Evidence for Initial Infestation by Mothocya parvostis (Isopoda: Cymothoidae) on Body Surface of Juvenile Japanese Halfbeak, Hyporhamphus sajori (Beloniformes: Hemiramphidae)

Abstract

Mothocya parvostis Bruce, 1986 (Isopoda: Cymothoidae) is a parasite that mainly infests the branchial cavity of Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846) (Beloniformes: Hemiramphidae) (pelagic marine fish). Although it has generally been considered to parasitize the branchial cavity throughout its life, early studies suggested that mancae (first free swimming stage) initially attach to the body surface of the host. In this study, H. sajori juveniles were collected off Ohkurokamishima Island, Hiroshima, Japan, and examined for cymothoid infestation. Of the 57 individuals that were examined, 5 were parasitized, all on the ventral surface of the fish bodies. One manca was genetically identified as M. parvostis based on the sequence of cytochrome c oxidase subunit I (100% or 99.81% identity with known sequences). These findings support the earlier claim that mancae infest the body surface of H. sajori. Due to the host’s slender body and small branchial cavity, early-stage parasites may remain external in the initial phase of infestation.

Mothocya parvostis Bruce, 1986 (Isopoda: Cymothoidae) infests the branchial cavity of fish [1], including the Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846) (pelagic marine fish), as its definitive final host. Cymothoids feed on fish tissue and blood and are dependent on their hosts for nutrition. An increase in juvenile mortality rate due to cymothoids was reported [2]. Elucidating the cymothoid life cycle is important to understand this potential threat to fish. The life cycle of M. parvostis was reported earlier than that of many other cymothoid species [3]. Inoue [3] reported that mancae (first free swimming stage) of these parasites were released from the brood pouch of female M. parvostis and infested the body surface of the host H. sajori, where they become young males. Some of these males then moved into the opercular cavity of the fish and changed sex to female. In addition, M. parvostis has been reported to use juveniles of some fish species as optional intermediate hosts when it cannot parasitize juvenile H. sajori [4,5].

However, in many cymothoid species, it has been thought that the infestation site remains almost unchanged as their life cycle progresses [6,7,8,9,10,11,12]. Among cymothoids, reports of cases such as Mothocya lineata (Miers, 1876), where parasites change their attachment sites depending on their life cycle stage, are extremely rare [1,13]. Therefore, the relocation of the infestation site proposed by Inoue [3] has been largely ignored in subsequent studies (e.g., Smit et al. [14]), and M. parvostis has been treated as a species that parasitizes the branchial cavity throughout its life [1,15,16].

In this study, H. sajori juveniles were collected from the surface waters around oyster-culturing rafts off Ohkurokamishima Island, Etajima, Hiroshima Prefecture, Japan (see Kawai et al. [17]), on 30 May 2019, using paper cups. The presence or absence of cymothoids was confirmed and photographed. The cytochrome c oxidase subunit I (COI) sequence of the parasite was analyzed for species identification because most cymothoid species are morphologically difficult to identify, except in the case of adult females. DNA extraction and PCR amplification were performed following the protocol outlined by Fujita et al. [18]. The PCR product was sent to Eurofins Genomics (Tokyo, Japan) for sequencing using the dye terminator method. The sequence was submitted to GenBank (accession number: PX108758).

Of the 57 individuals checked, 5 were found to be infested by cymothoid mancae or juveniles (Figure 1). No cases of multiple parasites infesting a single fish were observed. The infestation site was the ventral surface of the body in all individuals (Figure 1). The bodies of the live cymothoids were translucent, and the digestive tract appeared to contain fish blood (Figure 2). One cymothoid parasite was removed from an H. sajori juvenile and fixed in 99.5% ethanol for observation and molecular analyses. The fixed cymothoid was identified as a manca because it lacked pereopod 7 (Figure 3). After observation, the fish were released.

Comparison of the COI sequence of the specimen with the sequences of M. parvostis used for redescription by Fujita et al. [15] (LC763834–LC764100) showed 100% or 99.81% similarity, with an E-value of 0.0, supporting its identification as M. parvostis.

Fujita et al. [19] reported that when M. parvostis parasitizes juveniles of black sea bream, Acanthopagrus schlegelii (Bleeker, 1854), it first attaches to the surface of the host around the mouth, taking advantage of the host’s feeding behavior, and then moves to the branchial cavity. However, all five individuals obtained in this study were parasitizing the same site (the ventral surface of the body), and what appeared to be fish blood was visible in their digestive tract (Figure 2). Therefore, it is unlikely that they were newly attached and had not yet entered the branchial cavity, or that they accidentally adhered to the body surface due to handling during collection. At least in the case of mancae, the claim by Inoue [3] that M. parvostis infests the body surface was proven partly. Hyporhamphus sajori has a slender body shape, and the branchial cavities of their juveniles are small (Figure 1), so parasites may remain on the body surface because they cannot enter the branchial cavity.

Similarly, there are cases in which mancae of Cymothoa pulchrum Lanchester, 1902, which normally parasitizes the buccal cavity of fish, grow into juveniles on the body surface of fish under captivity [20]. Many aspects of the behavior of Cymothoidae in the early stages remain unclear, and further observations of many species are necessary.