Podocerus jinbe Tomikawa, Yanagisawa, and Vader, 2019

Tomikawa, Ko, Yanagisawa, Makio, Higashiji, Takuo, Yano, Nagisa & Vader, Wim, 2019, A New Species of Podocerus (Crustacea: Amphipoda: Podoceridae) Associated with the Whale Shark Rhincodon typus, Species Diversity 24, pp. 209-216 : 210-215

Genus Podocerus Leach, 1814 Podocerus jinbe Tomikawa, Yanagisawa, and Vader , sp. nov. [New Japanese name: Jinbē-doronomi] ( Figs 1 –5)

Material examined. Holotype: male 6.0 mm (NSMT- Cr 26048). Paratypes: male 5.1 mm (NSMT-Cr 26049), male 6.0 mm (NSMT-Cr 26050), female 3.7 mm (NSMT-Cr 26051). Further materials: 357 males and 291 females (KT private collection).

Description. Male (holotype, NSMT-Cr 26048). Body

( Fig. 1A, B ; paratype males NSMT-Cr 26049–26050) weak- rounded.

ly rugose, without dorsal carina; pereonite 7 and pleonite 1 Antenna 1 ( Fig. 2A ) length 0.4 times as long as body with dorsal setae. Head ( Fig. 1C ; paratype male NSMT-Cr length; length ratio of peduncular articles 1–3 1.0: 1.8: 1.4; 26049) dorsally smooth; rostrum short; lateral cephalic lobe peduncular article 1 subquadrate, with long plumose setae Fig. 5. Podocerus jinbe sp. nov., paratype female 3.7 mm, NSMT-Cr 26051. A, basis to dactylus of gnathopod 1 (some setae omitted), medial view; B, basis to dactylus of gnathopod 2 (some setae omitted), medial view. on posterodistal corner; peduncular articles 2 and 3 with 11 slender setae; posterior margin of dactylus 2-dentate with and 9 clusters of long plumose setae on posterior margins, short setae. Gnathopod 2 ( Fig. 3B, C ) basis 2.3 times as long respectively; primary flagellum 4-articulate, 0.3 times as as broad, concave anteriorly, lacking anterodistal setae; poslong as peduncular articles 1–3 combined, article 1 long, 3.1 terodistal corner of merus produced with simple and plutimes as long as article 2; accessory flagellum slender, 1-ar- mose setae; carpus indistinct, fused with propodus, with ticulate, 0.3 times as long as primary flagellum. Antenna 2 simple and plumose setae; propodus subovate, 1.7 times as ( Fig. 2B ) 1.5 times as long as antenna 1; peduncular article long as wide, anterior margin with 3 clusters and 1 pair of 4 with long plumose setae on posterior margin; peduncular short setae and seta, medial surface with numerous plumose article 5, 1.3 times as long as article 4, with short plumose setae, palm straight, about 60% of length of propodus, with and simple setae on posterior margin; flagellum 3-articulate, well-developed distal shelf, concave medially, shelf margin 0.3 times as long as peduncular articles 1–5 combined, ar- strongly crenulate, palm with subtriangular tooth and deticle 1, 2.3 times as long as articles 2 and 3 combined. fining tooth, lacking robust setae near palm defining tooth;

Upper lip ( Fig. 2C ) oval, ventral margin weakly concave, dactylus not reaching end of palm, with short seta on anwith minute setae. Left and right mandibles ( Fig. 2D, E ) teroproximal margin. with 5-dentate incisor; molar process small, non-triturative, Pereopods 3 and 4 ( Fig. 3D–F ) basis bearing anterodistal with a short plumose seta apically; accessory setal row with lobe; ischium subrectangular; merus slightly shorter than 3 setae; palp 3-articulate, length ratio of articles 1–3 1.0: 2.3: carpus, produced anterodistally; propodus longer than car- 1.5, article 1 bare, article 2 with 20 setae on ventral margin pus, with robust setae on posterior margin. Pereopod 5 (Fig. and submargin, ventral margin of article 3 lined with plu- 4A) basis with posterodistal lobe; merus with posterodistal mose setae, inner and outer surfaces of article 3 each with projection; length ratio of merus–dactylus 1.0: 1.1: 1.3: 0.6. 1 cluster of setae. Lower lip ( Fig. 2F ) outer lobe broad, setu- Pereopod 6 ( Fig. 4B ) basis subrectangular, length 1.4 times lose; inner lobes distinct. Maxilla 1 ( Fig. 2G ) inner plate in- as long as wide; posterodistal corner of merus produced; distinct; outer plate rectangular with 9 serrate robust setae; length ratio of merus–dactylus 1.0: 1.0: 1.3: 0.6. Pereopod palp article 2 bearing 4 robust and 9 slender setae distally. 7 ( Fig. 4C, D ) basis subquadrate, length 1.3 times width; Maxilla 2 ( Fig. 2H ) with broad inner and outer plates; outer merus produced posterodistally; length ratio of merus–dacplate longer than inner plate, bearing long plumose setae tylus 1.0: 0.9: 1.1: 0.5. on apical margin. Maxilliped ( Fig. 2I ) inner plate subrect- Coxal gill on gnathopod 2, broad distally, gills on pereoangular, apical margin with 2 small robust setae; outer plate pods 3–6 slender. slightly exceeding half of palp article 2, medial margin with Pleopods 1–3 ( Fig. 4E, F ) peduncle without setae, inner robust setae and long plumose setae; palp 4-articulate. distal corner with paired retinacula.

Gnathopod 1 ( Figs 1A , 3A ) coxa slender, subtriangular, Uropod 1 ( Fig. 4G ) biramous; peduncle 2.8 times as long longer than broad, anteroventral corner weakly produced, as broad, medial and lateral margins with 4 and 5 robust apically rounded; basis twice as long as broad, lacking an- setae, respectively, lacking ventromedial robust seta; inner terodistal setae; carpus 1.3 times as long as broad, ventral ramus 1.1 times as long as peduncle, with 9 medial and 3 margin weakly lobate; propodus subovate, length 1.4 times lateral robust setae; outer ramus 0.8 times of length of inner as long as wide, anterior margin with 6 clusters of slender ramus, with 2 robust setae on lateral margin. Uropod 2 (Fig. setae, posterior margin convex with 12 robust setae and 4H) biramous; peduncle 1.5 times as long as broad, with 1 medial and 2 lateral robust setae, ventromedial robust seta absent; inner ramus 1.7 times as long as peduncle, medial and lateral margins with 7 and 4 robust setae, respectively; outer ramus 0.8 times of length of inner ramus, bearing 3 lateral robust setae. Uropod 3 ( Fig. 4I ) uniramous, platelike; with 3 apical and 1 medial robust setae.

Telson ( Fig. 4J ) length 0.9 times of width, dorsal lobe with 3 long robust setae apically, lower margin with short lateral setae.

Female (paratype, NSMT-Cr 26051). Gnathopod 1 (Fig. 5A) basis slightly concave anteriorly, lacking anterodistal setae; propodus length 1.6 times as long as width, palmar margin of propodus bearing robust seta; posterior margin of dactylus 4-dentate. Gnathopod 2 (Fig. 5B) anterior margin of basis almost straight, lacking anterodistal setae; merus produced anterodistally, with plumose and simple setae; carpus free, distinct from propodus; propodus ovate, length 1.2 times width, palm convex, lacking distal shelf or teeth, with 4 robust setae near palmar corner.

Telson dorsal lobe with 4 long robust setae apically.

Etymology. The specific name is from the Japanese word for the whale shark Rhincodon typus ‘ Jinbē-zame ’; thus, it does not come from a Latin or Latinized word.

DNA Sequences. A sequence of COI ( LC466202 ; 658 bp) was determined from the paratype female (NSMT-Cr 26051) .

Distribution. Known only from Okinawa Island, Japan.

Remarks. Podocerus jinbe sp. nov. is morphologically similar to P. zeylanicus ( Walker, 1904) in having the following features: body without dorsal carinae; coxa of gnathopod 1 slender and subtriangular; carpus of male gnathopod 1 weakly lobate ventrally; propodus palmar margin of male gnathopod 1 with many robust setae; carpus of male gnathopod 2 indistinct, fused with propodus; propodus palm of male gnathopod 2 with distal shelf; basis of pereopod 3 lobate anterodistally; uropods 1 and 2 without ventromedial robust seta on each peduncle; and rami of uropods 1 and 2 with marginal robust setae, but lacking comb-like rows of robust setae. Podocerus jinbe sp. nov. differs from P.zeylanicus in the following features (features of P. zeylanicus in parentheses): relatively large body size, up to 6.0 mm in male (small, up to 4.0 mm in male); peduncular article 1 of antenna 1 0.6 (0.8) times as long as peduncular article 2; flagellar article 1 of antenna 1 3 (2) times as long as flagellar article 2; propodus of male gnathopod 1 subrectangular (ovate); basis of male gnathopod 2 concave anteriorly (almost straight); merus of female gnathopod 2 narrow, tapering distally (broad, rounded distally); inner ramus of uropods 1 and 2 with 9 (6) and 7 (5) robust setae on its medial margins, respectively; and telson apical lobe with 3 or 4 long robust setae (1 long and 2 short robust setae). Recently, it has been revealed that many species with wide distribution areas such as Nicippe tumida Bruzelius, 1859 contain morphologically similar cryptic species ( Matsukami et al. 2017). Podocerus zeylanicus has been recorded from various regions of the Indian Ocean and the South Pacific Ocean ( Walker 1904; Chevreux 1908; Ruffo 1969; Ledoyer 1986; Hughes 2013). However, there are slight morphological variations among populations, e.g., the shape of gnathopods and number of robust setae on rami of uropods ( Chevreux 1908; Ledoyer 1986; Hughes 2013). Adequate specimens from various regions will be needed to evaluate whether these variations are intraspecific or not.

This is the first record of an amphipod associated with the whale shark. In this study, more than 1000 individuals of P. zeylanicus were collected from the whale shark. Hughes (2016) reported rapid swarming behavior of P. chelonophilus , on the Gold Coast, Australia with many individuals observed crawling on a floating scuba diver at 3–5m depth. The numbers reached over 1000 within 5 minutes due to the recruitment of new individuals, a phenomenon noted to be a regular occurrence at the site ( Hughes 2016). It seems that the high density of attachment of P. jinbe to the whale shark found in the present study resulted from similar behavior. The host whale shark was weak, had no appetite, and ultimately died 3 months later after the first recognition of the presence of P.jinbe (MY personal observation). We have no information about the cause of the whale shark’s death. Species of Podocerus are filter-feeders gathering detritus in the sea with well-developed plumose setae on antennae ( Barnard et al. 1988), and the present species also bears such characteristic antennae. Thus, it is very unlikely that the individuals of P. jinbe found in this study were feeding directly on the whale shark. However, it is likely that respiratory problems were caused by recruiting large numbers of amphipods to the gill. The gill rakers of a whale shark may be suitable for inhabitation by P. jinbe to take advantage of feeding from the strong water currents caused by the feeding and breathing of the whale shark.

Figures

Fig. 1. Podocerus jinbe sp. nov., SEM photographs. A, paratype male 6.0mm, NSMT-Cr 26050; B and C, paratype male 5.1mm, NSMT-Cr 26049. A, habitus, lateral view; B, habitus, dorsal view; C, head, dorsal view.

Fig. 2. Podocerus jinbe sp. nov., holotype male 6.0mm, NSMT-Cr 26048. A, antenna 1 (some setae omitted), medial view; B, antenna 2 (some setae omitted), medial view; C, upper lip, posterior view; D, right mandible, lateral view (palp twisted); E, incisor of right mandible, lateral view; F, lower lip, anterior view; G, maxilla 1, anterior view; H, maxilla 2, anterior view; I, maxilliped (some setae omitted), anterior view.

Fig. 3. Podocerus jinbe sp. nov., holotype male 6.0mm, NSMT-Cr 26048. A, basis to dactylus of gnathopod 1 (some setae omitted; dactylus is strongly curved due to mounting angle), medial view; B, basis to dactylus of gnathopod 2 (some setae omitted), medial view; C, palmar margin of gnathopod 2 propodus, medial view; D, pereopod 3, medial view; E, basis and ischium of pereopod 3, medial view; F, pereopod 4.

Fig. 4. Podocerus jinbe sp. nov., holotype male 6.0mm, NSMT-Cr 26048. A, pereopod 5, medial view; B, pereopod 6, medial view; C, pereopod 7, medial view; D, basis and ischium of pereopod 7, medial view; E, pleopod 1, medial view; F, retinacula (coupling hooks) of peduncle of pleopod 1, medial view; G, uropod 1, dorsal view; H, uropod 2, dorsal view; I, uropod 3, dorsal view; J, telson, dorsal view.