An Updated Classification of the Recent Crustacea

1996). The order must be extremely ancient. If the
‘‘cladoceran’’ orders prove to be monophyletic, they
must be of extremely ancient origin. The most convincing
molecular evidence of affinity of the ‘‘cladoceran’’
orders is that in all four the V4 and V7
regions of the small subunit ribosomal RNA possesses
four helices, three of which are present in
Cyclestheria but are otherwise so far unique
(Crease and Taylor, 1998). Cyclestheria, long regarded
as a somewhat recalcitrant spinicaudatan,
has often been cast in the role of ancestor of the
‘‘Cladocera’’—without however demonstrating
how such different orders as the Anomopoda and
Haplopoda could have been derived from it. Although
the helices are very different in length and
primary sequences of their distal ends in the different
orders, their locations, secondary structures,
and primary sequences at their proximal ends are
conserved, which suggests homology. None of these
peculiarities is shared with the Spinicaudata, within
which order Cyclestheria was long included and to
which it is vastly more similar in morphology than
it is to any ‘‘cladoceran’’ order! According to some
investigators, evidence deduced from 18S ribosomal
DNA supports these relationships (Spears and
Abele, 2000). However, according to Dumont
(2000), ‘‘ongoing molecular work using the full sequence
of the 18S rDNA nuclear gene’’ not only
confirms the distinction of that order ‘‘but also suggests
that the Onychopoda might even be more
closely related to the Anostraca than with the cladoceran
orders Ctenopoda and Anomopoda.’’
Note, also, that the widely accepted 18S rRNA
phylogenetic tree of the Protozoa has now been seriously
questioned, and is probably unreliable (Phillippe
and Adoutte, 1998)!
With qualifications, some molecular evidence is
seductive and welcome, but is contradicted by other
molecular findings, and cannot gainsay either the
great morphological differences between the groups
concerned, or the failure to justify either the ‘‘Cladocera,’’
‘‘Conchostraca,’’ or ‘‘Diplostraca’’ by cladistic
analyses. To change the classification of these
animals on the basis of still-contentious molecular
evidence while ignoring the larger corpus of information
now accumulated, not only on morphology
but on morphology whose functional significance is
sometimes understood, and on life histories, would
merely upset what may indeed eventually prove to
be only an interim scheme, but one which for the
time being is perfectly serviceable. As Avise (1994)
notes, morphological and molecular evolution may
proceed at different rates, and the overall magnitude
of genetic distance between taxa is not necessarily
the only, or the best, guide to phylogenetic
relationships within groups.
The subclasses Sarsostraca and Phyllopoda seem
to be unnecessary. The latter name has also already
been a source of much confusion. A case can be
made for the Notostraca as being as distinctive as
the Anostraca, which alone renders grouping into
subclasses untenable.
Additional References
Avise, J. C. 1994. Molecular markers, natural history and
evolution. New York: Chapman and Hall.
Colbourne, J. K., and P. D. N. Hebert. 1996. The systematics
of the North American Daphnia (Crustacea:
Anomopoda): a molecular phylogenetic approach.
Philosophical Transactions of the Royal Society of
London 351B:349–360.
Dumont, H. J. 2000. Endemism in the Ponto-Caspian fauna,
with special emphasis on the Onychopoda (Crustacea).
Advances in Ecological Research 31:181–
196.
Phillippe, H., and A. Adoutte. 1998. The molecular phylogeny
of Eukaryota: solid facts and uncertainties. In
Evolutionary relationships among Protozoa, eds. G.
H. Coombs et al., 25–56. London: Chapman and
Hall.
Submitted by Geoffrey Fryer,
University of Lancaster, United Kingdom
BRANCHIOPODA
I am not sure that you should not include the Ilyocryptidae
in your classification. After all, it is a
quite serious action not to follow the advice of the
most important Recent taxonomist working in the
Cladocera that we have (N. N. Smirnov). Especially
since you follow so many other taxonomists in their
suggestions. You present no arguments for not doing
so. One could argue that an eventual splitting
of the Macrothricidae should await a phylogenetic
revision, but such a revision is likely not to appear
in due time. It is true that the change suggested by
Smirnov may not be based on phylogenetic criteria
(and the remaining macrothricids may still be paraphyletic),
but the same could be said about so
much of your classification anyway, as you mention
a couple of times.
I think when it comes to the lower level classification,
I believe it would be wise to follow the advice
of the people actually working on the taxa,
unless you have personal, strong arguments no to
do so. The case of the ‘Moinidae’ is different because
Fryer convincingly argues for their unity with
the rest of the Daphniidae. You could also cite his
1991 monograph on Daphniidae adaptive radiation
here.
The step you take concerning Cyclestheria is OK,
I think. It is understandable that you choose something
between the two alternatives. If we one day
decide to take the full step of the possible sister
group relation to the Cladocera, then a name is already
available by Ax (1999). He suggests the term
‘Cladoceromorpha.’ There are also a couple of new
molecular papers out on the issue that seem to support
Cyclestheria in the mentioned sister group position.
Submitted by Jørgen Olesen,
University of Copenhagen, Denmark
BRANCHIOPODA
The quotation from Fryer really encapsulates what
is wrong with the old ideas about crustacean phy-
104 Contributions in Science, Number 39 Appendix I: Comments and Opinions