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

The histopathology and the patterns of spatial integration of different typ es of trematode larvae (sporocysts, rediae) were examined in relation to th e stage of gonadal development of the host, the prosobranch semelparous mud snail Hydrobia ventrosa. Snail growth was analysed in relation to the type of infection (sporocyst or rediae, prepatent or patent). Seven taxa of larv al trematodes were found during this investigation: Maritrema subdolum, Mic rophallus sp,, Levinseniella brachysoma, Cryptocotyle concavum, Paramonosto mum alveatum, Asymphylodora demeli, and Himasthla sp. H. ventrosa reached m aturity 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 t he organs of the reproductive system (gonad, albumen gland, prostate) of H. ventrosa. In consequence, the affected organs, in particular the gonads, s howed severe histopathological alterations. Effects differed between the pr epatent and patent period. If infections appeared before meiosis of germ ce lls, the germinal epithelium of the gonadal tubules was still present. If i nfections appeared during meiosis of host germ cells, the germinal epitheli um 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 w ere histopathological alterations of the digestive gland observed. Growth r ates of snails with preparent infections were similar to growth rates of un infected 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, Giganti sm in infected H. ventrosa is interpreted as a side-effect of parasitic cas tration. (C) 1999 Elsevier Science B.V. All rights reserved.