Metopa dawsoni J.L. Barnard 1962
Creators: name: Tandberg, Anne Helene S.
Publisher: name: Zenodo
Year: 2009
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Stenothoidae
Metopa
Metopa dawsoni

description: Metopa dawsoni J.L. Barnard, 1962
Metopa dawsoni Barnard 1962: 139 –142, figs 10 and 11 Barnard 1964: 246
Type locality: Southern California (Barnard, 1962)
Material examined. Morphological examination: NMNH 106858-59, Southern California. Barnard No 6098. female, 5mm. 33°38’45”N, 118°14’45”W, 44,5m Feb1959. (from type locality, but not same lot as type, ID by JL Barnard)
See figures 5, 6 and 7.
Morphological description. Female, 5mm.
Head: epistome small and rounded, cephalic lobe rounded; eye 1/3 of head length, round and well defined. Antenna 1 (Fig. 5): long and slender, subequal to body length, peduncle article 2 longer than article 1, flagellum 13–15-articulate, minute thin and simple setules on each article; no accessory flagellum observed. Antenna 2 (Fig. 5): long and slender, slightly shorter than antenna 1; article 4 approximately as long as article 5, peduncle approximately 3x flagellum length; flagellum 8-articulate, each article with two minute simple setae. Mandible (Fig. 5): mandibular palp 3-articulate, article 1 short and article 3 minute; cross-section circular, long thin simple setae on two outer articles; lacinia mobilis and incisor well developed and serrate; no molar. Maxilla 1 (Fig. 5): inner plate with one distal simple seta; outer plate with four cuspidate, one conate and two simple setae and one smooth tooth apically, a row of simple setae along inner margin; palp 1- articulate, reaching 1.5x length of outer plate, with a series of serrate cuspidate teeth apically (sadly, this was not very well visible in the microscopic slide, and is missing in the illustration). Maxilla 2 (Fig. 5): outer plate in normal position to inner plate, slightly longer; both plates with simple setae apically: three on inner plate and 10 on outer. Maxilliped (Fig. 5): slender, inner plate partly divided, with one small simple seta apically on each lobe; outer plate a very small lobe reaching less than 1/5 length of palp article 1; palp 4-articulate, sparsely setose with a thin and short cushion at inner distal margin of article 3, article 4 with a dense row of thin short setae along inner margin.
Pereon: smooth. Gnathopod 1 (Fig. 6): simple; coxa subquadrate; basis linear with long simple setae along anterior margin; ischium subquadrate; merus distal margin not free, a cushion of short simple setae on distal margin and a few type A setae (Tandberg and Vader 2009) along apical margin; carpus long, a row of long simple setae along distal margin; propodus shorter and less broad than carpus, no palm, few long simple setae on distal margin; dactylus smooth, reaching 1/3 length of propodus, row of simple setae on distal margin. Gnathopod 2 (Fig. 6): coxa covering coxa 1, directed forwards, oval; merus long and thin; carpus with ridges along anterior margin, triangular, a row of type A setae along distal margin; propodus broader than carpus, palm nearly transverse, slightly oblique, nearly smooth, but with a sharp tooth at palmar corner, setae along palm, posterior margin straight, slightly longer than palm; dactylus as long as palm, smooth, weakly curved. Pereopods 3 and 4 (Fig. 6): simple and slender; coxa 3 subrectangular, coxa 4 triangular, ventral margin rounded, dactyli simple. Pereopod 5 (Fig. 7): basis slender; meral lobe 1/2 of carpus; dacylus reaching 1/2 length of propodus, simple; medium length simple setae along both margins of entire leg. Pereopods 6 and 7 (Fig. 7): stronger than P5; coxae small; bases posteriorly expanded; meral lobe nearly as long as or as long as carpus, respectively; dactyli reaching 1/2 length of propodus, smooth and with weakly crenulate anterior margin, respectively; medium length simple setae on both margins of both legs.
Urosome (Fig. 7): smooth. Epimeral plate 3 (Fig. 7): rectangular with rounded corner. Uropod 1 (Fig. 7): longer than uropod 2; biramous; peduncle longer than rami, rami subequal in length; peduncle one with short seta apically, inner ramus with two and outer ramus with three setae. Uropod 2 (Fig. 7): longer than uropod 3; biramous; peduncle slightly shorter than inner ramus; inner ramus longer than outer ramus; peduncle with three setae, rami each with two setae. Uropod 3 (Fig. 7): uniramous; ramus with two articles; peduncle shorter than ramus; peduncle and ramus each with two setae. Telson (Fig. 7): rounded, weakly boat-shaped, two robust setae on each side.
Ecology: lives at "medium depths” off south California/ Baja California from 12–185m, in greatest masses (appr. 1 specimen / m2) deeper than 50m. (Barnard 1964)
Distribution: only known from around the type locality (Barnard 1962) off Southern California: Pt Argüello, Baja Cristobál and Baja California.
Discussion: This species is very well described by Barnard. The main reason it is included here, in addition to giving a description of the mouthparts, which are not in Barnard’s description, is to make a complete redescription of all " type "/author-identified material of Metopa for a phylogenetic revision that is planned (Tandberg, Krapp-Schickel & Vader, in prep). Barnard (1962) notes an accessory flagellum "forming a minute bump" in the diagnosis of the male, but nothing is observed on antennae in female. This accessory flagellum was not observed in this study. As the specimen described here is a female, the specific morphology of gnathopod 2 for males will have to be extracted from Barnard (1962), where it is stated as having pereopod margin weakly crenulate with a deep excavation, dactylus not as long as palm, and merus being produced anteriorly.
Barnard (1962) proposes it very close to Metopa wiesei Gurjanova 1933, from which it differs by the “angle of projection of the last tooth on the finger-hinge process of male gnathopod 2” and the relative lengths of the carpus and dactylus of gnathopod 1 (M. dawsoni having a longer carpus and shorter dactylus than M. wiesei). It is also proposed to be close to Metopa alderi (Bate, 1857), but also from this species it differs by its long carpus of gnathopod 1, in addition to the “telsonic spines”. It is also separated from Metopa boeckii Sars, 1895 and Metopa robusta Sars, 1895 by the form of their gnathopods.
description: Published as part of Tandberg, Anne Helene S., 2009, A redescription of Metopa species (Amphipoda, Stenothoidae) based on the type material. 2. The United States National Museum of Natural History (NMNH), pp. 43-68 in Zootaxa 2309 on pages 49-53, DOI: 10.5281/zenodo.191826