Monstrilla longiremis Giesbrecht, 1893

(Figs. 5–6)

Monstrilla longiremis Sars, 1921, pl. V

Material examined. — One adult male from Kvalø, northeast Norway. Specimen deposited in the Sars Collection, Zoological Museum, University of Oslo, Norway (F21804), preserved in ethanol, badly damaged, body parts, including cephalothorax and the urosome, separated in vial.

Supplementary description of male from Kvalø. — Total body length of single specimen: 2.4 mm. Cephalothorax 0.74 mm long, 55% of total body length (Fig. 5 A). Oral papilla prominent, 43% of way back along ventral surface of cephalothorax (Figs 5 A, F). Cephalic region with conical anteroventral protuberance (arrowed in Fig. 5 A). Eyes and pigment cups weak. No sensilla observed on anterior part of cephalic region. Pair of usual nipple-like cuticular processes with convergent striae set to either side of oral papilla (Fig. 5 F). Antennules 5-segmented, representing about 54% of total body length (Figs 5 B–D). Terminal segment with distal elongation (Fig. 5 B, C). Many setae and spines missing or broken off. Summarized armature as observed here and depicted by Sars (1921: pl. VII): segments 1–5 armed with 1-0; 1-IV; 2-I; 2-III; 3-II setae (in Arabic numbers) and spines (in Roman numbers), respectively (Fig. 5 B–E); nomenclature of setal elements on the fifth segment (asterisk in Fig. 5 C, D) followed Huys et al. (2007).

Separated somites still bearing protopods of some swimming legs, but broken parts of leg rami loose in vial (Fig. 6 C). All swimming legs with 3-segmented rami and same armament pattern, except for leg 1 exopod with 5 rather than 6 elements on third segment (Fig. 6 G). Distal seta on third exopodal segments of legs 1–4 on leg 1 proximally naked, distally covered by spinules (Fig. 6 K,L). Third exopodal segment of legs 2–4 with sinuous external margin (Fig. 6 K, L). Inner seta on first exopodal segment of leg 1 absent (Fig. 6 K, L). Armature formula of legs 1–4 as usual in monstrilloids, as far as can be confirmed. Fifth legs represented by single lobe armed with single distal seta (Fig. 6 A, E, F; Sars 1921: pl. V). Fifth leg lobe cylindrical, reaching to about midlength of genital apparatus in lateral view (Fig. 6 A).

Urosome consisting of 4 somites. Genital complex represented by strong cylindrical shaft with distal pair of strongly divergent lappets, each armed with short, robust terminal spine (Figs. 5 G, 6A, B). Lappets moderately elongate and posteriorly directed in lateral view (Fig. 6 A, B). Anal somite about half as long as genital somite. Caudal rami subrectangular (Fig. 6 B). Each ramus with 4 setae (Sars 1921: pl. V); all caudal setae broken off in Kvalø specimen.

Remarks. — The present male of this species is as described and depicted by Giesbrecht (1893), Sars (1921: pl. V), and Huys and Boxshall (1991: fig. 2.5.11 C–E). Males of this species can be distinguished from all other known males of Monstrilla by the combination of: 1) the presence of a single distal seta on the fifth leg; 2) a genital complex with terminally diverging lappets each bearing a short, robust spine; 3) the relatively long antennule, equaling more than 50 % of total body length; 4) the presence of a distal elongation of the fifth antennular segment. The length of this elongation seems to be variable even among specimens from the same geographical region. It was illustrated by Huys & Boxshall (1991) from specimens of the Sars Collection that were collected at Hvalør and Bukken, Norway, in which the fifth antennular segment is relatively shorter than in the specimen from Kvalø (Fig. 5 B–D) and in Sars's (1921) illustration of this species.

The specimen from Kvalø examined herein was originally labeled by G. O. Sars as “ M. leucopis Sars?” (sic) when M. longiremis was known by him from different parts of Norway, including Kvalø (Sars 1921). Males of both species have similar body proportions, general antennular structure, and a single-lobed fifth leg with one distal seta. The single specimen found in the vial displays several differences from the specimen depicted by Sars (1921: pl. VIII) as M. leucopis. The latter has unbranched antennular elements b1–b3 (vs. branched in M. longiremis), no distal elongation of the fifth antennular segment, a character clearly present in M. longiremis (Giesbrecht 1893; Sars 1921: pl. V; Huys & Boxshall 1991) and in the specimen examined (see Fig. 5 B–D). Additionally, element 1 of the first antennular segment, apparently absent in the male of M. leucopis as depicted by Sars (1921 pl. VII), but present in the specimen examined. Furthermore, the two species have a differently formed genital complex, more robust and with long, slender terminal spines in M. leucopis (cf. Sars 1921: pl. VII); the fifth leg seems to be longer in M. longiremis than in M. leucopis (cf. Sars 1921: pls. V, VII); however, in the specimen from Kvalø, the fifth leg lobe is even longer than the appendage depicted by Sars (1921).