Cymbasoma tergestinum, Suárez-Morales & Goruppi & de Olazabal & Tirelli, 2017

Suárez-Morales, Eduardo, Goruppi, Alenka, de Olazabal, Alessandra & Tirelli, Valentina, 2017, Monstrilloids (Crustacea: Copepoda) from the Mediterranean Sea (Northern Adriatic Sea), with a description of six new species, Journal of Natural History (J. Nat. Hist.) 51 (31 - 32), pp. 1795-1834 : 1797-1800

Genus Cymbasoma Thompson, 1888

Cymbasoma tergestinum sp. nov.

( Figure 2 (a – h))

Material examined

Adult holotype female from station 1 located in the harbour of Trieste ( Figure 1 , Table 1), North Adriatic Sea, partially dissected, appendages mounted on slides in glycerine, sealed with Entellan®. Date of collection: 15 July 2015. Slide deposited in ECO-CHZ- 009521.

Description of adult female

Body moderately elongated, slender ( Figure 2 (a)); body length of holotype female 1.13 mm. Cephalothorax 0.65 mm long, representing 60 % of total body length.

Midventral oral papilla located at 26% of cephalothorax length. Pair of relatively large ocelli present, pigment cups well developed, medially conjoined, strongly pigmented; ventral cup and lateral cups equally sized ( Figure 2 (a, b)). Cephalic area with slightly produced, rounded ‘ forehead ’, ornamented dorsally with relatively deep curved striations arranged in a symmetrical, opposite pattern ( Figure 2 (d)); ventral surface of forehead with shallow transverse striations ( Figure 2 (b, c)) and with pair of sensilla ( Figure 2 (b)). Cephalic cuticular ornamentation including: (1) pair of unguiform scars between bases of antennules, (2) pair of symmetrical nipple-like processes on anterior ventral surface in preoral area, with transverse adjacent striation, (3) small, medial pair of papillalike processes, and (4) shallow cuticular ridges overlying anterior surface of oral papilla ( Figure 2 (b, c)).

Urosome consisting of fifth pedigerous somite, genital double-somite and anal somite, together representing 17% of total body length. Relative lengths of urosomites (fifth pedigerous, genital-double and free somites) as: 28.6: 47.6: 23.8 = 100, respectively. Fifth pedigerous somite with medial constriction and ventral peripheral striations ( Figure 2 (f)), fifth legs inserted on distal 1/3 of somite. Genital double-somite with smooth dorsal and ventral surfaces ( Figure 2 (e)), except for shallow ventral striations anterior to insertion of ovigerous spines. Anterior half of genital double-somite expanded, with rounded margins; posterior half with straight margins ( Figure 2 (a, f)). Caudal ramus subrectangular, 1.2-times longer than wide, armed with three subequally long, sparsely setulated caudal setae. Ovigerous spines paired, relatively short, representing 20% of total body length. Spines basally separated, slender, straight at their base and along shaft, without distal expansions.

Antennule length 0.26 mm, representing about 22% of total body length and 38% of cephalothorax length in the specimen examined; antennule 4-segmented, but lacking suture between segments 3 and 4 ( Figure 2 (d)). Relative length of distal antennulary segment: 46%. In terms of pattern described by Grygier and Ohtsuka (1995) for female monstrilloid antennulary armature, setae (Roman numerals) and spines (Arabic numerals), stout, remarkably short spiniform element 1 present on first segment; elements on second segment: 2d1 – 2, 2v1-3, and IId. Third segment with stout element 3 as long as spiniform elements of the 2v group; setal elements IIId and IIIv present. Segment 4 bearing elements 4d1,2, 4v1 – 2, element 4v1 longest of 4v-d group. Setae IVd, IVv, Vd and Vv present. Aesthetasc 4aes not observed in specimen. Element 5 spiniform, relatively slender. Subterminal elements b1-4 present, unbranched; elements 61 and 62 and 6aes present in apical position ( Figure 2 (d)).

Incorporated first pedigerous somite and succeeding three free pedigerous somites each bearing a pair of biramous legs. Pedigerous somites 2 – 4, together accounting for 22.5% of total body length in dorsal view. Legs 1 – 4 slightly increasing in size posteriorly. Intercoxal sclerites of legs 1 – 4 subrectangular, widest at base, tapering distally, frontal surface with subquadrate fields of minute spinules ( Figure 2 (g, h)). Bases of legs 1 – 4 articulating with large, rectangular coxa along oblique line; with outer basipodal seta; on leg 3, this seta is about 4-times longer than in the other legs, biserially setulated from proximal 1/3 and slightly thicker than those on the other legs ( Figure 2 (h)). Endopods and exopods of legs 1 – 4 triarticulated. Ramal 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, slender, and sparsely setulated. Spine on distal exopodal segment of leg 1 and 3 short (arrows in Figure 2 (g, h)). Outermost apical exopodal setae of legs 1 – 4 with inner margin ornamented with short setules, outer margin spinulose.

Armature formula of legs 1 – 4:

basis endopod exopod

leg 1 1 – 0 0 – 1;0 – 1;1,2,2 I-1;0 – 1;I,2,2

legs 2 – 4 1 – 0 0 – 1;0 – 1;1,2,2 I-1;0 – 1;I,1,2,2

Fifth legs medially separate, indistinctly bilobate, inner (endopodal) lobe inconspicuous, unarmed, represented by low inner rounded protuberance separated from fifth leg only at its distal end, lobe barely reaching midlength of outer lobe ’ s inner margin. Outer (exopodal) lobe subrectangular, slender, armed with three subequally long setae on distal position, innermost slightly shorter than adjacent setae ( Figure 2 (e, f)).

Male. unknown.

Type locality

Harbour of Trieste , North Adriatic Sea (45°36 ʹ 41.04 ” N, 13°47 ʹ 3.78 ” E) ( Figure 1 ) .

Etymology

The species name makes reference to the ancient name of Trieste (Tergeste), Adriatic Sea; the epithet has a neuter ending to match the genus name.

Remarks

This female monstrillid from Trieste was easily assigned to the genus Cymbasoma by its possession of three caudal setae and three urosomites (fifth pedigerous, genitaldouble and anal somites). Most of the known species of Cymbasoma have a fifth leg, bearing an outer lobe armed with two or three setae and an inner lobe developed to different degrees. The Trieste specimen is assignable to a group of species related to the C. rigidum complex ( Suárez-Morales 2006); the size and shape of the inner lobe has been used to separate species in this complex ( Suárez-Morales 2006; Suárez- Morales and McKinnon 2016). In our new species from Trieste, the inner lobe is weakly developed. This kind of short, poorly defined inner lobe has been depicted in records of C. rigidum ( Thompson 1888) by Scott (1904) from Scotland and Bernier et al. (2002) from the Gulf of St. Lawrence, Canada. Another variable character of the fifth legs in the group is the relative length of the inner terminal seta of the outer lobe; it can be nearly as long as the other setae, as in C. germanicum ( Timm, 1893) ( Suárez-Morales 2006) , and in reports of C. rigidum by Bourne (1890) and Scott (1904) or about half the length of the other two setae, as in Sars (1921), Wilson (1932), Sekiguchi (1982) and Bernier et al. (2002), all of them as C. rigidum . It is clear that this complex still contains many different species. This appears to be the case of the Trieste species described here; previous local records like that by Hure and KršiniĆ (1998) should be revised to determine its identity.

A fifth leg with a very weakly developed inner lobe, represented by a marginal expansion of the outer lobe or reaching less than halfway of inner margin of the outer lobe, is also present in C. thompsoni Giesbrecht, 1893 , C. reticulatum Giesbrecht, 1893 , C. tropica Wolfenden, 1906 , C. bali Desai and Krishnaswamy, 1962 , C. tirmizae Khan and Kamran, 1974 , and C. striifrons Chang 2012 , but also in the recently described Australian species C. curticrus Suárez-Morales and McKinnon 2016 and C. lentilum Suárez-Morales and McKinnon, 2016 . Cymbasoma reticulatum can be readily distinguished among other species of the genus by its reticulated cephalosome and antennules, a character absent in the new species. In C. reticulatum , C. bali and C. constrictum the three setae of the fifth leg outer lobe are equally long and wide ( Giesbrecht 1893; Desai and Krishnaswamy 1962, fig 10; Suárez-Morales and McKinnon 2016), differing from C. tergestinum , in which the innermost seta is slightly shorter and thinner than the other two. In C. thompsoni ( Giesbrecht 1893; Sars 1921), C. tropica ( Martin Thompson and Easterson 1983) , C. tirmiziae ( Khan and Kamran 1974) , C. striifrons ( Chang 2012) and C. lentilum ( Suárez-Morales and McKinnon 2016) , the innermost seta is clearly shorter than the other setae, in most cases reaching about halflength of adjacent seta, thus differing from the new species. In C. curticrus the innermost seta is only slightly shorter and narrower than the other fifth leg setae, thus resembling the pattern found in the new species from Trieste. In addition, the shape of the genital double-somite is different in these species; in C. reticulatum , C. tropica , C. bali , C. tirmiziae and C. striifrons the lateral margins are moderately produced and rounded ( Giesbrecht 1893; Desai and Krishnaswamy 1962; Martin Thompson and Easterson 1983; Chang 2012; Suárez- Morales and McKinnon 2016), whereas in the new species the anterior half is clearly expanded and the posterior half has straight margins ( Figure 2 (f)). In the Australian C. curticrus , the genital double-somite is also expanded anteriorly, but margins are angular, not rounded ( Suárez-Morales and McKinnon 2016). The antennule segmentation differs among these species, particularly in the fusion of segments 3 – 4. A complete suture is present in C. curticrus , C. bali and C. thompsoni and C. tirmiziae , whereas an incomplete suture is found in C. striifrons ( Chang 2012) ; these segments are fused in the new species ( Figure 2 (d)). Overall, the new species most closely resembles the Australian C. curticrus on the structure and armature of the fifth legs, but diverge in the shape of the genital double-somite, the antennule segmentation and in the body proportions: the cephalothorax represents 58% of the body in the new species vs 70% in C. curticrus (Suárez- Morales and McKinnon 2016). The unique combination of characters present in this specimen from Trieste justifies the proposal of a new taxon.

Figures

Figure 1. Location of zooplankton sampling stations in the Gulf of Trieste (Italy), where monstrilloid copepods were obtained during the surveyed period.

Figure 2. Cymbasoma tergestinum sp. nov., holotype adult female from Trieste. (a) habitus, dorsal view; (b) cephalic region, ventral view; (c) cephalic region, lateral view; (d) frontal ornamentation and antennule showing armature; (e) urosome and fifth legs, lateral view; (f) same, ventral view; (g) leg 1 with intercoxal sclerite; (h) leg 3 with intercoxal sclerite. Scale bars: 200 µm (a), 100 µm (d–f), 50 µm (b, c, g, h).