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Discussion
The monophyly of the in-group taxa, with Bremer support of 5, is well supported by five synapomorphies: the front with a median notch (2-0), the front without a median tooth (4-0), antennules folding transversely or obliquely (12-1), a distinct suture between male abdominal somites 4 and 5 (26-0), and the presence of a median sulcus on male thoracic sternite 4 (39-0). Transversely or obliquely folding antennules is unique.
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Table 2: Characters and their status used in the phylogenetic analysis. |
The Xanthoidea sensu lato is shown to be paraphyletic, consisting of seven major clades, A-G. The basic family-level classification currently in use is largely supported, although some subfamilies are herein elevated to family level and the Xanthoidea sensu lato is separated into several superfamilies (Table 4). Besides the relative position of the Mathildellinae (Clade G), each Clade A-F is construed as being of superfamily status. The results are partly consistent with the topology of Von Sternberg and Cumberlidge (2001), and in-group relationships are more fully resolved than in their analysis.
The analysis suggests that Clade A (Carpiliidae + Zanthopsidae + Tumidocarcinidae + Palaeoxanthopsidae) is the sister-group to the rest of the paraphyletic Xanthoidea. This clade, with a Bremer support of 2, is united by six synapomorphies: the absence of upper orbital fissures (7-19), the absence of a posterior carapace carina (11-1), the antenna situated out of the supraorbital angle (14-1), an occlusal surface of the dactylus of the major cheliped with a molar basal tooth (49-1), cylindrical propodi of pereiopods 2-5 (53-0), and the absence of corneous tips of pereiopods 2-4 (54-1). The analysis strongly supports recognition of the Carpiliidae as a basal lineage within the Xanthoidea as suggested by Coelho and Coelho Filho (1993), Von Sternberg et al. (1999), Von Sternberg and Cumberlidge (2001), and Wetzer et al. (2003). Števčić (2005) had placed the Carpiliidae within the Eriphioidea and the Zanthopsidae and Palaeoxanthopsidae within the Xanthoidea sensu stricto, positions not supported by our analysis. The good fossil record of all four families within Clade A, one of which extends into the Cretaceous (Palaeoxanthopsidae), one of which extends into the Paleocene (Zanthopsidae), and the remainder of which extend into the Eocene, supports the basal position of this clade. Guinot (1978) and Von Sternberg and Cumberlidge (2001) suggested a sister-group relationship between the Carpiliidae and Eriphiidae; however, the analysis rejects a Carpiliidae + Eriphiidae relationship, as Coelho and Coelho Filho (1993), Von Sternberg et al. (1999), and Wetzer et al. (2003) each suggested. The Carpiliidae is basal within Clade A, followed by the Zanthopsidae, and the most derived Palaeoxanthopsidae + Tumidocarcinidae. Within the Carpiliidae sternal sutures 4/5-7/8 of both sexes are complete, and these characters were regarded as plesiomorphic (Guinot, 1978; Števčić, 1995, 1998, 2005; Von Sternberg et al., 1999; Karasawa and Kato, 2003a). The Zanthopsidae + Palaeoxanthopsidae + Tumidocarcinidae clade lacks these plesiomorphic characters and has an incomplete sternal suture 4/5 of both sexes (characters 31 and 33).
The Pilumnoididae (Clade B) is the sister-group to the remainder of the in-group taxa. The Pilumnoididae (Clade B) and the Clades C-G + Portunoidea share three synapomorphies: the presence of a distinct suture between male abdominal somites 3 and 4 (25-0) and movable male abdominal somites 3-5 (27-0, 28-0). Guinot and Macpherson (1987) erected a new subfamily Pilumnoidinae for Pilumnoides and suggested that there is a close relationship between the Pilumnoidinae and Carpiliidae based upon thoracic sternum and cheliped characters. D’Udekem d’Acoz (1999) raised the Pilumnoidinae to full family status, whereas Davie (2002) included the Pilumnoidinae within the Goneplacidae. Most recently, Karasawa and Kato (2003b) supported the recognition of the Pilumnoididae, and suggested that the family was the sister to the Carpiliidae. However, the present analysis does not support a Carpiliidae + Pilumnoididae relationship. Števčić (2005) placed the Pilumnoididae within the Eriphioidea, which our analysis does not support. The Pilumnoididae (Clade B) lacks synapomorphies of Clade A and is derived as the sister to the remaining xanthoid (Clades C-G) + Portunoidea. Therefore, the analysis suggests that the Pilumnoididae warrants its own superfamily, diagnosed below.
Clade C (Xanthoidea sensu stricto) is weakly supported by only one synapomorphy (slender gonopod 1: 60-1). Clade C is a sister-group to a larger clade including the Clades D-G + Portunoidea, and both Clade C and this larger clade are unambiguously united by six characters: medially interrupted sternal sutures 4/5 and 5/6 of both sexes (31-1, 32-1, 33-1, 34-1), the presence of a medial line on male thoracic sternite 3 (37-1), and the absence of transverse sternal sutures 4/5-7/8 (38-1). Clade C contains the Pilumnidae, Domeciidae, Trapeziidae, Tetraliinae (which we consider as part of Trapeziidae), Panopeidae, Pseudo-rhombilidae, and Xanthidae sensu stricto. The Pilumnidae is the sister to the remainder of Clade C, which is supported by its Cretaceous fossil record. The Pilumnidae is supported by five synapomorphies, the absence of a median sulcus on male thoracic sternite 4 (39-1), the anterior end of the male sterno-abdominal cavity reaching to the anterior of thoracic sternite (42-1), a well-developed posterolateral prolongation of male episternite 7 (46-1), the presence of a hook-shaped apex of gonopod 1 (63-1), and the presence of a sigmoid gonopod 2 (68-1).
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Table 3: Input data matrix of 71 characters and 45 taxa. Missing characters states are shown by "?". |
The analysis supports the Domeciidae and Trapeziidae/Tetraliidae relationship suggested by Castro et al. (2004). Many previous works (Von Sternberg et al., 1999; Brösing, 2002; Castro et al., 2004) indicated that the Trapeziidae sensu lato is a rather advanced lineage within the polyphyletic Xanthoidea and is more closely related to 'higher' heterotremes and thoracotremes than to other xanthoids. Števčić(2005) placed the trapeziids, domeciids, and their relatives in a unique superfamily. However, our analysis does not support these contentions. The domeciid + trapeziid + tetraliine clade is derived as the sister to the panopeid + pseudorhombilid + xanthid clade (Jamieson, 1993; Schubart et al., 2000; Von Sternberg and Cumberlidge, 2001).
The Eucratopsinae, previously assigned to the Goneplacidae, was moved to the Panopeidae (Guinot, 1978; Martin and Abele, 1986; Schweitzer and Karasawa, 2004; Števčić, 2005), whereas Williams (1984) and Sakai (2004) retained the subfamily within the Goneplacidae. Hendrickx (1998) treated the Pseudorhombilinae, formerly included in the Goneplacidae, as a distinct family. Sakai (2004) synonymised the subfamily with the goneplacid Carcinoplacinae. In the present analysis, both the Eucratopsinae and Pseudorhombilinae are excluded from the Goneplacidae and are placed within the Xanthoidea sensu stricto (Clade C), and we concur with Hendrickx (1998) and Števčić (2005) in considering the Pseudorhombilidae as a family. We follow Schweitzer and Karasawa (2004) in placing the Eucratopsinae in the Panopeidae.
Hendrickx (1998) previously had noted that the relationship between the Pseudorhombilidae and Panopeidae was difficult to establish. A sister-group relationship between the Panopeidae and Xanthidae has been suggested by previous studies (Coelho and Coelho Filho, 1993; Von Sternberg et al., 1999), and our analysis supports this finding, suggesting the monophyly of the Panopeidae, containing the Eucratopsinae and Panopeinae and with the Panopeidae as a basal clade, followed by the more derived Pseudorhombilidae + Xanthidae sensu stricto clade.
Clade D (Platyxanthidae + Hypothalassiidae + Eriphiidae + Oziidae + Pseudoziidae) stands as the sister to Clades E-F (Goneplacidae + Hexapodidae) + Portunoidea. Clade D is weakly supported by only one character, an anteriorly tapered buccal frame (15-1). Two synapomorphies, an indistinct sternal suture 3/4 (30-2) and a long gonopod 2 (69-0), unite Clade D and the larger clade including Clades E-F + Portunoidea. Guinot (1978) had previously suggested that there was a close relationship between the Platyxanthidae and Eriphiidae, and Karasawa and Kato (2003a, b) had suggested that the Eriphiidae was the sister to the Pseudoziidae. Schweitzer (2005a) mentioned that the Carpiliidae, Zanthopsidae, Eriphiidae, Platyxanthidae, and Tumidocarcinidae comprise a natural group and might warrant their own superfamily. The analysis supports a Platyxanthidae + Eriphiidae + Pseudoziidae relationship, but the relationship of these three families together with the Carpiliidae and Zanthopsidae is not supported. Števčić (2005) erected the superfamily Pseudozioidea for the Pseudoziidae and Flindersoplacidae Števčić 2005, but our analysis does not support its superfamily status.
Hypothalassia is the sister-group of an unresolved polytomy consisting of the Pseudoziidae, Eriphiidae, and Oziidae. Hypothalassia lacks two synapomorphies, a weakly protruded front (5-1) and the absence of the upper orbital fissures (7-1), of the clade (Pseudoziidae + remaining members of Eriphiidae); therefore, it cannot be placed within the previously recognized eriphiid subfamilies. As a result, a new family is herein erected for Hypothalassia .
In the present analysis, the sister-group relationship of the clade (Pseudoziidae + Eriphiidae + (Oziinae + Dacryopilumninae + Menippinae)) is un-resolved; however, the Eriphiidae lack a synapomorphy (a long, filamentous gonopod 2: 70-1) of the monophyletic clade (Oziinae + Dacryopilumninae + Menippinae). The analysis suggests that the Eriphiidae containing the sole genus Eriphia be treated as a distinct family, which Coelho and Coelho Filho (1993), Guinot et al. (2002), and Ng and Clark (pers. comm. in Davie, 2002) have already suggested.
The monophyly of the clade (Oziinae + Dacryopilumninae + Menippinae) is supported by only one synapomorphy (70-1); even so, the Oziidae Dana, 1851a, i.e. Guinot et al. (2002), is considered to be a valid taxon and three subfamilies, Oziinae, Dacryopilumninae, and Menippinae, are here placed within it. The Oziinae is the sister to the Dacryopilumninae and Menippinae, and the Dacryopilumninae is derived as the sister to the Menippinae sensu stricto. The Dacryopilumninae is characterized by seven characters: the absence of a median notch on the frontal margin (2-1), the absence of frontal teeth (3-1), the absence of anterolateral teeth (8-1, 9-1), a completely closed orbital hiatus (10-1), the absence of a large molar tooth on the occlusal surface of the major cheliped (49-0), and the presence of the cheliped merus fused to the basis-ischium (51-1). The Menippinae is defined by one unique synapomorphy, a deep medial hollow on male thoracic sternite 5 (40-1). Ng et al. (2001) and Davie (2002) did not consider the Menippinae to be synonymous with the Oziinae, with which we concur. We also note that within the Eriphioidea, sternal sutures as well as features of the carapace, antennae, orbits, and gonopod 2 (Ng et al., 2001; Davie, 2002) are supported as useful phylogenetic characters. In the Hypothalassiidae, the Eriphiidae, and the Oziinae of the Oziidae, sternal sutures 4/5 and 5/6 of both sexes are incomplete, whereas in the Dacryopilumninae and Menippinae of the Oziidae, sutures 4/5 and 5/6 of females
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Fig. 1. Strict consensus tree of two most-parsimonious trees of 45 taxa. Length = 239, Consistency index = 0.4017, Retention index = 0.7534, Rescaled consistency index = 0.3026. Bremer support of each clade is indicated. Genera, subfamilies, families, and superfamilies listed at right follow the classification largely in the sense of Martin and Davis (2001), Davie (2002), and Poore (2004), reflecting the classification used to generate our list of included taxa. |
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Fig. 2. Strict consensus tree of two most-parsimonious trees of 43 taxa within the Xanthoidea sensu lato and Portunoidea. Length = 239, Consistency index = 0.4017, Retention index = 0.7534, Rescaled consistency index = 0.3026. Unambiguous Character state changes are given. Families and superfamilies at right reflect the new classification scheme proposed herein. |
Clade H (Progeryonoidea + Goneplacoidea + Portunoidea), with Bremer support of 2, is unambiguously united by four synapomorphies: a broadly triangular male abdomen (23-1), a wide thoracic sternum (29-1), a medially interrupted sulcus delimiting thoracic sternites 6 and 7 (35-1), and relatively long propodi of pereiopods 2-5 (52-1). The sister-group relationships within Clade H cannot be resolved as it is a polytomy. Karasawa and Kato (2003a, b) considered that the Goneplacidae was monophyletic based upon cladistic analysis; however, the present analysis suggests that the Goneplacidae sensu Karasawa and Kato is paraphyletic. The Portunoidea clade, with Bremer support of 2, is well defined by five synapomorphies: fused, immovable male abdominal somites 3-5 (27-1, 28-1); a spatulate dactylus of pereiopods 5 (55-1); strongly curved gonopods 1 (61-2); and the presence of a strongly curved, inflated base of gonopods 1 (62-1). The Mathildellidae, which had been considered by Karasawa and Kato (2003a) to be a goneplacid, is derived earliest within the clade, followed by the Geryonidae and Portunidae.
In previous work, the systematic placement of Progeryon has been variable. Bouvier (1922) originally placed Progeryon together with Geryon Krøyer, 1837, within his Galenidae. Balss (1957) and Guinot (1970 (1971)) assigned the genus to the Geryonidae; however, Manning and Holthuis (1981) questioned their opinions. Manning and Holthuis (1989) later suggested that Progeryon and Paragalene Kossmann, 1878, previously assigned to the Geryonidae, might well belong to a new family. Poupin (1996) and D’Udekem d’Acoz (1999) included Progeryon within the Xanthoidea, but did not place it into a family. Ng and Guinot (1999) suggested that Progeryon is a member of the goneplacid Carcinoplacinae. Most recently, Poupin (2003) included Progeryon within the Goneplacidae, but Karasawa and Kato (2003b) showed that Progeryon cannot be assigned to the Goneplacinae sensu Karasawa and Kato (2003a) (= Carcinoplacinae) and did not place it within any goneplacid subfamily. Števčić (2005) placed the genus within a tribe of the Geryonidae. The present analysis suggests that Progeryon warrants its own superfamily and family because the genus lacks the synapomorphies of Clade F (Gone-placidae + Hexapodidae) and the Portunoidea.
Clade F contains five subfamilies of the Goneplacidae, as well as the Hexapodidae, and, with Bremer support of 2, is well defined by five synapomorphies: a weakly protruded front (5-1), the anterior end of the male sterno-abdominal cavity reaching to the anterior thoracic sternite 4 (42-1), a well-developed posterolateral prolongation of male thoracic episternite 7 (46-1), the absence of dark-colored cheliped fingers (48-1), and the absence of corneous tips of the dactyli of pereiopods 2-5 (54-1). Within the clade, the Chasmocarcininae and Trogloplacinae are sister taxa nested as the most derived clade, preceded by the Hexapodidae, Carinocarcinoidinae, Goneplacinae, and the most basal Euryplacinae.
The monophyly of the Euryplacinae, with Bremer support of 4, is well supported by six synapomorphies: the presence of the basal article of the antenna reaching the front (13-0), the possession of a long telson (21-1), distinctly narrow male abdominal somites 4-6 (22-1), a slender, slightly curved gonopod 1 (60-1, 61-1), and a very short gonopod 2 (67-2). The Goneplacinae, with a Bremer support of 2, share two synapomorphies, thoracic sternite 8 visible posteriorly (45-1) and the possession of a truncated apex of gonopod 1 (64-1). The Carinocarcinoidinae + Hexapodidae + Chasmocarcininae + Trogloplacinae clade, with Bremer support of 3, is well united by four characters: the possession of completely fused, immovable male thoracic sternites 3-5 (25-1, 26-1, 27-1, 28-1). The Hexapodidae is derived as the sister to the Chasmocarcininae and Trogloplacinae and is united by six characters: an anteriorly tapered buccal frame (15-1), a relatively narrow male abdomen (23-0), short propodi of pereiopods 2-4 (52-0), possession of dactyli of pereiopods 2-4 with corneous tip (54-0), reduction of pereiopod 5 (58-1), and a short gonopod 2 (67-1), of which one is unique (58-1). The Hexapodidae + Chasmocarcininae + Trogloplacinae clade, with Bremer support of 4, is well supported by five synapomorphies: the absence of anterolateral spines (8-1), anterolateral and posterolateral margins not differentiated (9-1), absence of a carapace posterior carina (11-1), absence of endostomial ridges (16-1), and a narrow male abdominal somite 3 (24-1). A sister-group relationship of chasmocarcinines and trogloplacines is also well supported by three synapomorphies: the ischium of maxilliped 3 about equal to the merus (17-1), the presence of a supplementary plate on male thoracic sternite 8 (43-1), and male thoracic sternite 8 visible ventrally (45-1). The Carinocarcinoidinae + Hexapodidae + Chasmocarcininae + Trogloplacinae and Hexapodidae + Chasmocarcininae + Trogloplacinae clades each have high decay values and are well supported by unambiguous synapomorphies.
The Euryplacinae and Goneplacinae, basal within Clade F, lack the synapomorphies of the clade Carinocarcinoidinae + Hexapodidae + Chasmocarcininae + Trogloplacinae. Therefore, the Hexapodidae is not reduced to subfamily status within the Goneplacidae, but instead, the five recognized goneplacid subfamilies (Euryplacinae, Goneplacinae, Carinocarcinoidinae, Chasmocarcininae, Trogloplacinae) are elevated to full family status. Karasawa and Kato (2003a) had previously synonymised the Carcinoplacinae with the Goneplacinae, because in their phylogenetic analysis, the Carcinoplacinae cannot be clearly separated from the Goneplacinae.
Von Sternberg and Cumberlidge (2001) excluded the Goneplacidae from the Xanthoidea because the Goneplacidae was derived as the sister to the Portunoidea based upon their cladistic analysis, but they did not assign the family to any superfamily. Schweitzer (2005a) suggested that the Goneplacidae might warrant its own superfamily. The present analysis strongly supports the monophyly of the Euryplacidae, Goneplacidae, Carinocarcinoididae, Hexapodidae, Chasmocarcinidae, and Trogloplacidae by five syna-pomorphies; thus, these six families are placed with-in the superfamily Goneplacoidea.
The clade Portunoidea, with Bremer support of 2, is a monophyletic group defined by four synapomorphies: immovable male abdominal somites 3-5 (27-1, 28-1), a spatulate dactylus of pereiopod 5 (55-1), a strongly curved gonopod 1 (61-2), and an inflated, strongly hooked base of gonopod 1 (62-1). Karasawa and Kato (2003a) erected a new subfamily Mathildellinae within the Goneplacidae and suggested that the subfamily formed the most basal clade within the family. However, in the current analysis, the Mathildellinae is the basal-most lineage within the Portunoidea clade and lacks synapomorphies of the Goneplacoidea. Therefore, the Mathildellinae should be given full family status, and the Mathildellidae is here placed within the Portunoidea. The Mathildellidae is derived as the sister to the Geryonidae and Portunidae.
A sister-group relationship of the Geryonidae and Portunidae, with Bremer support of 2, is well supported by six synapomorphies: the basal article of the antenna reaching the front (13-0), a shallow sternal groove 3/4 (30-1), a male sterno-abdominal cavity reaching the anterior thoracic sternite 4 (42-1), chelipeds lacking dark-colored fingers (48-1), the occlusal surface of the dactylus of the major cheliped with a molar tooth (49-1), and dactyli of pereiopods 2-4 without corneous tip (54-1).
The clade Portunidae is defined by three synapomorphies: male thoracic sternite 8 visible ventrally (44-1), a well developed posterolateral prolongation of episternite 7 (46-1), and the presence of a penial groove of male thoracic sternite (47-1). The presence of a penial groove of male thoracic sternite 8 (Rodriquez, 1992) is a unique synapomorphy. Ng (2000) erected a new monotypic genus, Raymanninus , for Benthochascon schmitti Rathbun, 1931, and noted that there is a close relationship between Raymanninus and the geryonid genera based upon characters of the carapace, sternum, abdomen, and gonopods. Raymanninus (Portunidae incertae sedis; Ng, 2000) is the earliest-derived taxon within the portunid clade. The analysis suggests that both the Carcininae and Polybiinae are polyphyletic, as Von Sternberg and Cumberlidge (2001) and Schubart and Reuschel (2005) have already indicated; therefore, the systematics of both of these subfamilies needs to be re-evaluated.
A family of hydrothermal vent crabs, Bythograeidae Williams, 1980, was originally placed within its own superfamily Bythograeoidea, and suggested to be closely related to the Xanthidae sensu lato, Goneplacidae, and Portunidae (Williams, 1980). Guinot (1988) and Guinot et al. (2002) suggested that the Bythograeidae had close affinities with the marine Eriphiidae, Oziidae, 'Carcinoplacidae', and the freshwater Pseudothelphusidae Ortmann, 1893, based upon adult morphology. Tudge et al. (1998) proposed a close relationship between the Bytho-graeidae and the putative trapeziid genus Calocarcinus
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Fig. 3. The familial arrangement of the genera known from the fossil record previously assigned to the Xanthoidea sensu lato, based upon our classification, and their stratigraphic distribution. |
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Table 4: Superfamily and family arrangement of the Xanthoidea sensu lato proposed herein. † indicates a group that is extinct and extant; †† indicates an extinct group. |