An Updated Classification of the Recent Crustacea

ecognizable cephalic limbs, other than paired antennules
in one known stage, which makes elucidation
of their affinities very difficult) to other
Crustacea can be found in the above works as well
as in Boxshall and Lincoln (1987) and Huys et al.
(1993). Newman (pers. comm.) feels that, based on
the placement of the male and female genital apertures
and based also on the fact that the male
genital aperture empties at the end of a median intromittant
organ, tantulocarids are so closely related
to the Thecostraca that placement within the
subclass Thecostraca may be warranted. Certainly
they appear more closely related to the Thecostraca
than to any other maxillopodan group (W. Newman,
pers. comm.; J. Høeg, pers. comm.; and some
of the above references). However, for the present
classification, we have retained them as a separate
group within the Maxillopoda but not within the
Thecostraca. Separate status of the Thecostraca and
Tantulocarida was also suggested on morphological
grounds by Boxshall and Huys (1989a), although
we have not closely followed their proposed arrangement
(their fig. 6) for the organization of the
Maxillopoda.
The unusual and confusing life cycle of the tantulocarids
is now more completely known, thanks
to the work of Boxshall and Lincoln (1987) and
Huys et al. (1993). Based on these works and because
of the gap still remaining in the known tantulocarid
life cycle, the possibility that y-larvae (the
Facetotecta) might belong to this taxon has at least
been considered (M. Grygier, pers. comm.; see earlier
discussion under Facetotecta).
SUBCLASS BRANCHIURA
To our knowledge, this subclass, containing a single
order and family, has not changed since Bowman
and Abele (1982) (see also Gruner, 1996). Bill Poly
(pers. comm.) alerted us to the fact that, although
the order Arguloida is often credited to Rafinesque
(1815), Rafinesque employed only the term ‘‘Argulia’’
without treating it as a family or order. The
first person to use the name as an order was apparently
S. Yamaguti (1963, as Argulidea) (B. Poly,
pers. comm.). Bowman and Abele (1982) credited
the family name to Leach (1819) (as did Yamaguti,
1963, and Gruner, 1996). Although Leach’s usage
appeared after Rafinesque’s work, we have credited
Leach with recognition of the family and Yamaguti
(1963) for the order, despite Rafinesque’s original
(1815) use of ‘‘Argulia,’’ which of course became
the basis of both family and order names. Yamaguti
(1963) also established the family Dipteropeltidae,
and some subsequent workers (e.g., Overstreet et
al., 1992; Young, 1998) have continued to recognize
it, although we do not.
SUBCLASS PENTASTOMIDA
One of the most contentious changes in the new
classification is the inclusion within the Crustacea
Maxillopoda of the former phylum Pentastomida,
all members of which are, as adults, parasites in the
respiratory passages of vertebrates (see reviews by
Riley, 1986, and Palmer et al., 1993). An alliance
between pentastomes and branchiuran crustaceans
was first suggested on the basis of sperm morphology
some 29 years ago (Wingstrand, 1972; see also
Wingstrand, 1978; Riley et al., 1978; Grygier,
1983). Inclusion of pentastomes among the Crustacea
was actually considered but rejected by Bowman
and Abele (1982), who at the time felt that
insufficient evidence was available on that issue.
Ironically, it was Abele et al. (1989) (see also Abele
et al., 1992) who finally confirmed this relationship
(although some would debate whether this was
confirmed or not) by comparison of 18S rRNA sequences.
Additional supporting spermatological evidence
has accumulated since that publication (e.g.,
Storch, 1984; Storch and Jamieson, 1992). Storch
and Jamieson (1992) concluded that ‘‘a sister-group
relationship of pentastomids and Branchiura . . . is
confirmed’’ and that ‘‘the sperm of the pentastomebranchiuran
assemblage appear to be the most
highly evolved of the flagellate crustacean sperm.’’
Some modern invertebrate texts now treat the pentastomids
as crustaceans (e.g., Brusca and Brusca,
1990; Ruppert and Barnes, 1994). Brusca and Brusca
(1990) mention additional evidence such as similarities
in the type of embryogenesis, cuticular fine
structure, and arrangement of the nervous system.
Nevertheless, the amazing discovery of fossils
from Middle Cambrian limestones that are extremely
similar to extant pentastomes (Walossek
and Müller, 1994; Walossek et al., 1994) would
seem to cast doubt on placing them within the
Crustacea (see discussions in Walossek and Müller,
1994, 1998; also Almeida and Christoffersen,
1999) and certainly would argue against their being
maxillopods. If these fossils are indeed related to
modern-day pentastomids (an issue we feel is not
yet settled, but see Almeida and Christoffersen,
1999, for a dissenting opinion), then this finding
would dispel any notion that the pentastomes are
a recently derived group. Walossek and Müller
(1994) make the point that, if pentastomids are related
to branchiurans, then the morphology of the
two groups as well as their modes of development
have differed markedly for more than 500 million
years, such that present day similarities of their
sperm morphology might seem to carry less weight.
If the Cambrian fossils are indeed pentastomids—
appearing hundreds of millions of years before
most of their present day hosts were on the scene—
we must rethink whether we can accept such a major
divergence in body plan so soon after the Crustacea
itself appears in the fossil record. Thus, our
inclusion of them here represents an acceptance of
the available molecular and sperm morphology
data (for additional molecular support, see Garey
et al., 1996, and Eernisse, 1997) over apparently
sound fossil evidence to the contrary; this may
prove to be an error. Brusca (2000) suggests a way
to reconcile the issues if early pentastomids were
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