Maemonstrilla ohtsukai, Suárez-Morales & Mckinnon, 2014

Suárez-Morales, Eduardo & Mckinnon, A. David, 2014, The Australian Monstrilloida (Crustacea: Copepoda) I. Monstrillopsis Sars, Maemonstrilla Grygier & Ohtsuka, and Australomonstrillopsis gen. nov., Zootaxa 3779 (3), pp. 301-340 : 319-323

Maemonstrilla ohtsukai sp. nov.

( Figs 11–13 )

Material examined. Adult female holotype from Western Port Bay, Victoria, (38°35.344’S; 144°59.687’E; Station G of Kimmerer & McKinnon 1985) slides mounted in glycerine, sealed with Entellan ®, undissected, collected 5 January 1984 (cat. MTQW34271 ). Additional locality: Hamelin Pool Channel, Shark Bay , Western Australia (25°37.973’S; 113°43.846’E; Kimmerer et al., 1985). Date of collection: 16 June 1983. Note: Figures are based on a paratype specimen from Hamelin Pool Channel, that is now lost.

Description. Female: Mean total body length of specimens in type series 0.62 mm, range 0.49–0.82 mm (n=4), holotype 0.62 mm. Cephalothorax representing up to 64% of total body length ( Fig. 11A, C ), robust, not bulbous, 0.35– 0.52 mm long (n=4), holotype 0.39 mm. Cephalothorax with reticulate pattern of low ridges on dorsal and ventral surfaces, reticulation reaching pedigerous somites 2–4 and observed also on antennular segments. High reticular ridges only on “forehead” region ( Fig. 11 B, D ). Antennules relatively long, 0.12–0.18 mm (holotype 0.16 mm), representing 42% of cephalothorax length ( Fig. 11A, C ). Oral papilla relatively small, located anteriorly, about 20% of way back along ventral surface of cephalothorax ( Fig. 11C ). Pair of relatively large ocelli present, pigment cups weakly developed, separated by less than half eye diameter, unpigmented; ventral cup slightly larger than lateral cups. Three pairs of nipple-like cuticular processes on anterior ventral surface between antennule bases and oral papilla (arrowed in Fig. 11 B, C ).

Antennule four-segmented, with weak division between segments 3 and 4 ( Fig. 13F ). In terms of pattern described by Grygier and Ohtsuka (1995), element 1 present on first segment, biserially setulate; elements 2d 1, 2d 2, 2v 1, 2v 2, 2v 3, and IId present on second segment, element IIv broken off, socket observable. Third segment with elements 3, IIId, and IIIv. Segment four bearing elements 4d 1,2 and 4v 1-3 as well as IVd, IVv, Vd, Vm, and 5. Elements of groups 2v and 3 longer than those of groups 4d and 4v. Outer distal b 1-3 and also b 4-5 setae unbranched. Apically, only element 6 2 together with aesthetasc 6aes present, element 6 1 absent in all specimens examined.

First pedigerous somite incorporated into cephalothorax; this and succeeding three free pedigerous somites each bearing pair of biramous swimming legs. Pedigerous somites 2–4 together accounting for 38% of total body length in dorsal view. Dorsal surface of fourth pedigerous somite with row of four short pit setae (sensu Grygier & Ohtsuka, 2008) near posterior margin. Intercoxal sclerites of legs 1–4 sub-rectangular, without ornamentation on surface or along distal margin. Basis of legs articulating with rectangular coxa along diagonal line. Basis with thin, naked lateral seta on legs 1, 2, and 4; on leg 3, this seta thicker, biserially setulate, and 3.5 times longer than on other legs, reaching well beyond distal margin of exopodal ramus ( Fig. 13C ). Endopodites and exopodites of swimming legs 1–4 triarticulate ( Fig. 13A–D ). Ramus setae all lightly and biserially plumose except for spiniform outer setae on exopodal segments 1 and 3, that on third exopodal segment with both margins smooth. Outer margin of third exopodal segment of legs 1–4 with patch of minute spinules (arrowed in Fig. 13B–D ). Outer apical exopodal seta of swimming legs 1–4 with outer margin spinulose, inner margin lightly setulate ( Fig. 13E ). No inner seta on first exopodal segment of legs 1–4.

Armature formula of swimming legs:
	basis	endopodite	exopodite
leg 1	1-0	0-0;0-1;1,2,2	I-0;0-1;I,2,2
legs 2–4	1-0	0-0;0-1;1,2,2	I-0;0-1;I,1,2,2

Fifth legs paired, rod-like, with two setae, one distal, one subdistal ( Fig. 12D ); legs reaching to posterior margin of preanal somite. Urosome consisting of four somites: fifth pedigerous somite, compound genital somite with transverse suture at midlength, and two free postgenital somites, i.e. the preanal and anal somites ( Fig. 12 A– D ). Ventral surface of genital somite bearing ovigerous spines arising from low conical projection of anterior half of compound somite. Posterior half of genital compound somite with weakly developed ventral protuberance (arrowed in Fig. 12A ). Copulatory opening located on ventral surface at posterior base of ovigerous spine cone (arrowed in Fig. 12C ). Tips of ovigerous spines reaching to between legs 2 and posterior margin of cephalothorax. Spines cylindrical, smooth, and straight in proximal two-thirds; distal one-third either with moderately swollen bulbous process or tapering and curving distally ( Fig. 12A, C, D ). Caudal rami subrectangular, weakly divergent, approximately 1.8 times longer than wide, each ramus bearing six setae. Inner dorsal seta thinnest (seta VII of Huys & Boxshall 1991; arrowed in Fig. 12D ).

Etymology. This species is dedicated to Dr. Susumu Ohtsuka, Hiroshima University, Japan, for his remarkable contributions to the knowledge of the taxonomy and ecology of the marine Copepoda, including the description of the genus Maemonstrilla .

Diagnosis. Small species, of less than 1 mm in total body length. Cuticular ridges present on cephalothorax, antennules, and pedigerous somites. Reticulation absent from urosomites and caudal rami. Oral papilla small, with no particular features. Pigmented spots absent from body. Inner seta absent on first exopodal segment of legs 1–4; outer margin of coxa of legs 1–4 smooth. Posteroventral protrusion of genital compound somite weakly developed. Fifth leg uniramous, rod-like, armed with two distal setae.

Remarks. This species is assignable to the Maemonstrilla hyottoko species group as defined by Grygier and Ohtsuka (2008) mainly by its having a reticulate pattern of cuticular ridges on the cephalothorax, antennules, and pedigerous somites, no inner seta on the first exopodal segment of legs 1–4, and unbranched, rod-like fifth legs with two distal or subdistal setae.

By reference to Grygier and Ohtsuka’s (2008) key to the species of this species-group, this species is distinguished by its possession of a unique combination of features: 1) cuticular reticulation present on the cephalothorax, antennules, and pedigerous somites, but absent from the urosomites and caudal rami; 2) a small, ventrally directed oral papilla, not particularly long as in Mae. spinicoxa Grygier & Ohtsuka, 2008 or Mae. polka Grygier & Ohtsuka, 2008, nor flanked with “puffed cheeks” as found in Mae. okame Grygier & Ohtsuka, 2008, or bent as in some specimens of Mae. polka; 3) lacking pigmented spots throughout the body, thus being distinguished from Mae. polka (cf. Grygier & Ohtsuka 2008); 4) a weakly developed posteroventral protrusion of the genital compound somite, not spur-like or well-developed as in other Maemonstrilla of this group ( Grygier & Ohtsuka 2008); 5) a smooth outer margin of the coxa in legs 1–4, thus contrasting with the distinct coxal ornamentation patterns described for Mae. hyottoko, Mae. polka, and Mae. spinicoxa ( Grygier & Ohtsuka 2008); and 6) a body length of less than 1 mm (0.49–0.82 mm), thus differing from Mae. hyottoko (1.15–1.60 mm), Mae. polka (2.56– 2.66 mm), Mae. spinicoxa (1.48–1.70 mm), Mae. okame (1.20–1.67), Mae. simplex (1.80–1.98 mm), and Mae. turgida (A. Scott, 1909) (1.38–2.21 mm) ( Grygier & Ohtsuka 2008).

Figures

FIGURE 11. Maemonstrilla ohtsukai sp. nov., adult female. A) habitus, dorsal view; B) cephalic area of holotype showing cuticular reticulation, ventral view; C) habitus, lateral view; D) cephalic area of paratype specimen, ventral view. Scale bars: A, C= 100 µm, B, D= 50 µm.

FIGURE 12. Maemonstrilla ohtsukai sp. nov., adult female. A) urosome showing fifth legs, ovigerous spines, and weak posteroventral protuberance of genital compound somite (arrow), lateral view; B) urosome, ventral view; C) urosome showing ovigerous spines and position of genital pore (arrow), ventral view; D) same, another specimen, showing caudal rami with arrowed seta VII and fifth legs. Scale bars: A–D = 50 µm.

FIGURE 13. Maemonstrilla ohtsukai sp. nov., adult female. A) first swimming leg with coxal sclerite; B) second swimming leg with coxal sclerite; C) third swimming leg with coxal sclerite; D) fourth swimming leg; E) ornamentation of outer apical spiniform seta of third exopodal segment of second leg; F) right antennule, dorsal view. Scale bars: A, C–E = 100 µm, B = 25 µm.