Isobactrus Newell, 1947

Abé, Hiroshi, 2001, Phylogeny and character evolution of the marine mite genus Isobactrus (Acari: Halacaridae), Journal of Natural History 35 (4), pp. 617-625 : 618-620

Isobactrus Newell, 1947

[Type species: Isobactrus setosus (Lohmann, 1889) 5 Aletes setosus Lohmann, 1889 ]

Isobactrus Newell, 1947: 32 , 33, 68; 1953: 119±135; 1984: 30; Bartsch, 1972: 186, 197, 198; 1983: 197; 1989a: 29; Green and Macquitty, 1987: 22; Luxton, 1990: 407; AbeÂ, 1996: 66; 1998: 105, 106.

Diagnosis. Dorsal plates on idiosoma well developed and sometimes fused; ocular plate without setae; one of two corneae completely reduced; dorsum generally with ®ve pairs of setae; preocular setae sometimes turned into tiny projections; coxae fully incorporated into the body; lateral setae on third coxal region occasionally lacking; adjunctive and adanal setae completely lacking; ventral plates usually reduced to small coxal plates; genital plate undeveloped; genital and anal openings usually placed on slightly sclerotized membranous cuticle; genital opening of female ventrally; female with three pairs of perigenital setae and one or two pairs of subgenital setae; male with many long, ®liform perigenital setae and usually four pairs of subgenital setae; gnathosoma ventrad, not visible dorsally; rostrum with two pairs of rostral setae; palp four-segmented; second segment with one dorsal seta; third segment without seta; fourth segment with three setae; tarsus I with clavate solenidion and rudimentary famulus; tarsus II with clavate solenidion; tarsi without ventral setae; carpite moniliform or rod-like; all tarsi with two claws.

Remarks. The genus Isobactrus was originally established by Newell (1947) who designated Aletes setosus Lohmann, 1889 as the type species. Until the establishment of Isobactrus , the constituents: Aletes setosus , Rhombognathus cryptorhynchu s, Rhombognathus magnus and Rhombognathus (s. str.) levis , had been classi®ed within the genus Rhombognathus (s. str.) in Viets’s (1927a, 1927b, 1936) taxonomic system. Newell (1947) erected the genus Isobactrus on the basis of these four species in addition to his newly described species, I. hutchinsoni . Until now, 24 species have been described ( table 1).

Transformation series and characters

Morphological data were obtained from species descriptions and examination of actual specimens including types. The characters of the 27 transformation series used in the analysis are listed in Appendix I, terminology after Abe (1998). The data matrix erected from the discrete morphological characters observed in the transformation series is shown in Appendix II. Multi-characters observed in the outgroups are treated as polymorphism, while characters exclusive to the outgroups were not included in the analysis.

Cladistics

The data matrix (Appendix II) was subjected to maximum parsimony analysis using a PAUP 4.0b pre-release version (SwoOEord, 1996). According to the aforementioned cladistic relationship of the four rhombognathine genera ( AbeÂ,1998), Rhombognathus , Rhombognathides and Metarhombognathus were regarded as outgroups. Multistate transformation series were treated as unordered, using Fitch parsimony (Fitch, 1971). All transformation series were weighted by using a scale weighting option with base weight of 1 so that the total inūence of each transformation series is the same. Consequently, 14 transformation series were weighted 1, 11 transformation series 1/2 and two transformation series 1/3. Character optimization was executed under two diOEerent criteria, accelerated transformation (ACCTRAN) and delayed transformation (DELTRAN) (SwoOEord and Maddison, 1987). Heuristic search options were used: 100 replicates of random addition sequences of the taxa with tree-bisection-reconnectio n (TBR) branch swapping, holding ten trees at each step and saving all equally parsimonious trees. This analysis was followed by a successive-approximation s approach to character weighting based on the re-scaled consistency index (Farris, 1969, 1989) to evaluate the strongest transformation series and to choose among equally parsimonious trees ( Carpenter, 1988, 1994; Emerson and Hastings, 1998).