Megastygonitocrella, Karanovic & Hancock, 2009

Karanovic, Tomislav & Hancock, Peter, 2009, On the diagnostic characters of the genus Stygonitocrella (Copepoda, Harpacticoida), with descriptions of seven new species from Australian subterranean waters 2324, Zootaxa 2324 (1), pp. 1-85 : 47-49

Genus Megastygonitocrella gen. nov.

Diagnosis. Small to medium sized Ameiridae , with cylindrical habitus and no distinct demarcation between prosome and urosome. Integument with or without cuticular windows on cephalothorax and pedigerous somites; hyaline fringe of all prosomites smooth, those of urosomites smooth or finely serrated. First pedigerous somite incorporated into cephalothorax. Prosome ornamented only with large sensilla, urosome additionally ornamented with rows of small spinules. Genital somite free, partly or completely fused to first abdominal somite; genital field with single and relatively small copulatory pore, narrow copulatory duct and two small semicircular seminal receptacles; single small genital aperture covered by fused reduced sixth legs. Anal operculum convex, not reaching to posterior end of anal somite, smooth or ornamented with spinules near posterior margin. Caudal rami conical, as long as greatest width and slightly divergent; dorsal seta inserted near posterior margin and very close to inner margin; proximal lateral seta arising somewhat dorsolaterally at middle; distal lateral seta arising posterolaterally; inner apical seta at least as long as ramus and smooth; principal apical setae pinnate, with breaking plane. Antennula long and slender or short and stout, eight-segmented in female (although last two segments sometimes partly or completely fused) and tensegmented and geniculate in male; first segment with distal seta. Antenna composed of coxa, basis, twosegmented endopod and one-segmented exopod; exopod armed with one to three setae. Labrum with relatively narrow and convex cutting edge. Mandibula with narrow cutting edge and two-segmented palp; basis unarmed or armed with one seta, endopod with four to five apical setae. Maxillular endopod armed with single apical seta. Maxilla with single endite on syncoxa; endopod minute, armed with two apical setae. Maxilliped three-segmented, armed with at least one seta on syncoxa and one on endopod. All swimming legs with three-segmented exopod. Endopod of first leg three-segmented; endopod of second and third swimming legs two-segmented or one-segmented, while endopod of fourth swimming leg always one-segmented. All exopodal segments of about same length; first exopodal segment of all legs without inner seta; second exopodal segment of first leg without inner seta, that of other legs with inner seta; third exopodal segment of all legs with two outer spines, that of first, second and third legs without inner setae, that of fourth leg with one, two or no inner setae. First endopodal segment of first leg large, reaching to at least midlength of third exopodal segment, armed with short spiniform inner seta; penultimate endopodal segment of second and third legs (if present) always unarmed; ultimate segment of second leg armed with outer apical spine and inner apical seta or just with apical spine, that of third leg always with outer apical spine and inner apical seta and that of fourth leg always with single apical spine. Basis of first leg in male with inner spine transformed, smooth and inflated distally; only other form of sexual dimorphism in swimming legs involves somewhat shorter inner apical seta on ultimate endopodal segment of third leg in male. Fifth leg with maximum of four setae on exopod in female (and additional inner seta in male) and three elements on endopodal lobe; most species typically with armature variously reduced, to minimum of three setae on exopod and no elements on endopodal lobe, and displaying various fusion patterns (with maximum being completely fused to somite, represented by two small knobs). Sixth legs in male fused together medially and to somite, each armed with one or two setae.

Type species. Megastygonitocrella trispinosa ( Karanovic, 2006) comb. nov. [= Stygonitocrella trispinosa Karanovic, 2006 ].

Other species. Megastygonitocrella unispinosa ( Karanovic, 2006) comb. nov. [= Stygonitocrella unispinosa Karanovic, 2006 ]; Megastygonitocrella bispinosa ( Karanovic, 2006) comb. nov. [= Stygonitocrella bispinosa Karanovic, 2006 ]; Megastygonitocrella ljovuschkini ( Borutzky, 1967) comb. nov. [= Nitocrella ljovuschkini Borutzky, 1967 ]; Megastygonitocrella petkovskii ( Pesce, 1985) comb. nov. [= Stygonitocrella petkovskii Pesce, 1985 ]; Megastygonitocrella karamani ( Petkovski, 1959) comb. nov. [= Nitocrella karamani Petkovski, 1959 ]; Megastygonitocrella colchica ( Borutzky & Michailova-Neikova, 1970) comb. nov. [= Nitocrella colchica Borutzky & Mihailova-Neikova, 1970 ]; Megastygonitocrella dec sp. nov.; Megastygonitocrella ecowisei sp. nov.; Megastygonitocrella pagusregalis sp. nov.; Megastygonitocrella kryptos sp. nov.

Etymology. The genus name comes from the Greek adjective “megas” (meaning “large” and referring to the largest number of species in the group of genera studied here), prefixed to the existing genus name Stygonitocrella . Gender feminine.

Remarks. The genus Megastygonitocrella gen. nov. represents a closely related group of species of Stygonitocrella s. l., of which all, except one member, possess the same armature formula of the ultimate endopodal segment of the second to fourth legs (1.2.1). However, it is not just the number of armature elements that unites these species, but also their nature. All of these species have the ancestral outer subapical spine as their only element on the endopod of the second and fourth legs ( Table 2, characters 25 & 43), while the endopod of the third leg also has the ancestral outer apical seta (characters 35 & 36). The genera Stygonitocrella Reid, Hunt & Stanley, 2003 (as revised above) and Psammonitocrella Huys, 2009 also have only one apical element on the endopod of the second leg, but it represents the ancestral outer apical seta (character 26) rather than the outer subapical spine.

Most species of the genus Megastygonitocrella also have a one-segmented endopod on the second to fourth legs, except for M. colchica ( Borutzky & Michailova-Neikova, 1970) comb. nov., M. petkovskii ( Pesce, 1985) comb. nov. and M. karamani ( Petkovski, 1959) comb. nov. which have the endopod of the second and third legs two-segmented. The latter is also the only species with the armature formula 2.2.1 instead of 1.2.1. These two plesiomorphic characters were not enough to exclude M. karamani from the genus in our cladistic analysis and we think this species exhibits what the ancestor of this group looked like, at least in the characters associated with the swimming legs. That is why we think Neonitocrella Lee & Huys, 2002 is the closest living relative of Megastygonitocrella (see the Remarks section for Neonitocrella ). Unfortunately, Neonitocrella insularis ( Miura, 1962) , M. colchica , M. petkovskii and M. karamani are incompletely described (see Petkovski 1959; Miura 1962; Borutzky & Michailova-Neikova 1970; Pesce 1985) and many of their characters could not be compared or included in the cladistic analysis. Note that Pesce (1985) erroneously described the third leg of M. petkovskii as the second one and vice versa, which we attribute to a simple error during the dissection process. It seems that the segmentation of the endopod of the second and third swimming legs (effectively the loss of the penultimate segment) can be reduced in a single event (see also the Remarks section for the genus Psammonitocrella ), as M. colchica apparently is very closely related to M. ljovuschkini (Bortuzky, 1967) comb. nov. and the two species also live not far away from each other.

Members of the genus Megastygonitocrella are found in Australia (Pilbara region in Western Australia and Pioneer Valley in Queensland), Southern Europe ( Slovenia and the island of Lesbos, Greece) and the Caucasus (Western Gruzia and the Russian Krasnodarsk Region), which would suggest a Tethyan origin of this group. This zoogeographical connection is well recognized for many Australian stygofaunal elements ( Karanovic 2006; Humphreys 2008). All species of Megastygonitocrella have lost the inner seta on the second exopodal segment of the first leg, and have only two outer spines on all swimming legs and no inner armature elements on the third exopodal segment of the first, second and third legs.