Podosiridae, a new family of North Atlantic deep sea amphipod (Crustacea, Amphipoda)

Bellan-Santini (2007) described Podosirus vaderi from a vent community on the Mid-Atlantic Ridge in the Azores Triple junction zone. At the time she placed it in the family Eusiridae and pointed out similarities to the Podoceridae. The original description of Podosirus vaderi was based on an ovigerous female. Male characteristics are not known, but based on established trends among similar amphipods without apical robust setae on uropods 1 and 2, it is presumed that there is minimal sexual dimorphism. Because the female has the first gnathopod smaller than the second and a well developed male-like second gnathopod, it is probable that the male will have similar gnathopods. Whether calceoli occur in the male is also not known. Bellan-Santini (2007) originally placed Podosirus vaderi in the composite Eusiridae (as defined by Barnard & Karaman 1991). This family arrangement was not accepted by Bousfield & Hendrycks (1995, 1997) who re-established the original families Eusiridae, Calliopiidae and Pontogeneiidae. In the sense of Barnard & Karaman (1991) or Bousfield & Hendrycks (1995) Podosirus vaderi is not a eusirid because: maxilla 2 is without an oblique setal row on the inner plate which is much shorter than the outer plate; all coxae are similar in size; there are no lobes on coxa 5; there are dorsal carinae on pereonites 1–3; urosomite 1 is elongate; and the telson is entire. Likewise it is not a calliopiid because the body is subcylindrical; maxilla 2 inner plate is much shorter than the outer plate; coxae are all similar in size and pereopod 5coxa iswithout lobes; pereopods 3–4 merus is linear and much longer than the propodus; pereopods 5–7 basis is linear; uropods 1–2 are without apical robust setae and uropod 3 has styliform rami. It is not a pontogeneiid because all coxae are about the same size; carinae occur on the pleonites; there are no apical robust setae on the rami of uropods 1–2; uropod 3 rami are styliform; and the telson is entire. Podosirus vaderi is in fact most similar to amathillopsid amphipods (fig. 1). They share many core characters such as: similarly shaped heads, particularly the lateral cephalic lobes and the anteroventral corners; long mandibular palps with elongate third articles; long maxillipedal palps with stabbing dactyli; subcylindrical bodies; gnathopods 1 and 2 subchelate; pereopods 5–7 with basis linear; pleonites with dorsal carinae. The curved pereopods with elongate meri occur in both groups. Urosomite 1 is elongate in both taxa, but longer in Podosirus. Podosirus vaderi shares the following group of diagnostic character states with the Amathillopsidae: coxa 1 smaller than coxa 2, pereopods 5–7 of equal length, pleon with dorsal carina, uropod 3 outer ramus subequal in length with peduncle, telson entire. Podosirus cannot be placed within the Amathillopsidae, however, because of the following significant differences: accessory flagellum absent; gnathopod 1 carpus longer than propodus; gnathopod 1 smaller than gnathopod 2; no coxae acuminate; coxa 4 and 5 subequal in size and urosomite 1 elongate. The family Podosiridae fam. nov. is therefore erected here for Podosirus vaderi Bellan-Santini.