A revision of Acrenhydrosoma (Copepoda, Harpacticoida) with the establishment of Dyacrenhydrosoma gen. nov and Paracrenhydrosoma gen. nov and descriptions of two new species.

Acrenhydrosoma perplexa is redescribed from cotype and other material. It possesses only two setae on the antennal exopod, a mandibular palp with three setae, and a maxillulary coral endite partially fused to a basis which bears only three setae. The P1 endopod-2 has two armature elements and the P5 baseoendopod, in both sexes, has an outer sera borne on a submarginal peduncle. The male P5 exopod bears only two setae. The posterior margin of the preanal somite has a dorsal, dentate, pseudoperculum overlaying the operculum which is situated near the anterior margin of the anal somite. A new species, with a urosome structure similar to Acrenhydrosoma is described from mangrove forests in S.E. Asia and is placed in Dyacrenhydrosoma gen. nov. as D. breviseta sp. nov.. It can be distinguished from Acrenhydrosoma by the absence of a seta on the abexopodal margin of the antennal allobasis, a maxillulary coral endite (represented by 1 sera) completely fused to the basis which bears only two setae, reduced armature of the maxilla, short terminal setae without penicillate tips on Pi exopod-3, only two setae on P2-P4 endopod-2, a female P5 baseoendopod with only one inner spine and an exopod with only 2 setae. Material from northern Norway, attributed to Acrenhydrosoma perplexa by T. Scott (1903) is described and placed in Paracrenhydrosoma gen. nov, as P. normani sp. nov. It is very similar to Acrenhydrosoma maccalli but can be distinguished from it by features on the P5. A. maccalli and A. karlingi are moved to Paracrenhydrosoma which can be distinguished from Acrenhydrosoma by the absence of a pseudoperculum on the preanal somite, and also by an operculum situated in the posterior half of the anal somite, a broader, emarginate rostrum, a mandibular palp with four setae, a maxillulary coral endite (bearing two setae) completely separate from a basis bearing six setae, a P1 endopod-2 with three setae and a female P5 exopod with two setae.