Megastygonitocrella kryptos, 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 : 66-68

Megastygonitocrella kryptos sp. nov.

( Figs 16F–O )

Type material. Holotype, adult male dissected on one slide ( WAM C37346); Australia, Queensland, Pioneer Valley , bore 12600522, depth 15.26 m, 28 June 2003, leg. P. Hancock (jar no. PV37), 21°13’52”S 149°10’11”E.

Other material examined (TOPOTYPES). Two adult males mounted on one slide in toto ( WAM C37347); Australia, Queensland, Pioneer Valley , bore 12600522, depth 15.26 m, 27 June 2003, leg. P. Hancock (jar no. PV38), 21°13’52”S 149°10’11”E .

Description. MALE (HOLOTYPE). Body length, excluding caudal setae, 0.421 mm. Habitus, ornamentation of prosomites, rostrum, colour and nauplius eye similar to M. pagusregalis sp. nov. Hyaline fringe of all somites smooth.

Genital somite nearly twice as wide as long. Single large spermatophore ( Fig. 16L ) placed longitudinally inside fifth pedigerous and genital somites, both of which bear only four large dorsal sensilla each. Third and fourth urosomites ( Fig. 16N ) each ornamented with only six large posterior sensilla (four ventral and two dorsal); preanal somite without surface ornamentation. Anal somite ( Figs 16F & N ) ornamented with pair of large dorsal sensilla, two ventral cuticular pores, interrupted but straight row of large spinules at ventral 1/3 and posterior row of large spinules interrupted ventrally between caudal rami. Anal operculum ( Fig. 16F ) almost straight, not reaching to posterior end of anal somite, represents 42% of somite's width, completely smooth. Anal sinus smooth and widely opened.

Caudal rami ( Figs 16F & N ) long and very conical, 1.86 times as long as their greatest width (ventral view), slightly divergent, with space between them about one ramus width, without diagonal chitinous ridge dorsally; armed with seven setae (three lateral, three apical and one dorsal). Ornamentation consists of one spinule at base of dorsal seta, posterior ventral row of three small spinules and one dorsal cuticular pore. Dorsal seta inserted close to posterior medial margin, about 1.6 times as long as caudal ramus, triarticulate at its base and smooth. Proximal lateral seta arising somewhat dorsolaterally at midlength, 0.8 times as long as dorsal one and 1.6 times as long as distal lateral seta, which arises at 3/4 of ramus length. Inner apical seta slender, small, smooth, about 0.4 times as long as ramus. Both principal setae inserted slightly more dorsally than terminally, smooth and with breaking plane.

Antennula ( Fig. 16M ) not very long but slender, ten-segmented, but last two segments partly fused, not strongly geniculate, with geniculation between seventh and eighth segments, unornamented. Long aesthetasc on apical acrothek of fifth segment reaching slightly beyond posterior margin of appendage. Setal formula: 1.10.6.1.6.1.2.1.4.7. Majority of setae smooth and slender; three setae on fifth segment, one on sixth and one on seventh very short and spiniform. Outer (caudal) setae on ninth and tenth segments biarticulating on basal part; no setae with breaking plane.

Antenna, labrum, mandibula, maxillula, maxilla, maxilliped, second swimming leg ( Fig. 16H ) and third swimming leg very similar to M. pagusregalis sp. nov.

Armature formula of swimming leg 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 1/2/2 0/1/0 - -

First swimming leg ( Fig. 16G ) with inner spine on basis modified, smooth, curved outwards apically and very inflated.

Fourth swimming leg ( Figs 16I & O ) with one inner seta on third exopodal segment and with endopod slightly more elongated than in M. pagusregalis sp. nov.; all other details of armature, ornamentation and proportions similar to this species.

Fifth legs ( Figs 16J & N ) with distinct exopod and fused baseoendopods, but distinct from somite. Only ornamentation represented by single cuticular pore on anterior surface of baseoendopod. Exopod small, twice as wide as long, armed with four setae on both sides; innermost seta spiniform and bipinnate, other three smooth and slender; length ratio of exopodal setae 1: 0.7: 0.5: 1.2. Endopodal lobe unarmed. Outer basal seta 2.4 times as long as longest seta on exopod.

Sixth legs ( Figs 16K & N ) narrowly fused basally together and indistinct from somite, each armed with inner spine and outer seta of about same length.

FEMALE. Unknown.

Variability. Body length of males ranges from 0.421 mm to 0.433 mm (0.427 mm average; n = 3), while no females were collected and studied. One topotype male ( WAM C37347) has a somewhat shorter outer seta on the sixth leg ( Fig. 16N ), as well as the setae on the fifth leg exopod of slightly different proportions. All three males have no ornamentation whatsoever on the anal operculum and have their caudal rami of about the same length .

Etymology. The species name is a Greek adjective “kryptos” (meaning “hidden”) and refers to the fact that these three males were preliminarily identified as belonging to M. pagusregalis and only careful examination revealed their separate specific status.

Remarks. As mentioned in the Remarks section for the previous species, Megastygonitocrella kryptos sp. nov. is probably most closely related to the only other representative from Queensland, M. pagusregalis sp. nov., but they can be distinguished by a number of characters (see above).

Autapomorphic features of M. kryptos include its very narrow anal operculum and elongated, but conical, caudal rami. It is also the only known species in this group where the ancestral middle seta on the male sixth leg is absent, while the other two are present. The structure of the fifth leg resembles that of M. dec sp. nov., because it also is not fused to the somite and has a distinct exopod and an endopod that lacks armature. However, a closer examination shows that the ancestral innermost element ( Table 2, character 47) of the fifth leg exopod is absent in M. kryptos (just like in M. pagusregalis ), while it is present in M. dec . Only two other representatives of Stygonitocrella s. l. lack this armature element but both also have lost all the other exopodal and endopodal armature elements on the fifth leg: Stygonitocrella orghidani ( Petkovski, 1973) from Cuba and Reidnitocrella djirgalanica ( Borutzky, 1978) comb. nov. from Kyrgyzstan (see Petkovski 1973; Borutzky 1978; and above). These two species have very little in common with the genus Megastygonitocrella , which only shows that the reductions in the fifth leg originated independently a number of times and are not very suitable characters for generic or subgeneric definitions as was done by Suárez-Morales & Iliffe (2005).

Figures

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.