A classification of living and fossil genera of - Raffles Museum of ...

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HOW MANY SPECIES OF DECAPODA
ARE THERE?
Chace (1951) was the last authority to tabulate all genera
and species of then-known Recent decapod crustaceans,
based on a comprehensive file card system that he and
colleagues maintained at the United States National
Museum of Natural History. Chace estimated the known
number at 8,321 species, distributed among 1,000 genera,
with Brachyura accounting for 4,428 species (in 635
genera). According to Ng et al. (2008), this latter figure
compares quite favorably with their estimate of 4,120
known at that time (raised to 6,793 by 2008), and thus we
can place some confidence in Chace’s global decapod
species estimate of 8,321 known in 1951. More recent,
somewhat vague, published estimates put the number of
species somewhere upwards of 10,000 species, with only
Martin & Davis (2006) providing an accurate estimate of
7,905 species (but based on a database estimated to contain
only about half of all described crustacean species). Based
on our current effort, we put the total number of extant
species of Decapoda at 14,756 (in 2,725 genera), with
Brachyura accounting for 6,835 species, i.e. 46% (Table 1).
This implies that in the last 50 years, the number of
described species has nearly doubled! However, we are of
course still a long way from knowing the true global
diversity of Decapoda. The number of known fossil species
currently stands at 3,300. Here too, discoveries will
continue for some time to come, as new localities are
explored, more revisions are completed, and museum
collections are more thoroughly studied.
THE HIGHER CLASSIFICATION OF DECAPODA
The higher classification of the Decapoda is deeply rooted
in the early works of such luminaries as H. Milne Edwards,
Dana, Latreille and contemporaries, to which can be added
the often divergent opinions of Borradaile, Bate and others.
In the last few decades, a more stable classification has
emerged due to the more rigorous application of cladistic
techniques to both morphological and molecular data, as
well as to the advent of more sophisticated morphological
and molecular studies. Although the last word on the
subject has not been given (especially within the
Brachyura), we now appear to have broad consensus on the
validity of two suborders: Dendrobranchiata and
Pleocyemata, amply supported by genetic and
morphological studies. The Pleocyemata are herein further
divided into 10 infraorders, most of which have been
recognized for some time and appear uncontroversial
(Caridea, Stenopodidea, Astacidea, Achelata, Anomura,
Brachyura). In addition to those, we follow Dixon et al.
(2003), Ahyong & O’Meally (2004), and Bracken et al.
(2009) in affording infraordinal status to the glypheid
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lobsters (as infraorder Glypheidea). Following Scholtz &
Richter (1995) and Ahyong (2009), we also recognize the
infraorder Polychelida. We follow Robles et al. (2009) in
the recognition of two infraorders (Axiidea, Gebiidea) for
the thalassinidean families. Within the Brachyura, and
departing from Ng et al. (2008), we do not recognize the
section Podotremata, and instead follow Ahyong et al.
(2007) with three sections: Dromiacea, Raninoida,
Cyclodrippoida (see also Ahyong, Naruse et al., 2009).
Further notes on the classification employed within the
various groups are presented below.
We stress that our arrangement of taxa, both at higher
levels (families within superfamilies, sections, etc.) and at
lower levels (genera within families), is not agreed upon
by all workers.
Dendrobranchiata, Stenopodidea, Caridea. For the
Dendrobranchiata, apart from some additional genera, little
has changed since the comprehensive treatment in Pérez
Farfante & Kensley (1997), which in turn formed the basis
of the Martin & Davis (2001) classification. Recent and
ongoing molecular studies (e.g. Chan et al., 2008; Ma et
al., 2009) may require changes in the near future. The
stenopodideans, a relatively small group, also have
changed little since the Martin & Davis (2001) list,
although one family and several new genera have been
added. For the Caridea, an enormous and diverse
assemblage, our work is based on ongoing updates to the
Holthuis (1993) catalogue by S. De Grave and C. Fransen.
One difference pertains to the subfamilies in Atyidae,
which we do not recognize herein. Another difference
concerns the family status of several palaemonoid groups,
where we follow Bruce (1993) in recognizing eight
families. The order of superfamilies followed here is not a
reflection of phylogenetic order, and merely follows Chace
(1992) and Holthuis (1993). We acknowledge the recent
study by Bracken et al. (2009), which questioned the
validity of the superfamily arrangement within Caridea as
well as the monophyly of several families. Christoffersen
(1986, 1987, 1988a, 1988b, 1989, 1990) proposed an
alternative arrangement of caridean systematics (see
Holthuis, 1993, for a discussion). Although some of his
suggestions have been incorporated over the years (e.g. see
Martin & Davis, 2001), many others were rejected.
However, recent, as yet unpublished, work has forced
caridean workers to look more closely at some of
Christoffersen’s suggestions. Evidence that an extensive
rearrangement of caridean superfamilies and families is
needed is forthcoming in ongoing morphological and
molecular studies, but for now we adhere to the more
traditional classification.
Astacidea. The evolutionary relationships and taxonomy
of groups within the infraorder Astacidea are far from
settled and stable. Cladistic methods applied to
morphology (since the mid-1990s) and to molecular data
(since 2004) hold promise, but results often disagree even
at high taxonomic levels. For example, the extinct family