Isaeopsis K.H. Barnard, 1916

Pérez-Schultheiss, Jorge & Pardo, Luis Miguel, 2020, A new crab-associated amphipod of the genus Isaeopsis Barnard, 1916 (Amphipoda Senticaudata: Ischyroceridae) from southern Chile, Zootaxa 4861 (1), pp. 107-119 : 108-109

Genus Isaeopsis K.H. Barnard, 1916

Type species: Isaeopsis tenax K.H. Barnard, 1916 (by monotypy).

Species included. I. chiloensis n. sp.; I. tenax K.H. Barnard, 1916 .

Diagnosis. Ischyrocerini with antenna 1 accessory flagellum 1 or 2-articulate, with second article minute. Coxae 1–5 of equal depth, all as deep as, or slightly deeper than corresponding pereonite; coxa 2 of male similar to coxa 3. Coxa 4 without posterior excavation. Anterior lobe of coxa 5 as long as coxa 4. Male gnathopod 2 merus slightly longer than carpus; carpus subtriangular; propodus elongate, with palm concave, posteriorly with thumb or not; dactylus posterior margin smooth. Dactyli of gnathopods 1–2 with facial serrations. Pereopod 3–7 with propodus prehensile, subchelate; dactyli 5-7 with facial serrations. Uropod 3 biramous. Telson distally rounded, with a pair of dorsal, strong setae.

Remarks. Isaeopsis is the only known Ischyrocerini with subchelate pereopods. Another two species have been reported in Jassa , but these are exceptional for the genus, and differ from Isaeopsis by the flagellum of antenna 1 shorter than peduncle article 5 and the small carpus of pereopods 3–4, nearly as long as the anterodistal lobe of the merus ( Conlan 1990).

Inside Ischyrocerini , only the genera Microjassa Stebbing, 1899 , Myersius Souza-Filho & Serejo, 2014 , Veronajassa Vader & Myers, 1996 and Bathyphotis Stephensen, 1944 , have coxa 4 posteriorly excavated and the anterior lobe of coxa 5 short. In Isaeopsis and the remaining genera, coxa 4 is entire and as long as the anterior lobe of coxa 5.

Among these genera, there is a group ( Jassa Leach, 1814 ; Alatajassa Conlan, 2007 ; Ruffojassa Vader & Myers, 1996 ; Scutischyrocerus Myers, 1995 and Parajassa Stebbing, 1899 ) with the merus of pereopods 3–4 strongly produced anterodistally in a lobe, and with the carpus short, reduced, exceeding the merus lobe by half or more of its length. In Isaeopsis and allied genera ( Coxischyrocerus Just, 2009 ; most species of Ischyrocerus Krøyer 1838 ; Neoischyrocerus Conlan, 1995 ; Pseudischyrocerus Schellenberg, 1931 ; Tropischyrocerus Just, 2009 ; Paradryope Stebbing, 1888 and Ventojassa J.L. Barnard, 1970 ), the merus anterodistal lobe is short and the carpus is normal.

From this group of genera, Paradryope is easily distinguished by the cylindrical body, with small, short and nearly discontiguous coxae. The genera Pseudischyrocerus and Ventojassa differ from Isaeopsis in the linear second article of mandibular palp, not medially expanded, and by the long carpus of gnathopod 1, as long as or longer than propodus. In addition, the accessory flagellum of Ventojassa , has up to 4 articles. Coxischyrocerus differs from Isaeopsis in the small coxa 1, enlarged coxa 2, and the rounded setose lobe in the anterior margin of the ischium of male gnathopod 2.

Isaeopsis is most similar to the genera Ischyrocerus , Neoischyrocerus and Tropischyrocerus . Tropischyrocerus and Ischyrocerus lack facial serration in the dactylus of gnathopods 1–2 and pereopods 5–7. In addition, Tropischyrocerus has coxa 2 wider than deep and most species of Ischyrocerus have an articulated distal seta, partially immersed in outer ramus of uropod 3.

Finally, Isaeopsis shares the presence of a facial serration on the dactylus of gnathopods 1–2 and pereopods 5–7 with Neoischyrocerus , but differs in the prehensile pereopods and the structure of uropod 3. In Neoischyrocerus the maximum width of the peduncle and the length of the rami of uropod 3 does not reach 1/3 of the peduncular length, instead in Isaeopsis the maximum width of the peduncle is only little shorter than half of the length, and the length of the rami exceeds half of the length of the peduncle, at least in the new species.