Megastygonitocrella dec, 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 : 50-55

Megastygonitocrella dec sp. nov.

( Figs 10–11 )

Type material. Holotype, adult female dissected on one slide ( WAM C37348); allotype, adult male dissected on one slide ( WAM C37349); paratypes: one female dissected on one slide ( WAM C37350) and one male, two females and 11 copepodids preserved in 70% ethanol ( WAM C37351); Australia, Western Australia, Pilbara region, Yule River , bore GNHSLK1448B, 18 November 2003, leg. J. Cocking & M. Scanlon, 21°36’14”S 118°49’01”E.

Description. FEMALE (HOLOTYPE). Body length, excluding appendages and caudal setae, 0.497 mm. Preserved specimen colourless. Nauplius eye absent. Prosome ( Fig. 10A ) comprising cephalothorax and three free pedigerous somites, while urosome comprising fifth pedigerous somite, completely distinct genital somite and four abdominal somites. First two urosomites ( Fig. 10B ) narrower than previous and subsequent somites, which makes for unusually constricted habitus in dorsal view and distinct demarcation between prosome and urosome; prosome/urosome ratio 0.9 and greatest width at second pedigerous (first free) prosomite. Body length/width ratio about 5.1. Free pedigerous somites without pronounced lateral or dorsal expansions. Integument weakly chitinized and without cuticular windows. Rostrum ( Fig. 10A ) extremely small, membranous, not demarcated at base and ornamented with two dorsal sensilla on anterior margin.

Cephalothorax ( Fig. 10A ) with completely incorporated first pedigerous somite, quadriform in dorsal view, about as long as wide; represents 19% 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 with two large dorsal sensilla and two smaller lateral sensilla on each side; hyaline fringe smooth both dorsally and laterally ( Fig. 10A ). Large sclerotized joint ( Figs 10A–B ) present between fifth pedigerous and genital somites and visible both ventrally and dorsally.

Genital somite ( Fig. 10B ) approximately three times as wide as long (ventral view); ornamented only with four posterior dorsal sensilla. Hyaline fringe smooth and very narrow both ventrally and dorsally. Genital field with single small copulatory pore at about midlength, extremely short copulatory duct and two ovoid seminal receptacles, posterior part of which reaches well beyond copulatory pore, to about 2/3 of somite length. Relatively large genital aperture covered by fused reduced sixth legs, represents 46% of somite width. Third urosomite ( Figs 10A–B ) ornamented with 10 large posterior sensilla (two dorsal, four lateral (two on each side) and four ventral), two large anterior lateral sensilla (one on each side) and two lateral cuticular pores (also one on each side) at midlength; posterior margin of this somite represents widest part of urosome in ventral view; hyaline fringe smooth. Fourth urosomite with 10 large posterior sensilla (two dorsal, four lateral and four ventral), two lateral pores (one on each side) and two slightly arched, short ventrolateral rows of spinules at midlength. Preanal somite about as long as previous one, but narrower, 1.8 times as wide as long, ornamented only with two short rows of spinules similar to previous somite, but shorter and closer to each other. Anal somite ( Figs 10A–B ) ornamented with pair of large dorsal sensilla, two short ventral rows of spinules in anterior half and posterior row of spinules interrupted between caudal rami. Anal operculum unornamented, convex, not reaching to posterior end of anal somite, represents 50% of somite's width. Anal sinus smooth and widely opened.

Caudal rami ( Figs 10A–B ) short and conical, slightly shorter than their greatest width (ventral view), somewhat divergent, with space between them about one ramus width, with small diagonal chitinous ridge dorsally, armed with seven setae (three lateral, three apical and one dorsal). Ornamentation consists of two spinules each at base of dorsal and distal lateral seta, one ventral cuticular pore and posterior ventral row of seven or eight small spinules. Dorsal seta inserted almost at posterior end and close to inner margin, slender, smooth, 1.5 times as long as caudal ramus, triarticulate at its base. Proximal lateral seta arising somewhat dorsolaterally at 2/5 of ramus length, 0.8 times as long as dorsal one and 1.2 times as long as distal lateral seta, which arises at 4/5 of ramus length. Inner apical seta very slender and smooth, slightly shorter than ramus. Both principal setae sparsely pinnate at distal end, with breaking plane.

Antennula ( Fig. 10C ) eight-segmented, unornamented, slender, approximately twice as long as cephalothorax. Very long slender aesthetasc on fourth segment reaches beyond tip of appendage for more than length of last five segments combined and fused basally with large subapical seta; 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 setae smooth and without breaking plane; only three setae on eighth segment biarticulating on basal part. Length ratio of antennular segments, from proximal to distal end and along caudal margin, 1: 1.2: 1: 0.9: 0.6: 0.7: 0.4: 0.5.

Antenna ( Fig. 10D ) composed of coxa, basis, two-segmented endopod and one-segmented exopod. Coxa very short, unornamented. Basis about 1.4 times as long as wide, unornamented and unarmed. First endopodal segment 1.4 times as long as basis, 1.8 times as long as wide, unornamented and unarmed. Second endopodal segment longest, 1.4 times as long as first and 3.3 times as long as wide, armed laterally with two bipinnate spines flanking thin seta; apical armature consisting of five geniculate setae, longest one fused basally to additional smaller, slender and smooth seta; ornamentation consists of few spinules along anterior distal surface and two fringes on posterior surface. Exopod one-segmented, 0.6 times as long as basis and 2.3 times as long as wide, unornamented but armed with two setae; inner apical seta bipinnate, slender and 4.6 times as long as outer curved and smooth seta.

Labrum ( Fig. 10E ) not very large when compared to cephalothorax, trapezoidal, rigidly sclerotized, with relatively narrow and convex cutting edge, ornamented with one apical row of small spinules in between two subapical rows of strong spinules. Two ellipsoid fields of gustatory papillae visible on dorsal (posterior) surface.

Paragnaths not mounted properly to allow detailed observation.

Mandibula ( Fig. 10F ) very small, with wide cutting edge on elongated coxa, armed with numerous small teeth in between three ventral coarse teeth and one dorsal unipinnate seta. Palp uniramous, comprising basis and one-segmented endopod. Basis very elongated, unarmed and unornamented, 2.8 times as long as wide and 2.5 times as long as endopod. Endopod small and unornamented, about 1.9 times as long as wide; armed apically with five slender smooth setae.

Maxillula very similar to that of Gordanitocrella trajani gen. et sp.nov. ( Fig. 5I ).

Maxilla ( Fig. 10G ) small, unornamented, with proximal endite on syncoxa absent; distal endite well developed, highly mobile, armed with one pinnate spine and two smooth subequal setae. Basis drawn out into long claw, with shorter spiniform seta at its base. Endopod represented by minute segment, armed with two smooth, subequal apical setae.

Maxilliped ( Fig. 10H ) with short syncoxa ornamented with few large spinules and armed with single smooth seta near inner distal corner, 1.4 times as long as wide. Basis 2.4 times as long as wide and 1.7 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. 11B ), endopod of other swimming legs one-segmented ( Figs 11C–F ). 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(1)/2/2 0/1/0 - -

Intercoxal sclerite of all swimming legs small, with concave distal margin and without surface ornamentation. Praecoxae and coxae of all legs unarmed and unornamented. Basis of each leg unornamented; 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 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 of most legs with frilled membrane. All exopodal segments of about same length. First endopodal segment of first swimming leg ( Fig. 11B ) about three 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 smooth on inner side. Third exopodal segment of other legs with plumose inner apical seta and heterogeneously ornamented outer apical seta (plumose on inner margin, pinnate on outer; transitional stage between seta and spine). Apical endopodal element of second, third and fourth leg spiniform, but longer than segment; inner seta on endopod of third leg slender, plumose and about twice as long as apical element. Inner element on second exopodal segment short and spiniform, while all outer exopodal spines strong and bipinnate.

Fifth legs ( Figs 10B , 11G ) biramous but baseoendopods fused together medially, although distinct from somite. Endopodal lobe not recognizable, forming straight smooth hyaline fringe and without armature or ornamentation. Basal outer seta slender and smooth, on long setophore. Exopod a distinct segment, ovoid, small, about as long as its maximum width, unornamented but armed with four smooth setae; length ratio of exopodal setae from inner to outer side 1: 0.36: 0.18: 0.5. Longest exopodal seta strongest, 0.7 times as long as outer basal seta.

Sixth legs ( Fig. 10B ) completely fused together, forming simple operculum covering single gonopore, each armed with minute smooth seta.

MALE (ALLOTYPE). Body length, excluding caudal setae, 0.442 mm. Habitus ( Fig. 11H ), ornamentation of prosomites, rostrum, colour and nauplius eye similar to female. Hyaline fringe of all somites smooth. Body constricted at middle as in female, with fifth pedigerous somite narrower than fourth pedigerous or genital somites.

Genital somite twice as wide as long. Single large spermatophore ( Fig. 11H ) longitudinally placed inside fifth pedigerous and genital somites. Third and fourth urosomites with spinules and sensilla pattern same as in female; preanal somite without dorsal surface ornamentation as in female. Anal operculum ( Fig. 11H ) with posterior row of many minute spinules.

Caudal rami ( Fig. 11H ) slightly shorter and not divergent, but armature and ornamentation very similar to female.

Antennula ( Fig. 11I ) unornamented, long and slender, ten-segmented, not strongly geniculate, with geniculation between seventh and eighth segments. Long aesthetasc on apical acrothek of fifth segment homologous to aesthetasc on fourth segment in female. Smaller aesthetasc on tenth segment fused basally to two apical setae. Armature of first, ninth and tenth segments similar to female. Setal formula: 1.10.6.1.7.1.2.1.4.7. Majority of setae smooth and slender; two setae on fifth segment, one on sixth and one on seventh very short, spiniform and unipinnate. Outer (caudal) setae on ninth and tenth segments biarticulating on basal part; no setae with breaking plane.

Antenna ( Fig. 11J ), labrum, mandibula, maxillula, maxilla, maxilliped, second swimming leg ( Fig. 11L ) and fourth swimming leg ( Fig. 11N ) similar to female.

Basis of first swimming leg ( Fig. 11K ) with inner spine modified, smooth, inflated and apically transformed into a complex tridimensional structure, similar to twisted but widely open pincers.

Endopod of third swimming leg ( Fig. 11M ) with inner seta unipinnate and significantly shorter than in female.

Fifth legs ( Figs 11O–P ) similar to female, except exopod additionally armed with innermost bipinnate seta; second innermost seta (homologous to innermost seta on female exopod) also proportionally shorter. Outer basal seta shorter than in female.

Sixth legs ( Fig. 11Q ) narrowly fused basally together and indistinct from somite, each armed with two slender smooth setae; outer seta 1.8 times as long as inner one.

Variability. Body length of females ranges from 0.487 mm to 0.497 mm (0.492 mm average; n = 4), while only two males were collected and studied (0. 442 and 0.445 mm long). Both males exhibit asymmetry in the length of the innermost spiniform seta on the fifth leg exopod ( Figs 11O–P ). All males and females have the third exopodal segment of the fourth swimming leg with an inner seta present on one leg but absent on the opposite one ( Figs 11D–E ). It is interesting that all females have a smooth anal operculum, while both examined males have a posterior row of minute spinules.

Etymology. The specific name is dedicated to the Western Australian Government Department of Environment and Conservation (DEC), who collected the material and partly funded the preparation of this paper. It comprises an arbitrary combination of letters that can be treated as a Latin word and that should be conceived as a noun of feminine gender.

Remarks. The one-segmented endopod of the second to fourth swimming legs, armed only with the ancestral outer subapical spine on the second and fourth legs and additionally with an outer apical seta on the third (formula 1.2.1), leave no doubt that this species belongs to the genus Megastygonitocrella gen. nov. In fact, M. dec sp. nov. has the same segmentation and almost the same armature formula (the only difference being the number of inner elements on the third exopodal segment of the fourth leg) as the Caucasian M. ljovuschkini ( Borutzky, 1967) comb. nov. and six other Australian representatives: M. ecowisei sp. nov., M. pagusregalis 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 congeners by a constricted habitus, reduced armature of the antennal exopod, very long mandibular basis, completely free genital somite in the female and the unusually robust inner basal spine on the first swimming leg in the male. Megastygonitocrella dec can be easily distinguished further from M. ecowisei , M. trispinosa , M. bispinosa and M. unispinosa by its unarmed fifth leg endopodal lobe. It differs additionally from M. pagusregalis by having the fifth leg free (completely fused to somite in the latter species), a much more elongated antennula (very short in M. pagusregalis ) and different ornamentation of the urosomites. Finally, M. dec can be additionally distinguished from M. kryptos by the shape of its caudal rami, ornamentation of the anal operculum and the number of armature elements on the fifth leg exopod.

Figures

FIGURE 5. Gordanitocrella trajani gen. et sp. nov., holotype female: A—habitus, dorsal view; B—habitus, lateral view; C—urosome, ventral view; D—antenna; E—labrum; F—paragnaths; G—cutting edge of mandibula; H— mandibular palp; I—maxillula; J—maxilla; K—maxilliped. Scales = 0.1 mm.

FIGURE 10. Megastygonitocrella dec gen. et sp. nov., holotype female: A—habitus, lateral view; B—urosome, ventral view; C—antennula; D—antenna; E—labrum; F—mandibula; G—maxilla; H—maxilliped. Scales = 0.1 mm.

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.