An Updated Classification of the Recent Crustacea

SUBPHYLUM CRUSTACEA
Many of the questions considered most pressing today
have been asked for well over 100 years: Are
crustaceans a monophyletic group? How many major
clades, or classes, are there? Which is the most
primitive class? What are the relationships among
the classes? We cannot attempt to answer all of
these questions here, but below we offer a brief explanation
of how and why we arrived at the current
classification. In most cases, we provide some additional
information under the heading for each of
the various taxa (each of which is treated later). For
more in-depth discussions of the complex history
of attempts to classify the Crustacea, we refer the
reader to the following publications: Moore and
McCormick (1969), Schram (1986), Spears and
Abele (1997), Schram and Hof (1998), and especially
Monod and Forest (1996).
Are Crustaceans a Monophyletic Group?
The question of crustacean monophyly, the place of
the Crustacea within the Arthropoda, the question
of arthropod monophyly, and the relationships
among the many arthropod and crustacean groups
have been reviewed by several recent workers (see
especially Boore et al., 1995; Friedrich and Tautz,
1995; Telford and Thomas, 1995; Raff et al., 1994;
Fortey et al., 1997; Regier and Shultz, 1997,
1998b; Wheeler, 1998; Shultz and Regier, 2000;
Edgecomb et al., 2000). Broader questions concerning
whether crustaceans and other arthropods belong
in a phylum or larger clade called the Ecdysozoa
(see Garey et al., 1996; Aguinaldo et al.,
1997) are reviewed by Schmidt-Rhaesa et al. (1998)
and Garey (2000). We have not attempted to address
either of these issues (that is, the relationship
of crustaceans to other arthropods or the relationships
within the Ecdysozoa) and instead refer the
reader to the following publications and the papers
cited therein. Wheeler et al. (1993) presented a
combined analysis of morphological and molecular
data that strongly supported arthropod monophyly,
and this view was strengthened by Wheeler (1998).
Lake (1990) suggested arthropod paraphyly, while
Fryer (1997) presents several arguments in favor of
arthropod polyphyly. Strausfeld (1998) depicts insects
and crustaceans (both of which he feels may
be paraphyletic) as sister groups on the basis of
neuroanatomical data. Preliminary work on the
neurogenesis of compound eyes supports common
ancestry for crustaceans and insects as well (e.g.,
see Harzsch and Walossek, 2001, and references
cited therein). Friedrich and Tautz (1995) support
both arthropod monophyly and a crustacean–insect
sister group arrangement with DNA sequence data,
as do Boore et al. (1995, 1998), using mitochondrial
gene rearrangement data, and Wilson et al.
(2000), comparing the complete mitochondrial ge-
RATIONALE
nome of a malacostracan with that of Drosophila.
Regier and Schultz (1997, 1998a, b) also questioned
crustacean monophyly (their 1997 title suggests
crustacean polyphyly) based on EF-1� and
RNA polymerase II (Pol II); however, their results
were somewhat ambiguous, as there were no
strongly supported nodes, and support for a basal
Malacostraca was not high (J. Regier, pers. comm.).
Regier and Shultz also suggested (1997, 1998b), as
had other workers, that branchiopod crustaceans
may be more closely related to other arthropod
groups (hexapods and myriapods) than they are to
malacostracan crustaceans, although this too did
not have strong node support (what was strongly
supported was that branchiopods, and indeed all of
our six classes of crustaceans, grouped with hexapods
to the exclusion of myriapods, arguing against
the concept of the ‘‘Atelocerata’’ (hexapods � myriapods);
see also Popadić et al., 1996, and Shultz
and Regier, 2000). Another way of stating this is
that, if crustaceans are not monophyletic, then the
group that breaks them up is the Hexapoda and
not myriapods or chelicerates or groups outside Arthropoda.
The emerging field of developmental biology
(see references cited in the earlier section on
developmental genetics and crustacean classification)
also provides evidence that crustaceans and
insects are closely linked. Brusca (2000) nicely summarizes
the history of the controversy and the disparate
data sets. Two recent volumes address these
questions by way of collections of edited papers:
Fortey and Thomas (1997, Arthropod Relationships,
Chapman and Hall) and Edgecombe (1998,
Arthropod Fossils and Phylogeny, Columbia University
Press).
In the introduction to the latter volume, Edgecombe
notes that ‘‘the monophyly of Crustacea is
endorsed in every chapter that investigates the issue’’
(see also Edgecomb et al., 2000). Yet there
remains some doubt. We have found it advantageous,
at least for the project at hand, to treat the
group as monophyletic. We also note that there is
an abundance of fossil, morphological, and molecular
data that support this view. The ‘‘crown-’’ vs.
‘‘stem-group’’ approach as detailed by Walossek
and Müller (1990, 1998) is worth noting in this
regard; those authors consider the Crustacea monophyletic
and give several morphological characters
that uniquely define the group, while at the same
time they present interesting information on ‘‘stemline
crustaceans,’’ crustacean-like arthropods that
are not members of the crown group (their ‘‘Eucrustacea’’)
but that share at least some features
with true crustaceans. Other workers have argued,
some with more data than others, that the Crustacea
is paraphyletic (e.g., Moura and Christoffersen,
1996; García-Machado et al., 1999; Wilson et al.,
2000) or polyphyletic (e.g., Averof and Akam,
1995a, b) or that the question is, at best, unre-
12 � Contributions in Science, Number 39 Rationale