The failure to examine gonadal tissue of American eels, Anguilla rostrata, histologically could result in considerable errors in sexual identification. Two "organs of Syrski", usually considered to be the male testes, and several small, ribbon-like gonads, generally thought to indicate female ovaries, revealed contradictory results when sectioned histologically. Other sources of error in determining a representative sex ratio of eels were noted. In the Matamek River system, Quebec, the range in length at which gonadal formation commenced was considerable. Male eels were rare (5 of 613 fish), which is consistent with other reports for the northern limits of this species.
The objectives of this study were to describe (1) the osmotic environment and precise location of the Proterometra macrostoma redia in its snail intermediate host, (2) where retraction of the distome body into the cercarial tail occurs, and (3) the subsequent emergence path of the cercaria out of the snail. Snails, Elimia semicarinata , were collected from North Elkhorn Creek in Scott County, Kentucky and screened daily for patent infections. Live rediae were extracted from infected snails in either artificial pond water (APW) or artificial snail water (ASW) and monitored for changes in morphology and movement every hour over 5 hr at 22 C. Infected and control snails were simultaneously fixed and decalcified in Cal-Ex II, prepared for routine paraffin sectioning, and serial sections subsequently analyzed for rediae and cercariae location. Significantly (χ(2) = 42.45; 1 df; P = 0.0001) more rediae showed movement in ASW than in APW after 5 hr, suggesting a host compartment separate from the mantle cavity. Histological sections clearly showed rediae developing in close association with the snail digestive tract, within the peri-intestinal sinus of the snail, and isolated from the mantle cavity by a mantle membrane. Retraction of the distome body into the cercarial tail follows the emergence of the cercaria from the redia. Cercariae then enter the mantle cavity and emerge into fresh water through a siphon-like structure formed by the mantle collar of the snail.
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