An Updated Classification of the Recent Crustacea

APPENDIX I. COMMENTS AND OPINIONS
The following comments and opinions were provided
by colleagues (all of whom are listed in Appendix
II) after seeing the penultimate draft of the
classification. The authors wish to gratefully acknowledge
them for allowing us to reproduce their
remarks. References are listed after each comment
only if those references are not already listed in our
Literature Cited section. Some authors did not supply
full references; consequently, references may be
missing for some papers cited below.
CRUSTACEA (GENERAL)
The authors choose to treat the Crustacea as a
monophyletic group and thus find it justifiable to
produce an updated classification for organizing
museum collections and helping students of crustaceans
to search unfamiliar taxa. It should thus
become a useful taxonomic tool. I find much merit
in (1) the exposition of reasons for preferred arrangements
and (2) the attempt to introduce readers
to alternative opinions. The permanent drawback
of this compilation (considered by the authors)
is that taxa are not justified by diagnostic
characters.
As a means of reflecting some current phylogenetic
ideas on crustaceans, however, the present attempt
will be considered obsolete almost immediately
by some workers. The monophyly of the
Crustacea is far from settled. In fact, in my opinion,
it is very unlikely. The mandibulate arthropods are
traditionally divided into two grades (crustaceans
and tracheates), and it is obvious that the closest
relatives of the terrestrial tracheates should be
sought among aquatic crustaceans. If this scenario
is reasonable, the Crustacea become, in principle, a
nonmonophyletic grade-group. The Remipedia and
Malacostraca have been pinpointed as two successive
outgroups of the Tracheata (Moura and Christoffersen,
1996). If there is merit in such a proposal,
an incorrect assumption of monophyly could immediately
account for many discrepancies noted
among cladistic papers establishing the position
and internal relationships of the Crustacea. Researchers
striving for a phylogenetic arrangement
of the crustaceans should not exclude the terrestrial
descendants of crustaceans from their system. For
these reasons, rather than a practical, largely consensual,
and authority-based classification of the
Recent Crustacea, we need to reconstruct the system
of the Mandibulata (apparently the smallest
clade that includes all the so-called crustaceans, as
well as their myriapod and hexapod descendants).
Furthermore, apomorphic characters need to be
provided to distinguish acceptable monophyletic
taxa from unstudied, unknown, or unresolved traditional
taxa. Let me suggest that this become another
demanding, but long overdue, story.
Submitted by Martin L. Christoffersen,
Federal University of Paraíba, Brazil
BRANCHIOPODA AS PRIMITIVE
In regards to your first argument here, there are
three different sets of authors who cannot confirm
a branchiopod affinity for this taxon [Rehbachiella]
and consequently there in fact may be no Cambrian
branchiopods. The second part of your argument,
that there are neither Cambrian cephalocarids, nor
remipedes, is a non-sequitor. The late Ralph Gordon
Johnson used to say about the apparent age of
fossils ‘‘Things are always older than you think they
are.’’ An example of which relates to those Carboniferous
remipedes; there is in fact something in
the Silurian of Wisconsin, yet undescribed, that
may be a remipede. So, your first argument is weak.
Your second argument, derived from apomorphic
development, would seem to be valid, at least
under traditional assumptions. However, two
points might be mentioned in this regard. The
weakest point relates to the basic assumption of
anamorphy primitive. Certain aspects emerging
from developmental genetics might suggest an alternative;
however, this needs to be developed and
published (something I have not had time to do as
yet). Nevertheless, if we consider the matter in
strictly cladistic terms, if as you correctly state that
anamorphy is unique to branchiopods, within
Crustacea sensu stricto the issue of plesiomorphy is
not resolved—branchiopods have it, but non-branchiopods
(apparently) don’t. If you add outgroups
from the ‘‘other Mandibulata,’’ in an attempt to polarize
patterns of development, then if insects are
in fact a sister group to crustaceans, epimorphy
could be argued as plesiomorphic.
Third, the molecular data cited here is not being
employed properly by you. The distinctness of
branchiopods here in the papers you cite is stronger
than you indicate. For example, Spears and Abele
(1997) under certain assumptions actually pull
branchiopods into the hexapods, which possibly indicates
crustacean polyphyly. Of course you say
that branchiopods are (might be) closer to other
groups of arthropods—a fair judgment. If true, that
would indicate that the position of branchiopods
far exceeds that of a potential ‘‘basal group’’ of
crustaceans. Primitiveness under those circumstances
has nothing to do with it.
In short, you are wise not to create any additional
taxonomic categories. Moreover, your threepronged
argument would appear to be not clearly
drawn at all.
On the ancestral crustacean . . . you remark that
Schram and Hof (1998) obtain a clade Phyllopoda.
First, if you look at the paper carefully, we sometimes
get a phyllopodan clade, and sometimes
not—depending on the assumptions and inclusiveness
of the database employed. Contrary to Schram
(1986), I think Hof and I would state that the issue
of whether or not there is a monophyletic clade
Phyllopoda is indeed an open one—which is not
102 Contributions in Science, Number 39 Appendix I: Comments and Opinions