An Updated Classification of the Recent Crustacea
An Updated Classification of the Recent Crustacea
An Updated Classification of the Recent Crustacea
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elationship, although <strong>the</strong> view is certainly not universally<br />
shared (e.g., see <strong>the</strong> exchange between Olesen,<br />
1998, 2000, and Fryer, 1999, 2001), and <strong>the</strong>re<br />
is a large body <strong>of</strong> evidence suggesting that Diplostraca<br />
is nonmonophyletic. Additionally, <strong>the</strong>re is<br />
considerable doubt concerning <strong>the</strong> monophyly <strong>of</strong><br />
some <strong>of</strong> <strong>the</strong> groups we have included within it, such<br />
as <strong>the</strong> Cladocera. Fryer (1987a, 1995, 1999a, b)<br />
discusses <strong>the</strong> great morphological differences<br />
among <strong>the</strong> four groups traditionally placed in <strong>the</strong><br />
‘‘so-called Cladocera’’ and highlights <strong>the</strong> trenchant<br />
differences among <strong>the</strong>se taxa and <strong>the</strong> difficulty in<br />
reconciling <strong>the</strong>se forms within one taxonomic category.<br />
We should also point out that <strong>the</strong> ‘‘secondary<br />
shield’’ mentioned as unifying <strong>the</strong>se taxa (e.g., by<br />
Walossek, 1993; Olesen et al., 1997; Olesen, 1998)<br />
is, according to Fryer (1996b, 1999b), simply nonexistant,<br />
a misunderstanding <strong>of</strong> <strong>the</strong> nature <strong>of</strong> <strong>the</strong><br />
crustacean carapace. O<strong>the</strong>r characters that supposedly<br />
unite <strong>the</strong> ‘‘diplostracan’’ groups are similarly<br />
called into question by Fryer in a series <strong>of</strong> papers<br />
(1987a–c, 1995, 1996a, b, 1999b). In particular,<br />
after considerable work in attempting to reconstruct<br />
a primitive anomopod from which extant anomopods<br />
could have been derived and by so doing<br />
highlighting <strong>the</strong> great difficulties <strong>of</strong> any such exercise,<br />
Fryer (1995) argued against attempting to<br />
force such disparate taxa as Leptodora, Bythotrephes,<br />
and <strong>the</strong> superficially similar ctenopods into a<br />
taxon with <strong>the</strong> <strong>An</strong>omopoda, stating (pers. comm.)<br />
that ‘‘when those who make <strong>the</strong>se proposals can<br />
support <strong>the</strong>m by evolutionary series that involve<br />
animals that would work, I’ll pay more attention<br />
to <strong>the</strong>m.’’<br />
Within <strong>the</strong> Diplostraca, we have removed <strong>the</strong><br />
‘‘Conchostraca’’ (following to some extent <strong>the</strong> suggestions<br />
<strong>of</strong> Fryer, 1987c, and Olesen, 1998) in recognition<br />
<strong>of</strong> (1) <strong>the</strong> distinct nature <strong>of</strong> <strong>the</strong> Laevicaudata<br />
(Lynceidae), (2) <strong>the</strong> stark differences that separate<br />
Cycles<strong>the</strong>ria hislopi (sole member <strong>of</strong> <strong>the</strong> Cycles<strong>the</strong>riidae)<br />
from all o<strong>the</strong>r conchostracans, and<br />
(3) Cycles<strong>the</strong>ria’s possible affinities to <strong>the</strong> cladocerans<br />
on morphological and molecular grounds (see<br />
Martin and Cash-Clark, 1995; Olesen et al., 1997;<br />
Olesen, 1998; Spears and Abele, 1998, 2000). The<br />
fact that Cycles<strong>the</strong>ria differs significantly from o<strong>the</strong>r<br />
spinicaudate conchostracans, and probably to<br />
<strong>the</strong> extent that it should not be placed among <strong>the</strong>m,<br />
has also been highlighted (Martin and Cash-Clark,<br />
1995; Olesen et al., 1997; Olesen, 1999; Negrea et<br />
al., 1999). Thus, our resulting classification within<br />
<strong>the</strong> Diplostraca differs slightly from, and is in some<br />
ways a compromise between, <strong>the</strong> classification suggested<br />
by Olesen (1998) based on morphological<br />
characters and that suggested by Spears and Abele<br />
(2000) based on molecular data and is easily reconciled<br />
with <strong>the</strong> phylogeny proposed by Negrea et<br />
al. (1999). Our arrangement does not agree with<br />
<strong>the</strong> somewhat preliminary findings <strong>of</strong> Hanner and<br />
Fugate (1997) based on a relatively small segment<br />
<strong>of</strong> <strong>the</strong> genome.<br />
Removal <strong>of</strong> Cycles<strong>the</strong>ria from <strong>the</strong> Spinicaudata<br />
and placing it on an equal footing with <strong>the</strong> remaining<br />
Spinicaudata and with <strong>the</strong> Cladocera necessitated<br />
<strong>the</strong> creation <strong>of</strong> a separate suborder, <strong>the</strong> Cycles<strong>the</strong>rida,<br />
which we are crediting to Sars (1899)<br />
in keeping with ICZN article 50.3.1. Negrea et al.<br />
(1999) used <strong>the</strong> same spelling to refer to an order<br />
(Cycles<strong>the</strong>rida) within <strong>the</strong>ir superorder Conchostraca,<br />
thus indicating a closer affinity <strong>of</strong> Cycles<strong>the</strong>ria<br />
to <strong>the</strong> conchostracans ra<strong>the</strong>r than <strong>the</strong> cladocerans.<br />
We have not taken <strong>the</strong> bolder step <strong>of</strong> actually<br />
including <strong>the</strong> Cycles<strong>the</strong>riidae among <strong>the</strong> Cladocera,<br />
although <strong>the</strong>re is apparently evidence for<br />
this as well. Spears and Abele (1999a, b, 2000) note<br />
that, not only do 18S rDNA sequence data support<br />
<strong>the</strong> close relationships <strong>of</strong> Cycles<strong>the</strong>ria and <strong>the</strong> cladocerans,<br />
<strong>the</strong> two groups also share certain hypervariable<br />
regions <strong>of</strong> <strong>the</strong> gene that are not found in<br />
o<strong>the</strong>r branchiopods, and <strong>the</strong>se are potential synapomorphies.<br />
Ax (1999) first suggested <strong>the</strong> term<br />
‘‘Cladoceromorpha’’ for <strong>the</strong> clade containing Cycles<strong>the</strong>ria<br />
plus Cladocera. Papers by Crease and<br />
Taylor (1998) and Taylor et al. (1999) appear to<br />
<strong>of</strong>fer additional molecular support, and <strong>the</strong> phylogeny<br />
suggested by Negrea et al. (1999:196) supports<br />
such a clade as well, although <strong>the</strong>ir resulting classification<br />
<strong>of</strong> <strong>the</strong> Branchiopoda into five superorders<br />
does not.<br />
Sassaman (1995) presented fascinating insights<br />
into possible phylogenetic models for <strong>the</strong> conchostracan<br />
families based on <strong>the</strong> evolution <strong>of</strong> unisexuality<br />
in <strong>the</strong> group; he views lynceids as <strong>the</strong> sister<br />
group to all o<strong>the</strong>r families, while noting at <strong>the</strong> same<br />
time <strong>the</strong> unusual nature <strong>of</strong> <strong>the</strong> cycles<strong>the</strong>riids, which<br />
he posits as <strong>the</strong> sister group to <strong>the</strong> remaining ‘‘spinicaudatan’’<br />
families. Thus, in many ways, Sassaman’s<br />
(1995) phylogeny is consistent with our classification.<br />
Within <strong>the</strong> former ‘‘conchostracan’’ groups, <strong>the</strong><br />
spelling <strong>of</strong> <strong>the</strong> Lynceidae has been corrected (from<br />
Lyncaeidae, a typographical error in Bowman and<br />
Abele, 1982), and authorship for <strong>the</strong> family is now<br />
credited to Baird, 1845 (L. Holthuis, pers. comm.).<br />
Mark Grygier points out (pers. comm.) that ICZN<br />
Opinion 532 attributes <strong>the</strong> family name to Sayce,<br />
1902; however, <strong>the</strong>re are clearly earlier uses <strong>of</strong> <strong>the</strong><br />
family name Lynceidae (e.g., see review by Martin<br />
and Belk, 1988), and we are crediting <strong>the</strong> family<br />
name to Baird as noted above.<br />
Although <strong>the</strong> genera Imnadia and Metalimnadia<br />
at times have been suggested to represent distinct<br />
families (<strong>the</strong> Imnadiidae Botnariuc and Orghidan<br />
and <strong>the</strong> Metalimnadiidae Straskraba; see Marincek<br />
and Petrov, 1991; Roessler, 1991, 1995a, b; Orr<br />
and Briggs, 1999:8), most workers (e.g., Martin,<br />
1992; Sassaman, 1995) consider <strong>the</strong>m members <strong>of</strong><br />
<strong>the</strong> family Limnadiidae, as do we. Roessler’s (1991)<br />
erection <strong>of</strong> <strong>the</strong> family Paraimnadiidae was based on<br />
a species he described as Paraimnadia guayanensis,<br />
a junior synonym <strong>of</strong> Metalimnadia serratura (see<br />
Orr and Briggs, 1999). We also include among <strong>the</strong><br />
limnadiids <strong>the</strong> genus Limnadopsis and agree with<br />
18 � Contributions in Science, Number 39 Rationale