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

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Isaeopsis K.H. Barnard, 1916


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.