Trischizostoma raschi Boeck, 1861

Freire, Pablo R. & Serejo, Cristiana S., 2004, The genus Trischizostoma (Crustacea: Amphipoda: Trischizostomidae) from the Southwest Atlantic, collected by the REVIZEE Program, Zootaxa 645, pp. 1-15 : 8-10

Trischizostoma raschi Boeck, 1861 ( Fig. 4 )

Trischizostoma Raschii Boeck, 1861: 631 .

Trischizostoma Raschi — Sars, 1895: 31, pl. 120.

Trischizostoma nicaeense — Stebbing, 1906: 13 (partim); Barnard & Karaman, 1991: 536, figs. 87D, 89 O, 90B, 91H, 92H, 93C, 95U (not T. nicaeense (Costa, 1853)); Gurjanova, 1951: 156, fig. 31 (not T. nicaeense (Costa, 1853)).

Trischizostoma raschii — Sexton, 1908: 385, pl. 17, fig. 13; pl.18, 19, fig. 2–11; pl. 20; pl. 21, fig. 1–13, 15–18.

Trischizostoma raschi — Diviacco & Ruffo, 1989: 559, fig. 383.

Material examined. REVIZEE Program, R/V Thalassa col., stn. E­0507, 15º 08.595' S, 038º 40.638' W to 15º 07.158' S, 038º40.542' W, 1026 m, 1 ovigerous female, 39 mm, MNRJ 15139. Stn. E­0535, 19º 59.936' S, 039º 38.657'W to 19º 56.087'S, 039º 35.408 W, 1002 m, 1 ovigerous female, 29 mm, MNRJ 19251. Stn. E­0523, 19º 42.569'S, 038º 32.030' W to 19º 42.685'S, 038º 36.961' W, 922 m, 1 ovigerous female, 27 mm, MNRJ 15145.

Type locality. North Atlantic, off Norway coast.

Diagnosis. Eyes well developed. Maxilliped palp with 4 articles and longer than outer plate. Gnathopod 1 propodus of triangular. Coxa 2 with anterior margin acute. Coxa 4 posteriorly excavated, with postero­ventral lobe. Telson emarginate.

Description. Female, 29 mm ( Fig. 4 A). Rostrum reaching halfway of first article of antenna 1 peduncle. Eyes well developed. Antenna 1, article 1 of peduncle more than twice length of articles 2–3 together; flagellum with 9 articles; callynophore present. Antenna 2, article 5 slightly longer than article 4. Mandibles with distal incisors; palp 3­ articulate with marginal setae. Maxilla 1, inner plate 1/3 of outer plate length; outer plate with 8 distal setae and palp attached medially. Maxilla 2, inner plate slightly shorter than outer plate; outer plate with 3 lateral setae. Maxilliped, inner plate shorter than outer plate; palp with 4 articles and longer than outer plate.

Gnathopod 1 strongly subchelate ( Fig. 4 B); coxa reduced and partially covered by coxa 2; propodus inverted, robust, triangular; palm slightly convex with stout setae along all its extension and 3 pairs of longer setae distally. Gnathopod 2 ( Fig. 4 C), coxa subtriangular, with acute anterior margin; basis 2 X length of ischium, which is longer than carpus; dactylus reduced. Pereopod 3 ( Fig. 4 D), coxa longer than broad; basis longer than merus; carpus as long as propodus; propodus and dactylus with minute setae on posterior margin.

Pereopod 4, coxa posteriorly excavated, with postero­ventral lobe; basis as long as merus; carpus and propodus subequal in length. Pereopods 5–7, coxae broader than long; basis with posterior lobe; carpus and merus subequal in length; dactylus half length of propodus.

Uropods 1 and 2, peduncle subequal to outer ramus; inner ramus slightly longer than outer ramus. Uropod 3, peduncle slightly shorter than half of outer ramus; inner and outer rami subequal in length; outer ramus 2­articulate, with basal article 5 X length of distal article. Telson entire, emarginate ( Fig. 4 E).

Distribution. North Atlantic, Mediterranean Sea and Southwest Atlantic, off Brazilian coast (15º to 19º S).

Remarks. Trischizostoma raschi is similar to T. denticulatum , T. circulare , T. longirostrum , T. richeri , T. nicaeense and T. crosnieri in having the maxilliped palp longer than the outer plate and the telson entire. It differs from T. nicaeense (see Sexton, 1908), T. circulare , T. longirostrum and T. richeri in the shape of the anterior margin of coxa 2, which is acute in T. raschi and rounded in other species. It also differs from T. denticulatum in the triangular propodus of gnathopod 1, which is subquadrate in the latter.

Seemingly, T. raschi is very close to T. crosnieri in the acute anterior margin of coxa 2 and the triangular propodus of gnathopod 1. Lowry & Stoddart (1993) indicated that the number of stout setae on the palm of the gnathopod 1, the posterior lobe of the basis of pereopod 6 and shape of the telson as diagnostic characters between these species. However, the conspicuous distinction is in the telson, which is entire in T. raschi and cleft in T. crosnieri.

Some characters may change according to the development of the individual. Juveniles of T. raschi , removed from a mature female, have the telson cleft, a relative longer propodus of gnathopod 1 and the palm without setae when compared with the adult forms ( Fig. 4 F). Detailed examination in some juveniles (30 specimens) suggests that the setae on the palm of gnathopod 1 appeared at the same time and not by addition in each distinct molt.

Figures

FIGURE 4. Trischizostoma raschi Boeck, 1861, female, 39 mm, MNRJ 151439. A, habitus; B, gnathopod 1; C, gnathopod 2; D, pereopod 3; E, telson; F, gnathopod 1 of juvenile, 4 mm, MNRJ 15143. Scales bars: A, 5 mm; B – D, 1 mm; E – F, 0.5 mm.