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Examination of numerous specimens of S. rivulatus from those inhabiting the coast of Yanbu (200 km north of the mangrove swamps) revealed that they were not naturally infected with any intestinal helminth parasite. Therefore, 70 specimens of nearly equal lengths (12-16 cm) were marked and transferred alive in June of 2010 to the lagoon to allow their infection with G. volubilis to be followed after ten weeks. To prevent fishes from escaping from the lagoon, a plastic net with narrow mesh size was used as a barrier at the lagoon mouth; the lagoon can therefore be considered as a natural aquarium. Through one week, 42 specimens (five-seven specimens/ day) of the marked fishes were caught from the lagoon by hand net. To avoid parasite postmortem or other migration along the gastrointestinal tract, each fish was killed and measured immediately after capture by a blow to the head and examined at a field laboratory (within 30-45 min after capture). Then the entire alimentary canal of each fish was immediately removed and, to record the exact position of individual parasites, the intestine was cut into ten equal sections; tied before cutting. Each section was opened and its contents examined under a dissecting stereomicroscope; individual parasites were examined to determine if they were alive or not, carefully teased out, re-examined alive in a saline solution, and the opened section was then shaken vigorously in a jar of saline to dislodge further worms and to remove mucus. Trematodes were placed in whirling hot water before fixation in hot 5% formalin under a slight coverslip pressure. This procedure gives specimens a uniform size and shape. The infrapopulation collected from each fish host was carefully counted, and its distribution in the intestine was recorded. Wholemounts were stained in alum carmine, cleared in terpineol and mounted in Canada balsam. All trematodes recovered were identified and their body lengths, round testes, round ovaries and oval eggs were measured (in micrometres) using a compound microscope with an eyepiece micrometer. The different stages of the parasite were classified according to Al-Jahdali & Hassanine (2012b); worms were categorized as newly excysted juveniles if they were closely similar to metacercariae, as immature if the sexual organs were little or moderately developed and the uterus contained no eggs, and as mature if the sexual organs were fully developed and the uterus contained eggs.
G. volubilis infrapopulations consisted of newly excysted juveniles (coming from new infections with metacercariae), immature and mature worms. These stages were only found in the anterior four segments (40 %) of the intestine of S. rivulatus, i.e. in a welldefined fundamental niche along the intestine of this fish. In all infrapopulations, immature and mature worms were found alive, while newly excysted juveniles were found alive or dead. In small infrapopulations (I-IX, slight densities), all newly excysted juveniles were alive. In larger infrapopulations (X-XXX, higher densities), the number of dead newly excysted juveniles gradually increased with infrapopulation size to include all juveniles in the largest infrapopulations (Table I). Dead juveniles (not included in infrapopulation size) were mostly found in second segment of the intestine and some were scattered along the posterior intestine (possibly carried out by the intestinal peristaltic to this region); their body walls seemed less transparent, their internal fluids exhibited no movement and their bodies were completely immobile. The relationship between number of dead newly excysted juveniles and infrapopulation size was strongly positive (R2 = 0.829, slope = 3.574, P
The distribution of newly excysted juveniles, immature and mature worms of Gyliauchen volubilis (in 30 infrapopulations; each represented by a bar) along the anterior four segments of the intestine of Siganus rivulatus (see also Table II).
In G. volubilis infrapopulations, the proportions of newly excysted juveniles, immature and mature worms showed a clear ascending order in each infrapopulation. This seemed to be normal, and was more probably due to the continuing accumulation of infections and the duration of each stage. In small infrapopulations (slight density), all stages of G. volubilis were found alive. In larger infrapopulations (high intensity), a differential mortality was only, and consistently, observed among the newly excysted juveniles, and significantly increased with infrapopulation size. Thus, newly excysted juveniles seemed to be adversely affected or more sensitive to crowding stress than immature and mature worms. This result strongly suggests density-dependent effects and intraspecific competition among juvenile worms and probably between them and immature and mature worms. Such a competition, which leads to individual mortality in high-density infrapopulations, may contribute to regulating the size of infrapopulations (Uznanski & Nickol 1982; Brown 1986; Kennedy 2006; Poulin 2007). However, failure of newly excysted juveniles to survive in high-density infrapopulations greatly supports the experimental finding of Uznanski & Nickol (1982) and Brown (1986) that parasite activation (excystment) is a density-independent process, but establishment and survival are apparently density dependent.
In gastrointestinal helminth parasites, density-dependent reductions in mean worm length (growth or fecundity) have been reported in several studies (e.g.Szalai & Dick 1989; Shostak & Scott 1993; Poulin 1999; Sasal et al., 2000; Richards & Lewis 2001; Dezfuli et al., 2002; Poulin et al., 2003; Hassanine & Al-Jahdali 2008). Such a decrease in length (fecundity) contributes to the regulation of the parasite population by the availability of infective stages for all infrapopulations (Poulin, 2007). Unlike these studies, the mean length of mature worms of G. volubilis seemed to be stable or less affected through all infrapopulations, since there were no significant relationships between mean worm length and infrapopulation size, and between numbers of mature worms and their mean lengths.
In hermaphroditic helminth parasites, cross-fertilization is preferred over self-fertilization (Esch & Fernandez, 1993; Combes, 1995; Trouvé et al., 1999; Brown et al., 2001), suggesting that cross-fertilization may result in substantial fitness benefits for parasites (Rohde, 1994; Wedekind et al., 1998; Lythgoe, 2000; Brown et al., 2001). These parasites have a special mating strategy available to them in the face of sexual selection, since they can adjust the ratio of resources allocated in mating to the male function versus the female function, depending on current selection pressures and environmental conditions such as mating group size (Charnov, 1982; Raimondi & Martin, 1991; Trouvé et al., 1999). However, theoretical (e.g., Charnov, 1996; Lively, 1990) as well as empirical studies (e.g., Wedekind et al., 1998; Trouvé et al., 1999; Schärer & Wedekind, 2001) revealed that the increase in resource allocation to the male function with mating group size is a consequence of local mate competition (Hamilton, 1967; Charnov, 1982), and indicates greater opportunities for cross-fertilization. These patterns were clearly observed in G. volubilis mating groups, where the mean testis size significantly increased and the mean ovary size significantly decreased with mating group size. However, the ratio mean testis size-mean ovary size, a reliable indicator of resource allocation to the male function and opportunities for cross-fertilization (Thomas & Poulin, 2003; Schärer, 2009) significantly increased with mating group size, and independent of the mean worm length. Moreover, mature worms in large infrapopulations were constantly found aggregated. Such aggregation increases intraspecific contact and hence facilitates cross-fertilization (Rohde, 1977). Combination of these results strongly suggests local mate competition in large mating groups or in large infrapopulations of G. volubilis.
In small infrapopulations of G. volubilis, mature worms were sluggish, scattered singly along the niche and never seen in mating pairs. Thus, mating opportunities are extremely rare, and the worms may be under strong selection for self-fertilization. According to Lüscher & Wedekind (2002), worms that seemed to avoid a potential mate either wait for a mating opportunity or reproduce by selfing. Sex allocation theory predicts that same-size animals should mate reciprocally, with pairs very different in size more likely to mate unilaterally (Angeloni et al., 2002). Mating behaviour of G. volubilis clearly followed this pattern, since in large infrapopulations the probability of mating reciprocally or unilaterally, depended on body size. Reciprocal insemination was favored when the two worms were small (nearly equal in size) and both adopting the male role, while unilateral insemination occurred when the two worms were distinctly differing in size and only one worm (the larger one) adopted the male role. Thus, sex allocation in G. volubilis seemed to be size dependent, i.e. larger worms were more biased toward male allocation and never seen together in mating pairs. Therefore, larger worms were sperm donors and preferred mating with smaller ones (sperm acceptors).
To describe the skeletal development and abnormalities in turbot Scophthalmus maximus, samples were collected every day from hatching to 60 days after hatching (DAH). A whole-mount cartilage and bone-staining technique was used. Vertebral ontogeny started with the formation of anterior haemal arches at 51 mm standard length (L(S) ) c. 11 DAH, and was completed by the full attainment of parapophyses at 169 mm L(S) c. 31 DAH. Vertebral centra started to develop at 63 mm L(S) c. 16 DAH and ossification in all centra was visible at 110 mm L(S) c. 25 DAH. The caudal fin appeared at 51 mm L(S) c. 11 DAH and ossification was visible at 206 mm L(S) c. 37 DAH. The onset of dorsal and anal fin elements appeared at 58 mm L(S) c. 15 DAH and 63 mm L(S) c. 16 DAH, respectively. Ossifications of both dorsal fin and anal fin were visible at 206 mm L(S) c. 37 DAH. The pectorals were the only fins present before first feeding, their ossifications were completed at 235 mm L(S) c. 48 DAH. Pelvic fins began forming at 72 mm L(S) c. 19 DAH and calcification of the whole structure was visible at 198 mm L(S) c. 36 DAH. In the present study, 24 types of skeletal abnormalities were observed. About 51% of individuals presented skeletal abnormalities, and the highest occurrence was found in the haemal region of the vertebral column. As for each developmental stage, the most common abnormalities were in the dorsal fin during early metamorphic period (stage 2), vertebral fusion during climax metamorphosis (stage 3) and caudal fin abnormality during both late-metamorphic period (stage 4) and post-metamorphic period (stage 5). Such research will be useful for early detection of skeletal malformations during different growth periods of reared S. maximus. 2012 The Authors. Journal of Fish Biology 2012 The Fisheries Society of the British Isles. 59ce067264
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