Plankton Benthos Res 15(4): 334–336, 2020 Plankton & Benthos Research © The Japanese Association of Benthology Note A new record of brachyurus Müller, 1776 (Laevicaudata: ) from the Shiretoko Peninsula, Northeast Japan

Norihito Takahashi*

Kukuma System Design, 524–15 Minehama, Rausu, Hokkaido 086–1751, Japan Received 24 September 2019; Accepted 5 July 2020 Responsible Editor: Kensuke Yanagi doi: 10.3800/pbr.15.334

Abstract: A new Japanese record of the Holarctic Lynceus brachyurus is presented with key morphological characteris- tics and habitat information. The rostrum, clasper, telson, and lamina abdominalis of this insular record are similar to those of the com- mon form of L. brachyurus in continental populations, and our specimens fall within the previously reported species variability.

Key words: Hokkaido, Holarctic, Smooth clam shrimp, Species variability, Temporary pool

Laevicaudatan clam shrimps are small freshwater crusta- pan, 44.121056°N, 145.084719°E). Research in the SFLF was ceans that inhabit inland astatic pools (Martin & Belk 1988, conducted under Permit No. 1604271 of the Kushiro Nature Rogers & Olesen 2016). These shrimps have a smooth, bi- Conservation Office, Ministry of the Environment. Collect- valved carapace, a large shield-like head with a rostrum, and ed shrimps were preserved in 70–80% ethyl alcohol. Speci- a tiny telson, as denoted in the name of the first described mens were dissected to examine morphological traits using species Lynceus brachyurus Müller, 1776. This species is stereo and optical microscopes (RAYMER SW-302 L and widespread in the Holarctic region and has previously been BX-3500TL). Illustrations of the specimens were drawn dur- reported in several countries, including Austria, Belgium, ing microscopy observations (Fig. 1). Examined specimens Canada, the Czech Republic, Denmark, Estonia, Finland, were deposited at the Hokkaido University Museum (ICHUM France, Germany, Kazakhstan, Hungary, Norway, Pakistan 5974–5975). (high altitude), Poland, Romania, Russia, Slovakia, Swe- Clam shrimps from the two new localities fall within the den, Turkey (high altitude), Ukraine, and the USA (Lund- species variability of L. brachyurus. The male rostrum (Figs. blad 1920, Daday 1927, Kaisila et al. 1963, Straskraba 1965, 1A–B) adopts the commonly reported “less inferomedial Martin & Belk 1988, Rabet et al. 2005, Ustaoğlu et al. 2005, projection” form (Martin & Belk 1988). The female rostrum Sidorovsky et al. 2014, Rogers & Olesen 2016). In the Japa- (Figs. 1C–D) displays an acute inferomedial projection. The nese archipelago, only Lynceus biformis (Ishikawa, 1895) has male clasper (Figs. 1H–I) comprises a typical arcuate endite previously been recorded and is found in rice paddy fields on 3 with a slender endopod and small, oblong endites. The tel- the main island Honshu (Ferrari & Grygier 2011). To date, son and female lamina abdominalis (Figs. 1E–G) bear similar there is no record of these shrimps from Hokkaido, which is characteristics to those reported in earlier studies (Sars 1896, the second largest island in the archipelago, located north of Daday 1927, Mattox 1939, Botnariuc 1947). Honshu. Both new localities are completely surrounded by dense The author collected 10 male (1.4–1.7 mm length) and forest. The PN snow-melt pool (Fig. 2A) appears annually 24 female (1.3–1.9 mm length) smooth clam shrimps on during spring in a mortar-shaped depression of the Horobetsu June 10, 2017, at Ponhoro-numa (PN; Shari, Hokkaido, Ja- riverside broad-leaved forest. The maximum diameter of the pan, 44.078156°N, 145.034221°E). Two female clam shrimps pool in early April is approximately 50 m. However, the pool (3.8–3.9 mm length) were collected on June 26, 2016, at disappears annually in late July, after which the fern spe- Shiretoko Five Lakes Forest (SFLF; Shari, Hokkaido, Ja- cies Thelypteris palustris grows abundantly in the area. In this locality, great numbers of small L. brachyurus (Fig. 2B) * Corresponding author: Norihito Takahashi; E-mail, amb13space@ appeared every year between 2015 and 2019, along with the gmail.com tadpoles of Rana pirica. In the SFLF, a small oval pool forms Holarctic clam shrimp from Shiretoko 335

Fig. 1. Lynceus brachyurus Müller, 1776. A) male head, left side view; B) male head, anterior view; C) female head, anterior view; D) female head, left side view; E) female telson and lamina abdominalis, dorsal view; F) male telson, left side view; G) male telson, ventral view; H) male left 1st thoracopod, superior view; I) male left 1st thoracopod, inferior view. All scale bars=0.25 mm.

(Fig. 2C), with an approximate length of 5 m. The presence of Japanese laevicaudatan species are separated by the Tsugaru L. brachyurus (Fig. 2D) was confirmed in this forest in 2016 Strait. However, if L. biformis really has Russian localities only. Numerous mosquito (Aedes spp.) larvae and cladocerans as reported in Yoon & Kim (2000), Hokkaido (including the (Daphnia spp.) also occur in this pool. study area) may also support this species. To date, there are This is the first record of a laevicaudatan clam shrimp few records of large branchiopods from Hokkaido (Kikuchi from Hokkaido and of Lynceus brachyurus (Japanese name: 1957, Takahashi et al. 2018 and this study), and further field Kita Tama-Kaiebi) in Japan. This finding reveals that two research and material based examinations are needed. 336 N.Takahashi

Fig. 2. Collection sites and shrimps. A) Ponhoro-numa (PN) pool; B) live Lynceus brachyurus; C) Shiretoko Five Lakes Forest (SFLF) pool; D) L. brachyurus specimen with right carapace removed.

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