An Updated Classification of the Recent Crustacea

solved (e.g., Regier and Schultz, 1997, 1998a, b;
Shultz and Regier, 2000), and we would be remiss
not to mention these dissenting opinions. Further
arguments for or against the monophyly of the
Crustacea (and also Arthropoda) can be found in
the reviews by Brusca (2000) and Giribet and Ribera
(2000).
Our treatment of the Crustacea as a subphylum
(of the Arthropoda) is therefore somewhat arbitrary.
Arguments could be (and have been) made
for recognizing the group as a distinct phylum, and
some workers refer to the Crustacea as a superclass
or class. Our choice of subphylum allowed us to
use classes within the group, which to us was more
manageable. Treating the Crustacea as a subphylum
implies monophyly of the Arthropoda. Although
this issue is not completely settled (see
above references and especially Fryer, 1997, in Fortey
and Thomas, 1997), most bodies of evidence of
which we are aware seem to indicate that the arthropods
are indeed a phylum (see summaries in
Raff et al., 1994; Telford and Thomas, 1995; and
Brusca, 2000) that includes the Crustacea.
How Many Classes Are There?
The history of higher level classification of the
Crustacea is briefly discussed in Holthuis (1993a),
Spears and Abele (1997), Schram (1986), Schram
and Hof (1998), and especially Monod and Forest
(1996). Some of the more notable schemes for crustacean
classification that have appeared subsequent
to the Bowman and Abele (1982) classification are
those of Schram (1986), Starobogatov (1986, with
English translation by Grygier in 1988), and Brusca
and Brusca (1990). Other workers have presented
phylogenies from which the reader can deduce alternative
classifications, even if no specific classification
is presented in the paper (e.g., Wilson,
1992).
Schram (1986) departed from Bowman and
Abele’s use of six classes by recognizing four
groups: Remipedia, Phyllopoda (which included the
branchiopods, cephalocarids [as Brachypoda], and
leptostracans), Maxillopoda (including tantulocarids,
branchiurans, mystacocaridans, ostracodes, copepods,
facetotectans, rhizocephalans, ascothoracidans,
acrothoracicans, and thoracicans), and Malacostraca
(containing both the hoplocarids and the
eumalacostracans). Schram’s (1986:542–544) classification
extends to the level of suborder and occasionally
infraorder. It is noteworthy not only for
attempting to derive a classification from his cladistic
analyses but also because of his inclusion of
a large number of fossil taxa. Unfortunately,
Schram (1986) also introduced, or employed, some
taxonomic names that have not been well accepted
(e.g., ‘‘Euzygida’’ for the stenopodidean shrimps;
‘‘Eukyphida’’ for the carideans; ‘‘Edriophthalma’’ to
contain the isopods and amphipods as distinct from
all other peracarids, etc.). Starobogatov (1986,
1988) recognized four groups as well, but the composition
of his four groups differs appreciably from
those of Schram and from those of all other previous
workers. Additionally, Starobogatov employed
some unusual names for his groupings (such
as Carcinioides for the malacostracans and Halicynioides
to accommodate some of the maxillopodan
groups) that are unlikely to receive wide recognition,
and his classification appears to be at
odds with most of the morphological and fossil
data (e.g., see Schram and Hof, 1998) as well as
with the molecular data (e.g., Spears and Abele,
1997). Brusca and Brusca (1990) recognized five
classes (Remipedia, Branchiopoda, Cephalocarida,
Maxillopoda, and Malacostraca), and in part because
this usage is in a major textbook, it has received
wide acceptance. Bousfield and Conlan
(1990, Encyclopaedia Britannica), whose classification
extends only to the ordinal level, followed
Schram’s lead for some groups of the Crustacea and
Bowman and Abele (1982) for others. Their classification
is noteworthy because of their attempt to
include fossil taxa as well and because of their laudable
attempt to estimate the number of families in
each order. Gruner (1993) treats the Crustacea as
a class, does not recognize the Branchiopoda or
Maxillopoda, and as a result includes 13 separate
subclasses. Apart from the somewhat unusual treatment
by Starobogatov, the number of proposed or
recognized classes seems to have depended mostly
upon whether the maxillopods are seen as a natural
assemblage and, if they are, whether the ostracodes
are within or outside of the Maxillopoda, and on
whether and how the Malacostraca should be divided.
In our classification, the subphylum Crustacea
includes six major groups, which we are treating as
classes: Branchiopoda, Remipedia, Cephalocarida,
Maxillopoda, Ostracoda, and Malacostraca. However,
this is somewhat misleading in that we are
also positing the Branchiopoda as the sister taxon
to all other crustacean groups. Thus, the ‘‘class’’
Branchiopoda should be accorded more weight
than the remaining classes, which together constitute
the sister group to the branchiopods in our
arrangement. Our treatment of crustaceans as being
comprised of six classes is quite conservative and
follows essentially the Bowman and Abele (1982)
classification. Perhaps the most salient problem is
our continued recognition of the Maxillopoda as a
valid class, when virtually all lines of evidence point
to its being an artificial assemblage (see discussion
under Maxillopoda). Thus, Wilson (1992) observed
that ‘‘the concept of the Maxillopoda is not supported
in any of the trees’’ and Spears and Abele’s
(1997) molecular analysis ‘‘fails to provide strong
support for a monophyletic Maxillopoda.’’ If we
eliminated the Maxillopoda as a class, as has Gruner
(1993) (and there are many lines of evidence
that suggest that this is the correct course), then we
would treat as distinct classes each of the currently
recognized ‘‘maxillopodan’’ subclasses (the Thecostraca,
Tantulocarida, Mystacocarida, and Copepo-
Contributions in Science, Number 39 Rationale 13