Monstrilla leucopis Sars 1921

Monstrilla leucopis Sars, 1921

(Figs. 1–4)

Monstrilla leucopis Sars, 1921, pl. VII

Monstrilla (?) danae T. Scott, 1900, p. 398–399, figs. 15–20 Monstrilla anglica; T. Scott, 1904, p. 246, figs.12–14

Material examined. — Five adult females from Kvalø, northeast Norway. Specimens deposited in the Sars Collection, Zoological Museum, University of Oslo, Norway. Specimens (F21803), ethanol-preserved, badly damaged, several body parts separated in vial, many setae missing, all specimens’ antennules broken off.

Description of adult females. — Total body length 2.84–3.26 mm (mean: 3.04 mm, n=4), measured from anterior end of cephalothorax to posterior end of anal somite. Cephalothorax representing between 58.9 % and 59.7 % of total body length. Oral papilla slightly protuberant (Fig. 1 A, B), located slightly less than 40% of way back along ventral surface of cephalothorax (Fig. 1 A–C). Ocelli and pigment cups inconspicuous. Cephalic segment with medial notch on “forehead”, visible in dorsal and ventral views (Fig. 1 D, E); sensilla not observed on cephalic area. Faint cuticular wrinkles arranged in loose parallel pattern near bases of antennules surrounding two longitudinal ventral rows each of three small, ear-shaped cuticular processes (Fig. 1 D); processes arranged in slightly different patterns in at least three specimens (Fig. 2 A–C). Other ventral cuticular ornamentation including usual nipple-like processes (scars) on anterior ventral surface above oral papilla; these processes radially ridged and furrowed (arrowed in Fig. 1 A, B). Pores and pits on body surface not described.

Antennules relatively long, slender, equal to 37% of total body length and 59% of cephalothorax length. Antennules four-segmented, intersegmental divisions inconspicuous, armed with at least 0; 1-V; 2-I; 10-VIII setae (Arabic numerals) and spines (Roman numerals), plus two aesthetascs (Fig. 2 D–I; cf. Sars 1921: pl. VII). Most setal elements broken off in each specimen; armature reconstructed on basis of adding all elements and setal sockets present in any specimen. In terms of Grygier and Ohtsuka’s (1995) terminology for female monstrilloid antennular armature, element 1 absent on first segment of all specimens examined; elements 2d1, 2d2, 2 v1, 2 v2, 2 v3, and IId on second segment (socket only of IId confirmed in specimens examined); with first four listed setal elements short in both antennules (Fig. 2 D, E, I). Third segment with elements 3, IIId, and IIIv, presented only by sockets in specimens examined (Fig. 2 F; cf. complete armature of segment depicted in Sars, 1921: pl. VII). Fourth segment bearing elements 4v 1, 4d1,2, and 4v 2–3 as well as setae IVd, IVv, Vm, and Vv (Fig. 2 D, E, G); elements 4aes, Vd, Vm, and 5 not observed in specimens examined, but depicted by Sars (1921: pl. VII). Subterminal elements b1–3 present (Sars 1921: pl. VII), unbranched. Aesthetasc 6aes as well as terminal elements 6 1 and 6 2 broken off in the specimens examined, but present in the species (cf. Sars 1921: pl. VII). Elements b4-b6 neither observed in the present specimens (Fig. 2 H), nor depicted by Sars (1921).

Cephalically incorporated first pedigerous somite and succeeding three free pedigerous somites each bearing a pair of biramous swimming legs. Swimming legs 1–4 slightly increasing in size posteriorly. Intercoxal sclerites of legs 1–4 rectangular, without ornamentation on surface or along distal margin. Basis of these legs articulating with large, rectangular coxa along diagonal posterior line. Basis with hair-like lateral seta on legs 1–4 (cf. Sars 1921: pl. VII), but this seta not observed in the present specimens (Fig. 4 B–D); this seta slightly thicker and longer on leg 3 than on other legs, lightly setulated from insertion (cf. Sars 1921: pl. VII). Endopodites and exopodites of swimming legs 1–4 triarticulated. Ramus setae all biserially plumose except spiniform outer seta (“spine”) on exopod 1 and 3. Inner seta of first exopodal segment absent on legs 1–4 (Fig. 4 B–D, Sars 1921: pl. VII). Outermost apical exopodal setae (also a “spine”) of swimming legs 1–4 with inner margin bearing sparse row of short, hair-like setules, outer margin lightly spinulose to tip (Fig. 4 B,C). Outermost seta of third endopodal segment of legs 2–4 modified, thick-walled, stout (Fig. 4 C). Third endopodal segment of leg 1 with only four setae (Fig. 4 B, F) in at least two specimens.

Fifth legs medially conjoined at expanded base, unsegmented, each consisting of relatively large, strong outer (exopodal) lobe tapering distally (Figs. 3 A, B, 4A). Lobe armed with two equally long exopodal setae, reaching level of distal margin of anal somite (cf. Sars 1921: pl. VII); all setae of fifth legs broken off in females examined.

Urosome consisting of four somites: fifth pedigerous bearing fifth legs, genital double somite, free postgenital somite, and free anal somite. Fifth pedigerous somite representing about 4.5% of total body length, about as long as two postgenital somites together. Genital double somite relatively long, representing between 6 % and 6.5 % of total body length; ratio of its length and lengths of two succeeding somites 56.2: 27: 16.8 = 100. Double somite with cuticular striae near outer distal margins, ventral surface with rounded protuberance on anterior proximal margin (Fig. 3 E), this being more prominent in one specimen than in others (Fig. 3 C). Anterior half of genital double somite with outer margins expanded laterally (Fig. 3 D). Ovigerous spines on proximal half of ventral side of double somite; spines paired, arising separately from low ventral expansion of somite surface (Fig. 3 A). Ovigerous spines relatively short, about 19.5 % of total body length and 32% as long as cephalothorax. One specimen showing adhering egg clusters along spines covered by thin, gelatinous sheath (Fig. 1 A); tips of spines separated from each other, with slight distal narrowing (Fig. 3 A).

Caudal rami subrectangular, not divergent, approximately 1.7 times longer than wide, most setae broken off from specimens examined, but Sars (1921: pl. VII) observed three terminal subequal setae and two on outer margin, one of the latter greatly reduced.

Remarks. — Monstrilla leucopis belongs to a group of species of Monstrilla with relatively long, slender, and poorly segmented antennules in the female. This group includes M. clavata Sars, 1921; M. longiremis Giesbrecht, 1893; M. grandis Giesbrecht, 1891; M. conjunctiva Giesbrecht, 1902; M. longicornis Thompson, 1890; M. cf.

helgolandica Claus, 1863 (sensu Sewell 1949, not Sars 1921); M. elongata Suárez-Morales, 1994 and M. grygieri Suárez-Morales, 2000. Aside from M. elongata, with antennules representing ca. 35% of cephalothorax length (Suárez-Morales 1994), all these species have relatively long antennules (60%–98% of total body length) (cf. Suárez-Morales, 1994, 2000; Giesbrecht 1893; Sars 1921; Sewell 1949). In M. leucopis the antennule length is at the lowest 1/3 of this size range. Among these species, only M. leucopis, M. conjunctiva, M. elongata, and M. cf. helgolandica have a female fifth leg represented by a single lobe with two terminal setae. Only the former two species share with M. leucopis the absence of antennular element 1 on the first antennular segment and the presence of unbranched antennular elements b1–3 (Sewell 1949; Suárez-Morales 1994). These three species (M. leucopis, M. conjunctiva, M. elongata) also share the absence of an inner seta of the first exopodal segment of the first leg and they have a similar pattern of caudal setation, with five setae, the terminal outermost one being reduced (cf. Sars 1921; Sewell 1949; Suárez-Morales 1994).

In the description of M. elongata from the western Caribbean, Suárez-Morales (1994) recognized a close resemblance between this species and M. leucopis. Based on the general resemblance between M. conjunctiva and M. leucopis, several authors (Sewell 1949; Isaac 1974, 1975; Razouls et al. 2009) considered the latter as at least a possible junior synonym of the former. Monstrilla conjunctiva was originally known only from the male, a specimen from deep Antarctic waters (Giesbrecht 1902). A male and female recorded from tropical Indian Ocean surface waters in the Nicobar Islands were questionably assigned to this species (Sewell 1949), and the general resemblance of the female to Sars’s (1921) M. leucopis probably motivated the decision to propose this synonymy. However, because of obvious biogeographic considerations and the fact that cooccurrence of two monstrilloid specimens of two genders in a sample is no indicator of conspecificity (Suárez-Morales 2001), it is clear that the Nicobar females reported by Sewell (1949) are not conspecific with M. conjunctiva from the Antarctic area. In fact, Sewell (1949) doubted his designation of these females as M. conjunctiva as it was called “unknown” in the text and written as “ M. (?) conjunctiva only in the figure caption. Hence, a morphological comparison between these females is not necessary since the Nicobar females most probably represent an undescribed species. During this study, unique characters were revealed that distinguish M. leucopis from all other known species of Monstrilla. These include the presence of modified setae on the endopods of legs 2–4. These stout, thick-walled setal elements clearly differ from the usual setae and are easily detectable. This kind of modification has not been described previously among the Monstrilloida. Another interesting character is the reduction of the number of elements (to four) on the third endopodal segment of leg 1, the usual pattern including five setae. This reduction was observed in at least two of the five specimens examined.

Based on all the evidence recounted above, it is suggested that M. leucopis is a valid species with unique characters; it is known only from Norway but there are some other regional records that could be assignable to this species. In his works on the Scottish copepods, T. Scott (1900, 1904) reported (as Monstrilla (?) danae and M. anglica, respectively) specimens that resemble M. leucopis. These were noted by Sars (1921) and later on by Sewell (1949), who also stated that Scott’s (1904) figures were different from those published in 1900 and that only the latter illustrations represented a specimen barely distinguishable from M. leucopis. I agree with Sewell’s (1949) comment about the conspecificity of Scott’s (1900) female M. anglica and M. leucopis.

The male of M. leucopis was described by Sars (1921) from the same locality in Norway (Kvalø). Unfortunately, the sample labeled as the male of M. leucopis in the Sars Collection contained a male specimen of M. longiremis. There are no other specimens in the Sars Collection labeled as M. leucopis (Å. Wilhelmsen, in litt.); hence, the single male specimen of this species mentioned and depicted by Sars (1921) is considered lost. Considering Sars’s (1921) figures of the male M. leucopis, it is clear that it has a strong resemblance to the male of M. conjunctiva described by Giesbrecht (1902) from the Antarctic Ocean and the one from the Indian Ocean assigned to this species by Sewell (1949). Clearly, these two males (Nicobar and Norway) are likely to represent distinct species because it is biogeographically inconceivable that they are conspecific with the Antarctic M. conjunctiva. In the light of the morphological evidence supporting the validity of M. leucopis, a comparison is made between the Nicobar and the Antarctic males. In the holotype male of M. conjunctiva (Giesbrecht 1902), the cephalothorax is relatively shorter (53% of total body length) than in the Nicobar male (61%). The genital somite of the holotype appears to be relatively smaller than that of the male from Nicobar as depicted by Sewell (1949); in the former, this somite represents 33% of the urosome, vs. 50% in the Nicobar male. The distal elements 1 and 2 of the fifth antennular segment (sensu Huys et al. 2007) appear to be subequal in the Nicobar male whereas element 2 is distinctly longer in M. conjunctiva (cf. Giesbrecht 1902: figs. 1, 2). The terminal seta of the single fifth leg lobe is clearly longer in the Nicobar male, it reaches well beyond the distal margin of the caudal rami, whereas it does not reach this margin in M. conjunctiva (cf. Giesbrecht 1902: fig. 3). In lateral view, the accessory subterminal spiniform element of the genital complex is inserted more anteriorly than in the Nicobar male (cf. Sewell 1949).