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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APPENDIX I. COMMENTS AND OPINIONS<br />
The following comments and opinions were provided<br />
by colleagues (all <strong>of</strong> whom are listed in Appendix<br />
II) after seeing <strong>the</strong> penultimate draft <strong>of</strong> <strong>the</strong><br />
classification. The authors wish to gratefully acknowledge<br />
<strong>the</strong>m for allowing us to reproduce <strong>the</strong>ir<br />
remarks. References are listed after each comment<br />
only if those references are not already listed in our<br />
Literature Cited section. Some authors did not supply<br />
full references; consequently, references may be<br />
missing for some papers cited below.<br />
CRUSTACEA (GENERAL)<br />
The authors choose to treat <strong>the</strong> <strong>Crustacea</strong> as a<br />
monophyletic group and thus find it justifiable to<br />
produce an updated classification for organizing<br />
museum collections and helping students <strong>of</strong> crustaceans<br />
to search unfamiliar taxa. It should thus<br />
become a useful taxonomic tool. I find much merit<br />
in (1) <strong>the</strong> exposition <strong>of</strong> reasons for preferred arrangements<br />
and (2) <strong>the</strong> attempt to introduce readers<br />
to alternative opinions. The permanent drawback<br />
<strong>of</strong> this compilation (considered by <strong>the</strong> authors)<br />
is that taxa are not justified by diagnostic<br />
characters.<br />
As a means <strong>of</strong> reflecting some current phylogenetic<br />
ideas on crustaceans, however, <strong>the</strong> present attempt<br />
will be considered obsolete almost immediately<br />
by some workers. The monophyly <strong>of</strong> <strong>the</strong><br />
<strong>Crustacea</strong> is far from settled. In fact, in my opinion,<br />
it is very unlikely. The mandibulate arthropods are<br />
traditionally divided into two grades (crustaceans<br />
and tracheates), and it is obvious that <strong>the</strong> closest<br />
relatives <strong>of</strong> <strong>the</strong> terrestrial tracheates should be<br />
sought among aquatic crustaceans. If this scenario<br />
is reasonable, <strong>the</strong> <strong>Crustacea</strong> become, in principle, a<br />
nonmonophyletic grade-group. The Remipedia and<br />
Malacostraca have been pinpointed as two successive<br />
outgroups <strong>of</strong> <strong>the</strong> Tracheata (Moura and Christ<strong>of</strong>fersen,<br />
1996). If <strong>the</strong>re is merit in such a proposal,<br />
an incorrect assumption <strong>of</strong> monophyly could immediately<br />
account for many discrepancies noted<br />
among cladistic papers establishing <strong>the</strong> position<br />
and internal relationships <strong>of</strong> <strong>the</strong> <strong>Crustacea</strong>. Researchers<br />
striving for a phylogenetic arrangement<br />
<strong>of</strong> <strong>the</strong> crustaceans should not exclude <strong>the</strong> terrestrial<br />
descendants <strong>of</strong> crustaceans from <strong>the</strong>ir system. For<br />
<strong>the</strong>se reasons, ra<strong>the</strong>r than a practical, largely consensual,<br />
and authority-based classification <strong>of</strong> <strong>the</strong><br />
<strong>Recent</strong> <strong>Crustacea</strong>, we need to reconstruct <strong>the</strong> system<br />
<strong>of</strong> <strong>the</strong> Mandibulata (apparently <strong>the</strong> smallest<br />
clade that includes all <strong>the</strong> so-called crustaceans, as<br />
well as <strong>the</strong>ir myriapod and hexapod descendants).<br />
Fur<strong>the</strong>rmore, apomorphic characters need to be<br />
provided to distinguish acceptable monophyletic<br />
taxa from unstudied, unknown, or unresolved traditional<br />
taxa. Let me suggest that this become ano<strong>the</strong>r<br />
demanding, but long overdue, story.<br />
Submitted by Martin L. Christ<strong>of</strong>fersen,<br />
Federal University <strong>of</strong> Paraíba, Brazil<br />
BRANCHIOPODA AS PRIMITIVE<br />
In regards to your first argument here, <strong>the</strong>re are<br />
three different sets <strong>of</strong> authors who cannot confirm<br />
a branchiopod affinity for this taxon [Rehbachiella]<br />
and consequently <strong>the</strong>re in fact may be no Cambrian<br />
branchiopods. The second part <strong>of</strong> your argument,<br />
that <strong>the</strong>re are nei<strong>the</strong>r Cambrian cephalocarids, nor<br />
remipedes, is a non-sequitor. The late Ralph Gordon<br />
Johnson used to say about <strong>the</strong> apparent age <strong>of</strong><br />
fossils ‘‘Things are always older than you think <strong>the</strong>y<br />
are.’’ <strong>An</strong> example <strong>of</strong> which relates to those Carboniferous<br />
remipedes; <strong>the</strong>re is in fact something in<br />
<strong>the</strong> Silurian <strong>of</strong> Wisconsin, yet undescribed, that<br />
may be a remipede. So, your first argument is weak.<br />
Your second argument, derived from apomorphic<br />
development, would seem to be valid, at least<br />
under traditional assumptions. However, two<br />
points might be mentioned in this regard. The<br />
weakest point relates to <strong>the</strong> basic assumption <strong>of</strong><br />
anamorphy � primitive. Certain aspects emerging<br />
from developmental genetics might suggest an alternative;<br />
however, this needs to be developed and<br />
published (something I have not had time to do as<br />
yet). Never<strong>the</strong>less, if we consider <strong>the</strong> matter in<br />
strictly cladistic terms, if as you correctly state that<br />
anamorphy is unique to branchiopods, within<br />
<strong>Crustacea</strong> sensu stricto <strong>the</strong> issue <strong>of</strong> plesiomorphy is<br />
not resolved—branchiopods have it, but non-branchiopods<br />
(apparently) don’t. If you add outgroups<br />
from <strong>the</strong> ‘‘o<strong>the</strong>r Mandibulata,’’ in an attempt to polarize<br />
patterns <strong>of</strong> development, <strong>the</strong>n if insects are<br />
in fact a sister group to crustaceans, epimorphy<br />
could be argued as plesiomorphic.<br />
Third, <strong>the</strong> molecular data cited here is not being<br />
employed properly by you. The distinctness <strong>of</strong><br />
branchiopods here in <strong>the</strong> papers you cite is stronger<br />
than you indicate. For example, Spears and Abele<br />
(1997) under certain assumptions actually pull<br />
branchiopods into <strong>the</strong> hexapods, which possibly indicates<br />
crustacean polyphyly. Of course you say<br />
that branchiopods are (might be) closer to o<strong>the</strong>r<br />
groups <strong>of</strong> arthropods—a fair judgment. If true, that<br />
would indicate that <strong>the</strong> position <strong>of</strong> branchiopods<br />
far exceeds that <strong>of</strong> a potential ‘‘basal group’’ <strong>of</strong><br />
crustaceans. Primitiveness under those circumstances<br />
has nothing to do with it.<br />
In short, you are wise not to create any additional<br />
taxonomic categories. Moreover, your threepronged<br />
argument would appear to be not clearly<br />
drawn at all.<br />
On <strong>the</strong> ancestral crustacean . . . you remark that<br />
Schram and H<strong>of</strong> (1998) obtain a clade Phyllopoda.<br />
First, if you look at <strong>the</strong> paper carefully, we sometimes<br />
get a phyllopodan clade, and sometimes<br />
not—depending on <strong>the</strong> assumptions and inclusiveness<br />
<strong>of</strong> <strong>the</strong> database employed. Contrary to Schram<br />
(1986), I think H<strong>of</strong> and I would state that <strong>the</strong> issue<br />
<strong>of</strong> whe<strong>the</strong>r or not <strong>the</strong>re is a monophyletic clade<br />
Phyllopoda is indeed an open one—which is not<br />
102 � Contributions in Science, Number 39 Appendix I: Comments and Opinions