Rhizophoracepon magnagibbus, Williams & Boyko & Tri, 2023

Williams, Jason D., Boyko, Christopher B. & Tri, Ngo Van, 2023, Description of a new genus and species of bopyrid (Isopoda: Epicaridea: Bopyridae) from the pinnotherid crab, Plenotheres coarctatus (Bürger, 1895), associated with mangrove clams from Vietnam, Raffles Bulletin of Zoology 71, pp. 531-541 : 535-538

Rhizophoracepon magnagibbus , new species

( Figs. 1 –4)

Type material examined. Holotype: mature female (5.9 mm TL; ZRC 2023.0051), from left branchial chamber of female Plenotheres coarctatus (Bürger, 1895) (7.7 mm CL, 10.4 mm CW; ZRC 2022.0054), from mangrove clam, Geloina (= Polymesoda ) coaxans (Gmelin, 1791) ( Cyrenidae ), Dong Rui Mangrove forest, Tien Yen district, Quang Ninh Province, northeastern Vietnam, from Ba Che Market, Ba Che Town, coll. V. T. Ngo, 9–13 March 2022. Allotype: mature male (3.4 mm TL; ZRC 2023.0052), same locality data and host as holotype. Paratypes: mature female (5.7 mm TL), mature male (2.5 mm TL) ( ZRC 2023.0053), from left branchial chamber of female P. coarctatus (7.8 mm CL, 9.7 mm CW; ZRC 2022.0054), same locality data as holotype; mature female (7.1 mm TL), mature male (3.8 mm TL) ( ZRC 2023.0054), from right branchial chamber of female P. coarctatus (7.1 mm CL, 11.3 mm CW; ZRC 2022.0054), same locality data as holotype.

Additional material examined. Female P. coarctatus (8.9 mm CL, 11.5 mm CW) ( ZRC 2022.0055 ) with right branchial chamber swollen but empty, same locality data as holotype .

Etymology. The species name is a combination of magna - (large) and - gibbus (hump), in reference to the extremely large bulge that the bopyrid makes in the carapace of the host. Used as an adjective.

Description. Female holotype (ZRC 2023.0051) length 6.2 mm; maximum width (across pereomere 5; not including extended oostegites) 4.0 mm; head length 0.9 mm; head width 1.8 mm; pleon length 1.8 mm. All segments of body distinct ( Fig. 2H ).

Head subovate, frontal lamina thin and extending to lateral margins of head. Eyes absent. Antennules and antennae with two and five articles each, respectively, terminally setose ( Fig. 3A ). Barbula with two slender, subequal, falcate pointed lateral projections on each side, middle region smooth ( Fig. 3B ). Maxilliped with prominent anterior segment, large thick palp present, plectron triangular, short and acute ( Fig. 3G ).

Pereon broadest across pereomeres 3 and 4. Coxal plates on pereomeres 1–7 rounded with crenulate surface and small thin projections ( Figs. 2D, H , 3H ), dorsolateral bosses and tergal projections absent. Mid-dorsal projections on pereomeres absent ( Fig. 2I ). Oostegites enclosing brood pouch ( Fig. 2E ), oostegite 1 ( Fig. 3E, F ) with subcircular anterior article, subequal in size to posterior article, internal ridge digitate, posterior article rounded with distolateral triangular projection. Pereopods subequal in structure, posterior pairs slightly longer than anterior ( Fig. 3C, D ). All pereopods with elongate meri and ischia, and blunt dactyli.

Pleon with six segments, first five pleomeres each bearing a pair of biramous digitate pleopods and uniramous digitate lateral plates, surfaces crenulate ( Fig. 3I–L ). Endopodites, exopodites, and lateral plates subequal in size. Terminal pleomere ending in uniramous uropods, similar to but broader than fifth pleopods, surface smooth, margins slightly crenulate ( Fig. 3K, L ).

Male allotype ( ZRC 2023.0052 ) length 3.4 mm; maximum width (across pereomere 3) 1.2 mm; head length 0.3 mm; head width 0.6 mm. All body regions and segments distinct ( Figs. 2F , 4A) .

Head ovate, distinctly separated from first pereomere ( Figs. 2F , 4A). Minute eyes near posterolateral corners (Fig. 4A). Antennules of three articles each; antennae visible beyond margins of head in dorsal view, of six articles each; both bearing setae on two distalmost articles (Fig. 4C).

Pereomeres 2 and 3 subequal in width, patches of pigmentation on pereomeres 2–7 (Fig. 4A). No pereomeres with mid-ventral tubercles. Pereopods subequal in size and structure except anterior three pairs with longer dactyli (Fig. 4B, D, E).

Pleon with six pleomeres, 1 and 2 subequal in width, others each narrower than preceding one ( Figs. 2G , 4B). First five pleomeres with low, rounded pleopods bearing nipple-like extension, mid-ventral tubercles present on pleomeres 1–4 (Fig. 4B). Pleomere 6 without uropods, posterolaterally extended into two flat, broad, smooth rami separated by large triangular median indentation ( Figs. 2F, G , 4A, B).

Remarks. A total of nine specimens (smallest 9.0 mm CL, 9.4 mm CW; largest 11.8 mm CL, 13.2 mm CW) of Plenotheres coarctatus (Bürger, 1895) were collected at the type locality between 9–13 March 2022, four of which bore a male and female pair of bopyrid isopods in the branchial chamber or showed evidence of prior infestation. The parasitized side of the carapace is greatly deformed ( Figs. 1 , 2A–C ). For example, in one host specimen (ZRC 2022.0054) the width of the carapace is ~7% wider (7.1 mm versus about 7.6 mm) when the extension caused by the parasitic isopod is added and thickness is ~58% greater (5.7 mm versus 9.0 mm).

Ecology. The Dong Rui Mangrove forest is a brackish water environment (ca. 24.5–29.1‰; Nguyen et al., 2020). From a collection in this region (March 2013, Tien Yen District, Quang Ninh province, Vietnam), the pea crab P. coarctatus was found in approximately 3% of larger mangrove clams (20 pea crabs found in 25 kg of dissected clams of sizes 22–25 individuals/kg) and smaller clams (five pea crabs found in 5 kg of dissected clams of sizes 30–35 individuals/kg). Overall prevalence of hyperparasitic isopods was 20% (5 of the total 25 pea crabs infested) in this sample. However, none of the pea crabs from the larger clams were infested with hyperparasitic isopods, whereas all five pea crabs from the smaller clams were infested with hyperparasitic isopods (significantly different from a predicted equal distribution: p <0.001 Fisher Exact Probability Test). The larger and smaller clams were from the same clutch (i.e., same age). We cannot definitely state the pea crabs from the 2013 collection were infested with Rhizophoracepon magnagibbus , new genus, new species, as other yet undiscovered bopyrids may infest this host, but based on the locality and host, this is the most likely hyperparasitic isopod present.

Figures

Fig. 1. Live images of the host crab Plenotheres coarctatus (Bürger, 1895) and the parasitic isopod Rhizophoracepon magnagibbus, new genus, new species. A, Plenotheres coarctatus, oblique lateral view showing swelling of left branchial chamber caused by R. magnagibbus (male parasite can be seen through swelling by the paired, opaque white dots to side of dark mid-line); B, Plenotheres coarctatus, dorsal view showing swelling of left branchial chamber caused by R. magnagibbus.

Fig. 2. Light microscope images of the host crab Plenotheres coarctatus (Bürger, 1895) (ZRC 2022.0054) and the parasitic isopod Rhizophoracepon magnagibbus, new genus, new species. A, P. coarctatus, dorsal view showing swelling of left branchial chamber caused by R. magnagibbus (ZRC 2023.0053); B, P. coarctatus (Bürger, 1895), dorsal view showing swelling of left branchial caused by R. magnagibbus (ZRC 2023.0054); C, P. coarctatus, posterior view showing vaulted left branchial caused by R. magnagibbus (ZRC 2023.0054); D, paratype female, dorsal view (ZRC 2023.0053) from host shown in A; E, paratype female, ventral view from host shown in A; F, allotype male, dorsal view (ZRC 2023.0052); G, allotype male, ventral view (ZRC 2023.0052); H, holotype female, dorsal view (ZRC 2023.0051); I, paratype female and male pair, lateral view (ZRC 2023.0054), from host shown in B, C. Scale bars = 2.5 mm (A–C), 1.25 mm (D, E, H, I), 1 mm (F, G).

Fig. 3. Rhizophoracepon magnagibbus, new genus, new species, paratype females (ZRC 2023.0054). A, left antennule (A1) and antenna (A2); B, left barbula; C, left pereopod 1; D, left pereopod 7; E, left oostegite 1, outer view (dashed lines show close up of setae on posterior lobe); F, left oostegite 1, inner view (dashed lines show close up of setae on anterior lobe); G, left maxilliped, outer view; H, coxal plates and tergal projections on pereomeres 5–7; I, pleopod and lateral plate of pleomere 1, left side, dorsal view; J, pleopod and lateral plate of pleomere 5, left side, dorsal view (arrow shows where lateral plate was attached); K, uropods plus pleopod and lateral plate of pleomere 5 from left side, dorsal view; L, uropods plus pleopod and lateral plate of pleomere 5 from left side, ventral view. (cp = coxal plates, en = endopod, ex = exopod, lp = lateral plate, tp = tergal projections, Uro = uropods). Scale bars = 100 mm (A, C, D), 500 µm (B, E–L), 25 µm (insets of E, F).