Prodistomum waltairensis (Madhavi, 1972) Bray & Gibson 1990, n. syn.

Prodistomum waltairensis (Madhavi, 1972) Bray & Gibson, 1990

Synonyms: Opechona waltairensis Madhavi, 1972; Hamacreadium karachiensis (Bilqees & Masood, 1975) n. syn.; Hamacreadium rastrellii (Bilqees & Masood, 1975) n. syn.

Records. From the Indian mackerel, Rastrelliger kanagurta (Cuvier) (Perciformes: Scombridae), off Visakhapatnam, Andhra Pradesh, India by Madhavi (1972) as O. waltairensis. From R. kanagurta off Karachi, Pakistan by Bilqees & Masood (1975) as H. karachiensis and as H. rastrellii. From R. kanagurta off Visakhapatnam, Andhra Pradesh, India by Madhavi & Lakshmi (2011).

Remarks. Bilqees & Masood (1975) distinguished H. karachiensis and H. rastrellii by lengths of the body, oesophagus and pre-pharynx, sucker ratio, testes shape and egg size. Here they are considered to represent a single species. Although H. karachiensis is described with greater variation in body length (990–1,590 µm), it cannot be distinguished from H. rastrellii using this character (1,480–1,640 µm long). The same is true for oesophageal length (120–280 vs 110–180 µm), and egg size overlaps almost entirely (57–63 × 22–29 vs 53–66 × 26–39 µm). The testes are “distinctly follicular” vs “irregular, lobed, branched or follicular”. Hamacreadium rastrellii does appear to have a longer pre-pharynx and a smaller sucker width ratio, but these differences are interpreted as intraspecific here.

This species clearly does not belong in Hamacreadium because it is small (maximum length 1,590 µm), the testes are tandem, the ovary medial, the cirrus-sac entirely sinistral and protruding distinctly into hindbody, the genital pore lateral to the ventral sucker, the oesophagus exceptionally long, the pre-pharynx variable, and the vitelline field strongly restricted about the caeca. Furthermore, R. kanagurta is a planktivorous species (Madhavi & Lakshmi 2011) and has been thoroughly examined for parasites (Madhavi & Lakshmi 2011 and references therein). Madhavi & Lakshmi (2011) recently examined 184 individuals in the Bay of Bengal, off India, and found no opecoelids, but three lepocreadiid species: Opechona bacillaris (Molin, 1859) Dollfus, 1927, P. waltairensis and Prodistomum orientale (Layman, 1930) Bray & Gibson 1990. In a similar study off the Yemeni Coast in the Red Sea, Al-Zubaidy & Mhaisen (2014) found infections of O. bacillaris and P. orientale, but not P. waltairensis, in 182 R. kanagurta. Of the three species, the specimens of Bilqees & Masood (1975) agrees most closely, and is considered here synonymous, with P. waltairensis, because the gonads are lobed and the vitelline follicles and excretory vesicle extend to the intestinal bifurcation.

Being lepocreadiids, species of Opechona Looss, 1907 and Prodistomum Linton, 1910 possess tegumental spines and remnant eye-spot pigment; O. bacillaris, P. waltairensis and P. orientale are further characterised by an external (to the cirrus-sac) seminal vesicle, and O. bacillaris and P. waltairensis also possess a pseudo-oesophagus (Bray & Gibson, 1990). None of these characters was described by Bilqees & Masood (1975), although arguably these are details which may have been overlooked, especially given the poor quality of the specimens examined.

Finally, the specimens described by Bilqees & Masood (1975) are also similar to Parapolylekithum karachiense Bilqees, Ghazi, Khan & Khatoon, 2004 emend., for which Bilqees et al. (2004) proposed a new genus in the Allocreadiidae, based on two specimens from another scombrid, the Indo-Pacific king mackerel, Scomberomorus guttatus (Bloch & Schneider) [as Cybium guttatum (Bloch & Schneider)], also collected off Karachi. Bilqees et al. (2004) did not justify the inclusion of Parapolylekithum Bilqees, Ghazi, Khan & Khatoon, 2004 in the Allocreadiidae, which is considered by most authors to consist of only freshwater species with distinctive life-cycles (Caira & Bogéa 2005; Cribb 2005b). Parapolylekithum karachiense most likely represents a species of Opechona or Prodistomum.