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