An Updated Classification of the Recent Crustacea

(based primarily on additional strong larval evidence
that has accrued since the Van Dover et al.
(1986) paper) that eumedonids are simply a subfamily
of the Pilumnidae (see also Lim and Ng,
1988). Indeed, Ng (1983) considered it a pilumnid
subfamily, as have several other workers (reviewed
by Števčić et al., 1988). Yet Chia and Ng (2000)
continue to recognize the family. For now, we have
continued to treat the Eumedonidae as a separate
family with clear affinities to the Pilumnidae, and
thus we have placed it with the pilumnids among
the xanthoids.
Recognition of Halimede as different from other
pilumnids goes back at least to the time of Alcock
(1898), who recognized the ‘‘alliance’’ Halimedoida.
More recent workers (e.g., Serène, 1984:11)
have recognized the Halimedinae as a subfamily of
the Pilumnidae. Although Bella Galil (pers. comm.)
feels that the genus Halimede differs sufficiently
from other xanthoids to warrant recognition of a
separate family, the Halimedidae, we are not aware
of any formal treatment or description of the family
and how it differs from the other pilumnid groupings.
At least some workers (e.g., R. von Sternberg,
pers. comm.) would place the Hexapodidae in the
Thoracotremata instead of among the xanthoid
families in the Heterotremata; von Sternberg also
suggests, based primarily on characters of the orbits,
that the Goneplacidae may be more closely
related to portunids than to other xanthoid families
(see also Sternberg and Cumberlidge, in press).
Concerning phylogeny of xanthoid crabs, Rice
(1980, 1983) and Martin (1988) have postulated,
based on larval features (zoeal and megalopal), that
the ‘‘Group III’’ larvae (e.g., Homalaspis, Ozius,
Eriphia) might be primitive; Martin et al. (1985)
suggested that pilumnids might be the least derived
assemblage. Guinot (1978) felt that pilumnids and
panopeids were more derived than the other groupings.
In the current classification, we have simply
listed the families alphabetically within the Xanthoidea.
Superfamily Potamoidea
The higher taxonomy of the freshwater crabs has
long been in a state of disarray, and there has been
little agreement among authors as to the number of
superfamilies and families (e.g., see Cumberlidge,
1999, for a review; Bott, 1970a, b; Pretzmann,
1973; Ng, 1988, 1998; Sternberg et al., 1999; Peter
Ng, pers. comm.; Neil Cumberlidge, pers. comm.).
Up to 3 superfamilies and 12 families are recognized,
depending on the author and also on how
far back in the literature one goes. Available higher
classifications of the freshwater crabs are based
largely on morphological data and, until recently
(Rodríguez, 1992; Sternberg, 1997; Sternberg et al.,
1999; Sternberg and Cumberlidge, in press), few
have been based on cladistic analyses. Many early
freshwater crab systematists considered all the
world’s freshwater crabs to comprise a single
monophyletic family, Potamidae. Others (Bott,
1970a, b; Pretzmann, 1973) recognized 11 families
and 3 superfamilies, arguing that the group is polyphyletic
(or at least paraphyletic) and that similarities
represent convergent adaptations of different
lineages to similar habitats. Investigations over the
past two decades (e.g., Rodríguez, 1982; Ng, 1988;
Guinot et al., 1997; Cumberlidge, 1999) have questioned
the validity of several families, and these
studies continue to reveal the fundamental artificiality
of Bott’s (1970a,b) 11-family taxonomic arrangement.
However, in the absence of a robust
phylogenetic study, most authors (including Bowman
and Abele, 1982) have adopted their own variant
of Bott’s classification (albeit reluctantly), and
this format is followed here.
Underlying the above taxonomic instability is the
unresolved question of the monophyly of the freshwater
crabs. A growing body of recent research
(Rodríguez, 1992; Sternberg, 1997; Sternberg et al.,
1999; Sternberg and Cumberlidge, in press) has falsified
the monophyly of the entire group and supports
paraphyly with two main lineages. The first
lineage includes the Trichodactylidae, which may
be descended from some portunoid stock (see
above under superfamily Portunoidea), and thus
represents an independent line from any of the ‘‘potamoid’’
stock. The second lineage includes the rest
of the freshwater crab families. The work of Sternberg
et al. (1999), Cumberlidge and Sternberg
(1999), Abele et al. (1999), Spears et al. (2000),
and Sternberg and Cumberlidge (2000a) indicates
that the nontrichodactylid freshwater crabs (all of
which are heterotremes) appear to be most closely
related to a marine crab clade that includes ocypodids,
grapsids, and possibly pinnotherids, with
the grapsids providing the best candidate for a sister
taxon (an odd result in light of the fact that
currently the potamoids are treated as heterotremes
whereas the grapsoids are thoracotremes). The hypothesis
suggested by Sternberg et al. (1999), that
most families of freshwater crabs form a single
clade composed of New and Old World lineages, is
a departure from the traditional view of the freshwater
crab relationships and may lead to further
alterations of the higher classification of the group.
Some of the more recent evidence (see especially
Abele et al., 1999; Spears et al., 2000) seems to
indicate that the freshwater crabs may have arrived
via two (and possibly more) invasions. One point
of agreement seems to be that the New World pseudothelphusids
represent a separate clade from the
Old World potamoids. These New World crabs
have long been thought to represent an independent
lineage (sometimes referred to as the Pseudothelphusoidea;
see below) from the rest of the world’s
freshwater crabs (see also Sternberg and Cumberlidge,
1999). However, even this idea is somewhat
controversial concerning whether the trichodactylids
belong to the New World clade or represent a
separate, independent invasion. Sternberg et al.
(1999), citing the works of Magalhães and Türkay
54 Contributions in Science, Number 39 Rationale