Metacyclops arnaudi (G. O. Sars, 1908 )

Tang, Danny & Knott, Brenton, 2009, Freshwater cyclopoids and harpacticoids (Crustacea: Copepoda) from the Gnangara Mound region of Western Australia, Zootaxa 2029, pp. 1-70 : 33-41

Metacyclops arnaudi (G. O. Sars, 1908)

( Figs 17–21)

Material examined. Lake Yonderup, Yanchep National Park, Western Australia (31°33'23''S, 115°41'09''E): 15 ♀ (1 dissected and mounted on slide) and 1 copepodid ( AM P.78917–P.78918), 22 August, 2007; pool at Loch M c Ness, Yanchep National Park , Western Australia (31°32'57''S, 115°40'49''E): 2 ♀, 22 August, 2007; swamp near Carpark Cave , Yanchep National Park , Western Australia (31°33'11''S, 115°41'08''E): 49 ♀ (18 damaged; 5 dissected and mounted on one slide each), 7 ♂ (1 damaged; 2 dissected and mounted on one slide each) and 19 copepodids, 1 August, 2007 .

Redescription of female. Total body length (measured from anterior margin of cephalothorax to posterior margin of caudal rami), based on 7 specimens, 890–990 µm, mean 936 µm; body width 290–320 µm, mean 307 µm. Prosome ( Fig. 17A) composed of cephalothorax and 3 free pedigerous somites; all prosomites densely ornamented with cuticular pits (not figured); posterior margin of prosomites smooth. Urosome ( Figs 17A–B) comprised of fifth pedigerous somite, genital double-somite and 3 free abdominal somites. Genital double-somite longer (150–160 µm; mean 156 µm) than wide (120–130 µm; mean 124 µm), with numerous cuticular pits (not figured); posterior margin smooth dorsally and serrate ventrally; seminal receptacle not observed. Postgenital somites 1–2 ( Figs 17B, 18A) with few cuticular pits (not figured); posterior margin uneven dorsally and smooth ventrally. Anal somite ( Figs 17B, 18A) with numerous cuticular pits (not figured) and spinular row along posterior margin of ventral surface extending to dorsal surface; anal cleft with 3 transverse spinular rows.

Caudal ramus ( Fig. 17B) longer (80–90 µm; mean 83 µm) than wide (22.5–25 µm; mean 24.6 µm), with 6 setae (seta I absent) and numerous cuticular pits (not figured). Spinules present at insertion point of setae II and III. Seta II unilaterally spinulate; seta VII with few setules apically; remaining setae bilaterally spinulate. Setae III and VI subequal; seta VII longer than setae III and VI. Middle terminal setae with proximal breaking plane.

Antennule ( Fig. 18B) 11-segmented, with armature and ancestral segmentation pattern in brackets as follows: 7+ae (I– V), 4 ( VI – VII), 6 ( 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 proximoventral spinular row; segments 1–8 with cuticular pits (not figured). Some specimens from swamp near Carpark Cave with partial articulation(s) on posterior margin of antennule as follows: on segment 2 in 3 specimens , segment 4 in 1 specimen, segment 6 (arrowed in Fig. 18B) in 3 specimens and segments 3 and 6 in 1 specimen.

Antenna ( Figs 18C–D) 4-segmented, comprising coxobasis and 3-segmented endopod; all endopodal segments with cuticular pits on frontal surface (not figured). Coxobasis armed with 2 distomedial barbed setae and long distolateral exopodal seta; frontal surface with proximal row of minute spinules and short spinular row on lateral margin; caudal surface with several groups of spinules as figured. Proximal endopodal segment with inner spinulate seta and large patch of spinules along lateral margin. Middle endopodal segment bears 9 medial setae and 2 spinular rows on lateral margin. Terminal endopodal segment with proximal and distal spinules and 7 setae.

Labrum ( Fig. 18E) with long setules on anterior surface, central toothed process and 11 teeth between 2 blunt protrusions on distal margin.

Mandible ( Fig. 18F) composed of coxa and reduced palp. Coxal gnathobase with large quadridentate tooth, 3 bicuspidate teeth, 2 unicuspidate teeth, long unicuspidate tooth with proximal accessory process, barbed seta and row of strong spinules. Palp with 3 apical setae (2 long and plumose, 1 short and unilaterally barbed).

Maxillule ( Fig. 18G) composed of praecoxa and 2-segmented palp. Praecoxal arthrite bears 7 medial elements (1 spinulate, 1 with large proximal spinule, 5 naked), 3 distal unicuspidate teeth and distal spinulate seta. Coxobasis with proximal seta and 3 terminal setae. Endopod 1-segmented, armed with 3 distal setae.

Maxilla ( Fig. 19A) indistinctly 5-segmented, composed of praecoxa, coxa, basis and 2-segmented endopod. Praecoxa unornamented, with 2 spinulate setae on endite. Coxa with cuticular pits along lateral margin and longitudinal spinular row on anterior surface; proximal and distal endites with 1 and 2 spinulate setae, respectively. Basis drawn out into large claw furnished with large spinules along middle part of inner margin and cuticular pits on distal end; accessory armature composed of naked seta and large spinulate spine; few cuticular pits present on outer margin. Proximal endopodal segment partially fused to basis, armed with 2 strong setae. Distal endopodal segment bears 3 setae.

Maxilliped ( Fig. 19B) 4-segmented, comprising syncoxa, basis and 2-segmented endopod. Syncoxa with 3 medial spinulate setae and outer row of stout spinules. Basis with patch of large spinules along outer margin, row of long spinules on anteromedial surface and 2 medial spinulate setae. Proximal endopodal segment with long spinules on anterior surface and long inner spinulate seta. Terminal endopodal segment unornamented, bears 3 long setae.

Legs 1–4 biramous ( Figs 19C–D, 20A–E), with 2-segmented rami. Armature on rami of legs 1 to 4 as follows (Roman numerals = spines; Arabic numerals = setae):

Leg 1 ( Fig. 19C) intercoxal sclerite naked. Coxa highly ornamented with spinules and pores on anterior surface and lateral spinular row on posterior surface. Basis with mediodistal triangular process, setules along inner margin, few pores near insertion point of endopod and spinules on anterior surface and at insertion point of endopod and inner spine. Outer seta setulate; inner spinulate spine extends to distal margin of terminal endopodal segment. Proximal exopodal segment with pores and distal spinular row on posterior surface; outermost seta on terminal segment with lateral row of spinules and inner row of setules; insertion point of all spines with small spinules; both segments with setules along inner margin. Proximal endopodal segment with setules along lateral margin and proximal group of pores and distal spinular row on anterior surface; distal endopodal segment with setules along outer and proximomedial margins and long, thin spinules at insertion point of outer seta and apical serrate spine. All spines serrate.

Leg 2 ( Fig. 19D) similar to leg 1, except as follows: (1) coxa with additional distolateral spinular row and considerably less developed spinular row on distal margin; (2) basis lacks inner spine and bears additional lateral spinular row and triangular process near rounded inner distal angle; (3) terminal exopodal segment with pores on posterior surface and homogeneously ornamented outermost seta; (4) terminal endopodal segment with additional seta, indentation on mid-lateral margin and relatively smaller spinules at insertion point of outer seta and apical spine; and (5) rami with minute spinule(s) at insertion point of most setae.

Leg 3 ( Fig. 20A) identical to leg 2, except terminal endopodal segment with pores on posterior surface and 2 apical, subequal spines. Two specimens collected from swamp near Carpark Cave with formula 1,I+1,4 on terminal endopodal segment ( Fig. 20D).

Leg 4 ( Figs 20B–C) ornamented as in leg 3, except posterior surface of coxa with more complex ornamentation and proximal endopodal segment with pores and distal spinular row on posterior surface. Outer spine on terminal endopodal segment about 0.84 times as long as apical and inner spine. Two specimens, as mentioned above, with formula 1,I,3 on terminal endopodal segment ( Fig. 20E).

Leg 5 ( Figs 17A, 20F) held laterally, visible in dorsal view; composed of protopod completely fused to somite and free exopod segment. Protopodal seta long, with few setules apically. Exopod subquadrate, bears 2 apical elements; spine about 0.33 times as long as adjacent seta and 1.3 times as long as exopodal segment.

Leg 6 ( Fig. 20G) rudimentary, represented by weakly spinulate seta and 2 short naked spines on genital operculum.

Male. Total body length (measured from anterior margin of cephalothorax to posterior margin of caudal rami), based on 3 specimens, 700–730 µm, mean 720 µm; body width 200–210 µm, mean 203 µm. Prosome ( Fig. 21A) composed of cephalothorax and 3 free pedigerous somites. Urosome ( Fig. 21A) comprised of fifth pedigerous somite, genital somite and 4 free abdominal somites. All somites ornamented with cuticular pits (not figured). Genital somite wider than long (mean 83 × 93 µm). Caudal ramus longer than wide (mean 57 × 20 µm), armed and ornamented as in female.

Antennule ( Fig. 21B) 17-segmented, digeniculate, with armature and ancestral segmentation pattern as follows: 7+3ae (I–V), 4 (VI–VII), 1 (VIII), 2+ae (IX), 1 (X), 2 (XI), 2 (XII), 2 (XIII), 2+ae (XIV), 2 (XV), 2 (XVI), 2 (XVII), 2+ae (XVIII), 3 (XIX–XX), 3 (XXI-XXIII), 3 (XXIV-XXV), and 6+ae (XXVI–XXVIII). Cuticular pits present on segments 1–3 and 11–15. Short, highly chitinized spine present on segments 9 and 12. Short, spinulate seta present on segments 11–14. Short, blunt element present on segment 14. Modified anvil-shaped element(s) present on segments 14 and 15.

Free exopod segment of leg 5 ( Fig. 21C) with inner spine slightly shorter than segment and 0.25 times as long as adjacent seta.

Leg 6 ( Fig. 21D) represented by 3 elements on genital operculum; outer setulate seta longest of 3 elements; middle seta unilaterally spinulate; inner spine ½ length of middle seta.

Remarks. We attribute our specimens to Metacyclops arnaudi (G. O. Sars, 1908) , as they conform, in general, to G. O. Sars’ (1908) original description of two adult females collected from a swamp at St Arnaud in Victoria, Australia. Admittedly, there are a number of differences between our specimens and Sars’ description as follows (with features of Sars’ material given first followed by our material): the length/width ratio of the caudal rami is 6.2 versus 3.4; the second endopodal segment of the antenna bears eight as opposed to nine inner setae; the inner spine on the basis of leg 1 extends to the middle rather than to the distal end of the second endopodal segment; and the armature formula on the second endopodal segment of legs 3 and 4 is 1,I+1,4 and 1,I,3, respectively, as opposed to 1,II,4 and I,I,I+2. Although the last set of differences is taxonomically significant, it is premature in this case to attribute a new specific status to the Yanchep specimens based entirely on the leg dissimilarities described above given that the armature pattern on the endopods of legs 3 and 4 of those specimens was variable. As depicted in Figures 20D & E, legs 3 and 4 in two individuals from our samples are structurally identical to those of Sars’ specimens. Regrettably, the type material of M. arnaudi is no longer extant (see Hamond 1987) so the discrepancies highlighted above between our specimens and Sars’ material remain unresolved. Nonetheless, we are confident that our specimens are conspecific with M. arnaudi , particularly considering that: a) the Western Australian population matches Sars’ material with respect to the proportions of the urosomal somites, segmentation of the antennule, structure of the bases of legs 1 to 4, armature pattern on the exopod of legs 1 to 4, and most importantly, the position and structure of leg 5; and b) variations in the proportion of the caudal rami and armature of the natatory legs are known to occur in the Cyclopidae ( Reid 1992; Dumont 2006).

Although M. arnaudi was redescribed by Kiefer (1967) based on specimens collected by Dr. Ian A. E. Bayly from salt lakes in Victoria, this material was later revealed by Fiers (2001) to represent a new genus and species, Meridiecyclops baylyi Fiers, 2001 . The latter author also noted that the taxonomic affinities between M. arnaudi and his newly established genus Meridiecyclops Fiers, 2001 were difficult to establish, as the male of M. arnaudi has never been described and the only illustrations available for comparison were those of G. O. Sars (1908). For these reasons, Fiers decided to retain M. arnaudi in the genus Metacyclops Kiefer, 1927 , which was adopted by Karanovic (2004a, b). Our description of both sexes of this enigmatic species revealed that it is indeed a member of Metacyclops ; it cannot be included in Meridiecyclops given that the male maxilla, as well as the female genital double-somite and basis and proximal endopodal segment of leg 4, are not modified.

Metacyclops arnaudi has been recorded previously, often as Microcyclops arnaudi , in aquatic faunal investigations in Victoria ( Geddes 1976; Morton & Bayly 1977) and Western Australia ( Geddes et al. 1981; Bayly 1982; Halse et al. 2000b, 2002a). We must also add that this cyclopoid species was also reported, as Microcyclops arnaudi , from freshwater pools in Victoria, South Australia and Tasmania by Morton (1977). We strongly believe the specimens reported in Morton (1977), Morton & Bayly (1977), Geddes et al. (1981) and Bayly (1982) are conspecific with our material for the following reasons. Firstly, Morton’s (loc. cit.) relatively detailed description of M. arnaudi is congruent, particularly regarding the relative proportions of the caudal rami and structure of leg 5, with our taxonomic account. Even though the armature of legs 1 to 4 given in Morton’s account was identical to that of G. O. Sars’ (1908) material, Morton also noted, with respect to leg 4, that “the outer seta and distalmost inner seta may sometimes (not uncommonly) be transformed into spines”. Secondly, the cyclopoid specimens collected by Geddes et al. (1981) and Bayly (1982), which included M. arnaudi (listed as Microcyclops arnaudi ) among others, were identified by Mr. David Morton [see Methods section in Geddes et al. (loc. cit.) and Acknowledgement section in Bayly (loc. cit.)]. Lastly, we have examined three adult female M. arnaudi specimens lodged in the Crustacea collection of Museum Victoria (Reg. No. J12213 – Microcyclops arnaudi collected on July, 1979, from a semi-permanent pond in South Portland, Victoria) that: a) were identified by Mr. David Morton; and b) are morphologically similar to our Western Australian material. The Museum Victoria specimens vary slightly from our samples, however, by having relatively more cuticular pits on postgenital somites 1 and 2 and a relatively longer inner apical spine on the terminal endopodal segment of leg 4. It is now apparent that M. arnaudi , as currently recognised by us, is a somewhat variable species that occurs throughout the southern half of Australia.

The presence of three spines on the distal exopodal segment of legs 1 to 4 is, as previously noted by Karanovic (2004a), a synapomorphy of M. arnaudi , M. monacanthus ( Kiefer, 1928) , M. margaretae ( Lindberg, 1938) , M. trispinosus Dumont, 1981 , M. pilanus Karanovic, 2004 , M. laurentiisae Karanovic, 2004 , M. kimberleyi Karanovic, 2004 and M. pilbaricus Karanovic, 2004 . This small assemblage of species, termed the “ trispinosus ”-group by Karanovic (2004a), exhibit a Gondwana distribution: M. monacanthus was described from New Zealand, M. margaretae from India, M. trispinosus from Africa and the remaining five species from Australia (G. O. Sars 1908; Kiefer 1928; Lindberg 1938; Dumont 1981; Karanovic 2004a, b). Among members of the “ trispinosus ”-group, M. arnaudi has cuticular pits on the surface of the body somites, caudal rami and certain appendages, an exopodal seta on the antenna, nine inner elements on the middle endopodal segment of the antenna and one apical spine on the distal endopodal segment of leg 4 in common with M. pilanus and M. laurentiisae . Metacyclops arnaudi can be easily distinguished from these two closely related taxa by having an aesthetasc on the first antennulary segment and leg 5 armed with a spine that is longer than the free exopodal segment in the adult female. It is worth noting here that this characteristic aesthetasc is present on all three Museum Victoria specimens. It is unclear whether this element is present or absent in G. O. Sars’ (1908) and Morton’s (1977) specimens, as it was not depicted (nor the other aesthetascs invariably present on the distal antennulary segments of cyclopid species) in Sars’ illustration of the antennule and the antennulary armature pattern was not given in Morton’s description.