Metopa

Tandberg, Anne Helene S. & Vader, Wim, 2009, A redescription of Metopa species (Amphipoda, Stenothoidae) based on the type material. 1. Zoological Museum, Copenhagen (ZMUC), Zootaxa 2093, pp. 1-36 : 2-3

Metopa - through time

When Krøyer (1842) originally described Leucothoe clypeata and L. glacialis , he placed them in the genus Leucothoe , emending its genus diagnosis. Goës (1866) and Spence Bate (1862) moved both species to Montagua . But Boeck (1872) placed Montagua in synonymy with Stenothoe , both genera lacking a mandible palp, and erected the new genus Metopa , with M. clypeata as type species, for species with a short 3-articulate mandible palp (3rd article very short) and a 1-articulate palp on Maxilla 1, in his words: "Mandibulae palpo brevi, 3articulato; articulo 3tio fere obsoleto. Maxillae 1mi paris palpo 1articulato. Reliqva cum genere Stenothoe ferme conveniunt". In the following years, many further Metopa species were described, usually without further examination of the mouthparts (see e.g. Sars 1892).

About 50 years after the erection of Metopa, Schellenberg created the monotypic genus Prometopa for the antarctic P. tuberculata ( Schellenberg, 1926) . This separation has been discussed ( Just, 1980; Barnard & Karaman, 1991), but that is not the theme for this paper. The cutting from Metopa started already with Sars (1892), who erected Metopella , for which Gurjanova (1938) selected Metopa longimana Boeck, 1870 as type species. Gurjanova (1938) also erected the genus Mesometopa , for which she chose Metopa esmarki Boeck, 1870 as type-species; later Metopa neglecta was moved by Shoemaker (1955) to the same genus. Barnard & Karaman (1987) erected the genus Torometopa , with Metopa crenatipalmata as type, for a series of southern species, and in Barnard & Karaman (1991) they erected the genus Hardametopa , for which they chose Metopa nasuta Boeck, 1870 as type, and at the same time moved Metopa carinata to the same genus.

An example of the problems that arise from new species being described without descriptions of mouthparts can be found in the fate of Sars’ species Metopa rubrovittata Sars, 1883 . When Barnard (1962) erected the new genus Stenula , he chose Stenothoides latipes Chevreux & Fage, 1925 as the type for his new genus, which was diagnosed by article 2 of P5 slender, article 2 of P6 and P7 broad, and both mandible palp and maxilla 1 palp uniarticulate. Lincoln (1979) moved M. rubrovittata to Stenula , and then synonymized this taxon with S. latipes on the basis of the mouthparts, as the mandibular palp of Metopa is 2- to 3-articulate.

Today the accepted diagnosis of Metopa is in Barnard & Karaman (1991):

Antenna 1 lacking nasiform process on article 1. Accessory flagellum absent or vestigial. Palp of

mandible 2- to 3-articulate; palp of maxilla 1-articulate. Inner plate of maxilla 2 ordinary. Inner

plates of maxillipeds mostly fused together or well separated (type). Gnathopods 1–2 subchelate,

different from each other in size and shape: gnathopod 1 small, almost simple (variable), article 4

incipiently chelate; article 5 elongate, barely lobed. Pereopod 5 with rectolinear article 2, pereopods

6–7 with expanded, lobate article 2. Pereonite 4 short. Pleonites 4–6 free; pleonite 3 lacking dorsal

process; pleonite 4 not extended posterodorsally. Telson ordinary, flat.

This differs from Boecks original diagnosis most notably in the change of the mandibular palp having to be 3-articulate (with a rudimentary third article) to the palp being 2- to 3-articulate.

As it appears from the redescriptions below, Metopa clypeata is not a very "typical" Metopa . Metopa has on several occasions been thought polyphyletic (most notably in Krapp-Schickel & Koenemann (2006), but also Barnard & Karaman (1991)), and if the genus is split, the name will of course follow the type species. A phylogenetic analysis of Metopa will come in a later paper, since this will have to rely on redescriptions of most of the species.