742 J. Parasitol., 93(4), 2007, pp. 742–749  American Society of Parasitologists 2007 PROXIMATE FACTORS AFFECTING THE LARVAL LIFE HISTORY OF ACANTHOCEPHALUS LUCII (ACANTHOCEPHALA) Daniel P. Beneshand E. Tellervo Valtonen Department of Biological and Environmental Science, P.O. Box 35, FI-40014 University of Jyva ¨ skyla ¨ , Finland. e-mail: dabenesh@cc.jyu.fi ABSTRACT: The growth and eventual size of larval helminths in their intermediate hosts presumably has a variety of fitness consequences. Therefore, elucidating the proximate factors affecting parasite development within intermediate hosts should pro- vide insight into the evolution of parasite life histories. An experimental infection that resulted in heavy intensities of an acan- thocephalan (Acanthocephalus lucii) in its isopod intermediate host (Asellus aquaticus) permitted the examination of parasite developmental responses to variable levels of resource availability and intraspecific competition. Isopods were infected by ex- posure to egg-containing fish feces, and larval infrapopulations were monitored throughout the course of A. lucii development. The relative rate of parasite growth slowed over time, and indications of resource constraints on developing parasites, e.g., crowding effects, were only observed in late infections. Consequently, the factors likely representative of resource availability to larval parasites (host size and molting rate) primarily affected parasite size in late infections. Moreover, at this stage of infection, competitive interactions, gauged by variation in worm size, seemed to be alleviated by greater resources, i.e., larger hosts that molted more frequently. The relatively rapid, unconstrained growth of young parasites may be worse for host viability than the slower, resource-limited growth of larger parasites. Many parasite life history traits, e.g., growth rates, are pre- sumably determined by the level of host resource consumption. Therefore, the exploitation strategies employed by larval hel- minths in their intermediate hosts should be reflected in parasite growth rates and body sizes. The potential fitness benefits linked to larger larval size may include better establishment success in the definitive host (Rosen and Dick, 1983; Steinauer and Nickol, 2003), higher adult fecundity (Fredensborg and Poulin, 2005), and less developmental time to sexual maturity (Poulin, 1998). Consequently, selection should promote faster larval growth and greater ultimate size. However, the direction- al selection on parasite size is probably stabilized by the general need to maintain host viability until transmission occurs (Laf- ferty and Kuris, 2002). This evolutionary tradeoff has been modeled to predict growth strategies of larval helminths in their intermediate hosts (Parker et al., 2003). Specifically, it was suggested that growth patterns may not be entirely a function of resources, but instead they may reflect flexible, adaptive life history strategies. That is, individual para- sites in the presence of competing conspecifics decrease their size, and thus the parasite burden on the host, not as a response to limited resources, but to maintain host viability. Parker et al. (2003) attempted to address the predictions of their model by ex- amining data taken from the literature on experimental infections of copepods with pseudophyllidean cestodes, but too little infor- mation was available to make comparisons robust. Recent exper- imental work, though, suggests that the larval growth of the ces- tode Schistocephalus solidus may vary in an adaptive manner (Mi- chaud et al., 2006). Like cestodes, there is a paucity of information on the growth strategies of acanthocephalans in their intermediate hosts. Intermediate host size and the presence of competitors are known to affect acanthocephalan development in some systems (Awachie, 1966; Uznanski and Nickol, 1980; Pilecka-Rapacz, 1986; Dezfuli et al., 2001; Poulin et al., 2003; Steinauer and Nick- ol, 2003), but the pervasiveness and magnitude of these phenom- ena are poorly known. Therefore, further empirical work is nec- essary to assess theoretical expectations and reach general conclu- sions concerning life history strategies of parasites in intermediate hosts. Received 6 October 2006; revised 14 January 2007; accepted 15 Jan- uary 2007. To elucidate the evolutionary forces shaping parasite growth in their intermediate hosts, the proximate factors affecting par- asite development must be understood. Resource availability, possibly represented by host size, condition, and/or growth, is one such factor presumably affecting the rate of parasite ontog- eny. Another factor is the number of conspecifics present. The response of developing parasites to variable resource pools and infection intensities can thus provide insight into the effects of competition on parasite life history strategies. An experimental infection with an acanthocephalan (Acanthocephalus lucii) in its isopod intermediate host produced high infection intensities (Benesh and Valtonen, 2007b). This provided an opportunity to examine parasite growth under conditions of high yet variable levels of intraspecific competition. MATERIALS AND METHODS Animal collection, maintenance, and experimental infection The collection site, maintenance of isopods, and experimental infec- tion protocol were described previously (Benesh and Valtonen, 2007b). Briefly, adult isopods (5 mm) of the species Asellus aquaticus were individually exposed to European perch (Perca fluviatilis) feces con- taining acanthocephalan (A. lucii) eggs. The exposure was terminated after 10 days, and the course of infection was monitored over a period of 101 days. Data collection Isopod molting was followed throughout the experiment, and the date of observed molts was recorded. Isopods were checked daily to deter- mine survival. The sex and length of isopods were recorded upon death. Dead isopods were dissected, and parasites from infected isopods were counted and measured. Parasites were examined with a compound mi- croscope, and measurements were taken using an ocular micrometer. During the early stages of the experiment, i.e., before 50 days or so, the length and width of individual worms was measured to the nearest 0.004 mm. Later in the experiment, because worms were much larger, the length and width of individual worms were measured to the nearest 0.01 mm. When possible, worms were sexed. After 101 days postex- posure (PE), all surviving isopods were killed and dissected. Data analyses The volume of individual worms was used as a measure of worm size. If worms were greater than or equal to 1 mm in length, they were consid- ered cylindrical in shape and their volume was calculated using the formula (lw 2 )/4, where l is worm length and w is worm width. Worms less than 1 mm were approximately ovoid in shape, and their volume was computed using the formula (lw 2 )/6. For each infected isopod, 4 variables were