Trischizostoma longirostrum Chevreux, 1919

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 : 5-8

Trischizostoma longirostrum Chevreux, 1919 ( Fig. 3 )

Trischizostoma longirostre Chevreux 1919: 575 ; 1927: 43, pl. 1.

Material examined. REVIZEE Program, R/V Thalassa col., stn. E­0509, 15º 47.725' S, 038º 21.973' W to 15º 46.378' S, 038º 24.839' W, 2076m. 12/ 06/ 2000; 1 female, 47 mm, ovigerous, MNRJ 15141. Stn. E­0528, 19º 45.258' S, 039º 03.003' W to 19º 47.581' S, 038º 59.827' W, 1237m, 29/06/2000, 1 female with oostegites, 25 mm, MNRJ 15142. Stn. E­ 0 535, 19º 58.936' S, 039º 38.657' W to 19º 56.087' S, 039º 35.408' W, 1002 m, 01/07/2000, 1 female with oostegites, 39 mm, MNRJ 15143.

Type locality. Northeast Atlantic, between Dakar and Cabo Verde Islands. Diagnosis. Eyes present. Maxilliped palp with 4 articles and longer than outer plate. Gnathopod 1 propodus oval and anteriorly excavate. Gnathopod 2, coxa with anterior margin rounded; ischium subequal to carpus in length. Pereopod 3 basis 2 X length of propodus. Telson entire, distally truncate.

Description. Female, 39 mm ( Fig. 3 A). Rostrum reaching middle of first article of antenna 1 peduncle. Eyes well developed. Antenna 1, first article of peduncle longer than articles 2–3 and shorter than callynophore. Antenna 2, peduncle article 4 subequal in length and broader than article 5. Mandibles with small distal incisor; palp 3­articulate and bearing marginal setae. Maxilla 1, inner plate 1/3 length of outer plate, with 8 distal setae, palp reduced and attached on median part of outer plate. Maxilla 2, inner and outer plates subequal in length; outer plate with 4 lateral setae. Maxilliped, inner plate shorter than outer plate; palp with 4 articles and longer than outer plate.

Gnathopod 1 ( Fig. 3 B), strongly subchelate; coxa reduced and partially covered by coxa 2; propodus inverted, oval, anteriorly excavated; palm with stout setae in all its extension and 3 pairs of longer setae distally. Gnathopod 2 ( Fig. 3 C), coxa with anterior margin rounded; ischium shorter than basis and subequal to carpus; propodus less than half­length of carpus; dactylus reduced. Pereopod 3 ( Fig. 3 D), coxa longer than broad; basis 2 X length of propodus; carpus slightly longer than propodus; dactylus less than halflength of propodus; carpus, propodus and dactylus covered with minute and stout setae on posterior margin. Pereopod 4, coxa posteriorly excavated; basis longer than carpus and merus together; propodus as long as carpus; dactylus less than half­length of propodus. Coxae 5–7 broader than long. Basis of pereopods 5–7 longer than broad and with posterior lobe. Pereopods 5 and 7, carpus as long as merus, and longer than propodus. Pereopod 6, carpus as long as propodus and slightly shorter than merus.

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

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

Remarks. Trischizostoma longirostrum is part of the group of species with maxilliped palp longer than outer plate, anterior margin of coxa 2 rounded, and telson entire, which includes: T. richeri Lowry & Stoddart 1994 , T. nicaeense Costa 1853 , T. macrochela Vinogradov 1990 , and T. circulare . Among these species, T. longirostrum is very close to T. richeri in the oval shape of propodus of gnathopod 1. The distinctions between them are on carpus of gnathopod 2, which is subequal to ischium in T. longirostrum and longer than ischium in T. richeri ; basis of pereopod 3, which is twice the length of propodus in T. longirostrum and 3 X the length of propodus in T. richeri ; the anterior margin of propodus of gnathopod 1, which is excavated in T. longirostrum ; and the telson shape, which is distally truncate in T. longirostrum and distally acute in T. richeri .

Chevreux (1927) used the length of rostrum as a diagnostic character. However, this character is variable in different stages of the individual within this genus. Examining juvenile specimens of T. raschi and T. denticulatum showed that the rostrum is relatively longer when compared with adults forms.

A curiosity observed was the absence of eyes in the largest individual (47 mm) found within our samples. Smaller specimens (25 mm and 39 mm) had weak eyes.

Modification on the ending of the specific name is proposed. The name Trischizostoma (from the Greek stoma, mouth) has a neuter gender that needs to be followed by the correct adjective declination um ( Brown, 1956).

Figures

FIGURE 3. Trischizostoma longirostrum Chevreux, 1919, female, 47 mm, MNRJ 15141. A, habitus; B, gnathopod 1; C, gnathopod 2; D, pereopod 3; E, telson. Scales bars: A, 5 mm; B – D, 1 mm; E, 0.5 mm.