In this paper, we describe two new species of dajid isopods found enveloping the antennules of decapod hosts: Akrophryxus acinaces sp. nov. parasitizing Pycnoplax surugensis (Rathbun, 1932) (off Pacific coast of central Japan, approximately 200 m depth) and A. pallipalicus sp. nov. parasitizing Parapalicus armatus Castro, 2000 (South China Sea, approximately 410 m depth). Females of both species exhibit the characteristic spheroid shape found in the genus Akrophryxus Williams and Boyko, 2021. Females of the three species in Akrophryxus can be distinguished by several characters, including form of the pleon and oostegite 5. Males can be distinguished by head/pereomere 1 to pereomere 2 ratio and posterolateral margin shape, and pereopod 6 dactylus and ischium size. The three species of Akrophryxus not only differ in morphological characters but parasitize species belonging to three different heterotreme brachyuran families. The first described cryptoniscus larval stage for any species in the genus is described for A. acinaces sp. nov. We also describe a new cryptoniscoid, Chimaeroniscus spheramator gen. et sp. nov., the first hyperparasite described from any dajid host (A. pallipalicus sp. nov. on the host Pa. armatus). This hyperparasite is a putative egg predator of its dajid host and exhibits a unique combination of generic level characters including: pleon markedly narrower than pereon, antennule with large teeth on basal segment and with setal brush, 3 or 4 coxal teeth on all pereomeres, pereopods 3–7 isomorphic and dissimilar from pereopods 1 and 2, telson with smooth distomedial projection, and uropodal exopods half-length of endopods. Finally, we address some taxonomic issues within Cryptoniscoidea, moving two genera (Neritoniscus Schultz, 1977 and Cryptocisus Schultz, 1977) known only by cryptoniscus larval characters from Cryptoniscoidea incertae sedis to Dajidae and synonymize Cryptonus Schultz, 1977 with Holophryxus Richardson, 1905.
A giant phyllosoma larva, assigned to the final stage, was collected from off Peru. This specimen was determined to be Projasus bahamondei George, 1976 by DNA barcoding, and morphological description was given for the first time in the genus Projasus George and Grindley, 1964. The present phyllosoma was characteristic of having very wide cephalic shield (about 2.3 times of thorax width), median rostrum, eminent setose exopods on the second and third maxillipeds, and five stout spines on basial endite of maxillule. Projasus phyllosoma has morphological characteristics of both ‘Silentes’ and ‘Stridentes’ groups in the family Palinuridae.
We describe a new species of the rare and deep-sea family Myxasteridae from Japanese waters. This is the first record of this family in the western North Pacific. The new species, Asthenactis agni n. sp., differs from all congeners in having seven arms with an R/r ratio of 7.2, primary radial plates with convex distal edges, and two to three actinolateral spines that are up to 4.7 mm long.