Stephos fernandoi, Suarez-Morales, Eduardo, Gutierrez-Aguirre, Martha A., Cervantes-Martinez, Adrian & Iliffe, Thomas M., 2017

Suarez-Morales, Eduardo, Gutierrez-Aguirre, Martha A., Cervantes-Martinez, Adrian & Iliffe, Thomas M., 2017, A new anchialine Stephos Scott from the Yucatan Peninsula with notes on the biogeography and diversity of the genus (Copepoda, Calanoida, Stephidae), ZooKeys 671, pp. 1-17 : 1-4

Stephos fernandoi sp. n. Figures 1, 2, 3, 4

Material examined.

Holotype. One adult ♀, collected on 6 July 2014 from the anchialine cave of Cenote Tres Potrillos, Cozumel Island (20°27'3.2"N, 86°59'14.4"W), Quintana Roo, Mexico. Specimen dissected on slide deposited in the collection of Zooplankton of El Colegio de la Frontera Sur (ECOSUR) in Chetumal, Mexico, under number ECO-CHZ-09411. Allotype: one adult ♂, collected on same date and site, specimen dissected (ECO-CHZ-09412). Paratypes: four dissected adult ♀♀, one dissected adult ♂, slides (ECO-CHZ-09413), two undissected ♀♀, eight undissected ♂♂ (ECO-CHZ-09414), and three undissected ♀♀, three ♂♂ (USNM-1422288), all from same date and site, ethanol-preserved, vials.

Descriptions.

Female. Mean length of prosome: 0.343 mm (n = 13); total length including caudal rami = 0.475 mm (n = 13). Body with typical calanoid shape, relatively robust in lateral and dorsal views, prosome 5-segmented, widest at first pedigerous somite (Figs 1A, 4A, B). Cephalosome and first pedigerous somite completely separate, fourth and fifth pedigerous somites fused, with posterolateral corners rounded, moderately produced, symmetrical (Fig. 4A). Rostrum weakly developed, represented by small medial expansion, rostral points absent (Fig. 4C). Urosome 4-segmented, representing 31% of total body length. Genital double-somite relatively long, almost 40% of urosome, barrel-shaped, symmetrical, weakly expanded mid-ventrally, expansion associated with genital field (Fig. 2J). Single gonopore opening ventrally at proximal 1/3 of somite; adjacent ventral surface of somite ornamented with 4 slender spiniform elements (arrowed in Fig. 4E) inserted at each side of simple, transverse genital operculum. Anal somite shortest of urosome, subrectangular, about 10% of urosome length, cuticular ornamentations absent on dorsal and ventral surfaces (Figs 2J, 4F).

Caudal rami subrectangular, symmetrical, length/width ratio = 1.6-1.7, armed with 6 caudal setae (II-VII) (Fig. 4F). Inner margin naked except for displaced dorsal seta (VII) inserted on proximal 1/4 of inner margin, seta reaching beyond distal margin of ramus (Fig. 4F). Caudal seta I absent, seta II (Fig. 4F) reduced, inserted near base of seta III. Terminal setae III-VI well developed. All ramal setae biserially plumose.

Antennule (Fig. 1 B–D) 24-segmented, reaching posterior margin of preanal somite. Armature per segments as follows: segmental number (ancestral segment, setae (s) + aesthetasc (ae)): 1(I-II, 3s); 2(III-IV,4s + ae), 3(V, 2s), 4(VI, 2s), 5(VII, 2s), 6(VIII, 1s + ae), 7(IX, 2s), 8(X-XI, 3s), 9 (XII, 1s+ae), 10 (XIII, 1s), 11(XIV, 2s + ae), 12(XV, 2s), 13(XVI, 2s + ae), 14(XVII, 1s), 15(XVIII, 1s), 16(XIX, 1s), 17(XX, 1s), 18(XXI,1s + ae), 19 (XXII,1s), 20(XXIII,1s), 21(XXIV,2s +1s), 22(XXV,1s +1s), 23(XXVI, 1s +1s), 24(XXVII-XXVIII, 3s + ae) (Figs 1 B–D). One of the setal elements on segment 12 spiniform (arrow in Fig. 4D). Distal segment with apical acute process present in some specimens (Fig. 1D).

Antenna (Fig. 1E) biramous, with exopod longer than endopod. Coxa armed with one seta. Basis with two distal subequal setae on medial margin. Endopod 2-segmented, first segment long, cylindrical, with short seta inserted at 2/3 of medial margin; distal portion of terminal segment with two lobes, proximal lobe with 8 setae; distal lobe with single short, lateral seta plus five long terminal setae. Exopod indistinctly 7-segmented, first segment with one long seta, second segment longest, armed with three setae, one proximal, one medial and one on distal position. Segments 3-6 with 1, 2, 1, 1 setae, respectively. Distal segment with crown of three long, terminal setae, subequal in length and diameter.

Mandible (Fig. 1G) with gnathobase armed with four large monocuspid ventral teeth plus three smaller bicuspid teeth, dorsal monocuspid tooth, and short dorsal seta. Serial teeth distinctly separated from large ventralmost tooth by diastema. Palp biramous (Fig. 1F), basis robust, armed with four subequal setae inserted on medial margin. Endopod short, 2-segmented; proximal segment with two short and one long setae, outer margin protuberant; distal segment subrectangular, with 10 setae, one reduced. Exopod indistinctly 5-segmented, armed with 1, 1, 1, 1, 2 setae.

Maxillule (Fig. 1H) with praecoxal arthrite bearing nine spiniform marginal setae. Coxal epipodite with nine setae, coxal endite with two setae. Basis with proximal endite bearing four setae, distal basal endite armed with five setae. Endopod reduced, not articulated to basis, indistinctly 3-segmented, proximal segment with four setae, second segment with two setae, distal segment with six. Exopod oblong, with ten subequal setae.

Maxilla (Fig. 1I) indistinctly 6-segmented including precoxa, coxa, allobasis and 3-segmented endopod. Praecoxal and coxal endites with 5, 3, 3, 3 setae, distal coxal endite with two stout spinulated setae. Basal endite of allobasis with 3 setae, incorporated endopodal segment with single seta. Free endopodal segments armed with 1, 1, 3 setae.

Maxilliped (Fig. 2A) indistinctly nine-segmented, precoxa and coxa partially fused, precoxa unarmed, with cluster of spinules. Coxa with three groups of setae, proximal endite with 1 seta, middle endite with two, distal with two. Basis ornamented with row of short spinules; armed with 3 setae, one shorter than the rest. Endopod six-segmented, armed as follows: 2, 4, 4, 2, 2, 4. Basal and endopodal setae slender, distally attenuated.

Legs 1-4 (Fig. 2 B–E) biramous, increasing in size posteriorly. First swimming leg (Fig. 2B) with three-segmented exopod and one-segmented endopod; coxa subrectangular, with short outer coxal seta not reaching distal margin of basal segment; row of spinules at insertion of coxal seta. Basipod with long, recurved inner plumose seta reaching beyond distal margin of third exopodal segment; outer basipodal seta slender. Endopod with outer knob ornamented with 1-3 minute apical setules (Fig. 2B, G). First exopodal segment with row of spinules. Outer spine on third exopodal segment elongate, spine shorter in some specimens (arrowed in Fig. 2F). Second leg with two-segmented endopod (Fig. 2C), legs 3 and 4 with three-segmented exopods and endopods, with articulate setae (Fig. 2D, E). Armature formula of legs 1-4 as in Table 1.

Fifth legs (Fig. 2I) reduced, symmetrical, uniramous, two-segmented with proximal segment cylindrical, distal segment proximally globose, forming long spiniform bipinnate apical process (Figs 2I, 4G, H).

Male. Body slightly longer than female, average total length: 0.493 mm (n =10); length of prosome: 0.31 mm (Fig. 3A). Rostrum as in female. Urosome 5-segmented, representing 32% of total body length. First urosomite symmetrical; anal somite shortest. Caudal rami relatively short, symmetrical, caudal setae as in female.

Left and right antennules 24-segmented, lacking geniculation, slightly longer than in female when extended posteriorly; antennulary armature as in female. Mouthparts and swimming legs 1-4 as in female.

Fifth legs (Figs 3B, 4 I–L) uniramous, asymmetrical. Left leg five-segmented, about as long as right counterpart; proximal segment widest of ramus, with inner margin expanded. Second, third, and fourth segments elongate, fourth with triangular plate on distomedial angle; distal segment with three terminal lamellae tapering distally plus subdistal subtriangular process, and with short seta inserted proximally on medial margin (Fig. 4J). Right fifth leg (Fig. 3B) four-segmented, first and second segments cylindrical, robust, unarmed. Third segment elongate and tapering. Fourth segment very slender and bifurcating distally into “C” -shaped structure furnished with 6-8 peg-like elements along bifurcation (Fig. 4L). End of subdistal process acute, opposite end with apical leaf-like expansion (Fig. 4K).

Etymology.

The new species is named after the Mexican carcinologist Dr. Fernando Alvarez (Instituto de Biología, UNAM, Mexico), who has significantly contributed to the knowledge of the Mexican crustacean fauna, particularly from caves and anchialine habitats.

Remarks.

The new species was included in the diverse stephid genus Stephos based on its possession of the following characters: 1) cephalosome and first pedigerous somite separate, pedigers 4-5 partially fused, 2) female urosome 4-segmented, male five-segmented; 3) caudal rami with 4 terminal setae ( III–VI), dorsal caudal seta VII inserted on inner margin; 4) antennules 24-segmented in male and female, lacking geniculation in male; 5) female leg 5 uniramous, one or two-segmented, distal segment tapering, ornamented; 6) male fifth legs uniramous, strongly asymmetrical, modified into grasping organ, left leg five-segmented, with complex distal segment, right leg slender ( Bradford-Grieve 1999).

Based on the morphology of the male fifth legs, Bradford-Grieve (1999) divided the species of Stephos into four distinct groups. The new species S. fernandoi can be assigned to "group IV" by its possession of a male right leg 5 with a narrow fourth segment. Currently, this group includes nine species: S. pentacanthos Chen & Zhang, 1965 from off China, S. tsuyazakiensis Tanaka, 1966 from Japan, S. rustadi Strömgren, 1969 from Norway, S. morii Greenwood, 1977 from Australia, S. pacificus Ohtsuka & Hiromi, 1987 from Japan, S. angulatus Bradford-Grieve, 1999 from New Zealand, S. marsalensis Costanzo, Campolmi & Zagami, 2000 from Italy, S. vivesi Jaume, Boxshall & Gràcia, 2008 from the Balearic Islands, and S. goejinensis Moon, Yeon & Venmathi Maran, 2015 from Korea (see table 1 in Bradford-Grieve 1999; Jaume et al. 2008; Moon et al. 2015).

The new species is the only one in this group with a right leg 5 ramus combining a distal segment (segment 4) with diverging processes set at right angles with acute tips plus a series of peg-like elements along the longest process (Fig. 4L). It differs from S. angulatus because in this species, the processes are both apically rounded and the segment lacks the peg-like elements observed in the new species. The left ramus has a similar structure in both species, with segment 4 bearing a distal lobular process ( Bradford-Grieve 1999, fig. 8; Fig. 4L) and three distal lamellae, but the new species has an additional subdistal process (Figs 3B, 4J). In S. marsalensis , the distal segment of right male P5 is unbranched ( Costanzo et al. 2000, fig. 4d), thus diverging from the bifid condition found in S. fernandoi ; also, the left leg has five lamellate hyaline processes on the distal segment vs. only three such processes in the new species. The anchialine S. vivesi has a left leg with eight narrow lamellae and a relatively simple, spatulate distal segment of the right leg with two proximal processes ( Jaume et al. 2008, fig. 2 b–d), thus diverging from the pattern observed in the new species. The fifth leg of the new species differs from that of S. geojinensis in the number of lamellae on the distal segment of the left leg, three (Fig. 4J) vs. seven long plus 13 short lamellae, and right leg with distal segment bifurcate, with both branches subequally long (Fig. 3B) vs. outer branch extremely long, inner branch reduced ( Moon et al. 2015, fig. 4d). The same kind of distally asymmetrical right fifth leg is present in S. tsuyazakiensis ( Tanaka 1966, fig. 1o), thus differing from the new species. In S. morii , the right leg distal segment has a strong inner bulb-like process ( Greenwood 1977, fig. 4g) which is absent in the new species; also, the left leg terminal segment has a long, distinctive spiniform process on subdistal position which is not present in S. fernandoi (Fig. 3B). In S. rustadi , the right leg distal segment is chela-like, with an expanded inner margin and the left leg is clearly shorter that its right counterpart and has three distinctive hook-like processes ( Strömgren 1969, fig. 3f), thus diverging from the fifth leg structure of the new species. In S. penthacanthos , the right fifth leg has a spiniform process on the outer margin and the terminal segment is modified into a long, slender claw-like process, curved inwardly ( Chen and Zhang 1965, fig. 20.5), thus differing from the spatulate distal segment described in the new species. In S. pacificus , the right fifth leg distal segment is relatively simple, represented by an elongate, narrow unbranched structure ( Ohtsuka and Hiromi 1987, fig. 3f), different from the pattern observed in the new species, clearly branched (Fig. 3B).

The female fifth leg of S. fernandoi differs from that known in most species of Stephos , which has a medial seta and/or a row of spinules on the distal segment. It most closely resembles the fifth legs of the two species of Miostephos , M. cubrobex from Cuba ( Bowman 1976, fig. 13) and M. leamingtonensis from Bermuda ( Yeatman 1980, fig. 5), both with an attenuated unarmed distal segment. The new species differs in the genus characters (i.e. three-segmented female urosome, six-segmented left male fifth leg, reduced male right fifth leg strongly resembling the female fifth leg) ( Bowman 1976). In species of Stephos , the female genital double-somite has widely different patterns of ornamentation on the ventral and/or lateral surfaces, including rows of spinules with both symmetrical and asymmetrical arrangements ( Ohtsuka and Hiromi 1987; Bradford-Grieve 1999; Costanzo et al. 2000), lack of surface ornamentation, as in S. canariensis ( Boxshall et al., 1990) or S. grieveae ( Kršinić, 2015) or a highly modified, strongly asymmetrical somite as in S. exumensis (Fosshagen, 1970). The new species has a unique pattern combining a symmetrical genital double-somite with an ornamentation pattern represented by a set of 4 spiniform elements at each side of the genital operculum; this pattern has not been observed in any other species of Stephos .