Maemonstrilla protuberans sp. nov.

(Figs 18–20)

Material examined. Adult female holotype from Davies Reef, Queensland, Australia (19°7.34’S; 146°53.024’E), partially dissected, ethanol-preserved, slide mounted in glycerine, sealed with Entellan ®. Date of collection: 5 October, 1985. Slide deposited in collection of MTQ, Australia (cat. MTQW34273) .

Description. Female: Total body length of holotype 0.57 mm. Cephalothorax robust in dorsal view, representing up to 58% of total body length (Fig. 18A, B). Cephalothorax covered with pattern of minute spinules mixed with light reticulation. Reticulation observed only on anterior dorsal and ventral surfaces of cephalothorax and on some antennular segments (Figs. 18A, B, 19C). Antennule relatively long, 0.22 mm, representing 35% of total body length and 61% of cephalothorax length (Fig. 18A). Oral papilla conical, ventrally projecting, relatively large, located anteriorly, about 20% of way back along ventral surface of cephalothorax (Fig. 18C). Pair of relatively large ocelli present, pigment cups weakly developed, separated by about half eye diameter, unpigmented; ventral cup slightly smaller than lateral cups. Forehead rounded, protruding medially. Preoral surface with ventral rounded protuberance (arrowed in Fig. 18C) and two pairs of nipple-like cuticular processes with adjacent pattern of striae (Fig. 18C, D).

Antennule four-segmented, with weak divisions between segments 2–3 and 3–4 only marked by constrictions (Fig. 19C). High reticular ridges on outer margin of segments 3 and 4 (arrowed in Fig. 19C). Element 1 present on first segment, reaching to proximal one-third of second antennular segment. Elements 2d 1, 2d 2, 2v 1, 2v 2, 2v 3, and IId present on second segment. Third segment with elements 3, IIId, and IIIv; element 3 strongly developed. Segment four bearing aesthetasc 4aes and elements 4d 1,2 and 4v 1,2, with 4v 1 being remarkably long; elements IVd, IVv, Vd, Vv, Vm, and 5 also present. Most elements of groups 2v, 2d and 3 longer than those of group 4, but element 4v 1 only slightly longer than most elements of 2v,d groups and shorter than element 3. Outer distal b 1-3 branched, elements b unbranched. Apically, only 6 2 present, but socket of element 6 1 observed on both antennules.

5-6

First pedigerous somite incorporated into cephalothorax; this and succeeding three free pedigerous somites each bearing pair of biramous swimming legs. Pedigerous somites 2–4 relatively short, together accounting for only 22% of total body length. Intercoxal sclerites of legs 1–4 low, wide, without ornamentation on surface or along distal margin. Basis of legs articulating with coxa along diagonal line. Basis with thin lateral seta on legs 1– 4; on leg 3, this seta thicker, biserially setulate and 4 times longer than in legs 1 and 4, 2.5–3 times longer than in leg 2, reaching distal margin of second segment of exopodite (Fig. 20C). Endopodites and exopodites of swimming legs 1–4 triarticulate (Fig. 20A–D). Ramus setae all lightly and biserially plumose except spiniform outer seta on exopodal segments 1 and 3 and inner seta of first exopodal segment, these all being short and slender. Outer apical exopodal seta of swimming legs 1–4 with outer margin spinulose, inner margin naked (Fig. 20A–D). Inner seta on first exopodal segment of legs 1–4 absent, no remnant being observed.

Armature formula of swimming legs as in Maemonstrilla ohtsukai .

Fifth legs paired, rod-like, with two setae, one distal, one subdistal (Fig. 19A, B). These legs short, not reaching posterior margin of genital compound somite (Fig. 19A). Urosome consisting of four somites: fifth pedigerous somite, compound genital somite with incomplete transverse suture, and two free postgenital somites, i.e. the preanal and anal somites (Fig. 19A, B). Ventral surface of genital double somite bearing ovigerous spines arising from conical projection of ventrally protuberant anterior half. Posterior half of genital compound somite straight, without ventral process. Copulatory opening located on ventral surface at posterior base of ovigerous spine cone (arrowed in Fig. 19A). Ovigerous spines relatively short, reaching to between legs 2 and 3, partially concealed by relatively small egg mass (Fig. 18B). Spines cylindrical, smooth, and straight, tapering and diverging distally, not swollen or bulbous distally (Fig. 19A). Caudal rami subquadrate, weakly divergent, approximately 1.3 times longer than wide, each ramus bearing six setae. Inner dorsal seta (seta VII of Huys & Boxshall 1991) thinnest (arrowed in Fig. 19A).

Male: unknown.

Etymology. The specific epithet makes reference to the ventral preoral protuberance, a feature not previously described in any species of Maemonstrilla .

Diagnosis. Cuticular reticulation limited in extent, as in Mae. spinicoxa. Antennule relatively long, representing more than 60% of cephalothorax length. Oral papilla conical, straight, long. Ventral rounded preoral protuberance present. Pigmented spots absent from body. Inner seta on first exopodal segment of legs 1–4 absent; outer margin of coxa of these legs smooth. Posteroventral margin of genital compound somite straight, with no process.

Remarks. Maemonstrilla protuberans is assignable to the Maemonstrilla hyottoko species group as defined by Grygier and Ohtsuka (2008) mainly by its possession of a lightly reticulate cephalothorax and antennules mixed with a dense spinulose pattern, the absence of an inner seta on the first exopodal segment of legs 1–4, and unbranched, rod-like fifth legs with two setae. The ornamentation pattern observed in this species has some resemblance with that described for Mae. spinicoxa, but it is more complex in Mae. spinicoxa (Grygier & Ohtsuka 2008, fig. 14A). Both species share a similar shape of the antennules, including the constrictions marking the divisions between segments 2–3 and 3–4 and the high reticulation ridges on the outer margins of segments 3 and 4, but in Mae. spinicoxa the antennule is only 37% of the cephalothorax length and Mae. protuberans lacks the large denticles on the proximal outer coxal lobes that are distinctive of Mae. spinicoxa. The ventral preoral protuberance of Mae. protuberans has not been reported in any other species of this genus, all of which have a straight or smooth preoral area (Grygier & Ohtsuka 2008).The length of the antennule, representing more than 60% of the cephalothorax length, distinguishes this species from all other Maemonstrilla, in which the antennule length ranges between 29% and 51% of the cephalothorax length (Grygier & Ohtsuka 2008). In contrast to the other species of the Maemonstrilla hyottoko group, this new species has a straight, unmodified posteroventral margin of the compound genital somite. This has been considered a defining character of the M. turgida group (Grygier & Ohtsuka 2008), although the posteroventral protrusion of M. ohtsukai sp. nov. is only weakly developed. Maemonstrilla protuberans has a moderately developed, ventrally directed oral papilla, not long as in Mae. spinicoxa and Mae. polka nor either flanked with “puffed cheeks” as in Mae. okame or bent anteriorly at midlength as in some specimens of Mae. polka. Its body length (0.57 mm) is similar to that of Mae. ohtsukai (0.55–0.82 mm), thus distinctly smaller than most of the other known species of the genus, which are all well over 1 mm long (Grygier & Ohtsuka 2008). These two species are the smallest members of Maemonstrilla currently known.