The life cycle of Hexangium sigani Goto & Ozaki, 1929 (Digenea: Microscaphidiidae) from the Red Sea.

The microscaphidiid Hexangium sigani Goto & Ozaki, 1929 was found in the intestine of Siganus rivulatus, a siganid fish permanently resident in a lagoon within the mangrove swamps on the Egyptian coast of the Gulf of Aqaba. Intra-molluscan stages of this trematode (mother sporocysts, rediae and cercariae) were found in the gonads and digestive gland of Nassarius pullus (Gastropoda: Nassariidae), a common snail in the same lagoon. Consequently, the life cycle of H. sigani was elucidated under natural conditions: eggs are directly ingested by the snail; mother sporocysts and rediae reach maturity 5-7 and 16-17 weeks post-infection; rediae contain 18-26 developing cercariae; fully developed cercariae are monostome, without penetration glands, emerge from the snail during the night 18-19 weeks post-infection and rapidly encyst on aquatic vegetation (there is no second intermediate host); encysted metacercariae are not progenetic; 2-day-old metacercariae encysted on filamentous algae fed to S. rivulatus developed into fully mature worms 5-6 weeks post-infection. The cycle was completed in about 24 weeks. The intra-molluscan stages are very similar to those of Dictyangium chelydrae Stunkard, 1943, the only described intra-molluscan stages of any microscaphidiid. However, they are also similar to those of the family Mesometridae. The present study of H. sigani describes the first complete microscaphidiid life cycle, and implicitly supports the phylogenetic relationship of this family with the Mesometridae inferred from molecular phylogenetic studies.

The life cycle of Sclerocollum saudii Al-Jahdali, 2010 (Acanthocephala: Palaeacanthocephala: Rhadinorhynchidae) in amphipod and fish hosts from the Red Sea.

The rhadinorhynchid Sclerocollum saudii Al-Jahdali, 2010 was found in the intestine of its type host, Siganus rivulatus Forsskål & Niebuhr, 1775, a siganid fish permanently resident in a lagoon within the mangrove swamps found on the Egyptian coast of the Gulf of Aqaba (between 28°7'N and 28°18'N). Larval forms of this acanthocephalan (acanthors, acanthellae and cystacanths) were only found in Megaluropus agilis Hoek, 1889 (Crustacea: Gammaridae), a benthic amphipod abundant on algae and seagrasses in the lagoon. So, this life cycle of S. saudii was elucidated under semi-natural conditions: embryonated eggs of S. saudii were directly ingested by the amphipod and hatched in its intestine; the released acanthor penetrated the intestinal epithelium in 12-18 h to reach the connective tissue serosa, where it remained for about 3 days, then penetrated the intestinal wall and remained attached to its outer surface for 4 days. It then detached and dropped free in the amphipod haemocoel and transformed into an oval acanthella, growing for 16 days to reach the cystacanth stage. The cystacanth at 46 days post-infection was infective to fish (excysted in its intestine as an active juvenile). Male and female juveniles reached maturity 17 and 23 days post-infection. Recently copulated females first appeared 26 days post-infection and all females seemed to be copulated at 28 days post-infection; partially and fully gravid females first appeared 31 and 35 days post-infection. Mature males and fully gravid females started to die off naturally 31 and 43 days post-infection and were totally expelled from the fish intestine by 42 and 52 days post-infection. The cycle was completed in 89 days and is similar to other known palaeacanthocephalan life cycles, but has its own characteristics.

Infrapopulations of Sclerocollum saudii Al-Jahdali, 2010 (Acanthocephala: Cavisomidae) in the rabbitfish Siganus rivulatus (Teleostei, Siganidae) from the Saudi coast of the Red Sea.

In infrapopulations of helminth parasites, density-dependent effects, through some form of intra- and interspecific competition, play an important role in shaping and regulating the infrapopulations. The mechanisms responsible for these processes have often been observed in laboratory studies and rarely studied under natural conditions. Here, 24 natural infrapopulations (77-447 individuals) of the acanthocephalan Sclerocollum saudii Al-Jahdali, 2010 from the fish Siganus rivulatus consisted of cystacanths, newly excysted juveniles, immature and mature worms, distributed in a well-defined fundamental niche (anterior 60% of the intestine). Each of these stages exhibited a significantly different longitudinal distribution within this niche. In small infrapopulations, cystacanths and newly excysted juveniles were found in the sixth 10% of the intestine, immature worms in the fifth 10% and mature worms in the anterior 40% of the intestine. However, their proportions followed a clear ascending order in each infrapopulation, and the female-male ratios of both immature and mature worms were distinctly female-biased. In large infrapopulations, mature worms existed partially in the site of immature ones, where a differential mortality among immature females was constantly observed. However, the proportions of immature worms increased significantly and those of mature worms decreased significantly, the mean lengths of immature and mature females decreased dramatically and the female-male ratios were distinctly male-biased. The mean sizes of immature and mature males seemed stable through all infrapopulations. The distribution of mature males and females suggests intense male-male competition for access to females, and reveals that larger females are copulated prior to the smaller ones. The results are statistically significant and suggest that infrapopulation self-regulation is through density-dependent mechanisms, in which immature females may play a key role.