Prochelator Hessler, 1970

Brix, Saskia & Bruce, Niel L., 2008, Prochelator tupuhi sp. nov., the first record of Desmosomatidae Sars, 1897 (Crustacea: Isopoda) from New Zealand waters *, Zootaxa 1866, pp. 482-492 : 483-485

Prochelator Hessler, 1970

Synonymy: Prochelator Hessler, 1970: 27 –28; Mezhov, 1986: 139; Kussakin, 1999: 278; George, 2001: 1840; Brenke, Brix and Knuschke, 2005: 180.

Type species: Eugerda lateralis Sars, 1899 ; original designation Hessler (1970).

Composition: P. angolensis Brenke, Brix & Knuschke, 2005; P. abyssalis Hessler, 1970 ; P. hampsoni Hessler, 1970 ; P. incomitatus Hessler, 1970 ; P. lateralis ( Sars, 1899) ; P. litus Hessler, 1970 ; P. s a r s i George, 2001; P. uncatus Hessler, 1970 ; P. k u s s a k i n i Mezhov, 1986; P. serratum ( Fresi & Schiecke, 1969) ; P. tupuhi sp. nov.

Diagnosis: Pereopod 1 large, carpo­chelate, dactylus and propodus forming movable counterpart to large spine­like claw­seta on distal end of carpus. Inferior margin of carpus of pereopod 1 with one midventral seta and a slender seta distally proximal to claw­seta. Pleotelson with posterolateral spines.

Generic remarks: The genus Prochelator was erected by Hessler (1970) who transferred the type species from Eugerdella to Prochelator and described five new species of Prochelator . Desmosoma serratum was recently transferred to Prochelator by Brenke et al. (2005). Five publications have subsequently defined the genus ( Hessler 1970, Mezhov 1986, Kussakin 1999, George 2001 and Brenke et al. 2005). The present diagnosis differs from the diagnosis presented by George (2001) in including species with uniramous uropods. In Desmosomatidae , the uropods are biramous in the plesiomorphic condition but the exopod is reduced and is completely absent in approximately half of the described species. Thinking about Hessler´s (1970) statement about reduction of the exopod, the loss of the exopod might be homoplasious within the family. Although the phylogenetic significance of these reductions is questioned due to the presence or absence in species of the same genus, the reduction may serve well as species­level diagnostic character, but not as a generic character. Precise definition of the apomorphies is important, as the genera Prochelator and Chelator closely resemble each other. According to Hessler (1970) the prefix “Pro­” refers to the fact, that Chelator is probably derived from a species of Prochelator . But accepting phylogenetic sytematics and the concept of monophyly, a present­day taxon cannot be assumed to be derived from another present­day taxon, but rather both are derived from a common ancestor.

To assess the taxonomic position of Prochelator and Chelator , a detailed discussion of all characters (possible apomorphies) and a phylogenetic analysis would be necessary. The principal character shared by Prochelator and Chelator is the chela of pereopod 1. Chelator can be distinguished from Prochelator by the ventral projection at the base of the claw­seta of the former. This projection is the only distinguishing character that can be identified when comparing generic diagnoses of both genera.

The ventral setae on the carpus of pereopod 1 were so uniform between the species that Hessler (1970) assigned to Prochelator , that he gave them the formal designation of “accessory setae”. Hessler (1970) used this term for Prochelator but not for Chelator . He describes the ventral row of small setae on the lower margin of the carpus in species of Chelator as a row of setae, of which none can be labelled as “accessory seta”. The term “accessory seta” is confusing and may be synonymised with the term “major seta” ( Hessler 1970). Both terms are not used in the present paper; instead the setal types on the articles of the pereopods are described. A single midventral seta on the lower margin of the carpus is unique to Prochelator . This midventral seta can be used to distinguish Prochelator and Chelator , in addition to the projection at the base of the claw­seta listed as the only distinguishing character in the paragraph above.

Other desmosomatid species with a chelate pereopod 1 can be distinguished from Prochelator and Chelator by apomorphies. Species included in Oecidiobranchus possess a propodus that is more enlarged than the carpus with the dactylus folding against the ventral margin of the propodus. Unique to Oecidiobranchus is the small branchial chamber ( Hessler 1970). Disparella possess a large cephalic spine at the point of insertion of the antennula. Such a cephalic spine is also found e.g. in Prochelator lateralis and P. uncatus , but not in any species of Chelator . The distinguishing apomorphy of Disparella , which separates the genus from Prochelator , is the elongate slender propodus (more than 3.5 times long as wide) and the setation on the ventral margin of the carpus of pereopod 1. Disparella possess a well defined row of setae behind the claw­seta. Such a row is not found in Prochelator or Chelator . Pereopods 2–4 of Disparella species are obviously setose (carpus> 20 setae, propodus> 8 setae in adult) while in Prochelator the carpus of the anterior pereopods have no more than 10 setae in a row (except P. hampsoni ).

Although the comparison of characters shows that Prochelator and Chelator are very similar, Prochelator and Chelator should be treated as separate genera based on the following reasoning: it is not possible to homologize the setae on the ventral margin of the carpus of pereopod 1, and no transformation series can be postulated explaining how the midventral setae ( Prochelator ) and the few small setae ( Chelator ) originated.

The uniramous uropod in P. incomitatus and P. angolensis may have evolved convergently because the loss of the exopod is an apparently homoplasious character state as it occurs in genera that are otherwise widely disparate; the body form of females differs clearly between the two genera; posterolateral spines on the pleotelson are always present in females of Prochelator while in females of Chelator these posterolateral spines are absent. Furthermore, females of Chelator are more compact and the pereonites are rounded, while species of Prochelator have a different profile from lateral view. While in some species of Prochelator the posterior pereonites and the pleotelson are flattened, in species of Chelator the pleotelson is as high as pereonite 5, the rounded pleotelson form giving the body outline a more compact look than in Prochelator . In Chelator the first four body segments are more compact than in Prochelator (the first pereonite in Chelator twice as high as pereonite 5).