Genus: Hexangium Goto et Ozaki, 1929

(syn: ArthurLoossia Nagaty, 1954)

Hexangium was erected by Goto and Ozaki (1929) as a new genus within the trematode family Angiodictyidae Looss, 1902 to include H. sigani Goto et Ozaki, 1929, collected by them from the intestine of the siganid fish Siganus fuscescens off Misaki and Takamatsu, Japan. In H. sigani, the body is elongate, the oral sucker is absent (the structure surrounding the mouth has long been accepted as a pharynx rather than an oral sucker (Blair 2005)), the ventral sucker is absent, the oesophagus is long and ends in an oesophageal bulb, the intestinal caeca extend posteriorly close to the testes which are situated diagonally in the posterior region of body, the genital pore is situated midway between the pharynx and the intestinal bifurcation, the ovary is round and immediately post-testicular, the vitelline follicles are arranged singly in rows extending lateral and medial to the caeca for almost their entire length, and each of the two arms of the excretory vesicle divides into three long collecting ducts.

Subsequently, two new species of Hexangium have been described from siganid fishes off the Philippines, H. affinum Tubangui et Masilungan, 1944 from the intestine of Amphacanthus javus by Tubangui and Masilungan (1944) and H. secundum Annereaux, 1947 from the intestine of Teuthis concatenata by Annereaux (1947). However, Yamaguti (1958) placed Hexangium in the Hexangiinae Yamaguti, 1958, a new subfamily within the family Angiodictyidae, and, based on the striking similarity, he considered Arthur loossia Nagaty, 1954 a synonym of Hexangium, transferring the type and the only species of the former, A. loossi Nagaty, 1954, to Hexangium as a new combination. This species was collected from the intestine of the labrid fish Pseudoscarus harid and the siganid fish Teuthis oramen in the Red Sea off Egypt. Hexangium affinum, H. secundum and H. loossi are very similar to each other and to the type-species, H. sigani, and only differ from it in having symmetrically rather than diagonally arranged testes. Therefore, Razarihelisoa (1959) and Velasquez (1961) expressed doubt about the validity of these species, suggesting that they might be conspecific and synonyms of H. sigani.

H. elongatum Manter, 1963 was described from the intestine of an acanthurid fish, Naso sp., off Fiji. He distinguished this species from H. sigani by the absence of an oesophageal bulb, the long intestinal caeca which extend to near the posterior end of body,the tandemly arranged testes and by the excretory collecting ducts which are three in number.

During the years 1964–2004, several authors (e.g. Fischthal & Kuntz 1964 1965, Al-Yamani & Nahhas 1981, Geets & Ollevier 1996, Sey et al. 2003, Dzikowski et al. 2003) agreed with Razarihelisoa (1959) and Velasquez (1961) and continued to consider H. affinum, H. secundum and H. loossi as synonyms of H. sigani.

H. leptosomum Machida et Uchida, 1990 was described from the intestine of the acanthurid fish Naso vlamingii, off Japan. They distinguished this species from the two valid species of Hexangium, H. sigani and H. elongatum, by the combination of many characters including the absence of an oesophageal bulb, the long intestinal caeca which extend to the posterior end of body, the tandemly arranged testes and by the genital pore which is located posterior to the intestinal bifurcation. However, they observed that in H. elongatum, the anterior region of the ventral surface is concave and acts as an accessory attachment organ.

H. brayi Hassanine et Gibson, 2005 was described from the intestine of the siganid fish Siganus luridus in the Red Sea off Sharm El- Sheikh, Egypt. They distinguished this species from H. sigani and H. elongatum by body shape, which is distinctly pyriform, the terminations of the intestinal caeca which are distinctly saccular, the vitelline follicles which are confined to the intercaecal field and by the arms of the excretory vesicle which divide into two long collecting ducts. They also agreed with previous authors in considering H. affinum, H. secundum and H. loossi as synonyms of H. sigani.

In a comprehensive revision of the Microscaphidiidae Looss, 1900, Blair (2005) considered the Angiodictyidae as its synonym, and because the relationships among many of the microscaphidiid genera are unclear, he did not accept and did not recognise the subfamilies within this family. Accordingly, Hexangium was transferred to the Microscaphidiidae. He also transferred H. elongatum to a new genus, named Parawardula Jones et Blair, 2005, within the family Mesometridae Poche, 1926, since in this species the anterior region of the ventral surface is concave and acts as an accessory attachment organ (Machida & Uchida 1990), the oesophageal bulb is absent, the intestinal caeca almost reach the posterior end of the body and the testes are tandemly arranged. All of these characters, especially the first, are essential in distinguishing the Mesometridae from Microscaphidiidae. However, he erected Pseudohexangium Blair, 2005 as a new microscaphidiid genus for H. leptosomum, in which the oesophageal bulb is absent, the intestinal caeca almost reach the posterior end of the body, the testes are tandemly arranged and the genital pore is situated posterior to the intestinal bifurcation.

Accordingly, two species of Hexangium are currently recognized, i.e. H. sigani Goto et Ozaki, 1929 (type species) and H. brayi Hassanine et Gibson 2005.