Akrophryxus acinaces, Williams & Boyko & Moritaki, 2022

Williams, Jason D., Boyko, Christopher B. & Moritaki, Takeya, 2022, Two New Species of Spheroid Ectoparasitic Isopods (Epicaridea: Dajidae) Attached to the Antennules of Brachyuran Crab Hosts, with Description of a New Genus and Species of Hyperparasite (Epicaridea: Cryptoniscoidea), Species Diversity 27 (2), pp. 227-241 : 229-232

publication ID	https://doi.org/ 10.12782/specdiv.27.227
publication LSID	lsid:zoobank.org:pub:BE0F7ECC-B2B4-41D9-A62B-287D0ECB1C6A
persistent identifier	https://treatment.plazi.org/id/29693B32-7CD9-436D-8ED6-28638A354ADF
taxon LSID	lsid:zoobank.org:act:29693B32-7CD9-436D-8ED6-28638A354ADF
treatment provided by	Felipe
scientific name	Akrophryxus acinaces
status	sp. nov.

Treatment

Akrophryxus acinaces View in CoL sp. nov.

[New Japanese name: Tsumenaga-kani-no-hana-chochin] ( Figs 1–5 View Fig View Fig View Fig View Fig View Fig )

Material examined. Holotype: mature female ( SMBLV0649 ; 4.5 mm diameter) with developing eggs, attached to right antennule of Pycnoplax surugensis (Rathbun,1932) (type host, SMBL-V0653 ; female, 26.2 mm CL, 35.1 mm CW); 34°08′49.0″N, 136°33′25.0″E, Kumano-nada Sea off Kii Nagashima , Kihoku , Mie prefecture, Pacific coast of central Japan (type locality), approximately 200 m depth, bottom trawl net, fishing trawler Jinsho-maru, 3 June 2020 (fixed 14 September 2020). GoogleMaps

Allotype: mature male ( SMBL-V0650 ; 0.65 mm) from female holotype; same data as for holotype. GoogleMaps

Paratypes: four cryptoniscus larvae ( SMBL-V0651 ; 1 on SEM stub, rest in ethanol) from ovigerous female holotype; one mature female ( SMBL-V0652 ; 4.0 mm diameter) attached to left antennule of same host specimen as holotype, same data as for holotype GoogleMaps .

Etymology. The species name is a noun in apposition derived from the Latin for “scimitar” in reference to the strongly recurved and elongate dactyli of the posterior pairs of pereopods of the male.

Derivation of Japanese name. We propose the new Japanese name Tsumenaga-kani-no-hana-chochin which refers to elongate dactyli of the male (“Tsumenaga” means “long claws”) and appearance of the female on the host (“hana-chochin” means “snot bubble”).

Distribution. Known only from the type locality and type host.

Description of female. Body spheroid, length and width nearly equal (maximal diameter in lateral view= 4.5 mm), filled with developing eggs in ovary ( Figs 1 View Fig , 2A–F View Fig ). Cephalon externally indistinguishable from pereon, without eyes. Antennules absent, antennae each as oblong flat plate lateral to oral cone, covered with minute scales (not shown) ( Fig. 3E, F View Fig ). Oral cone rounded ( Figs 2E View Fig , 3B View Fig ), mouthparts indistinct. Maxillipeds ovate with recurved digitiform extension ( Fig. 3G, H View Fig ), extending into groove of oostegite 1. Pereopods 1–5 subequal in size and shape ( Figs 2E, F View Fig , 3A–C View Fig ); recurved dactylus short with setae and scales on ventral surface, propodus, carpus and merus fused, ischia and bases stout ( Fig. 3D View Fig ). Oostegite 1 largest ( Fig. 3B, I, J View Fig ), broadly ovate with small posterior accessory lobe, broad lobe medially divided in lateral view ( Fig. 3I View Fig ); oostegites 2–4, progressively larger, thin and closely applied to each other ( Fig. 3K View Fig ); oostegites 2 and 3 subrectangular ( Fig. 3K, L View Fig ), expanded posteriorly with small setae distally, oostegite 4 ( Fig. 3M View Fig ) subquadrate, small setae on medial and posterior margins. Oostegite 5 absent or extremely reduced. Pleon modified (see Remarks) as oblong, narrow, thickened plate ( Figs 1D–F View Fig , 2A–D View Fig ) partially surrounding host antennule ( Fig. 2G View Fig ) with two large circular medial holes: largest surrounding antennule of host and closest to mouthparts of parasite, smaller hole farthest from mouthparts of parasite ( Fig. 3E View Fig ). Larger hole flanked by two minute perforations ( Fig. 2B, C View Fig ).

Description of male. Body recurved ventrally ( Fig. 4A View Fig ), when flattened approximated 0.65 mm in length. Cephalon fused with pereomere 1 ( Fig. 4A View Fig ), anterior margin rounded and inflated dorsoventrally, posterolateral margins evenly rounded; lacking eyes, cephalic slits present. Antennules reduced, broadly conical, apparently of two segments, terminal segment minute with setae ( Fig. 4A View Fig ); antennae not observed. Oral cone triangular ( Fig. 4A View Fig ). Pereomeres 2–6 distinct, 4–6 subequal in width, others slightly narrower ( Fig. 4A View Fig ); pereomere 7 fused with pleon, lateral margins recurved ventrally ( Fig. 4A View Fig ). Pereopods subequal in size and shape, all segments distinct, carpi rounded, ischia short, bases elongate ( Fig. 4A, C View Fig ); dactyli of pereopods 4–6 similar to 1–3 but with much longer dactyli extending past distal margin of carpi ( Fig. 4A, C, D View Fig ); pereopod 7 lacking. Pleon compact, rounded, all segments fused and fused with pereomere 7, rounded posteriorly; anal slit and pleopods lacking ( Fig. 4A View Fig ).

Description of cryptoniscus larva. Body tear-drop shaped ( Figs 4E View Fig , 5A, B View Fig ), length (not including long uropodal setae)=0.46± 0.03 mm (n=4), pereomere 4 widest ( Fig. 5A View Fig ), anterior and posterior segments slightly narrower. Median region of pereomeres 2–6 containing pair of ovalshaped testes ( Fig. 4E View Fig ). Cephalon anterior margin round, medial region of posterior margin convex, lateral regions concave, posterolateral margins extended posteriorly ( Fig. 4E View Fig ); eyes round, unpigmented. Body pigmentation lacking. Antennules of three articles each ( Fig. 5C View Fig ), basal article triangular with two stout simple setae, article 2 quadrate with four marginal simple setae and few low, rounded bumps, article 3 digitiform, inserted into article 2 distoventrally, less than half width of article 2, with three long distal simple setae, lateral “flagellum” with three or four long simple setae ( Fig. 5C View Fig ). Antennae of nine articles each (four peduncular and five flagellar) ( Fig. 4F View Fig ), all articles cylindrical, basal peduncular article broader, next segment longest, distalmost two segments subequal in size and approximately half the length of second segment; minute, distal simple setae on article 4; flagellar articles approximately half width of peduncular articles, with minute terminal simple setae, distalmost article with four long terminal simple setae and two short simple setae ( Fig. 4F View Fig ). Oral cone triangular, anteriorly directed, lacking oral sucker ( Figs 4E View Fig , 5A View Fig ). Pereomeres 1–7 with smooth coxal plates, each with distinct distomedial notch ( Fig. 5H View Fig ), and small indentation lateral to notch. Pereopods 1 and 2 ( Fig. 5A, C, D View Fig ) each with short, slightly curved dactylus; propodus semi-spherical, inner margin folded, bearing two flat, multifid setae and one stout simple seta; carpi triangular, distally expanded with single distal multifid seta; meri and ischia rounded, bases cylindrical. Pereopods 3–5 ( Fig. 5A, E–G, I View Fig ) with more elongate curved dactyli, propodi more elongate and narrower, inner margin of each folded, bearing two flat, multifid setae and one stout simple seta ( Fig. 5I View Fig ); carpi triangular, distally expand- ed with single distal multifid seta; meri and ischia rounded, bases cylindrical. Pereopod 6 ( Fig. 5G View Fig ) dactylus subequal in length to those of pereopods 3–5, propodus more elongate and narrower than those of pereopods 3–5, inner margin slightly folded, bearing three stout, multifid setae; carpus triangular, with distal setae; merus and ischium rounded, basis cylindrical. Pereopod 7 ( Fig. 5G, J View Fig ) subequal to pereopod 6 but with elongate dactylus extending to distal margin of merus. Pleon ( Figs 4G View Fig , 5A View Fig ) with five pairs of biramous pleopods, endopods cylindrical, exopods triangular, both with long terminal plumose setae ( Fig. 4G View Fig ). Pleotelson not observed (obscured by pleomeres). Uropods biramous ( Fig. 4G View Fig ), composed of wide sympod and cylindrical endopod slightly shorter than spatulate exopod; endopods and exopods terminally setose ( Figs 4H View Fig , 5K View Fig ).

Remarks. As discussed by Williams and Boyko (2021) for other spheroid dajids, the female of A. acinaces sp. nov. has a body showing fusion of pereomeres and pleon (i.e., lacking segmentation when viewed externally) and the plate or shield-like structure surrounding the host antennule is interpreted as the pleon. The female of A. acinaces sp. nov., can be distinguished from that of the type species, A. milvus as well as A. pallipalicus sp. nov. by the following characters: modified pleon with two holes and subtriangular in lateral view ( A. milvus with three holes and trapezoidal in lateral view; A. pallipalicus sp. nov. with two holes and trapezoidal in lateral view), two small perforations lateral to hole closest to mouthparts (no holes present in A. milvus or A. pallipalicus sp. nov.), and oostegite 5 absent or extremely reduced (present and well-developed in A. milvus ; absent or reduced in A. pallipalicus sp. nov.). The male of A. acinaces sp. nov., can be distinguished from that of A. milvus and A. pallipalicus sp. nov. by the following characters: head/pereomere 1 approximately as wide as pereomere 2, posterolateral margins expanded (head/pereomere 1 narrower than pereomere 2, posterolateral margins not expanded in A. milvus ; head/ pereomere 1 approximately as wide as pereomere 2, posterolateral margins expanded in A. pallipalicus sp. nov.), dactylus of pereopod 6 elongate, reaching beyond distal margin of carpus (dactylus of pereopod 6 short, reaching at most midpoint of propodus in A. milvus and A. pallipalicus sp. nov.), ischium of pereopod 6 less than twice as long as wide (less than twice as long as wide in A. milvus ; more than twice as long as wide in A. pallipalicus sp. nov.). The cryptoniscus larval stage is only known from A. acinaces sp. nov.

Video evidence showed the host Py. surugensis repetitively used both its third maxillipeds and chelipeds, unsuccessfully, to remove the dajids attached to its antennules (see Appendix 1). Decapods are known to groom their appendages ( Bauer 1981, 1989) for removal of fouling organisms and parasites ( Ritchie and HØeg 1981). However, the female dajids were not dislodged by these attempts, showing the tenacity of their attachment to the antennules through the use of their pereopods and also because the whole body of the female dajid encapsulates the host antennule. Although isolated alive for 80 and 103 days, respectively, for the specimens on the left antenna of the host (SMBL-V0652) and right antennae of the host (SMBL-V0649), no larvae were released. Only the holotype female was paired with a male; this female had developing eggs (0.10± 0.01 mm, n=10; across long axis) and also four cryptoniscus larvae within the marsupium.

The cryptoniscus larva of A. acinaces sp. nov. lacks an oral sucker and this character state is also found in the cryptoniscus larvae of the closely related spherical dajid T. clypeus . The presence of oral suckers on cryptoniscus larvae has long been used as a de facto synapomorphy for Dajidae , e.g., Tattersall (1911) and Schultz (1977), but there is a growing body of evidence that the presence/absence of a sucker is variable within the family. In some species it appears the sucker is more easily broken off ( Taberly 1957); however, none of the four cryptoniscus larvae of A. acinaces sp. nov. had a sucker or even trace of one. We suggest it represents one of the dajid species where the structure is never developed in the cryptoniscus (see Williams and Boyko 2021 for a more detailed discussion of this issue).