Sigyn branchialis, An & Boyko & Li, 2015

An, Jianmei, Boyko, Christopher B. & Li, Xinzheng, 2015, A Review Of Bopyrids (Crustacea: Isopoda: Bopyridae) Parasitic On Caridean Shrimps (Crustacea: Decapoda: Caridea) From China, Bulletin of the American Museum of Natural History 2015 (399), pp. 1-85 : 72-75

Sigyn branchialis , n. sp.

Figures 21C, D , 22 , 23

Eophrixus branchialis An, 2006: 174 , fig. 89 (unavailable name).

MATERIAL EXAMINED: Infesting Alpheus digitalis De Haan, 1844 , holotype ♀ ( CIEAL 920301 ), Sanya, 18 ° 14′N, 109 ° 30′E, 23 March 1992. Allotype ♂ (CIEAL920301b), same locality as holotype. Female bopyrid parasiting right branchial chamber of host (fig. 21C, D).

DESCRIPTION: Holotype female (CIEAL 920301a): Length 4.67 mm (including brood pouch), head length 0.67 mm, head width 0.67 mm. pereon length 0.78 mm (fig. 22A, B). Head length equal to width, eyes absent (fig. 22A). Head with frontal lamina. Only a single pair of antennae of five articles present (fig. 22C). Maxilliped with much larger anterior article, without palp, plectron highly blunted and not visible as distinct structure (fig. 22D). Barbula (fig. 22E) not located at exact bottom of maxilliped, inclines left, with seven to eight acute lobes on margin. First pereomere concealed by head, only visible lateral to head. Second to seventh pereomeres distinct only on short side (fig. 22A). Brood pouch large and completely closed (fig. 22B). Oostegite 1 (fig. 22F–I) with distinct asymmetry, left side much larger than right side. Oostegite 1 of left side (fig. 22F, G) with nearly smooth internal ridge, posterolateral point long and sharp, directed laterally. Oo‐ stegite of right side (fig. 22H, I) with smooth internal ridge, posterolateral point blunt and posteriorly directed. First two pereopods of long side near head, third pereopod attached to ventral surface of brood pouch, opposite head (fig. 22B). Fourth to seventh pleopods crowded near pleon (fig. 22B). All pereopods of short side arranged in a line (fig. 22A). Pereopods of same size and structure, with long bases and blunt dactyli (fig. 22J, K).

Pleon of five pleomeres, first four pleomeres bearing uniramous flaplike pleopods and lateral plates. Fifth pleomere a bulging sphere, surface covered with tubercles (fig. 22L).

DESCRIPTION: Allotype male (CIEA920302): Length 1.64 mm, maximal width (across pereomere 4) 0.52 mm, head length 0.16 mm, head width 0.36 mm, pleon length 0.44 mm. All pereon segments distinct (fig. 22M, 23A). Allotype attached at pereon/pleon boundary of holotype. Body elongate, sides nearly parallel except for rounded head and posterior abdomen (fig. 22M). Head subovate, wider than long, fused with pereomere 1. Small dark eyes near posterolateral corners (fig. 22M). Antennule of three articles, terminally setose (fig. 23C). Antenna of five articles, much longer than antennule, terminal two articles setose (fig. 22B). Pereomeres subequal in width, with truncate and setose margins (fig. 23D). All pereopods with similar structure and proportions, carpi and meri fused (fig. 23D, E). Flap-shaped scales with serrated edges on surface of ischia, meri, and carpi (fig. 23D, F). Propodi with many flap-shaped scales embedded in dactyli (fig. 23E), scale surfaces not smooth but covered with microscales (fig. 23G). Pereon without midventral projections, but with many setae on ventral surface of pereon (fig. 23H). Pleon fused into single piece, without pleopods or uropods, terminally setose (fig. 23J).

ETYMOLOGY: The specific name, branchialis , refers to the type specimens found in the branchial chamber of the host.

HOST AND LOCALITY: Infesting Alpheus digitalis De Haan, 1844 (Alpheidae) , Hainan Province, China.

REMARKS: The present specimens parasitize a caridean shrimp host, the female has a swollen brood pouch formed by the oostegites from one side of the body, asymmetrical oostegite 1, a highly distorted body, and the male has a fused pleon; therefore this species belongs to Hemiarthrinae . However, no genera in this subfamily described to date are known to infest the branchial chamber of their host shrimps. Most hemiarthrine species are ventral abdominal parasites, but a few are found parasitizing other parts of the shrimp hosts, such as the dorsal pleon ( Filophryxus dorsalis Bruce, 1972b ), or even embedded in the mouthparts ( Orophryxus shiinoi, Bruce, 1972a ). The closest position of a hemiarthrine species on a host to that found with Sigyn branchialis , n. gen., n. sp., is seen with Metaphrixus intutus Bruce, 1966 , which is positioned over the host branchial chamber and is attached underneath a raised flange derived from the first abdominal pleuron of the host. The branchial mode of parasitism is considered to be the ancestral condition for bopyrids (see Boyko et al., 2013) and Sigyn branchialis , n. gen., n. sp., with its branchial position and full complement of pereopods on both sides of the female body may represent the most primitive hemiarthrine known to date.

The present specimens differ sufficiently from other hemiarthrine genera and deserve generic status. The new genus is most closely related to Loki Markham, 1972 , Eophrixus Caroli, 1930 , Anisarthrus Giard, 1907 , and Cataphryxus Shiino, 1936 . Sigyn , n. gen., can be distinguished from those four genera by its mode of parasitism within the branchial chamber, female with uniramous pleopods and globular terminal pleomere, and male with fused pleon lacking any lateral notches or appendages. The differences are summarized in table 2. Additionally, the fourth pleopod of the female has a tubercle that may indicate an endopodite (fig. 22L).

There are two saclike organisms parasitizing the abdomen of the host shrimp (fig. 21C). The more anterior specimen is a shriv‐ eled sac with a distinct mantle opening, indicating that it is a rhizocephalan. The posterior specimen also appears to be a rhizocephalan, is ovate and filled with eggs, but there is no mantle pore. It is unclear whether

TABLE 2 Comparison of Sigyn n. gen. with Four Other Closely Related Hemiarthrine Genera

these represent two species or different stages of development of female externae of the same species. The only rhizocephalans known from alpheids are members of Thylacoplethus Coutière, 1902 (Cirripedia: Rhizocephala: Akentrogonida ), but neither of these belongs to that genus.

Figures

Fig. 21. Hemiarthrine bopyrids and hosts. A. Host of Eophrixus caudatus, n. sp. B. Female, male and eggs of E. caudatus. C. Host of Sigyn branchialis, n. gen., n. sp. (right branchial chamber bulge indicating bopyrid, arrows indicating rhizocephalans). D. Dorsal view of female of S. branchialis, n. gen., n. sp. with attached male.

Fig. 22. Sigyn branchialis n. gen., n. sp., holotype female (CIEAL920301a) (A–L): A. Dorsal view. B. Ventral view. C. Right antenna. D. Right maxilliped. E. Barbula and maxilliped. F. Left oostegite 1, external view. G. Left oostegite 1, internal view. H. Right oostegite 1, external view. I. Right oostegite 1, internal view. J. Left pereopod 4. K. Left pereopod 7. L. Ventral view of the pleon. Allotype male (CIEAL920301b) (M): M. Dorsal view. Scale: 1 mm (A, B); 0.15 mm (C, D); 0.35 mm (E); 0.25 mm (F, G, K); 0.20 mm (H–J); 0.4 mm (L); 0.3 mm (M).

Fig. 23. Sigyn branchialis, n. gen., n. sp., SEM image of male (CIEAL920301b) (A–J): A. Ventral view. B. Left antenna. C. Right antennule. D. First right three pereopods. E. Right pereopods 6, 7. F. Carpi and meri of left pereopod 2. G. Propodus of left pereopod 2. H. Ventral surface of pereopod 3. I. Terminal edge of pleon. Scale: 100 µm (A); 10 µm (B–F, H, I); 1 µm (G).