Mixocyclops mortoni, Tang & Knott, 2009

Tang, Danny & Knott, Brenton, 2009, Freshwater cyclopoids and harpacticoids (Crustacea: Copepoda) from the Gnangara Mound region of Western Australia, Zootaxa 2029, pp. 1-70 : 42-48

Mixocyclops mortoni sp. nov.

( Figs 22–25)

Type material. Holotype ♀ ( AM P.78919) and paratype ♀ (dissected and mounted on slide) ( AM P.78920), YN7 bore, Yanchep National Park, Western Australia (31°33'17''S, 115°41'13''E), 17 July , 1992.

Other material examined. Lot 51 Cave ( YN555 ), Yanchep , Western Australia (31°34'31''S, 115°42'10''E): 1 copepodid, 7 November , 2006; Gilgie Cave ( YN27 ), Yanchep National Park , Western Australia (31°34'07''S, 115°41'18''E): 1 damaged ♂ (dissected and mounted on slide) ( AM P.78921), 17 March , 1993; Edgecombe spring, Ellenbrook , Western Australia (31°47'39''S, 115°59'43''E): 1 ♀ ( AM P.78922), 9 October , 2006; Egerton spring, Ellenbrook , Western Australia (31°46'18''S, 115°58'51''E): 1 ♀ dissected and mounted on slide, 9 October, 2006 .

Description of female. Total body length (measured from anterior margin of cephalothorax to posterior margin of caudal rami), based on 2 specimens, 375–380 µm; body width 142.5–150 µm. Prosome ( Fig. 22A) composed of cephalothorax and 3 free pedigerous somites. Urosome ( Figs 22A–B) comprising fifth pedigerous somite, genital double-somite and 3 free abdominal somites. Genital double-somite wider (75–90 µm) than long (55–65 µm); seminal receptacle unilobate, situated in proximal ½ of genital double-somite. Anal somite ( Figs 22B–C) with spinular row along posterior margin and patch of spinules on anal cleft; anal operculum large, semi-circular, extends to proximal margin of caudal rami. Posterior margin of all somites smooth.

Caudal ramus ( Figs 22B–C) longer (22.5–25 µm) than wide (15–16.5 µm); bears 6 setae (seta I absent), 2 proximomedial spinules on dorsal surface and spinular row at insertion point of all elements except setae IV and V. Seta II pinnate, seta VII setulate, all others spinulate. Setae II and VI subequal in length. Seta III 2.9 times as long as seta VI. Seta VII about 2 times longer than ramus. Setae IV and V with proximal breaking plane.

Antennule ( Fig. 22D) 11-segmented, with armature and ancestral segmentation pattern in brackets as follows: 7 (I–V), 4 (VI–VII), 7 (VIII–XI), 2 (XII–XIII), 1+spine (XIV), 2 (XV–XVI), 3 (XVII–XX), 2+ae (XXI–XXIII), 2 (XXIV), 2+ae (XXV), and 7+ae (XXVI–XXVIII). First segment with proximal spinular row. Short spine on segment 5 blunt.

Antenna ( Fig. 23A) 5-segmented, comprising coxa, basis and 3-segmented endopod. Coxa naked and unarmed. Basis unornamented and armed with distomedial barbed seta; exopodal seta absent. Proximal endopodal segment with inner barbed seta and outer spinular row. Middle endopodal segment bears 5 medial setae and distolateral spinular row. Terminal endopodal segment with usual 7 apical setae and 2 groups of lateral spinules.

Labrum ( Fig. 23B) with 10 teeth along posterior margin and setules on anterior surface.

Mandible ( Fig. 23C) composed of coxa and highly reduced palp. Coxal gnathobase with tricuspidate tooth, bicuspidate tooth, 4 unicuspidate teeth, barbed seta and fine spinular row. Palp represented by sparsely pinnate seta.

Maxillule ( Fig. 23D) comprising praecoxa and 2-segmented palp. Praecoxal arthrite bears 6 medial elements, 3 distal unicuspidate teeth and distal seta. Coxobasis with proximal seta and 3 terminal setae. Endopod 1-segmented, armed with 3 setae.

Maxilla ( Fig. 23E) 5-segmented, composed of praecoxa, coxa, basis and 2-segmented endopod; all segments naked. Praecoxal endite with 2 spinulate setae. Proximal and distal coxal endites with 1 and 2 spinulate setae, respectively. Basis drawn out into large naked claw; accessory armature consists of weakly spinulate spine and naked seta. Proximal and distal endopodal segments each armed with 2 setae.

Maxilliped ( Fig. 23F) 4-segmented, comprising syncoxa, basis and 2-segmented endopod. Syncoxa with inner spinulate seta. Basis with medial spinulate seta and spinular row on lateral and anteromedial margins. Proximal endopodal segment with medial spinulate seta and spinular row. Distal endopodal segment unornamented and bears 2 unequal setae.

Legs 1–4 biramous ( Figs 24A–D), with 2-segmented rami. Armature on rami of legs 1 to 4 as follows (Roman numerals = spines; Arabic numerals = setae):

Leg 1 ( Fig. 24A) intercoxal sclerite naked and bilobed distally. Anterior surface of coxa with minute spinules along distal margin; posterior surface naked. Basis with small, mediodistal acute process, spinular rows at insertion point of endopod and inner distal angle pointed; outer plumose seta long, extends to terminal exopodal segment. Inner and outer margin of exopod and endopod, respectively, with setules. Proximal exopodal segment with distal spinular row on anterior and posterior surfaces. Proximal endopodal segment with distal spinular row on anterior surface. All spines denticulate, with small spinules at insertion point of each spine.

Leg 2 ( Fig. 24B) similar to leg 1, except basis with relatively shorter outer seta, spinules at insertion of outer seta and setules along inner margin, proximal exopodal segment without distal spinular row on anterior surface, terminal exopodal segment bears an additional spine and terminal endopodal segment bears 1 less inner seta.

Leg 3 ( Fig. 24C) similar to leg 2, except distal endopodal segment bears extra seta and distal exopodal segment bears indentation on mid-lateral margin, median transverse ridge on anterior surface (representing ancestral division between middle and terminal segments), spinular row on posterior surface and extra spine. Specimen from Egerton spring with discontinuous inner cuticle layer on medial margin of distal exopodal segment (arrowed in Fig. 25A).

Leg 4 ( Fig. 24D) identical to leg 3, except exopodal segments without spinular row on posterior surface and terminal endopodal segment bears 2 apical spines (outer spine ½ length of inner spine). Specimen from Egerton spring with lateral transverse suture and discontinuous inner cuticle layer on medial margin of distal exopodal segment (arrowed in Fig. 25B). Specimens from Egerton and Three Springs tumulus spring with outer apical spine ⅔ length of inner apical spine on terminal endopodal segment ( Fig. 25C).

Leg 5 ( Figs 22B, 24E) small, 2-segmented. Protopodal segment naked, expanded laterally and armed with long outer seta. Free exopodal segment 2 times longer than wide, bears long setulate seta and tiny medial spine.

Leg 6 ( Fig. 24F) represented by sparsely setulate seta, naked spiniform seta and spiniform process on genital operculum.

Male. Body measurements were not recorded as the single male specimen was damaged. The antennulary structure could not be described as the pair of antennules was damaged during dissection. Exopod of legs 3 ( Fig. 25D) and 4 ( Fig. 25E) 3-segmented, both with armature formula I-0; I-0; II,I,3. Leg 6 ( Fig. 25F) represented by naked seta and 2 unequal spinulate setae on genital operculum.

Etymology. This species is named after David W. Morton for his outstanding contribution to the knowledge of the Australian freshwater Cyclopidae .

Remarks. The genus Mixocyclops presently contains two representatives: M. crozetensis Kiefer, 1944 and M. minutus Chappuis, 1951 . The former taxon was established to accommodate three females and a male collected from a stream on Crozet Island ( Kiefer 1944), whilst the latter was described from a female specimen recovered after rehydration of a moss sample that was obtained originally from a waterfall along Guide River in Tasmania ( Chappuis 1951). Important features, such as the antenna and mouthparts of both species and the natatory legs of male M. crozetensis , were omitted from these descriptions. The structure of these appendages remains unknown, as both species have not been reported since their original discovery.

Judging from the cursory drawings of M. crozetensis and M. minutus , the former taxon differs from the latter by having a relatively longer caudal seta III, an additional element on the distal endopodal segment of leg 1 and the distal exopodal segment of legs 3 and 4, a relatively longer inner apical spine on the distal endopodal segment of leg 4 and apparently only one element on the distal segment of leg 5. Given that the structure and armature pattern of leg 5 has traditionally been used as a diagnostic feature of most cyclopid genera ( Kiefer 1927; Pesce 1996; Reid & Janetsky 1996), Lindberg (1954) argued that M. minutus should not be classified in the genus Mixocyclops , as it bears an apical seta and inner spine on the distal segment of leg 5 as opposed to just a seta on the same segment as in M. crozetensis . Lindberg (loc. cit.), however, failed to recognise that Kiefer (1944) admitted he could not determine with absolute certainty whether a tiny spine was present or not on the inner margin of the distal segment of leg 5. A minute inner spine was recently confirmed to be present on the distal segment of leg 5 in the type material of M. crozetensis by Dr. Frank Fiers (Royal Belgian Institute of Natural Sciences; in litt.). Based on this new information, the congeneric status between M. crozetensis and M. minutus is well founded.

The new species is included in Mixocyclops , as it shares a relatively broad genital double-somite, short caudal rami with seta VI shorter than seta III, 11-segmented antennule, legs 1 to 4 with 2-segmented rami, leg 1 basis without an inner seta and distal segment of leg 5 armed with an apical seta and small inner spine in common with M. crozetensis and M. minutus (see Dussart & Defaye 1995). Among these three species, the presence of four spines on the terminal exopodal segment of legs 3 and 4 is shared by Mixocyclops mortoni sp. nov. and M. crozetensis . Nevertheless, Mixocyclops mortoni sp. nov. can be easily distinguished from M. crozetensis by having two inner setae instead of one on the distal endopodal segment of legs 3 and 4. The suite of apomorphic characters present in M. minutus , such as two inner setae on the distal endopodal segment of leg 1 and three outer spines on the terminal exopodal segment of legs 3 and 4, suggest that this species diverged later from the common ancestor of Mixocyclops than M. mortoni sp. nov. and M. crozetensis .

Mixocyclops mortoni sp. nov. is not restricted to ground waters within the Gnangara Mound area, as we have in our personal collection several specimens of this species from Boranup Creek located in the Margaret River region of Western Australia (Tang & Knott, unpublished data) and examined conspecific material in the DEC collection that was obtained from Three Springs tumulus spring and Kodjinup Melaleuca Swamp in the northern and central section, respectively, of the Wheatbelt Region in Western Australia. Indeed, this species may be widespread in the southern half of Australia, as Morton (1977) also described a species, named Mixocyclops macaulae , collected from Sphagnum bogs on Mt. Baw Baw and Mt. Buffalo in Victoria that is similar, particularly with respect to the structure and armature of the caudal rami, antenna and legs 1 to 5 of the female and the segmentation of the outer ramus of legs 3 and 4 in the male, to the material upon which our description is based. Morton’s record, however, must be confirmed as his description was not supported by a complete set of illustrations.