Scina trispina, Gasca & Hendrickx, 2020

Gasca, Rebeca & Hendrickx, Michel E., 2020, Species of Scina Prestandrea, 1833 (Amphipoda, Hyperiidea, Scinidae) from western Mexico with the description of a new species from the Gulf of California, Zootaxa 4803 (2), pp. 329-344 : 337-340

Scina trispina sp. nov. ( Figs. 6‒8 )

Type material. Holotype ovigerous female, TALUD V, St. 5 (22°00’57”N, 106°40’00”W), December 13, 2000, 07:30 PM, from surface to ca. 1400 m ( TD, ca. 1600 m) ( ECO-CH-Z 10304 , specimen partially dissected).

Type locality. Gulf of California , Mexico, 22°00’57”N, 106°40’00”W .

Etymology. The species epithet refers to the distinctive presence of three small spines on the inner margin of the third uropod.

Diagnostic description. Based on holotype, female, 4.9 mm, ECO-CH-Z 10304. Antenna 1 1-segmented; basis of pereopods 1‒4 with unique width proportions: pereopod 1: 1, pereopod 2: 2/3, pereopod 3: 1/2; pereopod 4: 1/4; 3) pereopod 3 and pereopod 4 dactylus claw-like; pereopod 4 0.5 length of pereopod 5; one anterodistal denticle on basis of pereopod 5; pereopod 7 as long as basis of pereopod 6; 3 spines on the inner margin of uropod 1; and exopod insertion at distal 1/3 of uropods.

Description. Body. Female holotype. Body robust, laterally expanded, moderately hyaline, lacking keels; marsupium containing 10 eggs each of about 0.28 mm in diameter ( Fig. 6 ). Body length (anterior margin of head to tip of uropods) 4.9 mm including antenna, 3.7 mm excluding antenna. Pereion with head, 1.5 mm; head length about the same as first six pereonites; pleon length 0.8 mm; urosome length 0.3 mm, with uropods 1.4 mm.

Head. Antenna 1 ( Fig. 8E ) length 1.2 mm, about as long as head + first six pereonites; antenna tapering distally, triangular in transverse section, edges serrated (15 teeth), two apical teeth being longest; inner face with setae. Second segment about 5 times as long as basal article. Antenna 2 ( Fig. 8J ) poorly developed, represented by single cone-shaped segment, about 1/5 the length of antenna 1 basis (0.04 mm). Mouthparts concave, moderately globose. Maxillipeds ( Fig. 8C ) lobes broad, subequal in length; inner margins somewhat rough, with straight, parallel mar- gins. Maxillae 1 ( Fig. 8G ) with outer plate, inner plate, and palp. Outer plate relatively slender, tapering distally into acute apical tip, plate with one conspicuous stout short bristle on medial position and several slender spiniform elements subapically. Inner plate wide, rounded distal margin, furnished terminally with one long and several small bristles. Palp rounded, sac-shaped, shorter than plates, reaching about half-length of outer plate, with few terminal short bristles. Maxillae 2 ( Fig. 8H ) inner plate rounded, slightly tapering distally; outer plate about as wide as inner plate, but slightly longer; both plates with short terminal bristles. Mandible ( Fig. 8I ) distally truncate, with uniform small teeth, palp not observed.

Pereon. Pereopod 1 ( Fig. 7A ) basis with posterior margin more or less linear, anterior margin convex, approximately 2 x as long as wide; ischium and merus with thick short spine on postero-distal margin; merus with long seta in same position; carpus with row of 7 long setae on posterior margin plus 2 long and equal setae on antero-distal section; propodus with several long setae; dactylus straight, pointed, about 0.5 the length of propodus, directed backwards. Pereopod 2 ( Fig. 7B ) segments longer, slenderer and less setose than those of pereopod 1; basis about 2/3 of basis of pereopod 1 width; basis, ischium, merus and carpus smooth; ischium and merus relatively short; carpus and propodus similar in length (about 0.2 mm); propodus with long setae on anterior surface; dactylus straight, pointed, about 0.5 length of propodus and directed forward. Pereopods 3 and 4 ( Fig. 7C, D ) with basis of pereopod 3 about 0.5 as wide as basis of pereopod 1; basis of pereopod 4 about 1/4 as wide as basis of pereopod 1; except for the above, pereopod 3 and pereopod 4 similar in length and shape, no setae on segments, but small spines on posterodistal margins; propodus with spine close to base of dactylus; dactylus directed backwards. Pereopod 5 ( Fig. 7E ) about twice as long as pereopod 4, basis distally expanded; anterodistal angle produced, reaching 3/4 of ischium; with a strong denticle ending at process base; relatively small regular teeth on posterior border, last 2 longer and more conspicuous than the rest; all articles smooth; merus longer than carpus and this one longer than propodus; dactylus claw-like, with long setae at least 3 times as long as claw. Pereopod 6 ( Fig. 7F ) slender, long; basis about 62% of basis of pereopod 5 length; ischium very short; merus, carpus and propodus similar in length and breadth; dactylus claw-like with long distal seta more than 3 times as long as claw. Pereopod 7 ( Fig. 7G ) short, slender, approximately as long as pereopod 6 of basis; all segments straight, equally wide, unarmed. Dactylus claw-like, about 0.2 times as long as propodus.

Pleon. Uropod 1 ( Fig. 8A, B ) outer margin smooth; inner margin with 3 conspicuous teeth; outer ramus (broken off in holotype specimen) inserted at 1/3 distance from tip of uropod. Uropod 2 ( Fig. 8A ) margins smooth; outer ramus as long as 1/4 of endopod (broken in holotype), inserted at about 70 % of uropod length. Uropod 3 ( Fig. 8A ) margins smooth; outer ramus arising at about 1/3 of distance from tip of uropod; exopod long, about 0.6 times as long as endopod. Telson ( Fig. 8A ) triangular, about 0.6 times as long as double urosomite; base approximately 60% its length.

Remarks. The new species was identified as a species of Scina based on the following characters: 1) pereonites I and II not fused; 2) both pleon and pereion lacking dorsal spines and denticles; 3) 2-segmented female antenna 2; 4) oostegites with marginal setae; 5) coxal plates free; and 6) pereopod 7 shorter than pereopod 6 ( Vinogradov et al. 1996).

There are only two other species of Scina with long bristles on both pereopod 5 and pereopod 6 dactylus: S. setigera and S. parasetigera Zeidler, 1990 . The three species resemble each other also in terms of body size and proportions, among other characters. Scina trispina sp. nov. is very similar to S. parasetigera and S. setigera in the shape and size of the pereiopods and in the presence of a long bristle on pereopod 5 and pereopod 6. It also resembles S. setigera in the shape of the maxillipeds and in having a 1-segmented antenna. The new species, S. trispina sp. nov., differs from S. setigera in the telson length, the relative size of pereopod 6 and pereopod 7, the pereopod 7 proportion, in its possession of a single denticle on the anterodistal margin of basis of pereopod 5, and by basis of pereopod 4 being about 0.25 times as wide as basis of pereopod 1. It was not possible to determine if the proportion of the exopods on uropods 1–3 are distinct because those on uropod 1 and uropod 2 are broken off in the only specimen available. Only those on uropod 3 could be seen as similar. In the original description of S. setigera the uropod 1 possesses sparse small denticles along the endopodal inner margin ( Zeidler 1990), whereas S. trispina sp. nov. features three conspicuous teeth on the inner margin.

Scina trispina sp. nov. resembles S. parasetigera in the shape of maxilla 1 and 2, mandible, and in the pereopod 7 size, but differs from it in the shape of the maxillipeds, the 1-segmented and shorter antenna 2 (vs. longer, 2-segmented in S. parasetigera ), the presence of one anterodistal tooth on basis of pereopod 5, and the slender uropods. The new species also diverges from the other two species in having claw-like dactylus on both pereopod 3 and pereopod 4, vs. spine-like dactylus in S. setigera ( Wagler 1926, fig. 28) and S. parasetigera ( Zeidler 1990, fig. 5).

Figures

FIGURE 6. S. trispina sp. nov. Holotype female habitus, lateral view (ECO-CH-Z 10304). Scale bar, 0.5 mm (Photo: H. Bahena-Basave).

FIGURE 7. S. trispina sp. nov. Holotype adult female (ECO-CH-Z 10304). A-G. Pereopods 1-7 with details of dactylae. A, pereopod 1; B, pereopod 2; C, pereopod 3; D, pereopod 4; E, pereopod 5; F, pereopod 6; G, pereopod 7. Scale bar, 0.5 mm.

FIGURE 8. S. trispina sp. nov. Holotype adult female (ECO-CH-Z 10304). A, Urosome; B, uropods1 U1 (left and right); C, maxilliped; D, eye; E, antennae; F, habitus, lateral view with numbered pereonites; G, maxilla 1; H, maxilla 2; I, mandible; J, antenna 2. Scale bars: A, B, 0. 5 mm; C, 0.2 mm; D, G, H, J, 0.05 mm; E, 0.5 mm; F, 1.0 mm; I, 0.5 mm.