Caprella nojimaensis, Takeuchi & Shiraishi & Mimori, 2021

Takeuchi, Ichiro, Shiraishi, Momoko & Mimori, Ryosuke, 2021, A New Species of the Genus Caprella (Crustacea: Amphipoda: Caprellidae) Collected from a Gorgonian at 1497 m Depth off Boso Peninsula, Central Japan, Species Diversity 26, pp. 225-233 : 226-231

Family Caprellidae Leach, 1814 Genus Caprella Lamarck, 1801 Caprella nojimaensis sp.nov. ( Figs 2–4 )

Type localities. 34°46.710′N 139°50.311′E, 1497 m depth. Material studied. Holotype, 1 mature female, NSMT-

Cr 29015, 34°46.710′N, 139°50.311′E, 1497 m depth, 23

August 2012, R / V Natsushima , NT-12-22, HPD #1426, St. 3,

the gorgonian Calcigorgia gracilis , coll. R. Mimori.

Description. Holotype, mature female, body length, 20.69 mm, NSMT-Cr 29015. Head, 1.21 mm; pereonite 1, 1.65 mm; pereonite 2, 4.29 mm; pereonite 3, 4.19 mm; pereonite 4, 2.87 mm; pereonite 5, 4.09 mm; pereonite 6, 1.22 mm; pereonite 7, 1.18 mm. Pereonite 2 longest followed by pereonite 3 and pereonite 5.

Head and pereonites slender; eye distinctive. Pereonite 2 with paired mid-dorsal projections. Pereonite 4 with round anterolateral projection. Pereonite 5 with small anterolateral projection, paired mid-dorsal projections.

Antenna 1 slender, 0.95× body length; peduncle article 2 longest; peduncle article 3 straight, 0.80× peduncle article 2 length; flagellum 1.20× peduncular length, with 36 articles, proximal article composed of 5 articles. Antenna 2 slender; 0.45× antenna 1 length; peduncle article 2 to flagellum article 1 with dense plumous setae (swimming setae).

Mouthparts. Upper lip width 1.50× depth. Lower lip, inner lobe, setose. Mandible right incisor with 5 teeth, lacinia mobilis with 2 teeth followed by 2 bundle setose; molar distinct with molar flake. Mandible left incisor with 5 teeth, lacinia mobilis with 5 teeth followed by 3 bundle setose; molar large and distinct. Maxilla 1 outer plate with 7 stout apical setal-teeth; palp 2-articulate; article 2 5.0× article 1 with 11 apical to lateral robust or slender setae and single line of lateral setae. Maxilla 2 inner plate, oval, with approximately 30 setae; outer plate with approximately 30 apical setae. Maxilliped inner plate (basal endite) with setae on entire distal margin; outer plate (ischial endite) 2.0× inner plate (basal endite) with 9 stout setae, with approximately 10 setae on inner margin; palp 4-articulate, articles 2 and 3 longest, setose; dactylus falcate.

Gnathopod 1 robust, setose. Gnathopod 2 positions 0.20 from along anterior margin of corresponding pereonite; coxa vestigial; basis 0.60× length of pereonite 2, basis projection near distal margin; propodus ovate, large, length 2.0× width; palm proximal projection with 1 robust (grasping) seta, palm margin convex, smooth, with midpalmar projection, with narrow sinus, with broad distal shelf. Gill 3 elongate, 0.70× corresponding pereonite. Gill 4 elongate, 0.80× corresponding pereonite.

Pereopod 5 slender; basis shorter than propodus; propodus with 1 pair of robust setae from approximately 0.4 palm length from articulation with carpus, with approximately 10 setae along palm; dactylus curved. Pereopods 6–7 progressively longer than pereopod 5.

Behaviour and body colour. The host gorgonian Calcigorgia gracilis was scarcely attached to the rock face covered with sediments around the canyon. The seawater temperature and salinity at the collection site were 2.23°C and 34.54 psu, respectively.

The video taken by Hyper-Dolphin shows that the collected specimen of Caprella nojimaensis sp. nov. clung to the host using pereopods 5 to 7 in “upright” behaviour (Takeuchi and Hirano 1995) ( Fig. 5 ). The body colour of C. nojimaensis sp. nov. in situ was pale pink, the same as that of the host gorgonian, indicating the presence of protective colouration matching that of the host ( Fig. 5 ). The photo taken in the cold laboratory after the collection revealed that most body segments were pale pink, and mouthparts and gnathopod 1 were dark pink ( Fig. 6 ). Gnathopod 2 was pinkish in the distal part of basis and the grasping part of propodus and dactylus.

Careful observations of the video recording made at the collection site confirmed the presence of at least four large individuals of the same species of Caprella (three individuals on the gorgonian and one on the rock) in addition to the collected individual ( Fig. 7 ). This observation suggested that the present species widely dwells on the gorgonian and the rock surface of the canyon.

Etymology. The specific name nojimaensis is derived from the name of the collection site of the new species, Nojima Submarine Canyon.

Remarks. The genus Caprella is one of the most species diverse genera in the Amphipoda ( Crustacea: Malacostraca), containing more than 180 species (see Arfianti, et al. 2018; Horton et al. 2020). Caprella spp. are euryhaline marine invertebrates with 1 to 3 cm long body, mostly found in algal communities, on buoys in port areas, and in aquaculture facilities in the temperate region (see Takeuchi et al. 2001, 2003). Most species from this genus are known from the shallow waters (see Arimoto 1976; Takeuchi 1999; Guerra-García et al. 2013), and five species have been recorded until now from deep waters, at depths greater than 300 m, off central Japan and adjacent areas in the north-western Pacific ( Arimoto 1934, 1976; Takeuchi et al. 1989, 2019; Guerra-García and García-Gomez 2003; Lindsay and Takeuchi 2008).

Caprella nojimaensis sp. nov. is most closely related to C. gracillima Mayer, 1890 , a species reported more than 100 years ago from 96.9–182.9 m depth in mesophotic zone of the Tsugaru Strait, Korean Strait, and off Tokyo ( Mayer 1890, 1903; McCain and Steinberg 1970). Mayer (1890) reported three mature females of C. gracillima collected in 1882 from the Tsugaru Strait and in the northern coast of Korea facing the Sea of Japan (42°N, 130°30′E). The collection depth of those specimens was recorded as 109.7 to 182.9 m. Mayer (1890) provided a brief description of a small male, but made no notes about the collection record for this male individual. His description and the whole lateral view of antennae and body somites were based on a mature female with a body length of 20 mm. In addition, Mayer (1903) reported a small female of C. gracillima , which was collected with Caprella scaura Templeton, 1836 and Paraprotella secunda Mayer, 1903 at a depth of 96.9 m off Tokyo in 1902. Based on the description and figure of C. gracillima in Mayer (1890), C. nojimaensis sp. nov. differs from C. gracillima in: 1) antenna 2 with dense swimming setae vs. swimming setae sparse in C. gracillima ; 2) gnathopod 2 positions 1/5 from anterior margin of pereonite 2 vs. 1/ 2 in C. gracillima ; 3) pereonite 2 with paired small mid-dorsal projections vs. pereonite 2 smooth in C. gracillima ; and 4) pereonite 5 with small anterolateral projection and paired small mid-dorsal projections vs. pereonite smooth in C. gracillima .

The new species is unique in that it inhabits gorgonians at around 1500m depth, but its behaviour and morphology are similar to that of Caprella spp. inhabiting shallow waters. The presence of dense plumose swimming setae on antenna 2 suggest that C. nojimaensis sp. nov. filters the suspended matter flowing along the rocky substrate while assuming upright position. This behaviour is common in caprellids inhabiting shallow waters (see Caine 1977; Takeuchi and Hirano 1995).

The present study confirmed that distribution of the genus Caprella extends down the continental slope to a depth of around 1500 m in the Japan Trench near central Japan; this situation is similar to the continental slopes of the similar depth in Chishima/Kuril–Kamchatka Trench located north of the Japan Trench in the north-west Pacific (Vassilenko 1993). Caprella ungulina Mayer, 1903 was reported to attach to mouthparts of the lithodid crab Lithodes aequispina Benedict, 1894 and is distributed in Sagami Bay, central Japan, and the Sea of Okhotsk at a depth of 400– 410 m (Takeuchi et al. 1989). Guerra-García and García-Gómez (2003) described Caprella sabineae Guerra-García and García-Gomez, 2003 based on specimens collected from Sagami Bay, Japan, at 600 m depth during the Dr. Sixten Bock Japan Expedition in 1914. Onboard laboratory observations of Caprella subtilis Mayer, 1903 collected from 309 m depth in Sagami Bay, Japan, revealed that it attaches to swimming benthopelagic holothurians using pereopods 5–7 (or 6–7) in the “upright” position ( Lindsay and Takeuchi 2008). Thus, the species from the genus Caprella occupy a wide variety of habitats in the bathyal zone of the Japan Trench. Recently, Abyssododecas styx Takeuchi, Tomikawa, and Lindsay, 2016 (Phtisicidae) was reported from cold-seep sites at abyssal depths (5313 to 7322 m depth) in the Japan Trench (Takeuchi et al. 2016), which is located approximately 600–700 km north-east from the collection site of the new species.

Figures

Fig. 2. Caprella nojimaensis sp. nov., holotype, mature female, 20.69mm body length, NSMT-Cr 29015, collected from 1497m depth of Nojima Submarine Canyon, off Boso Peninsula, central Japan.

Fig. 3. Caprella nojimaensis sp. nov., holotype, mature female, 20.69mm body length, NSMT-Cr 29015, collected from 1497m depth of Nojima Submarine Canyon, off Boso Peninsula, central Japan. A, Antenna; G, gnathopod; P, pereopod. Scale bar: 1.0mm.

Fig. 4. Caprella nojimaensis sp. nov., holotype, mature female, 20.69mm body length, NSMT-Cr 29015, collected from 1497m depth of Nojima Submarine Canyon, off Boso Peninsula, central Japan. LL, Lower lip; MD, mandible; MX, maxilla; MXD, maxilliped; UL, upper lip; L, left; R, right. Scale bars: 0.10mm.

Fig. 5. Caprella nojimaensis sp. nov. (yellow arrow), holotype, mature female, 20.69mm body length, NSMT-Cr 29015, attached to the gorgonian Calcigorgia gracilis Matsumoto, van Ofwegen, and Bayer, 2019 at 1497m depth; Nojima Submarine Canyon, off Boso Peninsula, central Japan.

Fig. 6. Caprella nojimaensis sp. nov., holotype, mature female, 20.69mm body length, NSMT-Cr 29015, photographed at the cold laboratory of Tokyo Sea Life Park (Edogawa-ku, Tokyo, Japan).

Fig. 7. Four individuals of Caprella nojimaensis sp. nov. (red arrows) attached to the gorgonian and on rock and surrounding sediment at around 1500m depth in the Nojima Submarine Canyon, off Boso Peninsula, central Japan.