Eucyclops chihuahuensis Suárez-Morales and Walsh, 2009

Mercado-Salas, Nancy F., Suárez-Morales, Eduardo & Silva-Briano, Marcelo, 2015, Taxonomic revision of the Mexican Eucyclops (Copepoda: Cyclopoida) with comments on the biogeography of the genus, Journal of Natural History 50, pp. 25-147 : 69-72

Eucyclops chihuahuensis Suárez-Morales and Walsh, 2009

( Figure 28)

Description

Female. Average length excluding caudal setae = 640 µm. Prosome representing 59% of total body length, symmetrical in dorsal view. Urosomal fringes strongly serrate; posterior margin of anal somite with row of spinules. Genital double somite symmetrical, representing 11% of total body length; anterior third of genital double somite expanded laterally. Seminal receptacle with rounded lateral arms on posterior margin, typical of the serrulatus -complex. Anal operculum smooth, rounded. Length/width of caudal rami = 4.5; inner margin of caudal ramus naked; outer margin with strong spinules covering 63% with respect to the total length of ramus. Dorsal seta (VII) 0.4 times as long as caudal ramus and 0.8 times as long as outermost caudal seta (III). Ratio of innermost caudal seta (VI)/outermost caudal seta (III) = 1.3. Lateral caudal seta (II) inserted at 76% of caudal ramus.

Antennule. Tip reaching midlength of third pediger. Armature per segment as follows: 1(8s), 2(4s), 3(2s), 4(6s), 5(2s), 6(1s+1sp), 7(2s), 8(3s), 9(2s+1ae), 10(2s), 11(2s+1ae), 12(7s +1ae). Two transverse rows of spinules on first segment, first row with strong spinules and adjacent second row with minute spinules. Spine on sixth segment reaching midlength of seventh segment.

Antenna ( Figure 28A–B). Coxa (unarmed), basis (2s+Exp), plus three-segmented Enp (1s, 9s, 7s, respectively). Basis with rows of spinules on frontal surface: N1(5), N2(5), N3(6), N4(5), N5(7), N15(4), N17(8), N18(4); on caudal surface: N7(4), N8(6), N9(7), N10(3), N11(7), N12(10), N13(13) N14(6), N20(10), N22(15). Caudal surface of Enp1 with B2(8).

Leg 1 ( Figure 28C–E). Frontal surface of intercoxal sclerite with row I bearing long spinules arranged in semicircular pattern on each side, caudal surface with row II bearing long hair – spinules, row I absent. Inner coxal seta biserially setulated, caudal coxal surface with spinule formula = A-B-C. Inner basal seta (basipodal spine) not reaching midlength of Enp3, 0.6 times as long as Enp. Length/width ratio Enp3 = 1.5, apical spine of Enp3 being 1.1 times as long as Enp3.

Leg 2 ( Figure 28F–G). Frontal surface of intercoxal sclerite with row I armed with hairs arranged in circular pattern; caudal surface with rows I and II continuous, both bearing long hair – spinules. Distal margin of intercoxal sclerite with two rounded chitinised projections. Inner coxal seta biserially setulated, caudal coxal surface with spinule formula = A-B-C-D. Length/width ratio of Enp3 = 1.9, apical spine of Enp3 1.3 times as long as Enp3. No modified setae present.

Leg 3 ( Figure 28H–I). Frontal surface of intercoxal sclerite armed with hairs arranged in circular pattern on each side; caudal surface with row I bearing long hairs (gap at middle section), rows II and III continuous, bearing long hairs. Distal margin with two rounded, chitinised projections. Coxa with strong biserially setulated inner coxal seta, proximal section with long hairs; distal section with strong spinules along bothmargins. Caudal coxal surface with spinule formula = A-B-C. Length/width ratio of Enp = 2.0, apical spine of Enp3 being 1.2 times as long as Enp3. No modified setae present.

Leg 4 ( Figure 28J–M). Distal margin with two low, rounded, chitinised projections. Frontal surface of intercoxal sclerite with row I bearing long hairs arranged in semicircular pattern, caudal surface of intercoxal sclerite with row I bearing long hairs, row II with long hair – spinules on outer margins and row III with long hair – spinules on outer margins. Frontal surface of coxa with row of small spinules at insertion of Bsp. Inner coxal spine with heterogeneous ornamentation; inner margin with long setules proximally and with strong spinules distally, outer margin with setules along proximal section and distally naked. Spinule formula on caudal surface = A-B-C + D-E-F-G-J. Length/width ratio Enp3 = 2.5, length ratio inner spine of Enp3/length Enp3 = 1.1; length ratio outer spine of Enp3/length Enp3 = 0.9; length ratio inner/outer spines Enp3 = 1.3. Lateral seta of Enp3 inserted at 68% of segment. No modified setae in Enp and Exp.

Leg 5. Free segment subrectangular, 1.5 times longer than wide, bearing one strong inner spine and two setae; medial seta 2.8 times longer than outer seta, twice as long as inner spine. Inner spine 2.2 times longer than segment.

Male. Unknown.

Remarks. When this species was described ( Suárez-Morales and Walsh 2009), it was related and compared to E. pseudoensifer ; however, even when the general shape and main proportions are similar to E. pseudoensifer , our analysis revealed that they belong to different groups. Eucyclops pseudoensifer was redescribed by Suárez-Morales and Walsh (2009) and new morphological data consulted in Dussart Collection (pers. obs. Mercado-Salas, 2012), mainly in reference to the ornamentation patterns of the swimming legs and antennal basis. Eucyclops pseudoensifer is not a member of the serrulatus - group because of the lack of N1 and N 2 in the antennal basis while both rows N1 and N2 are present in E. chihuahuensis , the former bearing long hair – spinules and N2 with small but strong spinules. This character places this species in the serrulatus -group. It appears to be more closely related to E. pectinifer and E. serrulatus than to its South American congeners E. pseudoensifer and E. leptacanthus . This species can be easily separated from E. serrulatus and E. pectinifer by the presence of a more complex ornamentation pattern in both the caudal and frontal surfaces of the antennal basis. Eucyclops serrulatus lacks rows N8, N9, N13, N18 and N22 which are present in E. chihuahuensis , and E. pectinifer lacks rows N13, N20 and N22 which are present in E. chihuahuensis . The three abovementioned species share a caudal surface of the P1 coxal sclerite with row I absent and row II bearing long hair – spinules, but differ in the intercoxal ornamentation of P2. In E. pectinifer and E. serrulatus row I on the caudal surface is absent while in E. chihuahuensis it is present and has long hair – spinules; the three species share a row II bearing long hair – spinules. Eucyclops chihuahuensis also differs from E. serrulatus and E. pectinifer in the ornamentation of the P3 intercoxal sclerite. In the first species all rows are armed with long hairs, while in the other two species these rows bear hair – spinules and strong spinules. The P4 intercoxal sclerite also differs among these three species: in E. serrulatus and E. pectinifer row I has long and strong spinules whereas E. chihuahuensis has long hairs. The coxal surface of E. chihuahuensis can be distinguished from that of E. serrulatus by the presence of row F, and from both E. serrulatus and E. pectinifer by the absence of row H. Based on the comparative analysis of these characters, we presume that previous records of E. pseudoensifer in Mexico could be assignable to E. chihuahuensis .