Digenetic trematodes of the Subfamily Aphanurinae Skrjabin & Guschanskaja, 1954 infecting the commercially important fish Thryssa malabarica (Bloch, 1795) (Clupeiformes, Engraulidae) from the brackishwater bodies of Kerala, India

Three species of digenetic trematodes of the Family Hemiuridae Looss, 1899, Subfamily Aphanurinae Skrjabin & Guschanskaja, 1954 with their prevalence, intensity of infection and mean abundance are reported Aphanuroides lethrini, Aphanurus stossichii and Aphanurus tuberculatus. A total of 26 specimens of Thryssa malabarica (Bloch, 1795) were examined for digenean infection. Prevalence of infection of A. lethrini, A. stossichii and A. tuberculatus were 11.53%, 69.23% and 15.38%, respectively, the intensity of infection were 1.6, 3.1 and 2.5 and the mean abundance were 0.19, 2.15 and 0.38, respectively. The prevalence, intensity and mean abundance of infection were at the maximum level in A. stossichii infection. Recovery of these parasites from T. malabarica represents a new host record and the first report from Kerala. The characteristic features of the recovered digeneans are discussed.


Introduction
Our knowledge on trematode parasites of brackishwater fishes and their distribution is limited. The brackishwater environment represents an unique ecosystem of high fertility, that supports a rich natural fishery which is of considerable significance in the rural economy of developing countries (Tampi, 1973). The Malabar anchovy Thryssa malabarica (Bloch, 1795), a marine, estuarine group fish (Mahima, 2014) is an important food fish in Kerala.
Previous studies on the digeneans of Thryssa spp. inhabiting the marine habitat have been reported by Ahmad (1980), Hafeezullah (1980b), Gupta and Jain (1992), Madhavi (2011), and Sailaja and Madhavi (2012). The present study on the digenetic trematodes of the Subfamily Aphanurinae infecting the commercially important fish T. malabarica from the brackishwater bodies of Kerala forms the first report from Kerala.
The purpose of this study is to expand our knowledge of the Family Hemiuridae Looss, 1899, Subfamily Aphanurinae Skrjabin & Guschanskaja, 1954 endohelminths of T. malabarica from the brackishwater bodies of Kerala.

Materials and methods
Fishes were collected from various brackishwater bodies of Northern Kerala during the period from June 2018 to June 2019. Specimens were immediately brought to the laboratory and examined for digenetic trematodes under a stereozoom dissecting microscope. A total of 26 specimens of T. malabarica were examined. Adults recovered were carefully removed and cleaned in 0.75% saline in separate watch glasses and studied under Nikon ECLIPSE Ni-U phase contrast research microscope with or without vital staining. Permanent whole mounts were prepared after fixing them in 5% formalin under slight cover glass pressure and staining with acetocarmine, following the procedure outlined by Cantwell (1981). Identification of parasites was done following Gibson (2002). Specimens were measured using the Nikon NIS Elements Imaging software. All measurements are in micrometers (µm), as range followed by mean in parentheses. Drawings were made with the aid of Nikon Y-IDT drawing tube attached to the Nikon ECLIPSE Ni-U microscope and the details were added free hand from observations made on live specimens. Photographs were taken with a Nikon Y-TV55 camera. Prevalence, intensity of infection and mean abundance were estimated following Bush et al. (1997).

Three species of digenetic trematodes
Aphanurus lethrini, A. stossichii, and A. tuberculatus were found infecting the intestine of the fish T. malabarica.
Collection localities: Mattool and Ezhom of Kannur District and Chandragiri backwaters of Kasargod District.
Period of collection: June 2018 to June 2019.

Aphanuroides lethrini Nagaty and Abdel-Aal, 1962
Remarks: The species is characterized by highly plicated body surface. Ventral sucker is near the anterior extremity and is larger than oral sucker. Pharynx is small. Caeca terminates near posterior extremity. Testes are oval, two in number and located at the middle third of body. Ovary is post testicular and seminal vesicle is long, bipartite and attenuated anteriorly.

Mean abundance
body. Vitellaria are post ovarian and have two compact lobes. Eggs are large and oval. Rudiments of ecsoma are clearly visible. Three out of 26 T. malabarica were infected with A. lethrini and the prevalence of infection is 11.53%. Five A. lethrini were recovered from three infected fishes so the intensity of infection is 1.6 and five A. lethrini were recovered from a total of 26 fishes so the mean abundance is 0.19 (Table 1).

Aphanurus stossichii (Monticelli, 1891) Looss, 1907 (Figure 5)
Remarks: This species is characterised by anteriorly tapered, elongate and plicate body. Forebody is short; oral sucker is small and ventral sucker is large & muscular. Pre pharynx is absent. Caeca reach up to the posterior extremity. Testes are tandem. Seminal vesicle lies anterior to the anterior testis. Sinus sac is tubular, narrow. Ovary is post testicular. Vitellarium is single, large, post ovarian and slightly larger than ovary. Uterine coils reach up to the posterior extremity. Eggs are thin shelled. Rudiments of ecsoma are absent. Eighteen out of 26 T. malabarica were infected with A. stossichii and the prevalence of infection is 69.23%. Fifty six A. stossichii were recovered from 18 infected fishes and the intensity of infection is 3.1 and 56 A. stossichii were recovered from a total of 26 fishes, so the mean abundance is 2.15 (Table 1).
Collection localities: Mattool and Ezhom of Kannur District and Chandragiri backwaters of Kasargod District.

Aphanurus
tuberculatus (Hafeezullah, 1981) (Figure 6) Remarks: Body is elongate; plications can be seen more conspicuously in the anterior region extending up to the vitelline area. Oral sucker is smaller than the ventral sucker. Caeca are long, extending up to the posterior end of body. Seminal vesicle is elongate with thick muscular wall and found between the anterior testis and the posterior region of the ventral sucker. Testes are almost equatorial. Tubular sinus sac extends from the anterior border of the ventral sucker to the posterior end of the oral sucker and encloses knob -like sinus organ with fine tubercles. Ovary is post testicular. Vitellarium is post ovarian and indented anteriorly. Eggs are elongate and numerous.
Four out of 26 T. malabarica were infected with A. tuberculatus so the prevalence of infection is 15.38%. Ten A. tuberculatus were recovered from four fishes and the intensity of infection is 2.5 and 10A. tuberculatus were recovered from a total of 26 fishes so the mean abundance is 0.38 (Table 1).
Collection localities: Mattool and Ezhom of Kannur District.