Monstrillopsis planifrons, Delaforge, Aurelie, Suarez-Morales, Eduardo, Walkusz, Wojciech, Karley Campbell, & Mundy, C. J., 2017

Delaforge, Aurelie, Suarez-Morales, Eduardo, Walkusz, Wojciech, Karley Campbell, & Mundy, C. J., 2017, A new species of Monstrillopsis (Crustacea, Copepoda, Monstrilloida) from the lower Northwest Passage of the Canadian Arctic, ZooKeys 709, pp. 1-16 : 2-6

Monstrillopsis planifrons sp. n. Figs 2, 3, 4

Material examined.

Adult female holotype from Dease Strait (69°1.5'N; 105°20.2'W), partially dissected. Selected appendages (legs 1-4) and cephalothorax and urosome on semi-permanent slides, mounted on glycerine. Date of collection: 02 June 2014. Plankton sampled underneath ice-covered water column. Slides deposited in the Collection of Zooplankton at El Colegio de la Frontera Sur (ECOSUR), in Chetumal, Mexico (ECO-CH-Z-09535).

Description.

Female. Body length of holotype specimen measured from anterior end of cephalosome to posterior margin of anal somite = 1.92 mm. Cephalothorax (incorporating first pedigerous somite) approximately 1.1 mm long, representing 58% of total body length (Fig. 2B). Oral papilla located at 20% of way back along ventral surface of cephalothorax. Pair of relatively large ocelli present, pigment cups moderately developed, medially conjoined, strongly pigmented; ventral cup larger than lateral cups (Fig. 2A). Cephalic area with conspicuous, protruding “forehead” process with flat, coarse anterior margin ornamented with transversely arranged cuticular ridges at its base (Figs 2A, 3C). Protruding frontal process with pair of sensilla inserted at each side. Cephalic ventral ornamentation including: 1) a pair of small papilla-like structures between antennule bases (paired arrows in Fig. 3C), 2) preoral pair of nipple-like processes on anterior ventral surface; processes with adjacent wrinkles, rounded in shape; processes connected medially by transverse wrinkles (Fig. 3C). Cephalothorax covered with small papilla-like cuticular processes arranged randomly on ventral, lateral and dorsal surfaces (Figs 2B, 3B, C). Other ventral cuticular ornamentation including shallow striae on lateral and anterior surfaces of oral cone. Pedigerous somites 2-4 measuring 0.47 mm representing 24.7% of total body length; second pediger ornamented dorsally with three pairs of minute papilla-like processes on medial position (arrowed in Fig. 2B). Third pediger with pair of small sensilla on anterior half (Fig. 2B).

Urosome slender, consisting of fifth pedigerous somite, genital double-somite, and preanal and anal somites, together measuring 0.31 mm and representing 16% of total body length. Relative lengths of fifth pedigerous somite, genital double-somite, preanal and anal urosomites as: 31:38.2:14.1:16.7= 100, respectively (Fig. 2C, D). Genital somite longest of urosome, with pair of rounded expansions on posterolateral margins; surface smooth (Fig. 2C, D); somite with rounded ventral protuberance on anterior margin and with shallow suture on lateral surface (Fig. 3F). Ovigerous spines arising ventrally from proximal half of genital double-somite; spines paired, relatively short (0.62 mm), about 0.24% of total body length, posteriorly reaching distal margin of caudal setae (Fig. 2B). Spines basally separate, slender, straight at base and along shaft, both distally tapering into acute points; left spine slightly shorter (Fig. 2E, F). Specimen with a few eggs attached to ovigerous spines (Fig. 2G). Caudal ramus subrectangular, 1.9 times longer than wide, armed with four subequally long lightly setulated caudal setae (Fig. 2B).

Antennules relatively long, slender, not straight but clearly divergent (Fig. 2B). Antennule length = 0.59 mm, representing about 31% of total body length and 51 % of cephalothorax length; antennule indistinctly 4-segmented, segments 3-4 partially fused. Relative length of segments, from base to top as: 12.1; 29.2; 17.1; 41.6. Last segment distally tapering (Fig. 3A). Antennulary armature in terms of pattern described by Grygier and Ohtsuka (1995), including setae (Roman numerals), spines (Arabic numerals), and aesthetascs as: element 1 present on first segment, represented by relatively long, spiniform; elements on second segment: 2d1-2, 2v1-3 and long seta IId reaching slightly beyond distal end of antennule. Third segment with elements 3, IIId, and IIIv; element 3 remarkably long. Segment four bearing elements 4d1,2, 4v1-3, element 4v1 longest of group. Setae IVd, IVv, Vd, Vv, Vm, and 4aes present. Element 5 spiniform. Subterminal elements b1-5, branched; 6aes present. Apical elements 61 and 62 strong, spiniform; 61 twice as long as 62 (Fig. 3A). First segment of left antennule with an additional spine about half as long as element 1; supernumerary spine absent on right antennule.

Incorporated first pedigerous somite and succeeding three free pedigerous somites each bearing a pair of biramous legs. Legs 1-4 slightly increasing in size posteriorly, leg 1 being shortest. Intercoxal sclerites of legs 1-4 subrectangular, widest transversely, with rounded margins, with surface and posterior margins smooth; sclerites with decreasing size, that of leg 1 being largest (Fig. 4 AE–H). Basis of legs articulating with large, rectangular coxa along diagonal line. Basis of legs 1-4 with hair-like lateral seta (Fig. 4 A–D); on leg 3, this seta about 6 times longer and slightly thicker than those on the other legs (arrowed in Fig. 4C). Endopodites and exopodites of legs 1-4 triarticulated. Ramus setae all biserially plumose except spiniform outer seta on exopodal segments 1 and 3, and inner seta of first exopodal segment, these latter being short and sparsely setulated (Fig. 4 A–D). Spine on exopodal segment 1 of legs 1-4 long, almost as long as segment, reaching distal margin of succeeding segment. Long apical exopodal setae of legs 1-4 with inner margin setulated, inner margin lightly spinulose.

Armature formula of legs 1-4 as:

Fifth legs medially conjoined, indistinctly bilobed, inner (endopodal) lobe incospicuous, represented by small inner protuberance (arrows in Fig. 3E). Outer lobe large, robust, with rounded margins; lobe armed with three subdistal setae. Innermost seta shortest, all fifth leg setae biserially and lightly setulated (Fig. 2C).

Etymology.

The specific epithet, derived from the Latin adjective planus (flat) and the noun frons (forehead), makes reference to the flat, protuberant frontal margin on the cephalic area, which is distinctive of this species.

Remarks.

The Arctic species described herein is assigned to the genus Monstrillopsis owing to its possession of the combination of characters noted by Sars (1921) in his diagnosis of this genus: 1) two free somites posterior to the genital double-somite, 2) eye fully developed, 3) four-segmented antennules in the female, 4) oral papilla occurring near the anteriormost part of the cephalothorax: <20% of way back along cephalothorax, 5) bilobed female fifth leg, outer lobe armed with three setae, 6) furcal rami with four setae. For the Monstrillopsis genus, the number of female caudal setae is four, as in the present new species. However, males of some species can have more caudal setae (i.e., Monstrillopsis reticulata (Davis, 1949), M. zernowi Dolgopolskaya, 1948) ( Suárez-Morales et al. 2006), but except for the aberrant M. zernowi , with 5 caudal setae, the character is consistently present in the females.

Monstrillopsis planifrons sp. n. differs in several respects from its known congeners. Most species of Monstrillopsis of which females are known, like M. dubioides Suárez-Morales, 2004, M. ferrarii Suárez-Morales & Ivanenko, 2004, and M. chilensis , have affinities with M. dubia . This group of species share a relatively short cephalothorax (ca. 50-56% of total body length), four caudal setae, a well-defined digitiform inner lobe on the female fifth leg, an outer lobe armed with three long setae, antennule relatively short (30-35% of cephalothorax length) distinctly 4-segmented, and a relatively long genital double-somite (ca. 30% of urosome) with an expanded proximal half ( Suárez-Morales et al. 2006, 2008).

Monstrillopsis planifrons sp. n. differs from this group of species in having relatively longer antennules (53% of total body length), which is one of the most striking characters of this species; this pattern is similar to that depicted by Dolgopolskaya (1948, fig. 3.1) for M. zernowi . This aberrant species also resembles the new species in having fused antennulary segments 3-4. The new species clearly diverges from M. zernowi in the number of caudal setae: 5 vs. 4 in the new species. In addition, the fifth leg has a well-defined inner lobe ( Dolgopolskaya 1948, fig. 3.5), thus diverging from the weakly developed inner process found in the new species. Also, in M. zernowi the frontal anterior margin of the cephalic region is depressed, with a medial protuberance ( Dolgopolskaya 1948, fig. 3.2), thus differing from the produced, flat process that is present in M. planifrons sp. n. A relatively long antennule (47% of cephalothorax length) is also present in M. dubia ( Scott 1904, pl. XIII, fig. 14).

The antennule structure and armature are also a source of distinctive apomorphies of this new species. In several species of the genus the female antennulary segments 3-4 are clearly separate, as in M. dubia ( Scott 1904, pl. XIII, fig. 14), M. ferrarii ( Suárez-Morales and Ivanenko 2004, fig. 4B), M. dubioides ( Sars 1921; Suárez-Morales and Ivanenko 2004, fig. 9A), M. chilensis ( Suárez-Morales et al. 2006), and Monstrillopsis igniterra ( Suárez-Morales et al. 2008, fig. 2B). In the new species these segments are partly fused (Fig. 3A). The general armature of the antennules is similar in all the species examined, but the new species has some distinctive details: 1) the apical elements 61 and 62 (sensu Grygier and Ohtsuka 1995) are clearly unequally long, the former is about twice as long as element 62 (Fig. 3A). In the other species these elements are equally or subequally long ( Scott 1904; Sars 1921; Suárez-Morales and Ivanenko 2004; Suárez-Morales et al. 2006, 2008); 2) the new species has a unique character on the left antennule, with a supernumerary spine on the first segment, a character that has not been observed in any other monstrilloid, probably reminiscent of an ancestral armature/fusion pattern of this segment which is known to have only a single element ( Grygier and Ohtsuka 1995).

Monstrillopsis planifrons sp. n. has a distinctive genital double-somite, with a pair of large lateral protuberances on the posterior half of the somite, visible on dorsal and ventral views (Fig. 2C, D). In its known congeners the anterior half of the genital double-somite is expanded, either strongly as in M. igniterra ( Suárez-Morales et al. 2005, fig. 2D) and M. chilensis ( Suárez-Morales et al. 2006, fig. 4B) or weakly as in Monstrillopsis filogranarum ( Malaquin 1901, fig. 3), M. ferrarii and M. dubioides ( Sars 1921; Suárez-Morales and Ivanenko 2004, figs. 6A, 8C). Monstrillopsis dubia (sensu Scott 1904) has a short, robust genital double-somite (see Scott 1904: Pl. XIV, fig. 18), expanded on its proximal half.

Another distinctive character of the new species is its produced, flat corrugate forehead; this character is absent from all its known congeners, which have a simple, rounded forehead as in M. dubia (Scott, 1904, pl. XII, fig. 14), M. dubioides ( Sars 1921; Suárez-Morales and Ivanenko 2004, fig. 9E), M. chilensis ( Suárez-Morales et al. 2006, fig. 3A, B), M. igniterra ( Suárez-Morales et al. 2008, fig. 2D). In M. ferrarii the forehead is anteriorly produced and coarsely corrugate ( Suárez-Morales and Ivanenko 2004, fig. 3B), thus partially resembling that of the new species; however, the anterior margin is irregular in M. ferrarii and completely flat in M. planifrons sp. n. Also, in M. ferrarii the cephalic area has a pair of distinctive sculptured protuberances ( Suárez-Morales and Ivanenko 2004, fig. 3B, D) that are absent in the new species.

Additional differences of the new species with respect to its congeners include a weakly developed fifth leg inner lobe, which is remarkably reduced to a small rounded inner protuberance (arrowed in Fig. 3E); all the known females of the genus have a conspicuous, well-developed inner lobe, reaching beyond the mid-length of the outer lobe as in M. ferrarii ( Suárez-Morales and Ivanenko 2004, fig. 6D,E) and M. igniterra ( Suárez-Morales et al. 2008, fig. 2D), or slightly shorter, as in M. dubia ( Scott 1904, pl. XIV, fig. 17), M. chilensis ( Suárez-Morales et al. 2006) and M. dubioides ( Suárez-Morales and Ivanenko 2004, fig. 8C). In M. filogranarum , described and depicted by Malaquin (1901, fig. 3), the inner lobe is present but it is weakly developed, thus resembling the pattern found in the new species. These species can be easily distinguished by differences in the length of the anal somite, which is twice as long as the preanal somite in M. filogranarum ( Malaquin 1901, fig. 3) vs. equally long in the new species; in M. filogranarum the genital double-somite lacks expansions, thus diverging from the condition described in the new species.

In the new species the anal somite is about as long as the preceding urosomite; it shares this character with M. ferrarii ( Suárez-Morales and Ivanenko 2004, fig. 6E) and M. chilensis ( Suárez-Morales et al. 2006, fig. 2D) whereas in M. dubia ( Scott 1904, pl. XII, fig. 14, pl. XIV, fig. 18), M. dubioides ( Suárez-Morales and Ivanenko 2004, fig. 8E), and M. igniterra ( Suárez-Morales et al. 2008, fig. 2D) the anal somite is longer than the preanal somite.

Finally, the new species has the cephalothorax covered by small papilla-like structures; this kind of ornamentation has not been described in other species of the genus but it is known in several species of Monstrilla (i.e., M. wandelii Stephensen, 1913; M. elongata Suárez-Morales, 2001; M. pustulata Suárez-Morales & Dias, 2001). In light of these many differences, the erection of a new species for the specimen from the Canadian Arctic seems to be well justified.

Habitat.

The oceanography of the lower Northwest Passage is distinctive due to its relatively lower salinity, resulting from the four large rivers draining into the waterway ( Carmack and McLaughlin 2011). The salinity of the water column varied between 28.4 and 28. 8, the temperature of the water column was ca. -1.5 °C on the day of sampling. The depth of the sampling station was of 63 m and the water column was still ice and snow covered. Moreover, the region of Dease Strait has limited water exchange with its neighbouring water bodies, thus suggesting there is an accumulation of freshwater not only from rivers but also from ice melt ( McLaughlin et al. 2004, Campbell et al. 2016).