Report of Trematosoma rotunda (Ciliophora, Suctorea) as epibiont on harpacticoid copepod from western Indian coast

Suctorian ciliate Trematosoma rotunda (Allgén, 1952) is reported here for the first time as epibiont on a harpacticoid copepod collected from Mandovi river estuarine mouth, Goa, west coast of India, Arabian Sea. The description of the species, synonymy, data on species distribution and host taxa are also presented in this study.

Trematosoma rotunda (Allgén, 1952) a marine loricate suctorian ciliate described by Allgén (1952) as epibiont on marine nematodes Desmodorella tenuispiculum (Allgén, 1928) and Croconema stateni Allgén, 1928 collected in the Antarctic (Graham Land) and the Falkland Islands, was named as Acineta rotunda. In the same paper another suctorian species (Acineta ovoidea) inhabiting on nematodes was described. However, Curds (1985) synonymized A. ovoidea with A. rotunda showing that A. ovoidea was nothing but lateral view of A. rotunda. Dovgal (2002) transferred A. rotunda into genus Trematosoma Batisse, 1972 based on arrangement of its tentacles in two rows. Dovgal et al. (2009) reported this species on the nematode Pseudochromadora sp. collected near Ratnagiri, Maharastra State, central west coast of India (Arabian Sea), and provide detailed remarks on the systematics and nomenclature of this species. Fisher (2003) found epibiont ciliate on nematode Pseudochromadora cazca Gerlach, 1956, from the intertidal zone of North-Eastern Queensland, Australia; Dovgal et al. (2009a) identified this species based on Fisher's photo 'probably as Trematosoma rotunda (Allgén, 1952)'.
Bhattacharjee (2014), Ansari and Bhaduri (2016), Ghosh and Mandal (2019) reported the same species on the nematode host from Bay of Bengal, Indian Ocean. Baldrighi et al. (2020) found this species on nematode from NW Madagascar margin-deep sea pockmark and Gulf of Naples-shallow vent area.
We report here T. rotunda from Goa, west coast of India, Arabian Sea as an epibiont on harpacticoid copepod.

Material and Methods
Samples were collected from Mandovi river estuarine mouth (15°28'43.68"N and Long 73°41'41.28"E), Goa, west coast of India, Arabian Sea ( Figure 1) at 20m depth. Samples were sieved in the laboratory using 45 μm mesh size sieve. Infested meiofauna were sorted, mounted on a 50% glycerine slide and sealed with DPX. The systematic position of suctorian ciliates follows Dovgal (2002;2013). Measurements were carried out on four specimens using the program Toup View 3.7 for digital camera. Specimens are kept in the collection of the fourth author (MN) in CSIR-National Institute of Oceanography, Dona Paula, Goa.

Diagnosis
Marine loricate suctorian. Cell body short, rounded, laterally flattened, entirely fills up the lorica and clamped to their aperture border. Lorica delicate gently striated. Tentacles clavate, short, arranged in two groups of rows at the upper body surface. Macronucleus spherical or oval, centrally or slightly posteriorly located. Stalk short, thin, slightly ribbed.

Distribution and host specificity
Ratnagiri, Maharastra, India, Arabian Sea  It should be presumed that the species is widely distributed, whose spread may be limited possibly by salinity. The relatively small number of finds of the species is apparently associated with a lack of research interest to ciliate epibiosis. This species was predominantly found on nematodes. Chatterjee et al. (2019b) have previously mentioned about T. rotunda, which most likely epibiont on nematodes only. However, our new finding is not only the first report of this species from Goa, but the first its registration on representative of new host taxon -Harpacticoida Sars, 1903. The last finding permits to conclude that T. rotunda may inhabit on a wide range of meiobenthic marine invertebrates.