Monthly quadrat sampling of freshwater shrimps infested by the corallanid isopod Tachaea chinensis was conducted in Lake Biwa, central Japan, between April 2014 and September 2015. In total, 3,222 shrimps, including 2,786 Neocaridina spp., 309 Paratya improvisa, and 127 Palaemon paucidens were collected. Individuals of 431 T. chinensis were found from those hosts. The prevalence was 12.2% in Neocaridina spp., 5.5% in P. improvisa, and 24.4% in P. paucidens, respectively. All isopods found on hosts were immature stages and showed an annual emergence pattern; newly hatched juveniles were first sampled in August, increased the body length until May, and were not observed in June-July. To collect T. chinensis apart from the hosts, a cage trap filled with reed roots was put on the study site between June 2014 and September 2015. Isopods were collected in July and September. Specimens trapped in 2015 consisted of adult or premature individuals, and 39 out of 46 adult females collected were ovigerous. During an observation using snorkeling in July 2014, 29 adults were observed under stones and in withered Phragmites australis roots. Thus, T. chinensis reproduces in benthic habitats during summer. Subsequently, juveniles appear and grow via a series of molts on the hosts until the following year.
Gnathia limicola sp. nov. is described from Okinawajima Island, Ryukyu Archipelago, southwestern Japan. Burrows of this species were found in a small intertidal creek bank on a muddy tidal flat near mangrove trees. Adult males differ from those of other Gnathia species in the following features: (1) fine setae cover peduncle articles 1 and 2 of antenna 1, peduncle articles 1-3 of antenna 2, and the erisma of the mandibles; (2) the ventral frontal border of the cephalon is medianly notched, and the lateral parts extend beyond the dorsal frontal border; and (3) the penes are fused into a thin rectangular blade directed posteriorly. Adult females and praniza larvae were also distinguished morphologically from other Gnathia species. Based on field and laboratory observations, the mating behavior of this species appears similar to that of Paragnathia formica (Hesse, 1862), which inhabits salt marshes in Europe and North Africa.
Gnathia maculosa sp. nov. is described from males reared in a laboratory from larvae that were collected as ectoparasites on elasmobranchs, caught off Okinawa Island in the Ryukyu Archipelago, southwestern Japan. The species is most similar to G. trimaculata but it is distinguished from G. trimaculata by deeper and narrower dorsal sulcus, a narrower body, and the wider pylopod. Gnathia trimaculata, previously recorded from Great Barrier Reef, Australia, was also collected from elasmobranchs caught off Okinawa Island. The record of G. trimaculata from Okinawa indicates a wide range of the distribution of the gnathiids inhabiting elasmobranchs.
Gnathiid larvae were collected from the gill chambers of coastal sharks in southwestern Japan. Some were reared in a laboratory aquarium and successfully metamorphosed into adults. Morphological observations of the adult males identified three undescribed species, which are designated Gnathia albipalpebrata n. sp., G. parvirostrata n. sp., and G. dejimagi n. sp. on the basis of their larval morphologies and pigmentation patterns.
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