Sousa, W.P., 1983. Host life history and the effect of parasitic castration on growth a field study of Cerithidea californica Haldeman (Gastropoda: Prosobranchia) and its trematode parasites. Journal of Experimental Marine Biology and Ecology, 73, 273—296.

The concept, proposed by Sousa (1983), considers gigantism as a selectively neutral side effect of parasitic castration and predicts it, in contrast to Minchella’s (1985) hypothesis, in short-lived or semelparous snail hosts. Long-lived iteroparous species will rarely, if ever, exhibit enhanced growth following the infestation. This idea originates from the differences in patterns of allocation of resources to host growth, its maintenance (repair) and reproduction attributed to these life strategies. Much of the energy assimilated by short-lived snails is invested in reproduction, therefore parasitic castration of host liberates a large energy pool to be reinvested in somatic growth. Long-lived snails, their reproductive efforts being spread over several breeding seasons, invest extensively in maintenance to enhance the likelihood of their future reproduction and therefore would be expected to have less annual investment in reproduction. As a result of relatively low reproductive effort, parasitic castration of longer-lived snails frees a small amount of energy for reinvestment in additional growth. As a summary, Sousa’s concept considering energy equilibrium in a host—parasite system takes into account the following factors that influence the snail growth response to infestation. First, the annual amount of energy invested in host breeding which is one of the parameters of the specie’s life history. Second, the individual investment in breeding that varies with the individual host age. And finally, costs for host reparation of damages induced by parasitism determined by antagonism between host and parasite.
 

Hypothetical model of probability of growth alteration in a system of sexually matured snail - trematode parthenites as a function of snail longevity and pathogenicity of parthenites (Sousa' (1983) conception); + - enhanced growth; - - stunted growth; 1 - combination of short-lived snail host and low pathogenic parthenites, probability to increase the growth rate is high; 2 - combination of long-lived snail host and low pathogenic parthenites, probability of changing the growth rate tends to zero; 3 - combination of long-lived snail host and parthenites having relatively higher pathogenicity, probability to stunt the growth is high.