Host diversity and prevalence analysis
Out of 670 fish specimens examined, 395 individuals were found to be infected with
Argulus spp. parasites. The most affected species was Goldfish (249 no.), followed by Rohu and Catla species (104 and 21 no., respectively). The remaining fish species had lower rates of infection (Mrigal- 13, Subunkin-4, Milky white koi carp-3 and Tiger Oscar-1). This study revealed that
A. japonicus has a propensity to infect a diverse array of fish hosts, infecting 247 goldfish, 104 rohu, 21 catla, 13 mrigal, 4 subunkin and 3 milky koi carp. Additionally, it was observed that
A. foliaceus only infected a single tiger oscar (
Astronotus ocellatus), whereas
A. coregoni infected two goldfish individuals.
In the Indian context,
A. japonicus has been documented in many fish species, including
Cyprinus carpio (Sahoo et al., 2012), goldfish/Prussian carp
(Kumari et al., 2019) and Himalayan snow trout,
Schizothorax richardsonii (Tandel et al., 2021). Saha and Bandyopadhyay (2015), were the first to report the infestation of
A. coregoni,
A. japonicus, and
A. foliaceus, in Red Can Oranda Gold Fish (
Carassius auratus auratus) in West Bengal. Similarly, in our present study,
A. japonicus has been found to infect a wide range of host species with various infestation rate including goldfish (90.20%), milky koi carp (50%), mrigal (46.43%), rohu (38.24%), Catla (36.21%) and subunkin (27%), whereas,
A. foliaceus and
A. coregoni infected only tiger oscar (50%) and goldfish (13.33%), respectively (Fig 1 and 2), showing low host diversity than
A. japonicus. However, there was no significant difference in terms of prevalence rates between the host species and
Argulus spp. (
P>0.05) (Fig 2). This is the first record of
A. japonicus infecting milky koi carp and
A. foliaceus infecting tiger oscar in India. This phenomenon could potentially be attributed to the transmission of
Argulus spp. infections among fish hosts in the same aquatic system or to the introduction of infected fish into a culture environment.
Pathological changes
Acute hemorrhagic inflammation of the epidermis, increased mucosal production, scale spillage and fin corrosion were identified at the sites of
Argulus infestation
(Alas et al., 2010). These were consistent with the pathological findings in the present study (Fig 3).
Morphological description of Argulus spp.
The morphological characteristics previously described were used to differentiate
A. coregoni, A. foliaceus and
A. japonicus based on the morphology of the abdominal lobes, respiratory regions and length of the cephalothoracic carapace as described by
Noaman et al., (2010) and
Wafer et al., (2015). As per
Noaman et al., (2010), A. coregoni exhibits pointed abdominal lobes, posterior lobes of the cephalothoracic carapace do not extend beyond the commencement of the abdomen, similar to
A. foliaceus. However, in
A. foliaceus it has rounded abdominal lobes with the posterior emargination not extending to the midline. Additionally, adults of
A. coregoni has a body length of 12 mm. In this context, it is observed that the fully developed female specimen of
A. coregoni displays a greater magnitude in terms of physical dimensions and spermathecae are filled with sperms when compared with both
A. japonicus and
A. foliaceus as per
Shimura (1981) (Fig 4-IIA, B). Conversely, the posterior lobes of the cephalothoracic carapace in
A. japonicus extend beyond the commencement of the abdomen. Furthermore, it has been observed that the abdominal lobes in
A. japonicus have a more pointed shape compared to those in
A. foliaceus, as reported by
Noaman et al., (2010) and
Wafer et al., (2015). These were the best visual cues used for the distinction among
A. coregoni, A. foliaceus and
A. japonicus in the current study (Fig 4-I and II). The female isolates of
A. japonicus, A. foliaceus and A. coregoni reported in the present study have a body length and width of 6-8 and 3-5 mm, 4-8 and 3-5 mm and 7-12 and 4-6 mm respectively.
In the adult gravid female specimens of
A. japonicus, several morphological variations were observed in the body length, width, carapace appearance, shape and appearance of the abdominal lobes (Table 1 and Fig 4-I). Moreover, a shovel shaped peg on the posterior distal end of basis located on the coxa of the 4
th swimming legs is characteristics feature of male specimen in
A. japonicus (Sahoo et al., 2012), has also been reported in the present study (Fig 5a-c). All the male isolates of
A. japonicus in the present study were in the size of 4-5 (body length) and 3-4 mm (width) in size.
Based on the morphological variations and their sampling sites, three female isolates of
A. japonicus were designated as COF_AHE_P (01) (Udaipur), COF_AHE_P (03) (Lembucherra), and COF_AHE_P (06) (Agartala); and one female isolate each of
A. coregoni (COF_AHE_P (07)) (Agartala) and
A. foliaceus (COF_AHE_P (08)) (Agartala) (Table 2). However, there was no morphological variations in collected male specimens of
A. japonicus unlike the female counterparts. But, based on their sampling sites the male specimens have been designated as
A. japonicus COF_AHE_P (02) (Udaipur), COF_AHE_P (04) (Lembucherra), COF_AHE_P (05) (Agartala) and COF_AHE_P (09) (Agartala) (Table 2).
Molecular and phylogenetic analyses
The
Argulus samples obtained in this investigation was subjected to molecular characterization using the 18S rRNA genetic marker. The sequences of PCR product (650 bp and 1800 bp) of Argulus samples acquired in this study were subsequently analyzed using NCBI Gene Bank BLAST analysis demonstrated a high level of similarity, with six
A. japonicus isolates, namely COF_AHE_P(02) (OR418123.1), COF_AHE_P(03) (OR418124.1), COF_AHE_P(04) (OR418125.1), COF_AHE_P(05) (OR418126.1), COF_AHE_ P(06) (OR418127.1) and COF_AHE_P(09) (OR418130.1), exhibited sequence similarities of 99.33%, 99.32%. 99.49%, 99.83%, 97.77% and 99.49% respectively, when compared to
A. japonicus isolate A850 (OR687243.1) from Uttarakhand India.
A. coregoni isolate COF_AHE_P(07) (OR418128.1) and
A. foliaceus COF_AHE_P(08) (OR418129.1) exhibited a 100% and 99.66% sequence identity, respectively, when compared to the
A. coregoni (JQ740820.1) and
A. foliaceus (JQ740819.1) isolates from Iran. The level of similarity between
A. japonicus isolates COF_AHE_P(01) (OR237569.1) and
A. japonicus isolate MWS2 (KF747859) from China was found to be 96.70%. Using multiple sequence alignment, the nucleotide sequences were then aligned with the inclusion of 18S rRNA sequences from fifteenth and five different
Argulus species and one additional outgroup species. The phylogenetic tree illustrating the evolutionary relationship of the
Argulus spp. was depicted in Fig 6 A and 6 B.
The evolutionary pair-wise distances among the collected specimens of
A. japonicus (OR418123, OR418124, OR418125, OR418126, OR418127, OR418130, OR237569) in comparison with specimens from other regions of India (Uttarakhand) (OR687243) and China (KF747859), exhibited a range of 0.01% to 0.78% (Table 3). Similarly, for
A. coregoni it ranged from 0.0% (
A. coregoni COF_AHE_P (07) OR418128) to 0.01% (
A. coregoni Iran JQ740820) (Table 4). In case of
A. foliaceus the evolutionary pair-wise distances varied from 0.0% (
A. foliaceus COF_AHE_P (08) OR418129) to 0.03 % (
Argulus foliaceus isolate MWS7 18S Iran (KF747861) (Table 4) and between
Argulus spp. and the outgroup (
Lernaea cyprinacea Japan KP235363) exhibited a range of 0.01% to 0.91% (Table 3) and 0.01 % to 0.21% (Table 4). The present findings indicate that the morphological and phylogenetic characteristics of the
Argulus specimens under investigation corresponded to those of
A. japonicus,
A. foliaceus and
A. coregoni. Based on genetic diversity studies,
A. japonicus specimens showed variations in genetic makeup within individuals and across species, suggesting genetic drift due to stable or unstable environmental factors in each region. These parasites may have come from various nations or locations with different sampling circumstances. Another possible explanation for the variations could be mutations in the mitochondrial DNA of the isolates, as suggested by
Wadeh et al., (2010). Genomic and morphological variation among
A. japonicus (Fig 4-I) species may be influenced by environmental variables during climatic adversity. However, more research is needed to grasp these issues.