Megastygonitocrella pagusregalis, 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 : 62-66

Megastygonitocrella pagusregalis sp. nov.

( Figs 15 & 16A–E )

Type material. Holotype, adult female dissected on one slide ( WAM C37345); Australia, Queensland, Pioneer Valley , bore 12600065, depth 9.57 m, 07 August 2003, leg. P. Hancock, 21°19’37”S 149°00’06”E.

Description. FEMALE (HOLOTYPE). Body length, excluding caudal setae, 0.406 mm. Preserved specimen colourless. Nauplius eye absent. Habitus ( Figs 15A–B ) cylindrical, slender, without distinct demarcation between prosome and urosome; prosome/urosome ratio 0.9; greatest width at posterior part of cephalothorax, which is only slightly wider than first and second free prosomites. Body length/width ratio about 5.8; cephalothorax only slightly wider than genital double-somite (1.06 times). Free pedigerous somites without pronounced lateral or dorsal expansions. Integument weakly chitinized and without any cuticular windows. Rostrum ( Fig. 15A ) very small and membranous, not demarcated at base; ornamented with two dorsal sensilla on anterior margin.

Cephalothorax ( Figs 15A–B ) with completely incorporated first pedigerous somite, almost cylindrical in dorsal view, 1.2 times as long as wide; represents 21% of total body length. Surface of cephalic shield and tergites of three free pedigerous somites ornamented only with several large sensilla. Hyaline fringe of all prosomites narrow and smooth. Fifth pedigerous (first urosomal) somite ornamented only with four dorsal sensilla (two posteriorly and two at midlength); hyaline fringe smooth both dorsally and laterally ( Figs 15A– B ). Large sclerotized joint ( Figs 15A–C ) present between fifth pedigerous and genital double somites and visible both ventrally and dorsally.

Genital double-somite ( Fig. 15C ) approximately 0.8 times as long as wide (ventral view), only slightly wider at posterior part, with suture marking original segmentation clearly visible dorsally and laterally; ornamented with six large sensilla along suture, two posterior dorsal sensilla, four posterior ventral sensilla and one irregular row of relatively large posterior ventral spinules. Hyaline fringe smooth and very narrow. Genital field with single small copulatory pore, sclerotized narrow copulatory duct and two very small, ellipsoid seminal receptacles, posterior part of which not reaching to last third of copulatory duct. Copulatory pore situated at proximal third of double-somite length. Very small genital aperture covered by fused reduced sixth legs, represents 26% of somite width. Third urosomite ( Figs 15B–C ) ornamented with six large posterior sensilla (two dorsal and four ventral) and uninterrupted but irregular posterior ventral row of spinules; hyaline fringe smooth. Preanal somite significantly shorter than previous one, more than twice as wide as long and without surface ornamentation. Anal somite ( Figs 15C–D ) ornamented with pair of large dorsal sensilla, four ventral cuticular pores, interrupted and arched row of spinules ventrally at 1/3 and posterior row of somewhat larger spinules interrupted dorsolaterally and ventrally between caudal rami. Anal operculum convex, not reaching to posterior end of anal somite, represents 49% of somite's width; ornamented near posterior margin with six spinules of about same size as spinules on posterior lateral margin of somite. Anal sinus smooth and widely opened.

Caudal rami ( Figs 15B–D ) short and conical, shorter than their greatest width (ventral view), slightly divergent, with space between them more than one ramus width; with small diagonal chitinous ridge dorsally and armed with seven setae (three lateral, three apical and one dorsal). Ornamentation consists of two spinules each at base of dorsal and distal lateral setae, one dorsal and one lateral pore and posterior ventral row of five large spinules. Dorsal seta inserted nearly at posterior end and close to inner margin, 2.3 times as long as caudal ramus, triarticulate at its base and smooth. Proximal lateral seta arising somewhat dorsolaterally, 0.7 times as long as dorsal one and 1.7 times as long as distal lateral seta, which arises at 3/4 of ramus length. Inner apical seta very slender and small, smooth, about 0.8 times as long as ramus. Both principal setae inserted slightly more dorsally than terminally, with breaking plane and sparsely pinnate at distal end.

Antennula ( Fig. 15E ) eight-segmented, but ancestral seventh and eighth segments mostly fused, unusually short and stout, only about as long as cephalothorax, unornamented. Relatively slender aesthetasc on fourth segment reaches beyond tip of appendage for length of more than last four segments combined; much smaller and even more slender apical aesthetasc on eighth segment fused basally to two apical setae. Setal formula: 1.8.6.3.2.2.4.7. All segments, except second and eighth, wider than long. All setae smooth, slender and without breaking plane. Only four setae on eighth segment articulating on basal part. Length ratio of antennular segments, from proximal to distal end and along caudal margin, 1: 1.6: 0.7: 0.8: 0.6: 0.6: 0.3: 0.8.

Antenna ( Fig. 15F ) composed of coxa, basis, two-segmented endopod and one-segmented exopod. Coxa very short, unornamented. Basis about 1.3 times as long as wide, unarmed but ornamented with short row of spinules on anterior distal margin. First endopodal segment 1.5 times as long as basis, nearly twice as long as wide, unornamented and unarmed. Second endopodal segment longest, 1.4 times as long as first and 3.1 times as long as wide, armed laterally with two smooth spines flanking thin seta; apical armature consisting of five geniculate setae, longest one fused basally to additional smaller seta bearing proximal tuft of fine setules; ornamentation consists of few spinules along anterior surface and two fringes on posterior surface. Exopod one-segmented, 0.7 times as long as basis and 2.3 times as long as wide, unornamented but armed with three setae; inner (anterior) seta bipinnate, slender, arising apically and 1.5 times as long as other two setae, which are both slightly curved, spiniform and unipinnate at anterior distal part.

Labrum ( Fig. 15G ) not very large when compared to cephalothorax, trapezoidal, rigidly sclerotized, with relatively broad and convex cutting edge, ornamented with one apical row of smaller spinules in between two subapical rows of strong spinules. Two ellipsoid fields of gustatory papillae visible on dorsal (posterior) surface.

Paragnaths very similar to previous species, but not mounted satisfactorily enough to allow for drawing.

Mandibula ( Fig. 15I ) very small, with wide cutting edge on relatively short coxa, armed with numerous small teeth in between three coarse ventral teeth and one dorsal unipinnate seta. Palp uniramous, comprising basis and one-segmented endopod. Basis short, unarmed and unornamented, 1.4 times as long as wide and 1.2 times as long as endopod. Endopod small and unornamented, about 1.5 times as long as wide; armed with five slender smooth apical setae.

Maxillula ( Fig. 15J ) very small compared to body size, with large praecoxa; arthrite rectangular, not movable, unornamented, without smooth setae on anterior surface, armed with three minute setae on dorsal margin and four apical elements (probably three spines and one seta). Coxal endite slightly shorter than praecoxal arthrite, armed apically with one pinnate and curved and two slender smooth setae, all of about same length. Basis about as long as coxal endite, armed with four smooth setae apically and one subapically. Endopod a minute but distinct segment, armed with smooth apical seta.

Maxilla ( Fig. 15K ) small, unornamented, with proximal endite on syncoxa absent; distal endite well developed, highly mobile, armed with one pinnate spine and two smooth setae of about same length. Basis drawn out into long claw, with shorter spiniform seta at its base. Endopod represented by minute segment, armed with two smooth and equally long apical setae.

Maxilliped ( Fig. 15L ) unornamented, with short syncoxa armed with single smooth seta, 1.4 times as long as wide. Basis 2.4 times as long as wide and 1.8 times as long as syncoxa, unarmed. Endopod represented by long curved claw, ornamented distally with row of spinules along concave side; with thin seta at base.

All swimming legs with three-segmented exopod; endopod of first leg three-segmented ( Fig. 16A ), endopod of other swimming legs one-segmented ( Figs 16B–D ). Armature formula of swimming legs as follows (inner/outer element; inner/terminal/outer element):

Exopod Endopod

Segments 1 2 3 1 2 3

First leg 0/1 0/1 0/2/2 1/0 0/0 1/1/1

Second leg 0/1 1/1 0/2/2 0/1/0 - -

Third leg 0/1 1/1 0/2/2 1/1/0 - -

Fourth leg 0/1 1/1 0/2/2 0/1/0 - -

Intercoxal sclerite of all swimming legs small, with concave distal margin and without surface ornamentation. Praecoxae short and smooth. Coxa of third leg with two diagonal rows of spinules near outer margin, that of other legs smooth; all coxae unarmed. Basis of each leg ornamented with spinules near outer margin at base of exopod and with smaller ones at base of endopod; first leg additionally with several minute spinules at base of inner spine; armed with small outer spine on first and second swimming legs and smooth outer seta on third and fourth legs; first leg with stout spine on inner margin at about 3/5 of segment length. All exopodal and endopodal segments ornamented with strong spinules along outer margin and on outer distal corner; endopod and some exopodal segments with spinules along inner margin as well; inner distal corner of first and second exopodal segments with frilled membrane. All exopodal segments of about same length. First endopodal segment of first swimming leg ( Fig. 16A ) about 2.5 times as long as wide and reaching to 2/3 of second exopodal segment; endopod significantly longer than exopod. Endopod of second and third swimming legs about as long as first exopodal segment; that of fourth leg somewhat shorter. Apical armature element(s) on first leg geniculate, pinnate on outer (concave) side and plumose on inner side. Inner apical seta on third exopodal segment of other legs plumose; outer apical seta plumose on inner margin, pinnate on outer (transitional stage between seta and spine). Apical endopodal element of second, third and fourth legs short and spiniform; inner seta on endopod of third leg slender, smooth and about as long as apical element. Inner element on second exopodal segment short and spiniform, while all outer exopodal spines strong and bipinnate.

Fifth leg ( Figs 15C , 16E ) fused almost completely to somite, represented by two unornamented but armed ventrolateral knobs. Inner knob, representing exopod, armed with three subequal, smooth and slender setae. Outer knob represents outer part of basis and armed with single long unipinnate seta; this seta much longer than any seta on inner knob, reaching to 3/4 of genital double-somite length. Inner part of each baseoendopod absent, fused together into smooth and slightly concave hyaline fringe.

Sixth legs ( Fig. 15C ) completely fused together, indistinct, forming simple operculum covering single gonopore, without ornamentation or armature.

MALE. Unknown.

Variability. Only one female was collected and studied, and no asymmetric features were observed.

Etymology. The species name is a combination of a Latin noun “pagus” (meaning “district”, “state”) and a Latin possessive adjective “regalis” (meaning “royal”) and is a rough translation of Queensland, the Australian state in which this species was found.

Remarks. As mentioned in the Remarks section for Megastygonitocrella dec sp. nov., M. pagusregalis sp. nov. belongs to a group of very closely related congeners, all with the same segmentation and almost the same armature formula of the swimming legs: M. ljovuschkini ( Borutzky, 1967) comb. nov., M. ecowisei sp. nov., M. dec sp. nov., M. kryptos sp. nov., M. trispinosa ( Karanovic, 2006) comb. nov., M. bispinosa ( Karanovic, 2006) comb. nov. and M. unispinosa ( Karanovic, 2006) comb. nov. However, it is easily distinguished from all these species by its unusually short antennula and the fifth leg completely fused to the somite. It differs from each species individually by many more characters and we will avoid repeating those differences already stressed in the Remarks sections of other species.

Megastygonitocrella pagusregalis is probably most closely related to the only other species from Queensland, M. kryptos sp. nov., and it is unfortunate that the former species is known only from females, while the latter is known only from males, so some characters cannot be compared. Actually, in our preliminary identification of the Queensland material, we entertained for a short time the possibility that they belonged to the same species because of their very similar habitus and swimming legs. However, differences in the ornamentation of the urosomites and anal operculum as well as the shape of the anal operculum and caudal rami all point towards their separate specific status. Also, the fifth leg is not fused to the somite in M. kryptos , although the two species have homologous armature elements on this appendage (that of male M. kryptos with an additional innermost seta as a normal form of sexual dimorphism in this group of freshwater ameirids). The third exopodal segment of the fouth swimming leg carries one inner seta in M. kryptos and none in M. pagusregalis , but this character is known to be variable in some species (see, for example, Figs 11E–F ).

Figures

FIGURE 11. Megastygonitocrella dec gen. et sp. nov., A–G, holotype female; H–Q, allotype male: A—left caudal ramus, lateral view; B—first swimming leg; C—endopod of second swimming leg; D—third swimming leg; E—third exopodal segment of right fourth leg; F—left fourth swimming leg; G—right fifth leg; H—habitus, dorsal view; I— antennula; J—exopod of antenna; K—basis of first swimming leg; L—endopod of second swimming leg; M—endopod of third swimming leg; N—endopod of fourth swimming leg; O—right fifth leg; P—left fifth leg; Q—left sixth leg. Scales = 0.1 mm.

FIGURE 15. Megastygonitocrella pagusregalis gen. et sp. nov., holotype female: A—habitus, lateral view; B—habitus, dorsal view; C—urosome, ventral view; D—anal somite and left caudal ramus, lateral view; E—antennula; F—antenna; G—labrum; H—paragnaths; I—mandibula; J—maxillula; K—maxilla; L—maxilliped. Scales = 0.1 mm.

FIGURE 16. Megastygonitocrella pagusregalis gen. et sp. nov., A–E, holotype female; Megastygonitocrella kryptos gen. et sp. nov., F–M, holotype male; N & O, topotype male (WAM C37347; 0.427 mm): A—first swimming leg; B— second swimming leg without second and third exopodal segments; C—third swimming leg; D—fourth swimming leg; E—left fifth leg; F—anal somite and caudal rami, dorsal view; G—basis of first swimming leg; H—endopod of second swimming leg; I—endopod of fourth swimming leg; J—left fifth leg; K—left sixth leg; L—spermatophore; M— antennula; N—urosome, ventral view; O—third exopodal segment of fourth swimming leg. Scale = 0.1 mm.