AN UPDATED PHYLOGENY OF THE CONCHOSTRACA CLADOCERA CLADE (BRANCHIOPODA, DIPLOSTRACA)

Transcription

1 AN UPDATED PHYLOGENY OF THE CONCHOSTRACA CLADOCERA CLADE (BRANCHIOPODA, DIPLOSTRACA) BY JØRGEN OLESEN 1 ) Humboldt Universität zu Berlin, Institut für Biologie, Vergleichende Zoologie, Philippstrasse 13, D Berlin, Germany and Zoological Museum, University of Copenhagen, Universitetsparken15, DK-2100 Copenhagen, Denmark ABSTRACT An updated phylogenetic analysis of the Conchostraca and Cladocera (Branchiopoda, Diplostraca) is presented in response to a recently published critique of Olesen (1998). Some errors are corrected and some scorings are modi ed. Practically all earlier conclusions are supported. Among these are (1) monophyletic Diplostraca and Cladocera; (2) an uncertain status of the Conchostraca and Spinicaudata, probably they are both paraphyletic with respect to the Cladocera; (3) uncertain relationships between the four distinct taxa within the Cladocera, though some support was found for the Gymnomera (Haplopoda and Onychopoda). ZUSAMMENFASSUNG Als Antwort auf eine jüngst publizierte Kritik an Olesen (1998) wird eine aktualisierte phylogenetische Analyse der Conchostraca und Cladocera (Branchiopoda, Diplostraca) präsentiert. Einige Fehler der früheren Analyse werden korrigiert, einige Merkmals-Kodierungen modi ziert. Praktisch alle Schlußfolgerungen werden erneut unterstützt, so 1. die Monophylie der Diplostraca und der Cladocera, 2. der unsichere Status der Conchostraca und der Spinicaudata, beide Taxa sind wahrscheinlich paraphyletisch, und 3. die unsicheren Verwandtschaftsbeziehungender vier höheren Taxa innerhalb der Cladocera, obwohl insbesondere die Gymnomera (Haplopoda und Onychopoda) einige Unterstützung erfahren. INTRODUCTION Two years ago I presented the rst cladistic analysis of a wide spectrum of cladocerans and conchostracans (Branchiopoda, Diplostraca) (Olesen, 1998). The 1 ) jlolesen@zmuc.ku.dk c Koninklijke Brill NV, Leiden, 2000 Crustaceana 73 (7):

2 870 JØRGEN OLESEN information used was taken from the literature and from morphological studies conducted in relation to that study and analysed using the parsimony program PAUP (Swofford, 1993). The result was only partly successful, since some parts of the selected cladogram were not convincingly resolved. However, I decided to publish the result because earlier efforts to apply the cladistic approach to branchiopod phylogeny had included only the major taxa or just a few characters (e.g., Wingstrand, 1978; Walossek, 1993; Martin & Cash-Clark, 1995). A testable working hypothesis of the intrinsic phylogeny of the Conchostraca and Cladocera (Diplostraca) was thereby provided. A further reason for making a phylogenetic analysis of certain branchiopod taxa concerned the open state in which the branchiopod taxonomy was left by Fryer (1987a, b). He elevated two distinctly different subgroups of the Conchostraca (Spinicaudata and Laevicaudata) and four ditto of the Cladocera (Ctenopoda, Haplopoda, Anomopoda, and Onychopoda) to ordinal status which, together with the Anostraca and Notostraca, yielded the total number of eight extant branchiopod orders, arranged in a at structure with no supra-ordinal levels indicated. At the same time, a number of more or less well-established taxa, like the Phyllopoda (sensu Preuss, 1956), Diplostraca, Conchostraca, and Cladocera were suggested to be abandoned. The eight extant orders were then, together with two extinct orders, de ned in more detail than hitherto as a working base for future investigation, as said by Fryer (1987b). And, indeed, this paper has been a very useful working base for a number of workers (e.g., Martin, 1992; Walossek, 1993; Olesen, 1998). Seen from a phylogenetic perspective, it can sometimes be a useful and meaningful approach to split up a taxon with an uncertain intrinsic phylogeny (like the Branchiopoda) into the smaller taxa, which are each, with a high degree of certainty, monophyletic. Thereby, any future attempts at grouping them are given a better starting point, as has been done for the Branchiopoda by Fryer (1987b). From the diversity point of view, there are also good reasons for side-ranking the higher taxa of the Cladocera and Conchostraca with the Anostraca and Notostraca, since, for example, the four major cladoceran groups truly are as distinctive as are other orders of the Branchiopoda, as it is being said by Fryer (1987b), even though such reasoning always will contain an amount of subjectivity, as will any other attempt at using the Linnean hierarchical system. A at taxonomic scheme, as the one proposed by Fryer (1987b), can only be considered a starting point for future attempts to recombine the component taxa in high-level taxonomic categories. Such an attempt was made on a phylogenetic basis by Olesen (1998) who found support for a number of the supraordinal categories such as Diplostraca, Cladocera, and Gymnomera, whereas the Conchostraca turned out to be unsupported as did one of the orders in the scheme

3 BRANCHIOPOD PHYLOGENY 871 of Fryer (1987b), the Spinicaudata. Spears & Abele (2000) have meanwhile obtained largely the same results on the basis of molecular data. As a general response to the approach of Fryer (1987b), it is important to point out that differences are of no real importance when trying to establish a phylogeny. More important are the similarities, in particular, of course, those representing synapomorphies. The fact that the members of a certain taxon are very different (as in the Cladocera) does not exclude that they are the closest relatives to each other, as long as some similarities are also present. The only kind of morphological information that could convincingly indicate a polyphyletic origin of the Cladocera would be, for example, to identify similarities (synapomorphies) between cladoceran subgroups and other branchiopods (anostracans or conchostracans, for example) more convincing than those the cladocerans share with each other. No phylogeny re ecting a polyphyletic origin of the Cladocera has yet been proposed. One of the many advantages in using the cladistic approach in developing phylogenetic hypotheses, and in particular computer-based analyses, is the openness of the method. Since a character matrix is presented, it is easy for the reader to check the underlying assumptions and data when confronted with a surprising phylogeny. Hence, in some ways Fryer s (1999) critique of my analysis (Olesen, 1998) was welcome. Some errors were detected and some inaccurate scorings were identi ed. However, by focusing on minor weaknesses of the analysis, Fryer created a distorted picture of the value of the work. Many of the points raised by Fryer are not as clear-cut as pretended by him, and a number of his comments are the result of methodological misunderstandings. The intention of the present paper is, therefore, to clarify certain issues and to point out that practically none of my earlier conclusions needs to be changed on the basis of Fryer s (1999) critique. The best way to answer that criticism is to present an updated analysis of the matrix with the relevant characters corrected. CHANGES TO THE MATRIX OF OLESEN (1998) I have tried to present the arguments in this response to Fryer (1999) as clearly as possible, but it may in some cases be necessary to consult the respective earlier papers to see the background for the comments. Character numbering follows Olesen (1998). 1. Carapace as secondary shield Walossek (1993, 1995) suggested that the apparently similar ontogeny of the carapace in the Conchostraca and Cladocera constitutes a synapomorphy for these

4 872 JØRGEN OLESEN TABLE I Matrix: the updated character matrix. Most modi cations are mentioned in the text. Character numbering is the same as in Olesen (1998) Characters (52) Taxa (49) ANOSTRACA ? / NOTOSTRACA ? ? Laevicaudata Leptestheriidae Limnadiidae Cyzicidae 2 10/ Cyclestheriidae / ? Sida Latona Diaphanosoma Holopedium Polyphemus Podon Evadne Bythotrephes Cercopagis ???? Leptodora Daphnia / Simocephalus Scapholeberinae Ceriodaphnia Moina Moinodaphnia ???? Pleuroxus

5 BRANCHIOPOD PHYLOGENY 873 TABLE I (Continued) Characters (52) Taxa (49) Alonella Chydorus Pseudochydorus ???? Anchistropus ???? Eurycercus Saycia ? ???? Alonopsis ???? Acroperus Camptocercus Alona / Tretocephala ? ? ? ???? Rhynchotalona Graptoleberis Oxyurella ? ?? ???? Leydigia Monospilus / ? ???? Ilyocryptus 3 0/ ? / Acantholeberis ???? Macrothrix Ophryoxus Streblocerus Drepanothrix ? ???? Lathonura ???? Bosmina

6 874 JØRGEN OLESEN taxa, and this idea was accepted by Olesen et al. (1997) and Olesen (1998) (the type of ontogeny is brie y described below). Walossek suggested a new term, secondary shield, for this type of carapace, which was perhaps going too far, since arguments for homology with the carapace of the notostracans can still be found. Prompted by this suggestion, Fryer (1996) reviewed the branchiopod carapace and was of the opinion that the concept of a secondary shield is erroneous.... Furthermore, he claimed that the possession of such a shield as a synapomorphy for the Diplostraca is to base this unity on a non-existent character (Fryer, 1996: 1711), and this is repeated in Fryer (1999). However, I am far from convinced that Walossek (1993, 1995) was wrong. The carapace of some diplostracans appears to have a unique development where a more or less distinct suture separates the naupliar head section and the early growth zone of the carapace from each other. Such a carapace development is seen in Cyclestheria hislopi (Baird, 1859) (see Olesen, 1999) and in other spinicaudate conchostracans (e.g., Sars, 1886), in onychopod cladocerans, and in Leptodora. It is completely different from the regular carapace development of notostracans (e.g., Claus, 1873) and in the Upper Cambrian branchiopod fossil Rehbachiella, where the carapace is a simple posterior continuation of the naupliar head shield. The latter is most likely the plesiomorphic condition. Exactly how the laevicaudate conchostracans and certain cladocerans t into this distinction is as yet uncertain, but that is now in study. The unique and quite similar development of the carapace in a number of conchostracan and cladoceran taxa is, therefore, a valid candidate as a synapomorphy for these taxa (the secondary shield concept of Walossek, 1993, 1995), but the issue needs further exploration. The new states for this character are: 1. Carapace 0 = absent 1 = univalved 2 = bivalved, enclosing the whole body 3 = bivalved, enclosing the thorax and abdomen only 4 = modi ed into a dorsal broodpouch The change allows the carapace type of the Notostraca to be included in the character (state 1). 2. Growth lines of carapace Fryer (1999) comments that Alonopsis elongata Sars, 1862 practices carapace retention, and that not all species of Ilyocryptus do so. Fryer also states that

7 BRANCHIOPOD PHYLOGENY 875 as indicators of af nity growth lines are worthless, save for emphasizing the unity of the Spinicaudata (Fryer, 1999: 1041). The point is, that in a matrix of this kind it would be unfortunate to make an a priori distinction between carapace retention in different taxa. The result of the analysis shows that carapace retention in Monospilus, Alonopsis elongata, various species of Ilyocryptus, and the Spinicaudata, all four are convergences (which is what Fryer believes). An additional result (if Cyclestheria is the sister group of the Cladocera) is that the carapace retention of the Spinicaudata is a plesiomorphy, lost again in the Cladocera. The growth lines of all spinicaudatans would still be homologous, just lost once in a common ancestor of the Cladocera. The reason to consider this possibility at all is the evidence supporting Cyclestheria as the sister group to the Cladocera, something that would exclude the Spinicaudata from being monophyletic. 3. Carapace posterior margin This character was included in order to investigate the general similarities in carapace shape and morphology among some chydorid genera (Chydorus, Pseudochydorus, Anchistropus, Pleuroxus, and Alonella). These genera have a more spherical, in some cases globular, appearance than do other chydorid genera. However, this character is dif cult to code without overlapping with other taxa. It is, therefore, excluded for the moment. Fryer (1999: 1041) says that even if this character united these ve genera it would tell us nothing about their relationship to other branchiopods. A demonstration of any such import was never my intention. This sort of comment is repeated many times by Fryer and evidently re ects a misunderstanding of how much single characters can (or need to) contribute to the entire phylogeny of a taxon. 4. Carapace, plumose setae on ventral margin As Fryer (1999) points out, I erroneously coded Megafenestra as having plumose seta on the ventral margin. The character is deleted from the updated version of the matrix, and Scapholeberis and Megafenestra are included as Scapholeberinae since there can be little doubt about the monophyly of this taxon. Similarly to his comments on the previous character, Fryer (1999: 1041) states that even if these genera did share such remarkable setae this would tell us nothing about their relationship to other anomopods or about the relationship of this order to other groups. My response to this is the same as for the preceding character. 5. Rostral spine Leptestheriid and some juvenile cyzicid spinicaudatans have a small spine on the rostrum. The scoring for Cyzicidae is here changed to polymorphic.

8 876 JØRGEN OLESEN Fryer (1999: 1041) says that beyond suggesting a possible close relationship between these families, a small spine... is of no phylogenetic signi cance. Again, this is correct, but it is not relevant. Fryer is correct when he states that this character is inapplicable for the Haplopoda inasmuch as they have no rostrum. In a further comment on the same character, Fryer claims that several others are of this phylogenetically uninformative nature that refer to attributes of only one group,.... Fryer does not explain the uninformative nature of these characters, but as far as I can tell, he identi es two possible problems : (1) certain character states are present only in some taxa in the matrix (most of the characters he lists); and (2) some characters are occasionally inapplicable to certain taxa (few, if any, of the characters he lists). The rst is not a problem at all. To be informative, a character state necessarily must be present only in some taxa (more than one, less than all). This is probably another case of Fryer pointing with disapproval to the fact that many of the characters used simply associate members of wellestablished groups (Fryer, 1999: 1049). This is true, but it is necessary to include such characters in a phylogenetic analysis; otherwise the well-established groups may end up being unsupported by any synapomorphies. This is absolutely not the same as being uninformative, as Fryer maintains. Uninformative, as it is being used by Fryer, has no meaning in a phylogenetic context: the quali cation ought to be speci ed as either symplesiomorphic or homoplastic. 7. Compound eyes The comments of Fryer (1999) necessitate no change to the matrix. 8. Neck organ (dorsal organ), shape The comments of Fryer (1999) necessitate no change to the matrix. The morphological information used by Olesen (1998) was presented in Olesen (1996). 10. Dorsal keel No change of the matrix is needed. The result of the analysis suggests, as Fryer (1999) believes, that the dorsal keels of Cyclestheria and the two anomopod genera are convergent. 13. Antennules immobile This character refers to the rst antennae of Podon and Evadne (Onychopoda) being basally fused to each other and to the head surface, and hence immobile. This was noted by Mordukhai-Boltovskoi (1968) and repeated by Rivier (1998: 20). Fryer (1999) states that this character is phylogenetically uninformative (which is probably wrong) and uses as an argument that closely related anomopods differ in this respect [i.e., immobility]. In my view, this is not a valid argument for

9 BRANCHIOPOD PHYLOGENY 877 excluding a character from phylogenetic consideration. A character easily can be too varied in one taxon to be readily used, and, at the same time, provide solid information in another taxon. If we were always to exclude such characters, our stock of usable information would soon vanish. In this particular case, other taxa of the Onychopoda, like Cercopagis, have rst antennae that do not exhibit the fused condition described. Here, the antennules are small but they are still recognizable as real appendages, similar to those seen in a number of other cladocerans. This shows that the morphology of Podon and Evadne is a derived condition within the Onychopoda, and, therefore, must have been developed independently of that of certain daphniids. Later, in the same section, Fryer states that the antennules were primitively mobile; their condition in derived forms re ects functional needs as much as phylogeny. I believe that it is arti cial and irrelevant to distinguish between changes in morphology as either re ecting functional needs or re ecting phylogeny, at least in the rst part of an analysis. All similarities are potentially commonly derived (synapomorphies), no matter whether we can imagine a functional need for the modi cation or not. It is unwise to exclude characters from consideration just because we may perceive a functional purpose. 19. Antennae with two proximal setae Some phylogenetic information can be obtained from the number of setae on the antennal protopod of various cladocerans, although perhaps not in the way Olesen (1998) attempted. The character is excluded from the matrix for the moment. Fryer (1999) is correct in many of his comments on this character, but it is odd that he interprets the? in the matrix for some chydorid genera as indicating the absence of these setae (Fryer, 1999: 1044). He also misunderstands that this character pertains to the possession of exactly one pair of closely placed setae, and not to just a number of setae. The character was one of the many supporting characters for the Anomopoda, and its removal does not change the tree topology. 27. and 28. Claspers No further comments regarding the claspers will be presented here. The characters s scorings are kept unchanged. Additional discussion can be found in Olesen et al. (1997). 33. Trunk limbs 3 and 4, lter plates The scoring for this character is now modi ed to also include Ilyocryptus, although I am aware that this is likely to provoke a comment from Fryer similar to those given for characters 2 and 10.

10 878 JØRGEN OLESEN 34. Trunk limb 5, modi cation for posterior closing of inter-limb space as part of ltration cycle Fryer (1999) seems to make a case that there are problems in the scoring of this character (Fryer, 1999: 1046). He starts by stating that the specialization mentioned is an apomorphy for the Daphniidae, which is exactly what the analysis in Olesen (1998) showed. Fryer continues by arguing that the feeding mechanism in, for example, the Spinicaudata, is so different from that of the Daphniidae that no meaningful comparison of the action of the fth trunk limb is possible. But such a comparison is not at all the point. The function of this character in the analysis is only to point out the similarities among the members of the Daphniidae; that this family of cladocerans is different in this respect from other branchiopods only supports the validity of the character. In a further comment on this character, Fryer (1999) says that the character has no real meaning since one could nd more characters related to other limbs supporting the Daphniidae (my paraphrasing of Fryer s comments). However, it can hardly be an argument for the non-validity of a character that more characters could be identi ed in support of the same taxon. 36. Abdominal dorsal laments Fryer (1999) comments that the lack of dorsal abdominal laments in the Moininae, in his view, is a necessary correlate of the presence of a Nährboden in this taxon. This sounds likely, but it cannot change the a priori coding of this character. In the preferred cladogram there would be room for the hypothesis of Fryer. 37. Paired abdominal setae The scoring is changed for Leptodora, since it also has such a pair of setae. Probably it would be more correct to refer to them as telsonal setae, because this is where they appear to be attached. 41. Gut, anterior diverticula I agree that this character was weakly and erroneously coded in Olesen (1998). The intention was to point at the similarity between the simple, sac-like, anterior gut diverticula of certain anomopods. However, as pointed out by Fryer, the character thereby ignores the fact that different types of anterior gut diverticula are present in a number of other branchiopods. The character is excluded even though the organ is potentially informative phylogenetically, in a way yet to be clari ed.

11 BRANCHIOPOD PHYLOGENY Ephippium The presence of different types of ephippia in the Anomopoda and of an ephippium-like structure in Cyclestheria was scored the same way in Olesen (1998), even though it was admitted in the text that the homologies were uncertain. Fryer (1999) says that the structure is not exactly the same, but I am aware of no detailed study that documents this. In any case, the result of the phylogenetic analysis suggests that the structures have arisen independently (because of the disparate position of the taxa in the resulting tree), which is also what is believed by Fryer. The comment of Fryer necessitates no change to the matrix. OTHER CORRECTIONS TO OLESEN (1998) Unfortunately, the wrong cladogram was printed as g. 14 in Olesen (1998). The one shown is erroneously identical to his g. 15. The true consensus tree of the 20 equally shortest trees resulting from successive weighting had instead Cyclestheria in a sister group position to the Cladocera (as referred to in the Discussion in Olesen, 1998: 527) and it also included some other minor differences, of less importance now the matrix has been modi ed. A carapace has been indicated to be present in the early stage of Leptodora depicted in g. 4b (in Olesen, 1998), but this is wrong, since a carapace has not yet developed in this stage. GENERAL COMMENTS ON FRYER (1999) Fryer (1999) ends his commentary paper addressing Olesen (1998) by listing what he believes are inconsistencies in the deductions from the analysis (Fryer, 1999: 1048, 1049). However, not a bit of this is correct. He rst writes: Although the Spinicaudata and Conchostraca are not supported by the analysis, and while he has doubts about the unity of the Cladocera, Olesen (1998) includes all these groups in one taxon, the Diplostraca. Even if Fryer had got the premises correct (non-monophyly of certain included taxa; see below), problems in the monophyly of higher level taxa would not necessarily follow (in this case the Diplostraca). This would be the same as saying that if the Conchostraca are not monophyletic, then the Branchiopoda, or the Crustacea for that matter (not to go any further), could not be used as a valid taxon. Later in the same paragraph, he expresses the opinion that it is inconsistent to use the names of two taxa in the title of the paper (taxa Conchostraca and Cladocera) whose validity I, according to Fryer, claim to have reasons to doubt. It appears manipulative to say that I have doubts about the validity of the Cladocera

12 880 JØRGEN OLESEN (and these were not the words I used), and omit what else I said. In fact, I did nd support for the monophyly of the Cladocera, which is in contrast to Fryer (1987a, b). What I said in Olesen (1998: 525, 526) was, that the majority (but not all) of the supporting characters were reduction characters and the proposed monophyly may, therefore, not seem well supported. I also said (which is important, and what was omitted by Fryer, 1999) that an eventual polypheletic origin of the Cladocera (like the one claimed by Fryer, 1987a, b) can only be shown by identifying potential synapomorphies between separate cladoceran taxa and other crustaceans, such as conchostracans or other branchiopods. I further said, that such characters are not present or have not been identi ed, and the Cladocera are therefore (despite the large differences between the four orders) regarded as monophyletic until there is evidence to the contrary. Concerning the Conchostraca, Olesen (1998) did state that the Conchostraca are unsupported in the strict consensus tree (Conclusions, Olesen, 1998: 532). However, that same tree (his g. 13, virtually identical to g. 1 herein), upon which the discussion and the conclusion were based, gave no support for either a para- or a polyphyletic origin. The Laevicaudata, Cladocera, Cyclestheria, and the remaining spinicaudatans simply formed (and form) one large polytomy. The status of the Conchostraca (and the Spinicaudata) was, therefore, said to be uncertain (Conclusions, Olesen, 1998: 532). At that stage, I was uncertain whether the Conchostraca (or Spinicaudata) might perhaps, or not, eventually turn out to be convincingly supported. I thus had no basis for suggesting another classi cation involving the removal of the Conchostraca. As for the use of the name Conchostraca in the title, I found it preferable to be conservative and retain the name, until a well-supported phylogeny suggests differently. Hence, there is no inconsistency regarding use of taxon names in the title of Olesen (1998), as claimed by Fryer (1999). ANALYSIS OF THE MODIFIED MATRIX In contrast to the impression left by Fryer (1999), it has been necessary to make only very few changes to the matrix. The result of running the modi ed matrix in PAUP is largely the same as shown in Olesen (1998). Heuristic tree searches (search conditions the same as in Olesen, 1998: 524) yielded 1216 equally and most parsimonious trees with a length of 112 steps. This is approximately ve times as many, slightly shorter, trees than Olesen (1998) found, which is no surprising difference taking into account the modi cations of the matrix. The strict consensus tree of these trees (showing what is common amongst them) is, with one exception, the same as the strict consensus tree in Olesen (1998),

13 BRANCHIOPOD PHYLOGENY 881 Fig. 1. Strict consensus tree of 1216 most parsimonious trees at 112 steps. The only difference with g. 13 in Olesen (1998), is less resolution in the spinicaudatans(see text). upon which that paper s major conclusions were built. The only difference is, that the relationships among the spinicaudatans (not including Cyclestheria) are now left unresolved because of a change in the scoring of Rostral spine (character 5) in the Cyzicidae to polymorphic (not present in all taxa). Apart from this, all branching is identical to Olesen (1998) and all major conclusions are the same. These were the following (with some comments added): Diplostraca monophyletic. This has recently been supported by molecular data of Spears & Abele (2000).

14 882 JØRGEN OLESEN Conchostraca possibly paraphyletic, since the positions of both Cyclestheria and the Laevicaudata are unresolved. Spinicaudata possibly paraphyletic since the position of Cyclestheria is uncertain. Various types of molecular data suggest a position of Cyclestheria as the sistergroup of the Cladocera (Crease & Taylor, 1998; Taylor et al., 1999; Spears & Abele, 2000; see also below). Cladocera monophyletic. Different types of molecular data (Crease & Taylor, 1998; Taylor et al., 1999; Spears & Abele, 2000; see below) have recently supported this. Relationships among major cladoceran taxa unresolved, even though the Onychopoda and Haplopoda may constitute a monophyletic group (the taxon name would be Gymnomera). The supporting characters used need re-evaluation. Three major cladoceran taxa monophyletic (Onychopoda, Anomopoda, Haplopoda). Status of the Ctenopoda uncertain. Monophyly of three families within the Anomopoda is supported (Daphniidae, Chydoridae, and Bosminidae) while the Macrothricidae are likely not to be monophyletic. However, the information provided in Olesen (1998) for a possible division of this last family is weak, as already noted there and now repeated by Fryer (1999). The newly erected family Ilyocryptidae Smirnov, 1992 was not included in the scheme put forward by Olesen (1999), but perhaps it should have been, since cutting off distinct, clearly monophyletic taxa from a taxon that appears to be badly supported can be viewed as a sensible approach. However, the monophyly of the remaining macrothricids (if Ilyocryptus is taken out) would still be highly uncertain. Relationship among the families (Macrothricidae most likely not monophyletic) within the Anomopoda remains unresolved. I do not accept the suggestion of Fryer (1995) for the relationship of four families in the Anomopoda since no supporting characters were provided. Monophyly of the subfamily Aloninae within the Chydoridae apparently uncertain. RECENT WORKS ON BRANCHIOPOD PHYLOGENY USING MOLECULAR DATA Recently, a number of papers dealing with branchiopod phylogeny using molecular information have appeared (Hanner & Fugate, 1997; Crease & Taylor, 1998; Schwenk et al., 1998; Taylor et al., 1999; Spears & Abele, 2000). In relation to the phylogenetic questions addressed in the present paper, the following can be mentioned. The monophyly of the Diplostraca is supported by 18S rdna sequence data (Spears & Abele, 2000). A sistergroup relationship between Cyclestheria

15 BRANCHIOPOD PHYLOGENY 883 hislopi and the Cladocera is supported by 18S rdna sequence data (Taylor et al., 1999; and Spears & Abele, 2000). This relationship was additionally supported by a number of unique and homologous expansion regions in the 18S rdna gene (Crease & Taylor, 1998; Taylor et al., 1999; Spears & Abele, 2000), found only in Cyclestheria and the cladocerans examined. The Cladocera are supported by 18S rdna sequence data (Taylor et al., 1999; and Spears & Abele, 2000), and by the presence of a unique expansion region in the 18S rdna gene only found in the Cladocera (Crease & Taylor, 1998; and Taylor et al., 1999). A number of molecular based results concerning the intrinsic phylogeny of the Cladocera has also appeared, but these will not be outlined here (see Schwenk et al., 1998; and Taylor et al., 1999). As summarized in a combined tree in Taylor et al. (1999, g. 2b) and discussed by Spears & Abele (2000), this means that three principally different types of evidence are supporting a monophyletic taxon Cladocera with Cyclestheria as its sistergroup: morphology, presence of certain stem-loop structures (expansion regions) in the 18S rrna gene, and sequence data for the same gene. This is strong support and there can only be little doubt left about this phylogenetic hypothesis. Ax (1999) suggested the name Cladoceromorpha for the Cladocera + Cyclestheria, a suggestion that is followed here. CONCLUDING REMARKS Much of the criticisms raised by Fryer (1999) against Olesen (1998) are based on methodological misunderstandings, mostly related to mistaken ideas of how much single characters should contribute to an entire phylogeny. Fryer (1999) also distinguishes (wrongly, in my opinion), between morphological changes as either re ecting functional needs or re ecting phylogeny. I think this is an irrelevant a priori distinction, as all changes are potentially commonly derived (synapomorphies). Strangely, Fryer (1999) also appears to believe that if subgroups are not monophyletic (e.g., Conchostraca), then they cannot be included in a higher level taxon (in this case the Diplostraca). This should not hide the fact that Fryer (1999) pointed out some real errors, which I have corrected here in a modi ed version of the matrix. However, these changes had virtually no effect on the topology of the strict consensus tree of the most parsimonious trees, and practically all conclusions from Olesen (1998) are unchanged. It is important to stress that there is still some way to go before there is a completely satisfactory morphology based phylogeny for the Branchiopoda. A number of the characters used by Olesen (1998) need re-evaluation, and many

16 884 JØRGEN OLESEN other parts of the morphology of the Branchiopoda need to be explored for additional characters. It is correct when Fryer (1999) remarks that many of the characters used in Olesen (1998) only serve to hold together members of major taxa, but this only re ects the dif culties in locating characters that relate these taxa to each other, such as the major taxa within the Cladocera and the families within the Anomopoda. It must also be pointed out that it is necessary to evaluate the validity of what Fryer (1999) calls a well-established group by locating its supporting characters. Well-established groups cannot just be accepted a priori, which is why characters with a great likelihood of supporting them were included in Olesen (1998). As mentioned in the introduction to this commentary paper, a strength of the cladistic approach is the openness of the method. It provides an analytical tool for discussing and identifying in a precise way what is the morphological background for any disagreement in views on phylogeny. Instead of the disagreement just taking the form of forceful statements, it can now take the form of correction of errors, different interpretations of morphology, disallowal of characters, etc. The commentary paper of Fryer (1999) and the present response to it should document this in full. In my view, it is not crucial whether an analysis is computer-assisted or not. What is most important is to provide character support to all phylogenies presented, or for changes in the systematics of a group, for that matter, and also to closely consider and evaluate the consequences in terms of potential convergences or later reversals. I hope that this will be the case for all future phylogenetic studies on the Branchiopoda. As the alternative viewpoints addressed in the present paper show, a lot of work still remains to be done on branchiopod phylogeny. ACKNOWLEDGEMENTS Niel L. Bruce, Mark J. Grygier, and Stefan Richter kindly commented on the manuscript. REFERENCES AX, P., Das System der Metazoa II. Ein Lehrbuch der phylogenetischen Systematik: (G. Fischer, Stuttgart, Jena, Lübeck, Ulm). CLAUS, C., Zur Kenntnis des Baues und der Entwicklung von Branchipus stagnalis und Apus cancriformis. Abhandlungen der Königlichen Gesellschaft der Wissenschaften in Göttingen, 18: , pls CREASE, T. J. & D. J. TAYLOR, The origin and evolution of variable-region helices in V4 and V7 of the small-subunit ribosomal RNA of branchiopod crustaceans. Molecular Biology and Evolution, 15 (11):

17 BRANCHIOPOD PHYLOGENY 885 FRYER, G., 1987a. A new classi cation of the branchiopod Crustacea. Zoological Journal of the Linnean Society, London, 91: , 1987b. Morphology and the classi cation of the so-called Cladocera. Hydrobiologia, 145: , Phylogeny and adaptive radiation within the Anomopoda: a preliminary exploration. Hydrobiologia, 307: , The carapace of the branchiopod Crustacea. Philosophical Transactions of the Royal Society of London, (B) 351: , A comment on a recent phylogenetic analysis of certain orders of the branchiopod Crustacea. Crustaceana, 72 (9): HANNER, R. & M. FUGATE, Branchiopod phylogenetic reconstruction from 12S rdna sequence data. Journal of Crustacea Biology, 17 (1): MARTIN, J. W., Branchiopoda. In: F. W. HARRISON & A. G. HUMES (eds.), Microscopic anatomy of invertebrates, 9, Crustacea: (Wiley-Liss, New York). MARTIN, J. W. & C. CASH-CLARK, The external morphology of the onychopod cladoceran genus Bythotrephes (Crustacea, Branchiopoda, Onychopoda, Cercopagididae), with notes on the morphology and phylogeny of the order Onychopoda. Zoologica Scripta, 24: MORDUKHAI-BOLT OVSKOI, P. D., On the taxonomy of the Polyphemidae. Crustaceana, 14: OLESEN, J., External morphology and phylogenetic signi cance of the dorsal/neck organ in the Conchostraca and the head pores of the cladoceran family Chydoridae (Crustacea, Branchiopoda). Hydrobiologia, 330: , A phylogenetic analysis of the Conchostraca and Cladocera (Crustacea, Branchiopoda, Diplostraca). Zoological Journal of the Linnean Society, London, 122: , Larval and post-larval development of the branchiopod clam shrimp Cyclestheria hislopi (Baird, 1859) (Crustacea, Branchiopoda, Conchostraca, Spinicaudata). Acta Zoologica, Stockholm, 80: OLESEN, J., J. W. M ARTIN & E. W. ROESSLER, External morphology of the male of Cyclestheriahislopi (Baird, 1859) (Crustacea, Branchiopoda, Spinicaudata),with a comparison of male claspers among the Conchostraca and Cladocera and its bearing on phylogeny of the bivalved Branchiopoda. Zoologica Scripta, 25: RIVIE R, I. K., The predatory Cladocera (Onychopoda: Podonidae, Polyphemidae, Cercopagidae) and Leptodorida of the world. In: H. J. F. DUMONT (ed.), Guides to the identi cation of the microinvertebratesof the continental waters of the world, 13: (Backhuys Publishers, Leiden). SARS, G. O., Development of Estheria packardi as shown by arti cial hatching from dried mud. Archiv for Mathematik og Naturvidenskab,(B) 18 (2): 1-27, pls SCHWENK, K., A. SAND, M. BOERSMA, M. BREHM, E. MADER, D. OFFERHAUS & P. SPAAK, Genetic markers, genealogies and biogeographic patterns in the Cladocera. Aquatic Ecology, 32 (1): SMIRNOV, N. N., The Macrothricidae of the world. In: H. J. F. DUMONT (ed.), Guides to the identi cation of the microinvertebratesof the continental waters of the world, 1: (SPB Academic Publishing, The Hague). SPEARS, T. & L. G. ABELE, Branchiopod monophyly and interordinal phylogeny inferred from 18S ribosomal DNA. Journal of Crustacean Biology, 20 (1): SWOFFORD, D. L., PAUP: phylogenetic analysis using parsimony. Version 3.1. (Computer program distributed by Illinois State Natural History Survey, Champaign, Illinois). TAYLOR, D. J., T. J. CREASE & W. M. BROWN, Phylogenetic evidence for a single longlived clade of crustacean cyclic parthenogens and its implications for the evolution of sex. Proceedings of the Royal Society of London, (B) 266:

18 886 JØRGEN OLESEN WALOSSEK, D., The Upper Cambrian Rehbachiella and the phylogeny of Branchiopoda and Crustacea. Fossils and Strata, 32: 1-202, pls , The Upper Cambrian Rehbachiella, its larval development, morphology and signi - cance for the phylogeny of Branchiopoda and Crustacea. Hydrobiologia, 298: WINGSTRAND, K. G., Comparative spermatology of the Crustacea Entomostraca. 1. Subclass Branchiopoda. Det Kongelige Danske Videnskabernes Selskab, Biologiske Skrifter, 22: First received 2 March Final version accepted 4 April 2000.