Akrophryxus milvus, Williams & Boyko, 2021

Williams, Jason D. & Boyko, Christopher B., 2021, Out on a limb: novel morphology and position on appendages of two new genera and three new species of ectoparasitic isopods (Epicaridea: Dajidae) infesting isopod and decapod hosts, Zoosystema 43 (4), pp. 79-100 : 85-86

Akrophryxus milvus n. sp.

( Figs 3-5 , 11Q, R )

urn:lsid:zoobank.org:act:AC0E5AFB-69BB-434D-86A6-A679F9EACDBC

“sacculinid (? parasite attached to antennular fossa)” – Castro 2013: 443.

TYPE MATERIAL. — Holotype. Madagascar • MNHN-IU-2014-20314 ; ovigerous female (3.1 mm diameter), attached to left antennule of male Ethusa machaera Castro, 2005 (9.2 mm CL, 9.0 mm CW; MNHN-IU-2010-1997); Sta. CP 3595; 25°35’28.2012’’S, 44°15’25.2’’E; south of Pointe Barrow ; 821-910 m; coll. Bouchet et al. on the shrimper Nosy Be 11; ATIMO VATAE expedition, collected by trawling; 12.V.2010.

Allotype. Madagascar • MNHN-IU-2014-20315; mature male (740 µm L); same data as for holotype.

TYPE LOCALITY. — 25°35’28.2012”S, 44°15’25.2”E, Madagascar, south of Pointe Barrow, 821- 910 m.

TYPE HOST. — Ethusa machaera Castro, 2005 [Crustacea: Brachyura: Ethusidae ].

ETYMOLOGY. — The species name is derived from the Latin for “kite” in reference to the plate partially surrounding the host antennule that is reminiscent of the shape of a kite shield, defensive armor used primarily in Western Europe in the 10th-13th centuries. The gender is masculine.

DISTRIBUTION. — Known only from the type locality and type host.

DESCRIPTION

Female

Body spheroid, length and width nearly equal, filled with numerous eggs ( Figs 3A, B ; 4A , C-G). 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) ( Figs 4G ; 5G ). Oral cone rounded ( Fig. 4G ); mouthparts indistinct. Maxillipeds ovate with recurved digitiform extension ( Fig. 5H ). Pereopods 1-5 subequal in size and shape, without setae ( Figs 4G ; 5E, F ); dactylus short, recurved, propodus carpus and merus fused, ischia and bases stout. Oostegite 1 largest ( Figs 4G ; 5J ), broadly ovate with small posterior accessory lobe, broad lobe medially divided in lateral view ( Fig. 5K ); oostegites 2/3, 4, 5 progressively larger; oostegite 2 present, thin and closely applied to oostegite 3, lobes subequal in size and shape ( Fig. 5L ); oostegite 3 subtriangular ( Fig. 5L ), expanded posteriorly, medial and anterior finger-like extensions present with small setae distally, oostegites 4 and 5 subquadrate, small setae on medial and posterior margins ( Fig. 5M ). Pleon presumably modified (see Discussion) as oblong, narrow, thickened plate ( Fig. 4 A-G) partially surrounding host antennule with three circular medial holes: largest surrounding antennule of host and closest to mouthparts of parasite, smallest hole at midpoint of plate, intermediate sized hole farthest from mouthparts of parasite.

Male

Body not recurved ventrally ( Fig. 5A, B ). Cephalon fused with pereomere 1 ( Fig. 5A, B ), anterior margin rounded, posterolateral margins evenly rounded; lacking eyes, cephalic slits present. Antennules reduced, each apparently composed of three flattened disc-shaped segments ( Fig. 5C ); antennae each as single broad elongate lobes lateral to oral cone, flagella absent ( Fig. 5C ). Oral cone triangular ( Fig. 5B, C ). Pereomeres 2-6 distinct, 4-6 subequal in width, others slightly narrower ( Fig. 5A, B ); pereomere 7 fused with pleon, lateral margins recurved ventrally. Pereopods 1-6 subequal in size and shape, all segments distinct, carpi rounded, ischia short, bases elongate; dactylus, propodus and carpus with isolated marginal setae ( Fig. 5 B-D); pereopod 7 lacking. Pleon compact, rounded, all segments fused and fused with pereomere 7, rounded posteriorly; anal slit and pleopods lacking ( Fig. 5A, B ).

REMARKS

The female of Akrophryxus milvus n. gen., n. sp. is very different from those in all other dajids, excepting Telephryxus clypeus n. gen., n. sp. ( Table 1 ). The spheroid shape of the body and possession of a plate partially surrounding the host antennule are shared only by these two genera. The male of A. milvus n. gen., n. sp. is likewise very different from those in all other dajid genera ( Table 2 ), including T. clypeus n. gen., n. sp. in having only six pairs of pereopods with no trace of the seventh pair (pereopod 7 of two elongate segments in the sole species of Oculophryxus and of one or two stub-like segments in T. clypeus n. gen., n. sp.). The male of A. milvus n. gen., n. sp. also differs from those in most genera in having the pleon fused with pereomere 7 (distinct from pereomere 7 in males in all other genera except Holophryxus Richardson, 1905 and Oculophryxus ), and from all other genera in having a reduced antennule and an antenna with a single large segment (males of other genera with antennules of at least one well-developed segment and all with antennae segmented and with distal flagella).

The female of A. milvus n. gen., n. sp. can be distinguished from that of T. clypeus n. gen., n. sp. in lacking antennules and in having three medial holes on the narrow, triangular attachment plate partially surrounding the host antennule with no additional lateral holes (T. clypeus n. gen., n. sp. has flattened antennules and a broad, subquadrate attachment plate partially surrounding the host antennule and having two large medial holes, the largest with two additional small lateral holes). The male of A. milvus n. gen., n. sp. can be distinguished from that of T. clypeus n. gen., n. sp. in having only six pairs of pereopods (six fully-developed pairs plus a rudimentary seventh pair of one or two small segments in T. clypeus n. gen., n. sp.), a reduced antennule and a single segmented antenna (large ovate single segmented antennule and two segmented short antennae in T. clypeus n. gen., n. sp.), and the short and rounded pleon fused to pereomere 7 (pleon distinct from pereomere 7, elongate and tapered pleon in T. clypeus n. gen., n. sp.).

Figures

FIG. 3. — Akrophryxus milvus n. gen., n. sp.,holotype female,MNHN-IU-2014-20314, attached to Ethusa machaera Castro,2005,macropod images:A, dorsal view of host, showing parasite attached to left antennule; B, ventral view of host, lateral view of parasite showing attachment to left antennule. Scale bars: A,B: 1 mm.

FIG. 4. — Akrophryxus milvus n. gen., n. sp., holotype female, MNHN-IU-2014-20314: A, posterior view, proximal end of host antenna extending from body of parasite at top; B, host antennule, middle segment showing attachment sight of parasite with three holes created by mouthparts;C, lateral view, left side showing proximal end of host antenna extending from body of parasite; D, anterior view (opposite side to that shown in A; proximal end of host antenna facing viewer); E, lateral view, right side, outer membrane of parasite removed showing brood of developing eggs (dashed circles) and position of male shown by grey outline; F, lateral view, right side with eggs removed and showing position of host antenna (arrow indicates distal end); G, lateral view, right side with host antenna and eggs removed showing mouthparts (MO), antenna (A2), maxilliped (Ma) and pereopods (1 and 5 labeled). In figures A, C, D, and E, the three circular medial holes in pleon are labeled (hole surrounding antennule of host and closest to mouthparts, H1; hole at midpoint, H2; hole farthest from mouthparts, H3). Scale bars: A, C-G, 1 mm; B, 250 µm.

FIG. 5. — Akrophryxus milvus n.gen., n. sp., allotype male (A-D; MNHN-IU-2014-20315) and holotype female (E-M; MNHN-IU-2014-20314): A, dorsal view;B, ventral view; C, ventral view of anterior end showing left antennule (A1), left antenna (A2), mouthparts (MO) and first pereopods; D, right pereopod 6; E, pereopods 1-5 from left side (1 and 5 labeled with numbers); F, pereopod 3 from left side; G, antenna from left side, covered in scales (not shown); H, left maxilliped, ventral view, arrow shows digitiform extension that is inserted in fold of oostegite 1 in part K (shown by arrowhead); I, left maxilliped, lateral view; J, left oostegite 1, ventral view; K, left oostegite 1, lateral view showing fold where digitiform extension of maxilliped resides (arrowhead); L, left oostegite 3, anterior portion of oostegite 2 shown by arrowhead and in dashed line; M, left oostegite 5. In Figures 2G, H-K, and M asterisks show site of attachment for structures dissected from body of parasite. Scale bars: A, B, G, L, M, 100 µm; C, D, 25 µm; E, H-K, 250 µm; F, 50 µm.

FIG. 11. — Life-cycle diagram of the dajid isopod Prodajus ostendensis Gilson, 1909 parasitizing the opossum shrimp Gastrosaccus spinifer (Goës,1864): A, ovigerous female, ventral view. Eggs (shown in dashed circles) develop into early epicardium larvae and then molt once; B, mature epicaridium larvae released into the water column; C, mature epicaridium larvae find copepod intermediate host, attach and metamorphose into an ectoparasitic microniscus larva. Microniscus larva transforms into a cryptoniscus larva that detaches and is free-swimming; D, cryptoniscus larva settles on host mysid and invades marsupium; E, within the host marsupium the cryptoniscus larva transforms into a juvenile female; F, juvenile female, ventral view; G, another cryptoniscus larva (or larvae) settles and transforms into a dwarf male; H, host G. spinifer with ovigerous female P. ostendensis (in grey) occupying the marsupium; I, ovigerous female, lateral view with one dwarf male partially visible and eggs shown in dashed circles; J-M, developmental series of females of P. ostendensis, all in lateral view: J, cryptoniscus larva with oral sucker (OS); K, early juvenile female (second phase fide Gilson 1909); L, later juvenile female (third phase fide Gilson 1909); M, ovigerous female; N-R, proposed developmental series of females of Akrophryxus milvus n. gen., n. sp. and Telephryxus clypeus n. gen., n. sp.; N, cryptoniscus larva of T. clypeus n. gen., n. sp., dorsal view; O, hypothetical early juvenile female, lateral view; P, hypothetical later juvenile female of A. milvus n. gen., n. sp. attached to antennule of host (shown in grey), lateral view; Q, ovigerous female of A. milvus n. gen., n. sp. attached to antennule of host (shown in grey), lateral view; R, ovigerous female of A. milvus n. gen., n. sp. dissected to show attachment to antennule of host (shown in grey) with mouth (Mo) of parasite noted, lateral view (A, B, D-M, modified from figures in Gilson (1909); C, from G. O. Sars (1898)). Scale bars: A Q, R, 1 mm; E, 3 mm; N, 100 µm; rest not to scale.