An Updated Classification of the Recent Crustacea

and Boxshall (1996, lichomolgoid copepods),
Moura and Christoffersen (1996, ‘‘mandibulate’’
arthropods), Wilson (1996, isopods), Ahyong
(1997, stomatopods), Emerson and Schram (1997,
all arthropods), Hanner and Fugate (1997, branchiopods),
Spears and Abele (1997, several major
groups, review), Tshudy and Babcock (1997,
clawed lobsters), Tudge (1997b, anomurans), Walossek
and Müller (1997, Cambrian crustaceans and
their bearing on crustacean phylogeny), Wheeler
(1997, arthropods including crustaceans), Wills
(1997, all Crustacea), Jenner et al. (1998, hoplocarids),
Olesen (1998, conchostracans and cladocerans),
Schram and Hof (1998, all major groups,
extant and extinct), Shen et al. (1998, spelaeogriphaceans),
Strausfeld (1998, crustacean neurological
features), Taylor et al. (1998, mysidaceans and
other peracarids), Tucker (1998, raninoid crabs),
Wheeler (1998, all arthropod groups), Wills et al.
(1998, fossil and extant arthropod groups), Almeida
and Christoffersen (1999, pentastomids), Cumberlidge
and Sternberg (1999, freshwater crabs),
Huys and Lee (1999, laophontoidean harpacticoid
copepods), Sternberg et al. (1999, freshwater
crabs), Olesen (1999b, leptostracans), Spears and
Abele (1999b, crustaceans with foliaceous limbs;
2000, branchiopods), Walossek (1999, major crustacean
groups), Edgecomb et al. (2000, all major
arthropod groups), Negrea et al. (1999, branchiopods),
Shultz and Regier (2000, all major arthropod
groups), and Richter et al. (2001, cladocerans).
MOLECULAR SYSTEMATICS AND
CLASSIFICATION OF THE CRUSTACEA
Without doubt, the most exciting recent developments
in our understanding of crustacean relationships
have been in the realm of molecular systematics
and phylogenetics. Indeed, many of the cladistic
papers mentioned in the previous section are
based on molecular sequence data, which essentially
were not available at the time of the Bowman
and Abele classification. Molecular systematic studies
of arthropods have become so numerous that
Wheeler (1998) stated ‘‘the past decade has presented
us with nearly annual molecular analyses of
Arthropoda.’’ For the Crustacea, most of this work
has been championed by the laboratories of L. G.
Abele and T. Spears at Florida State University and
C. W. Cunningham at Duke University. This field,
as well as the field of developmental genetics
(which we barely touch upon here), is growing and
changing at a phenomenal rate. Many of the early
studies were based on relatively small sequences, so
it is not terribly surprising that there have been
some published results that appear unreasonable
based on our knowledge of morphology, embryology,
paleontology, and other sets of characters. As
we refine our selection of which genes to target,
improve our ability to extract and align increasingly
larger sequences, and devise better computational
algorithms, we might begin to see more agreement
between molecular results and more traditional
views of crustacean phylogeny, or at least
results that are less ambiguous. Or we may not. As
Spears and Abele (1997) state in the conclusion to
their review paper on the use of 18S rDNA data in
crustacean phylogeny, ‘‘Regrettably, in the crusade
for understanding relationships among crustaceans
and other arthropod lineages, the rDNA data represent
but a relic, and not the Holy Grail itself.’’
Yet despite this sobering conclusion, Spears and
Abele (1997) were able to make some very strong
statements concerning at least some crustacean
taxa. For example, Branchiopoda, Copepoda, Podocopida,
and Myodocopida are all clearly monophyletic;
the Malacostraca is clearly monophyletic
and includes the Phyllocarida (Leptostraca) (supported
also by Shultz and Regier, 2000); Maxillopoda
does not appear monophyletic (although certain
groups within it seem to be united); etc.
There are, of course, known problems associated
with some of these approaches (as one early example,
see the responses by Nielsen and others
(1989) to the article by Field et al. (1988) entitled
‘‘Molecular analysis of the animal kingdom’’). Fryer
(1997) points out several papers that question the
results and/or validity of recent studies of arthropod
phylogeny based on molecular data; Wägele
and Stanjek (1995) make the point that alignment
alone can be responsible for serious discrepancies
in analyses of such data. And of course the history
of a particular gene might not accurately reflect the
phylogeny of the species containing that gene (e.g.,
see Brower et al., 1996; Doyle, 1997; Maddison,
1997; Page and Charleston, 1998). Unfortunately,
the branchiopod genus Artemia, which has been
used for more molecular comparative studies than
any other crustacean genus, is not the best choice;
Maley and Marshall (1998) note that ‘‘brine shrimp
[have] long been known to produce artifactual
groupings.’’ Lake (1990) admitted that arthropod
paraphyly as indicated in his analysis may be a result
of long branch attraction caused by the inclusion
of Artemia and Drosophila; this problem was
mentioned also by Turbeville et al. (1991). It is also
disconcerting that, after so much money and effort
have been expended toward applying genetic data
to resolving the evolutionary roots of modern humans,
we still do not have a clear answer. Whether
Homo sapiens arose from a single African source
200,000 years ago or ‘‘multiple groups in Africa
and elsewhere’’ at least a million years ago is still
hotly debated (see Bower, 1999). How, then, are we
expected to place confidence in what the molecules
are telling us about the evolution of crustaceans
when our efforts, in comparison, have been so limited?
To summarize, we again quote Maley and
Marshall (1998): ‘‘To be confident in our hypotheses
of relationships among the animal phyla we
need to gather more DNA sequences, especially
from undersampled phyla; develop better methods
of DNA analysis on the basis of more realistic models
of DNA evolution; and develop independent
Contributions in Science, Number 39 General Introduction 7