylum Arthropoda.  The chelicerates evolved separately from the crustaceans and uniramids and should be grouped in their own phylum, Chelicerata.  However, it was determined that the crustaceans and uniramids share enough cladistically important characters to classify them together in the phylum we named Mandibulata.

 INTRODUCTION

 Buchsbaum (1938), Storer and Usinger (1965), Maddison (1995), and Berryman (1999) present the arthropods as a monophyletic group under Phylum Arthropoda.  This classification is based on the presence of paired jointed appendages, a segmented exoskeleton, and a ventral nerve chord.  Until the late 1970’s, this was a very common classification (Scheswohl et. al, 2001).  

 More recently though, Holt (2000), Manton (1977), Margulis and Schwartz (1998), and Scheswohl et. al (2001) propose polyphyletic classifications where the 3 groups chelicerates, crustaceans, and uniramids are separated into 3 separate phyla.  According to Colin Tudge (2000), the controversy surrounding the classification of arthropods is based on whether the groups’ shared features have been inherited from a common arthropod ancestor or have become comparable through convergent evolution.  Therefore, all taxonomic schemes for the arthropod taxa are currently being debated.

 Colin Tudge (2000) classifies the chelicerates in their own phylum, Chelicerata, and includes the crustaceans, myriapods, and insects in 1 phylum named Mandibulata.  Holt (2000), Margulis and Schwartz (1998) and Manton (1977) separate Phylum Chelicerata, Phylum Crustacea, and Phylum Uniramia based on characters that are currently considered more cladistically significant than those used for previous classifications.

 This paper is designed to determine, using cladistic analysis, how the arthropods, uniramids, chelicerates, and crustaceans, should be classified.  We hypothesize that the chelicerates, uniramids, and crustaceans should be combined into 1 phylum based upon shared characteristics.

 MATERIALS AND METHODS

Organisms Examined-

Table 1 lists the 25 genera used to examine the phylogenetic taxonomy of the groups chelicerates, uniramids, and crustaceans.  Of the 25 genera, we selected 7 from the group chelicerates, 7 from the group uniramids, and 7 from the group crustaceans.  We used Cephalofovea and peripatus 2 onychophorans, and Macrobiotus and Milnesium, 2 tardigrades, as out-groups.  Out-groups were used to distinguish plesiomorphic characters from the cladistically important characters, which are synapomorphic.  Out-group selection was based on presumed phylogenetic relationships, according to Margulis and Schwartz (1998).

Table 1.  This table shows the genera that were used in making the cladistic analysis.  An asterisk designates the out-groups.  It also shows the phylum to which each genus is assigned, according to the taxonomy of Holt (2001).  The genera were examined and their character states described according to the following references, as well as direct observation.  These sources are listed in the table: B=Bremner (1997), C=Cloudsley-Thompson (1968), D=Direct Observation, G=Gray (2002), H=Holt (2002), HR=Hopkin and Read (1992), HU=Hubble (1993), J=Jackman (1998), L=Lyon (2001), M=Manton (1977), MI=Miller (1999), MS=Margulis and Schwartz (2001), O=Olive (2001), SC=Schmitt (1965), S=Smith (2001), SM=Smith (1982), W=Waterman (1960), WA= Waggoner (1994), WR=Wright (2000)

 Characters Examined-

 Table 2 shows the characters and the character states used to describe the genera.  The 21 characters chosen dealt with the development, reproduction, physiology, structure, and ecology of the organisms examined and was based on the importance to the chelicerates, crustaceans, and uniramids.  We organized the characters into these five categories and weighted each accordingly.  We weighted developmental, reproductive and physiological characters with a higher importance because we expect them to be more highly conserved than structural and ecological characters.  Structural and ecological characters can more easily change or become lost through evolutionary changes.  Therefore, developmental, reproductive and physiological characters were given a weight of 2 and structural and ecological characters were given a weight of 1.

 Table 2.  This table shows the 21 characters used in the cladistic analysis of the taxa in Table 1, as well as the category in which each character was placed and its corresponding weight.  Each character is broken down into the state(s) that each genus exhibits. 

Construction of the Phylogram-

We constructed Figures 1, 2, and 3 using MacClade 4.0 (Maddison and Maddison, 2000), from the genera and their appropriate characters and character states as mentioned in Tables 1 and 2.  We weighted the developmental, reproductive, and physiological characters with a weight of 2 because we expect them to be more highly conserved among the taxa than the structural or ecological characters, which could more easily be changed or lost through evolution.  Using 25 genera and 21 characters, we compiled 3 equally parsimonious trees (Figures 1–3) of length 43.

RESULTS

Figure 1 has a tree length of 43 and shows 2 distinct clades, A and B.  Clade A contains the groups chelicerates, crustaceans and the onychophorans. The onychophorans did not behave as an out-group because they were included in the same clade as the chelicerates and crustaceans.  Some of the uniramids are represented in Clade B.  The genera Spirobolus and Pterotermes, both uniramids, are unresolved in this cladogram.

Figure 2 has a tree length of 43 and shows 3 distinct clades, A, B, and C.  Genera from both of the out-groups and the chelicerates are seen in clade A. The out-groups do not behave as out-groups, since they are included among the chelicerates.  Clade B includes the crustaceans.  Clade C contains the uniramids, except for Spirobolus, which is unresolved. 

Figure 3 has a tree length of 43 and shows 2 distinct clades, A and B.  The chelicerates are seen together in Clade A.  The crustaceans and the uniramids are shown as two separate groups within Clade B.  The out-groups behaved as out-groups because they are not included with any of the chelicerate, crustacean, or uniramid taxa.

                                                            A                                                              B

Figure 1: This cladogram was constructed using MacClade 4.0 (Maddison and Maddison, 2000).  The number of character changes between genera is indicated by the legend on the right.  This cladogram shows 2 distinct clades: A and B.  Clade A shows the chelicerates, the crustaceans, and the onychophorans, which is an out-group, together in the same clade.  Clade B shows some of the uniramids.  The genera, Spirobolus and Pterotermes, are unresolved.

                               A                                                  B                                      C

Figure 2: This cladogram was constructed using MacClade 4.0 (Maddison and Maddison, 2000).  The number of character changes between genera is indicated by the legend on the right.  This cladogram shows 3 distinct clades: A, B, and C.  Clade A shows the tardigrades and onychophorans, the two out-groups, together in the same clade as the chelicerates.  The crustaceans are seen in Clade B.  Most of the uniramids are seen in Clade C, except Spirobolus, which is unresolved. 

                                            A                                                      B

Figure 3: This cladogram was constructed using MacClade 4.0 (Maddison and Maddison, 2000).  The number of character changes between genera is indicated by the legend on the right.  This cladogram shows 2 distinct clades: A and B.  Clade A shows the chelicerates.  Clade B contains both the crustaceans and the uniramids. The out-groups were not included in any of these clades.

DISCUSSION

 Proposed Phylogeny-

 Figures 1-3 do not support our monophyletic hypothesis because the chelicerates, crustaceans, and, uniramids are not grouped together into 1 clade.  None of the figures support the polyphyletic hypothesis, separating the 3 arthropod groups into 3 phyla. 

 Figure 1 shows 2 clades, A and B.  Clade B contains the uniramids, except for Spirobolus and Pterotermes, which are unresolved.  The onychophorans did not behave as an out-group because they are included in Clade A with the chelicerates and crustaceans.  Therefore, we reject this figure. 

 Figure 2 shows 3 clades, A, B, and C.  Clade C contains the uniramids, except for Spirobolus, which is unresolved.  The out-groups are shown with the chelicerates in Clade A.  Since the onychophorans and tardigrades did not behave as out-groups we reject this figure.

 Figure 3 shows 2 clades, A and B.  The onychophorans and tardigrades behave as out-groups because they do not emerge with the arthropod taxa.  Clade B shows the crustaceans and uniramids evolving together.  The cladogram shows that these 2 groups have a common ancestor, which was a segmented animal with mandibles. They split into two groups within Clade B following the emergence of uniramous appendages and one pair of antennae in the uniramids from the crustaceans, which have biramous appendages and two pair of antennae.  The chelicerates are represented in Clade A.  The separation of the chelicerates from the groups in Clade B shows them evolving separately with the emergence of chelicerae and no antennae.  The cladogram does not support our monophyletic hypothesis, but rather shows the emergence of the arthropods from two evolutionary lines. 

 Proposed Taxonomy-

 Through an analysis of Figure 3, we propose that the chelicerates should be in their own phylum, Chelicerata, while the crustaceans and the uniramids should be united into 1 phylum, we named Mandibulata.  Our proposed taxonomy agrees with that proposed by Colin Tudge (2000).  This taxonomy does not agree with Holt (2000), Margulis and Schwartz (1998), or Manton (1977), who place the chelicerates, uniramids, and crustaceans into 3 separate phyla.  Our proposed taxonomy also differs from Buchsbaum (1938), Storer and Usinger (1965), and Maddison (1995), who place the arthropod groups into 1 phylum.

 The following is a listing of our proposed taxonomy.  We determined that the chelicerates should be classified as a separate phylum, Chelicerata, while the crustaceans and uniramids should be classified together under Phylum Mandibulata.

Phylum Chelicerata

             Nymphon

             Limulus

 Lactrodectus

             Ixodes

             Lycosa

 Scorpio

             Chelifer

Phylum Mandibulata

             Cambarus

             Homarus

             Cancer

             Pagurus

             Euphausia

             Artemia

 Milnesium

 Drosophila

 Anopheles

             Limenitis

             Pediculus

             Pterotermes

             Scolopendra

             Spirobolus

LITERATURE CITED

Berryman, Alan A. 1999. Phylum Arthropoda. http://classes.entom.wsu.edu/348/arthropoda.htm accessed 3 February 2002.

Bremner, Jim F. 1997.  Black widow spiders. http://www.desertusa.com/july97/du_bwindow.html accessed 2 February 2002.

Buchsbaum, Ralph. 1938. Animals Without Backbones. The University of Chicago Press, Chicago.

Cloudsley-Thompson, J.L. 1968. Spiders Scorpions Centipedes & Mites. Pergamon Press Ltd., London.

Gray, Mike. 2002. Wolf Spiders. http://www.amonline.net.au/factsheets/wolf_spiders.htm accessed 5 February 2002.

Holt, Jack R. 2000. Supplement: a classification of the animal kingdom and descriptions of phyla. Journal of Systematic Biology 7(1): http://comenius.susqu.edu/bi/202/Animals/default.htm  accessed 2 February 2002.

Hopkin, Stephen and Helen Read. 1992. The Biology of Millpedes. Oxford University Press, New York.

Hubbell, Sue. 1993. Broadsides From the Other Orders. Random House Inc., New York.

Jackman, John, R. 1998. Scorpions. http://entowww.tamu.edu/extension/bulletins/L-1678.html accessed 5 February 2002.

Lyon, W.F. 2001. Black widow spider. http://ohioline.osu.edu/hyg-fact/2000/2061A.html accessed 4 February 2002.

 Maddison, David R. and Wayne P. Maddison, 1995. Arthropoda. http://tolweb.org/tree/phylogeny.html accessed 5 February 2002.

 Manton, S.M. 1977. The Arthropoda. Oxford University Press, New York.

 Margulis L and K. Schwartz. 1998. Five Kingdoms, An Illustrated Guide to the Phyla of Life on Earth 3^rd edition. W.H. Freeman and Co. New York.

 Miller, William, R. 1997. Tardigrades: bears of the moss (about tardigrades). The Kansas Naturalist 42(3): pages not given.

 Myers, Philip. 1995. Phylum Arthropoda. http://animaldiversity.ummz.umich.edu/arthropoda.html accessed 3 February 2002.

 Olive, Peter. 2001. Reproduction and Life Cycles in Invertebrates. Nature Publishing Group. Macmillan Publishers Ltd., England.

 Scheswohl, Danielle, Brenda Clewell, Katherine Sauder, Brandon Ziegler, Travis Zook. 2001. Monophyly versus polyphyly in the superphylum arthropoda.  Journal of Systematic Biology 8 (1):  http://comenius.susqu.edu/bi/202/journal.htm  accessed 3 February 2002.

 Schmitt, Waldo L. 1965. Crustaceans. The University of Michigan Press and Ambassador Books Limited, United States.

 Smith, R.L. 1982. Pseudoscorpions. http://ag.arizona.edu/urbanipm/scorpions/pseudoscorpions.html accessed 3 February 2002.

 Smith, Vince. 2001. A rough guide to lice.  http://darwin.zoology.gla.ac.uk/~vsmith/index_guide.html accessed 6 February, 2002.

 Tudge, Colin. 2000. The Variety of Life. Oxford University Press Inc., New York.

 Waggoner, Ben. 1994. Introduction to the pycnogonida. http://www.ucmp.berkeley.edu/arthropoda/pycnogonida.html accessed 5 February 2002.

 Waterman, Talbot H. 1960. The Physiology of Crustacea. Academic Press, New York and London.

 Wright, Jonathan C. 2000. Onychophora (Velvet Worms). Nature Publishing Group. Macmillan Publishers Ltd., England.