Histopathological effects of larval trematode infections in mudsnails and their impact on host growth: what causes gigantism in Hydrobia ventrosa (Gastropoda: Prosobranchia)?

Abstract

Abstract The histopathology and the patterns of spatial integration of different types of trematode larvae (sporocysts, rediae) were examined in relation to the stage of gonadal development of the host, the prosobranch semelparous mudsnail Hydrobia ventrosa . Snail growth was analysed in relation to the type of infection (sporocyst or rediae, prepatent or patent). Seven taxa of larval trematodes were found during this investigation: Maritrema subdolum , Microphallus sp., Levinseniella brachysoma , Cryptocotyle concavum , Paramonostomum alveatum , Asymphylodora demeli , and Himasthla sp. H. ventrosa reached maturity at a size of 1.6–2.0 mm, after a lifespan of 9–10 months. Trematode infections were observed 1–2 months after maturation in snails larger than 2 mm. The parthenitae generation of all trematode species was located in the organs of the reproductive system (gonad, albumen gland, prostata) of H. ventrosa . In consequence, the affected organs, in particular the gonads, showed severe histopathological alterations. Effects differed between the prepatent and patent period. If infections appeared before meiosis of germ cells, the germinal epithelium of the gonadal tubules was still present. If infections appeared during meiosis of host germ cells, the germinal epithelium of the gonadal tubules was almost completely absent. The gonadal tissue was completely replaced by daughter sporocysts or rediae during the patent period. Sporocysts invaded also the accessory sex glands. In no instances were histopathological alterations of the digestive gland observed. Growth rates of snails with prepatent infections were similar to growth rates of uninfected snails. Growth of snails with patent infections was significantly enhanced. The increment in shell size per day was higher in snails infected with patent sporocysts than in snails infected with patent rediae. Gigantism in infected H. ventrosa is interpreted as a side-effect of parasitic castration.