An Updated Classification of the Recent Crustacea

acarida). Our treatment of the Thermosbaenacea as
true peracarids is in agreement with morphological
interpretations (e.g., Monod, 1984; Cals and Monod,
1988; Monod and Cals, 1988, 1999) and recent
molecular evidence (Spears and Abele, 1998).
Other workers (e.g., Newman, 1983; Sieg, 1983a,
b; Pires, 1987; A. Brandt, pers. comm.) have argued
for maintaining separate status from the other peracarid
groups (reviewed by Wagner, 1994). Wagner
(1994), whose extensive review we followed in the
current classification, also was of the opinion that
there is no real justification for excluding the Thermosbaenacea
from the Peracarida.
Within the Thermosbaenacea, two new families
have been described since 1982: Halosbaenidae
(Monod and Cals, 1988) and Tulumellidae (Wagner,
1994). The family Monodellidae was also recognized
by Wagner (1994), bringing the total to
four recognized extant families (up from one in
Bowman and Abele, 1982). Wagner’s (1994) thorough
treatment also suggests some phylogenetic relationships
among the thermosbaenaceans (as did
Monod and Cals, 1988). The Thermosbaenidae
and Monodellidae appear to be sister taxa, but the
position of the Tulumellidae was undetermined,
sometimes appearing as the sister group to the Halosbaenidae
and sometimes as part of the thermosbaenid
� monodellid clade (as in his ‘‘final proposed
phylogenetic tree’’; Wagner, 1994, fig. 498).
Thus, we have not attempted to phyletically order
the four recognized families at this time. See also
the recent review by Monod and Cals (1999),
where previous systematic arrangements (Cals and
Monod, 1988; Wagner, 1994) are briefly discussed.
ORDERS LOPHOGASTRIDA AND MYSIDA
Abele and Spears (1997) concluded, based on
rDNA studies, that the Peracarida (including the
Thermosbaenacea) is indeed a monophyletic assemblage,
but only if the Mysida are excluded. Jarman
et al. (2000) also would separate the Mysida, which
they felt are closer to the Decapoda, from the Lophogastrida.
Supporting evidence is also found in
the fact that all peracarids (again including thermosbaenaceans
but excluding Mysida) contain similar
hypervariable regions of 18S rDNA (Spears and
Abele, 1998). However, these distinctly peracarid
features appear to be present in the other mysidacean
group, the Lophogastrida. The inclusion of the
mysids (both Mysida and Lophogastrida) in the
Peracarida (e.g., as suggested most recently by
Richter and Scholtz, in press) has also been questioned
on morphological grounds. For example, as
noted above, Watling (1998, 1999a, b) feels that
the mysidaceans (i.e., both the Mysida and Lophogastrida
as the taxon Mysidacea) do not belong to
the Peracarida and are instead more closely allied
to the eucarids. Yet both groups of the Mysidacea
(Mysida and Lophogastrida) share some unique
and possibly synapomorphic morphological features
of the walking limbs (Hessler, 1982; see also
Hessler, 1985) and foregut (De Jong-Moreau and
Casanova, 2001) that suggest monophyly. Additionally,
Richter (1999; see also Richter and
Scholtz, in press) has shown that lophogastridans
and mysidans share unique morphological components
to the design of their ommatidia (although
these features also are shared with Anaspidacea and
Euphausiacea). The recent treatment by Nouvel et
al. (1999) treats the Mysidacea as monophyletic
(see also Richter, 1994, for further arguments in
favor of monophyly of the Mysidacea).
Are mysidaceans paraphyletic? Is it possible that
the Mysida fall outside the Peracarida sensu stricta
but that the Lophogastrida are true peracarids (ignoring,
for the moment, the larger question of
whether the Peracarida itself is monophyletic)? This
seems unlikely based on limb morphology (e.g.,
Hessler, 1982), and foregut morphology (De Jong-
Moreau and Casanova, 2001), and yet other workers
have noted significant differences between the
Mysida and Lophogastrida on morphological (and
now, it appears, on molecular) grounds. Several
other workers (e.g., G. Scholtz and S. Richter, pers.
comm.) commented on the distinct morphological
differences between the Lophogastrida and Mysida
and suggested that these taxa be elevated to ordinal
status and that the former Mysidacea that contained
the two be abandoned (but see also Richter,
1994, De Jong-Moreau and Casanova, 2001, and
Richter and Scholtz, in press, for arguments in favor
of monophyly). We have split the former order
Mysidacea, elevating each of the former mysid suborders
to order level, as have several other workers
before us, such as Schram (1984, 1986), and Brusca
and Brusca (1990:624, who note that an increasing
number of specialists have begun to treat the two
groups separately). This could be seen as a preliminary
for removing one or both of these groups
from the Peracarida, if the suggestions of Watling
(1998, 1999a, b) and Spears and Abele (1998) find
additional support in the future. However, we have
kept the two groups within the Peracarida for now.
Taylor et al. (1998) analyzed the relationships of
a group of fossil malacostracans (the Pygocephalomorpha)
that are possibly allied with mysids; one
of their conclusions was that the recent mysids and
lophogastrids do form a clade (albeit a somewhat
‘‘confused’’ one). Thus, our classification is most
similar to that of Brusca and Brusca (1990) in recognizing
both former ‘‘mysidacean’’ groups as orders
within the superorder Peracarida rather than
as suborders within the Mysidacea (as presented by
Nouvel et al., 1999). Casanova et al. (1998) examined
relationships of the two lophogastrid families
(Eucopiidae and Lophogastridae) based on
morphological and limited molecular data. Among
their conclusions was that the monogeneric eucopiids
(Eucopia) originated from within the Lophogastridae.
Authorities and dates for some taxa in the Mysida
have been changed to earlier workers and dates
(e.g., Mysida Haworth and Mysidae Haworth rath-
34 � Contributions in Science, Number 39 Rationale