An Updated Classification of the Recent Crustacea

chaeobrachyura would have been confusing, not
only because the constituency and alliances have
changed considerably from its original usage by
Guinot but because the entire group has been
moved to the other major crab clade. We also could
have used the older name Gymnopleura, established
by Bourne (1922) to accommodate the raninids
and still used by some modern workers (e.g.,
Dai and Yang, 1991). But we have now placed the
former tymoloids (now the Cyclodorippoidea) in
this subsection with the raninids (which may be a
mistake; see below). Hence, our use of the name
Raninoida for the subsection. We have credited this
higher taxon to the same authority (De Haan) who
established the family Raninidae. The other two
subsections (the subsections Heterotremata and
Thoracotremata), jointly constituting the sister
group to the Raninoida, are more or less as envisioned
by Guinot (1977, 1978, 1979). Our adoption
of Guinot’s scheme (minus the Podotremata)
has meant that many formerly recognized ‘‘tribes’’
or ‘‘sections’’ among the higher crabs have been removed.
This reflects not so much an advance in our
knowledge of which families are closely related but
rather knowledge concerning which ones are not.
For example, the formerly recognized Oxyrhyncha
appears to be an artificial assemblage (Števčić and
Gore, 1982; Jamieson, 1991a, b, 1994; Spears et
al., 1992), and there is no longer any justification
for recognizing the Oxystomata, Brachyrhyncha,
and other former sections or tribes (e.g., see Guinot,
1977, 1978; Spears et al., 1992; Števčić, 1998).
Thus, we have retained several of the crab superfamilies
but have removed many of the sections that
were found in the Bowman and Abele (1982) classification.
Yet acceptance of the sections Heterotremata
and Thoracotremata as natural monophyletic
lineages is by no means universal. For one thing,
Guinot herself never explicitly assigned every
known family to one of her sections, leaving some
families ‘‘orphaned’’ in her earlier publications.
And as noted above, these groups are admittedly
(Guinot 1977, 1978) ‘‘grades’’ rather than true
monophyletic lineages (or at least, if they are monophyletic,
this has yet to be demonstrated, although
there are preliminary data from morphology (see
below) and from 16S rDNA (Trisha Spears, pers.
comm.) that at least the Thoracotremata may have
some validity). While usage of these sections has
become relatively widespread, it is unfortunate that
many families were not explicitly mentioned by
Guinot, such that users of her classification have
been uncertain as to which families belonged to
which section. Schram (1986) provided a more
complete list of families (including some known
only from fossils).
Concerning monophyly of the Thoracotremata,
dissections of the male reproductive tract of a series
of freshwater crabs and some marine heterotremes
and thoracotremes (during a search for the sister
taxon of the freshwater crabs) has indicated that
the Thoracotremata is a monophyletic group
(Sternberg and Cumberlidge, 2001). One character
uniting the thoracotremes is that the distal tracts of
the vas deferentia pass through thoracic endosternite
8 and contact the male pleopods via apertures
on thoracic sternite 8. The situation in heterotremes
is different, with the vas deferens passing through
the musculature of endosternite 8 but also through
the coxa of pereiopod 5 such that the male sexual
tube contacts the pleopods via an aperture on the
coxopodite. According to Sternberg et al. (1999),
Sternberg and Cumberlidge (2001), and Cumberlidge
and Sternberg (pers. comm.), the Eubrachyura
(sensu Saint Laurent, 1980) are therefore defined by
females with sternal vulvae and males with sexual
tube outlets that open on the coxa of pereiopod 5.
The Thoracotremata constitutes a monophyletic
subset of the Eubrachyura characterized by male
sexual tube outlets that unambiguously open on the
sternum.
Within these last two subsections (Heterotremata
and Thoracotremata), many former subfamilies of
crabs, notably in the Xanthoidea and Majoidea and
some also in the Parthenopoidea, have been elevated
to family status based on the publications of several
workers (e.g., Serène, 1984, for xanthids; Hendrickx,
1995, for majids). This is an ongoing trend
that merely reflects our growing awareness of how
incredibly diverse these taxa are.
SECTION DROMIACEA
In an early version of the updated classification, we
had removed the dromiacean crabs from the Brachyura
and had placed them instead among the Anomura.
Larval characters have suggested this for
years (e.g., see Williamson, 1976, 1982; Rice,
1980, 1983; Martin, 1991), so much so that Williamson
(1988a, b) invoked an unusual hypothesis
of transspecific gene flow to account for it. Molecular
(18S rRNA) evidence brought to bear by
Spears et al. (1992) seemed to indicate that at least
some dromiaceans are indeed closer to the Anomura
than to the Brachyura sensu stricta based on
these preliminary data, and early studies of dromiacean
sperm morphology suggested their removal
from the Brachyura as well (Jamieson 1990,
1991a). Yet adult morphology has always suggested
that dromiids are true crabs (e.g., see Števčić,
1995), and moving the dromiids to the Anomura
would raise many additional questions. Should all
of the families associated with dromiids (i.e., the
former Dromiacea, including dromiids, dynomenids,
and homolodromiids) be moved to the Anomura,
even though larval and molecular evidence
are not in hand for all of them? Is the Dromiacea
in fact a valid, monophyletic grouping? If that
scheme were accepted, how many other ‘‘primitive’’
families should be moved? The fact that information
on larval, molecular, and sperm morphology
characters is still lacking for many members of this
assemblage, plus more recent molecular data
(Spears and Abele, 1999; T. Spears, pers. comm.),
50 Contributions in Science, Number 39 Rationale