Ericthonius didymus, Krapp-Schickel, 2013

Krapp-Schickel, T., 2013, New or amended data on Mediterranean Amphipoda: genera Dexamine, Ericthonius and Stenothoe, Zootaxa 3613 (2), pp. 125-145 : 131-136

Ericthonius didymus sp. n.

Fig. 5–8

Type material: Holotype male 4.5mm Laguna di Venezia , Malamocco, 1m depth. slide MVCr 7396-7397 (Museum of Verona).

Paratype female 6mm, same locality, slide MVCr 7398 ( Museum of Verona ) .

Additional material: a rich series in alcohol, in the Verona Museum.

Description (difference to E. argenteus in bold letters)

Adult males 3–4.5mm. Head elongate, lateral cephalic lobes strongly rounded (containing the oval dark eye). Rostrum small, short. Antennae: A1<A2; A1 peduncle art 1 somewhat shorter than the subequal arts 2,3; flagellum with 9 arts. A2: art 2 with short prolongation, art 4 <art 5, flagellum with 7 and more arts.

Mouthparts: Labrum (=UL) with acute epistome. Md palp art 2 the longest, art 3 spatulate, molar triturative. Mx1 inner plate triangular with few lateral setae. Mx2 inner plate with mediofacial setae. Labium (=LL) with acute mandibular process. Mxp art 4 (= dactylus) more than half length of art 3 (= propodus).

Gnathopods: Cx1 clearly smaller than Cx2; Gn1 basis unexpanded, slender, carpus triangular, propodus about 4/5 of carpus length, widest distally, posterior margin with many small indentations where bundles of setae are inserted. Cx2 broader than deep, with “stridulating” ridges on lower margin. Gn 2 basis bottle-shaped, elongate, narrowing proximally; merus posterodistally rounded; carpus about twice as long as wide, anterior margin smoothly convex, posterodistal tip acutely elongated, clearly longer than second tooth of bifurcation (which can be rounded or acute); propodus about twice as long as broad and clearly shorter than carpus, palm straight; dactylus slender, about 3/4 of propodus length, densely beset with long setae, also distally.

Peraeopods: P3, 4 basis bottle-shaped rounded, distally widest. Coxae 5, 6 beset with setae, anteriorly rounded, posterior part produced to narrow lobe, inferoposteriorly excavated. P5 twisted; basis posterior margin rather straight, posterodistal corner lengthened to narrow lobe defined by semicircular excavation near insertion of ischium. P6, 7 basis rectangular, with rounded posterodistal corner.

Uropods: U1,2 peduncle longer than subequal rami; inner ramus inner margin densely beset with short robust setae, outer ramus inner margin finely serrated. U3 ramus less than half length of peduncle.

Telson with cuticular hooks.

Ovigerous females 3.5– 6mm: Coxae densely beset with setae. Cx2 with stridulating ridges. Gn2 carpus anteriorly about half length of propodus, posteriorly acutely lengthened, length growing with age, tip reaching half of propodus posterior margin in large females. P5 subquadrate, posterodistal corner right-angled, not lengthened to lobe nor excavated.

Colour: brownish, with many dark patches.

Etymology. This species is very similar to E. argenteus and females are quite difficult to separate; the specific name stresses this fact, using the latizined form of didymos (Greek for twin) as noun in apposition.

Figures

FIGURE 5. Ericthonius didymus sp. n. A1, 2 = antennae; Hd = head; Mx1, 2 = maxillae; LL = lower lip; Md = mandible, Md' = mandible enlarged; Mxp = maxilliped, Mxp' = other maxilliped.

FIGURE 6. Ericthonius didymus sp. n. Mxp + Gn1 = maxilliped and first gnathopod in situ; Gn2 f = female second gnathopod; Gn1 m, Gn2 m = first and second male gnathopod.

FIGURE 7. Ericthonius didymus sp. n. Gn2 m = another second male gnathopod; P3–6 = peraeopods 3–6 in situ; P3, 4 = peraeopod 3, 4 in detail; Cx 5 f = coxa 5 female; P5 m = coxa 5 and peraeopod 5 of male; P5 f = peraeopod 5 of female.

FIGURE 8. Ericthonius didymus sp. n. P6 m, P6 f = peraeopod 6 of male and female; P6 ' = dactylus of P6 enlarged; P7 f = peraeopod 7 of female; Us = urosome; U1–3 = uropods 1–3; T = telson.