Alexandrella mandibulata, Berge & Vader, 2005

Berge, Jørgen & Vader, Wim, 2005, The amphipod genus Alexandrella (Amphipoda, Stilipedidae): taxonomic status, allometric growth and description of two new species, Journal of Natural History 39 (17), pp. 1327-1346 : 1335-1339

Alexandrella mandibulata n. sp.

( Figures 4–6 )

Holotype: female 50 mm, 55 ° 04–009S, 33 ° 57–599W, 3138–3239 m, collected on the RV Eltanin, 8 September 1963, cruise 9, station 722. Unique.

Distribution

Known only from the type locality in the Scotia Sea.

Diagnosis

Labrum strongly asymmetrically lobed. Mandibular incisors broad, irregularly toothed, right lacinia mobilis very small. Pereopod 1 with oostegite, pereopod 4 coxa rounded and only weakly excavate posteriorly. Pleonites 1–3 and urosomite 1 dorsally produced.

Description

Body. Rostrum weakly developed, eyes absent. Epistome unproduced. Body dorsally with a moderate projection posteriorly on pleonites 1–3. Urosomite 1 dorsally with a strong carina posteriorly and a deep excavation anteriorly. Coxae 1 anteriorly weakly expanded, not covering the head. Pereopods 5–7 and antenna 2 elongate.

Antennae. Antenna 1 shorter than antenna 2, peduncle article 1 longer than articles 2 and 3 combined, flagellum with more than 15 segments. Accessory flagellum small, inconspicuous, uniarticulate. Callynophore absent. Antenna 2 peduncle article 4 shorter than 5, flagellum elongate with 36 segments.

Mouthparts. Labrum longer than broad, strongly asymmetrically lobed, right lobe much larger than left, medially separated, right lobe distomedially concave. Labium distally broad and rounded, inner lobes absent. Mandibles powerful, hinge line lateral. Incisors irregularly toothed and broad. Lacinia mobilis present on both mandibles, right lacinia mobilis strongly reduced. Accessory setae-row weak, with four simple setae. Molar absent. Palp short, three-articulate, article 2 longer than 3. Article 2 distally with long and short setae, article 3 laterally with one row of setae and distally with long simple setae. Maxilla 1 palp two-articulate, powerful, distally broad but not transverse, distal and distolateral margin with short robust setae. Outer plate broad, setal teeth in a 6/2 arrangement, ST not cuspidate. Inner plate large, with two distal rows of setae; one row with long pappopectinate setae, second row with short simple setae, medially with two groups of simple setae. Maxilla 2 outer plate not as broad as inner, both plates distally with three distinct rows of setae. Maxilliped inner plate distally with one short nodular seta, distal and medial setae-rows parallel, setae short and simple. Outer plate broad, inner margin without setae, mediodistally weakly serrate, distally with one row of short robust setae. Palp fourarticulate, weakly setose, article 2 as long as articles 3 and 4 combined, article 4 pointed and powerful, distal margin cuspidate.

Pereopods and coxal plates. Coxa 1 large, weakly produced anteriorly, posterodistally with robust slender setae, coxae 2 and 3 long and broad, weakly longer than coxa 1, coxa 4 posterior lobe rounded, posterodistal margin weakly concave, distally rounded, coxae 5 and 6 subequal, distally with posterior lobe large, anterior lobe absent, coxa 7 distally convex and rounded. Pereopod 1 simple, basis with short simple setae on both anterior and posterior distal margins, ischium short, carpus broader than propodus, posterodistally not produced, propodus subrectangular, dactylus powerful and serrate. Pereopod 2 simple, basis with short simple setae on anterior margin, ischium short, carpus broader than propodus, posterodistally not produced, propodus subrectangular, dactylus powerful and serrate. Pereopod 3 shorter than 4, weakly setose, dactylus long and slender. Pereopods 5–7 not differentiated, elongate, progressively longer posteriorly, basis pereopod 7 concave posteriorly, posterodistally acute, dactyli long and slender.

Oostegites on pereopods 1–5, gills on pereopods 2–7.

Epimeral plate 1 small and rounded, epimeral plate 2 as large as 3, posterodistally weakly produced and acute, epimeral plate 3 posterodistally weakly produced, acute. Urosomite 1 dorsally with a deep excavation medially and distally with a large acute projection, urosomites 2–3, uropods and telson missing.

Etymology

This species is named based on the morphology of the mandibles which separates it from all other known stilipedid species.

Remarks

The present species is separated from all other stilipedid species mainly on the morphology of the mandibles and the labrum. The former has a relatively broad incisor, as do both A. australis and A. dentata , but it also has (irregular) teeth all along the margin of the incisor. As in A. australis , the right lacinia mobilis is reduced to a simple tooth in A. mandibulata . Furthermore, the left lobe of the labrum is distally concave, a character state not otherwise observed in this family.

Although many characters, such as the reduced right lacinia mobilis, the setation of the palp and inner plate of the first maxilla and the absence of inner lobes on the labium suggest a close relationship between A. australis and A. mandibulata , there is one character that distinctly suggests a sister group relationship between A. mandibulata and A. martae n. sp. (below); both possess oostegites on pereopod 1, a character state not previously recorded within the Amphipoda . In addition, both these species possess a smooth pereonite 7 (versus a dorsal projection as in A. australis ) and the projections on pleonites 1–3 are not so large as in A. australis and A. dentata .

The holotype of A. mandibulata , which so far is the only specimen known of this species, lacks both the telson and urosomites 2 and 3, including the uropods. However, the morphological characteristics of especially the mouthparts and the first pereopod are considered as being unique enough to validate naming this species.

Figures

Figure 4. Alexandrella mandibulata n. sp. Holotype.

Figure 5. Alexandrella mandibulata n. sp. Holotype.

Figure 6. Alexandrella mandibulata n. sp. Holotype.