Parasite infection and sand coarseness increase sand crab (Emerita analoga) burrowing time

Parasites with indirect life cycles require trophic transmission from intermediate hosts to definitive (vertebrate) hosts. Transmission may be facilitated if parasite infection alters the behavior of intermediate hosts such that they are more vulnerable to predation. Vulnerability to predation may also be influenced by abiotic factors; however, rarely are the effects of parasites and abiotic factors examined simultaneously. The swash zone of sandy beaches is a particularly harsh environment. Sand crabs (Emerita analoga) burrow rapidly in the swash zone to avoid predators and dislodgment. We examined prevalence and abundance of the acanthocephalan parasite Profilicollis altmani in sand crabs, and investigated the synergistic effects of sand grain size (an important abiotic factor), parasite infection, body size and reproductive condition on burrowing speed in females, from three California sites. More heavily parasitized crabs burrowed more slowly, making them potentially more vulnerable to predation by marine bird definitive hosts. Ovigerous females harbored more parasites than non-ovigerous females, but burrowed more quickly. All crabs burrowed slowest in the coarsest sand, and burrowing times increased with repeated testing, suggesting that it is energetically costly. Abiotic and biotic factors influence burrowing, and behavioral variation across sites may reflect the response to natural variation in these factors.     

滨鹬马蹄吸虫(复殖目:微茎科)的尾蚴和囊蚴期的季节动态

对科威特湾微茎科滨鹬马蹄吸虫幼虫期的中间宿主双带盾桑椹螺(Clypeomorus bifasciata)及小相手蟹(Nanosesarma minutum)的季节动态进行了研究。调查期超过一年,在检查的1 600只螺和415只蟹中, 11.8 %的螺感染了8种马蹄属线虫中的一种,且以滨鹬马蹄吸虫的感染占优势(9.9 %螺感染) ; 80 %的蟹感染滨鹬马蹄吸虫囊蚴。虽然一年四季两种宿主都会感染,但吸虫的流行和尾蚴(指成熟期感染)在夏季呈现高蜂。从螺体排出的尾蚴具有明显季节性,在此海湾必须要超过最低温度20℃。总的感染率在较大(较老)的螺里有所下降,显示吸虫影响宿主生存并随之影响宿主群体结构。囊蚴的感染丰度与蟹的个体大小有明显相关性;较大的蟹感染较多的囊蚴,显示宿主能耐受更多的吸虫。调查显示,囊蚴的感染率与蟹的大小或性别无相关性。囊蚴体外脱囊以及产卵吸虫的释放证明,成熟虫体终年存在于所有大小和性别不同的蟹里,显示从蟹到鸟的持续感染是可能的。总的来说,滨鹬马蹄吸虫在海湾的传播动态是由这两种无脊椎动物宿主来协调,并似乎是被一系列依赖于温度的活动控制,这些活动影响易感宿主种群及感染性幼虫期尾蚴和囊蚴的存在。     

Habitat preferences of two estuarine burrowing crabs Helice crassa Dana (Grapsidae) and Macrophthalmus hirtipes (Jacquinot) (Ocypodidae)

The horizontal and vertical distributions of two species of endemic, burrowing mud crabs Helice crassa Dana 1851 (Grapsidae) and Macrophthalmus hirtipes (Jacquinot 1853) (Ocypodidae) are described for the Avon-Heathcote Estuary (43°33′S: 172°44′E), Christchurch, New Zealand. Substratum preference is shown to be the most important factor influencing mud crab distribution, but lack of tolerance to salinities below 4‰ is also a significant factor preventing M. hirtipes from occurring at points close to freshwater input. Both species had similar sediment organic content and particle size requirements. Helice crassa was concentrated in well-drained, compacted sediments above mid-tide level, whilst Macrophthalmus hirtipeswas found in waterlogged areas below mid-tide level. This vertical separation is shown not to be caused by differential desiccation tolerances, but by feeding and burrowing adaptations related to these different substrata.     

Host sharing and host manipulation by larval helminths in shore crabs: cooperation or conflict?

Larval helminths of different species that share the same intermediate host and are transmitted by predation to the same definitive host may cooperate in their attempts to manipulate the behaviour of the intermediate host, while at the same time having conflicts of interests over the use of host resources. A few studies have indicated that intermediate hosts harbouring larval helminths have altered concentrations of neurotransmitters in their nervous system, and thus measuring levels of neurotransmitters in host brains could serve to assess the respective and combined effect of different helminth species on host behaviour. Here, we investigate potential cooperation and conflict among three helminths in two species of crab intermediate hosts. The acanthocephalan Profilicollis spp., the trematode Maritrema sp. and an acuariid nematode, all use Macrophthalmus hirtipes (Ocypodidae) as intermediate host, whereas Profilicollis and Maritrema also use Hemigrapsus crenulatus (Grapsidae). All three helminths mature inside gulls or other shore birds. There was a significant decrease in the mean volume of Profilicollis cystacanths as the intensity of infection by this parasite increased in H. crenulatus, the only host in which this was investigated; however, there was no measurable effect of other helminth species on the size of acanthocephalans, suggesting no interspecific conflict over resource use within crabs. There was, in contrast, evidence of a positive interspecific association between the two most common helminth species: numbers of Profilicollis and Maritrema were positively correlated among crabs, independently of crab size, in M. hirtipes but not H. crenulatus. More importantly, we found that the total number of larval helminths per crab correlated significantly, and negatively, with concentrations of serotonin in crab brains, again only in M. hirtipes; numbers of each parasite species separately did not covary in either crab species with serotonin or dopamine, the other neurotransmitter investigated in this study. The relationship with serotonin appears due mainly to numbers of Profilicollis and Maritrema and not to nematodes. This is the first demonstration of a potentially synergistic manipulation of host behaviour by different helminth species, one that appears host-specific; our results also point toward the neurobiological mechanism underlying this phenomenon.     

Assortative pairing in Gammarus insensibilis (Amphipoda) infected by a trematode parasite

We have investigated the influence of Microphallus papillorobustus (Trematoda) on the reproductive biology and mating patterns of its intermediate host Gammarus insensibilis (Amphipoda). Infected Gammarus species show altered behaviour which renders them more susceptible to predation by Charadriiform birds, the parasite's definitive hosts. In a natural population of G. insensibilis, mean parasite intensity was higher for unpaired individuals than for paired individuals. Fecundity was reduced in infected amphipods. Size-assortative pairing was significant, although infected males were found with smaller females compared to uninfected males of the same size. There was also a positive assortative pairing by parasitic prevalence. Vertical segregation between infected and uninfected individuals, male-male competition for access to uninfected females, and female choice may explain assortative mating for prevalence. This study provides the first empirical evidence that parasites can have a direct effect on patterns of mating in gammarids.     

Occurrence and maturation of Rhabdochona gnedini (Nematoda: Rhabdochonidae) in the barbels of the Sousa River, Portugal

Data on the seasonal occurrence and maturation of Rhabdochona gnedini Skrjabin, 1946 in its definitive host, the barbel Barbus bocagei, from the Sousa River, in northern Portugal, are presented. Monthly samples were taken from November, 1998 to October, 1999. A total of 178 fish specimens were inspected. R. gnedini occurred with high prevalences during whole year, attaining the highest values of intensity in summer (overall prevalence and mean intensity 85% and 31, respectively). A clear annual cycle of maturation was observed; females with mature eggs occurred in spring and summer. Unidentified Rhabdochona larvae were recorded from the trichopteran larvae Hydropsyche sp. (prevalence and intensity 29% and one to 34, respectively). These larvae are presumptive intermediate host for R. gnedini.     

Acanthocephala from lake fishes in Wisconsin: ecology and host relationships of Pomphorhynchus bulbocolli (Pomphorhynchidae)

Pomphorhynchus bulbocolli Linkins in Van Cleave, 1919, was considerably more common in fishes of the river-connected Tichigan Lake than of the landlocked Silver Lake, southeastern Wisconsin. It is reported from 17 species of principal, accessory, and occasional definitive hosts (new record in Moxostoma carinatum) and from 13 species of paratenic hosts (new records in Amia calva, Ictalurus punctatus, Lepomis cyanellus, and Pomoxis nigromaculatus). Infection patterns were influenced by fish species, feeding behavior, temperature, availability of intermediate host, type of water body, fish movement, and changes in fish host community. Host roles are not fixed but are often interchangeable. A seasonal cycle in prevalence, intensity, and maturation was evident, with greatest abundance and maturation during summer and recruitment during summer and autumn. Recruitment of new infections, development, and release of eggs, however, occurred all year. Sex ratio changed from near equal in new infections to one more highly in favor of females in older adults. Female fish were considerably more frequently and heavily infected than males. No relationship with fish age (size) was evident. Worms were mostly attached in posterior intestinal locations but initial establishment sites correlated with temperature. Translocation of P. bulbocolli due to competitive exclusion in concurrent infections was not observed. The significance of extraintestinal larval forms in the cycle of transmission was noted.     

Prevalence and mean intensity of infestation by Carcinonemertes carcinophila imminuta (Nemertea: Carcinonemertidae) in the gills of Callinectes danae and Callinectes ornatus (Decapoda: Portunidae) from São Sebastião,Brazil

The nemerteans of the genus Carcinonemertes live in association with decapod crustaceans. Juveniles of Carcinonemertes carcinophila imminuta Humes, 1942 are found in the gills of their hosts. A total of 281 specimens (52 males and 229 females) of Callinectes danae Smith, 1869 and 373 (332 males and 41 females) of Callinectes ornatus Ordway, 1863 were inspected for the occurrence of Carcinonemertes carcinophila imminuta in their gills. The prevalence and mean intensity of infestation by these nemerteans with regard to sex, maturity, condition of adult female crabs (ovigerous or non ovigerous), size (carapace width) and molt stage of hosts were evaluated. A prevalence of infestation by Carcinonemertes carcinophila imminutaof 39.1% was found in the population of Callinectes danae; the mean intensity of infestation was 12.0±2.7, while intensity ranged from 1 to 268. In this host species, significant differences in prevalence were observed between males and females, juveniles and adults and ovigerous and non-ovigerous adult females. Prevalence of infestation did not differ significantly among crabs in different molt stages. Mean intensity did not differ significantly between males and females nor among crabs in different molt stages. Significant differences in mean intensity were observed between ovigerous and non-ovigerous adult females of Callinectes danae. A positive correlation was observed between prevalence of infestation and the carapace width of Callinectes danae females. The prevalence of infestation in Callinectes ornatus was 8.0%; the mean intensity of infestation was 2.7±0.4 while intensity ranged from 1 to 9. Prevalence and mean intensity of infestation did not differ significantly between males and females and juveniles and adults. Prevalence did not differ significantly among crabs in different molt stages. No correlation was observed between the prevalence of infestation and the carapace width of males and females of Callinectes ornatus.     

Histopathological changes in gills of the estuarine crab Chasmagnathus granulata (Crustacea-Decapoda) following acute exposure to ammonia

Histopathological effects of ammonia on the gills of the estuarine crab Chasmagnathus granulata (Dana, 1851) were evaluated after acute exposure to ammonia concentrations around LC₅₀ value (17.85 Mm). Disruption of pilaster cells and a subsequent collapse of gill lamellae were the main effects observed. Ephitelial necrosis and hyperplasia were also detected. Significant (P<0.05) increases in pCO₂ and lactate, and significant decreases of pO₂ were detected in the haemolymph of ammonia-exposed crabs. These changes suggest that the observed histopathological damage affected gas exchange, possibly leading to death.     

On the ecology and distribution of Pseudoterranova decipiens C (Nematoda: Anisakidae) in an intermediate host, Hippoglossoides platessoides, in northern Norwegian waters.

The distribution of Pseudoterranova decipiens C helps demonstrate the ecological basis of this genetically defined sibling species. In northern Norwegian waters the major fish intermediate host is Hippoglossoides platessoides. Overall prevalence, mean intensity and intensity range in H. platessoides were 15%, 16.5 and 1-165, respectively. Outside the range of its only known definitive host, the seal Erignathus barbatus, the parasite was not found in the same intermediate host.     

Species-specific prevalence of mermithid parasites in populations of six congeneric host caddisflies of Ecnomus McLachlan, 1864 (Trichoptera: Ecnomidae)

Mermithid nematodes are entomophagous parasites and, despite being present in diverse aquatic insects, studies of caddisflies acting as definitive hosts are few and the ecological impacts on host populations are largely unknown. During a four-year study in a stream in southeastern Australia, parasitic mermithid worms were commonly found inside adult caddisflies, but only species of the genus Ecnomus McLachlan, 1864 (Ecnomidae). Ecnomus were the definitive host and parasite prevalence ranged from <1% to nearly 50% across six species. Species-specific prevalence varied little between years and was typically higher in males than in females. Parasite intensity ranged from 1 to 6 (median = 1), but did not vary between species or sexes. Infected hosts could fly, but were castrated and died when worms emerged. High prevalence and virulence (reproductive failure and death of the host) suggests that these parasites could potentially play a role in the population dynamics of some Ecnomus spp.     

Prevalence and intensity of Haemobaphes diceraus (Copepoda: Pennellidae) from shiner perch,Cymatogaster aggregata (Embiotocidae)

The prevalence and intensity of the hematophagous pennellid copepod Haemobaphes diceraus were monitored over a 10-mo period in shiner perch Cymatogaster aggregata at Pipers Lagoon, Nanaimo, British Columbia. The prevalence and mean intensity of metamorphosed adult female H. diceraus (n = 421) were 10.0% and 1.2 (+/-0.5 SD), respectively. The majority (97.9%) of infected fish had single infections, reflecting the possibility of intensity-dependent parasite-induced mortality, rejection of additional parasites, or both. Transforming females were detected throughout the year; however, there was no detectable seasonal pattern of colonization. Neither copepodids nor adult males of H. diceraus were observed on the gills of shiner perch, and this was consistent with the hypothesis that an intermediate host harbors these stages. Males of Haemobaphes sp. infected the gills of bay pipefish Syngnathus griseolineatus with a prevalence and mean intensity of 56.0% and 6.8 +/- 3.7, respectively. Transmission of H. diceraus to shiner perch probably occurs in inshore protected areas, where shiner perch ecologically overlap with the probable intermediate host of H. diceraus, the bay pipefish.     

Larval helminths in intermediate hosts: does competition early in life determine the fitness of adult parasites?

Density-dependent effects on parasite fitness have been documented from adult helminths in their definitive hosts. There have, however, been no studies on the cost of sharing an intermediate host with other parasites in terms of reduced adult parasite fecundity. Even if larval parasites suffer a reduction in size, caused by crowding, virtually nothing is known about longer-lasting effects after transmission to the definitive host. This study is the first to use in vitro cultivation with feeding of adult trematodes to investigate how numbers of parasites in the intermediate host affect the size and fecundity of adult parasites. For this purpose, we examined two different infracommunities of parasites in crustacean hosts. Firstly, we used experimental infections of Maritrema novaezealandensis in the amphipod, Paracalliope novizealandiae, to investigate potential density-dependent effects in single-species infections. Secondly, we used the crab, Macrophthalmus hirtipes (Ocypodidae), naturally infected by the trematodes, M. novaezealandensis and Levinseniella sp., the acanthocephalan, Profilicollis spp., and an acuariid nematode. These four helminths all develop and grow in their crustacean host before transmission to their bird definitive host by predation. In experimental infections, we found an intensity-dependent establishment success, with a decrease in the success rate of cercariae developing into infective metacercariae with an increasing dose of cercariae applied to each amphipod. In natural infections, we found that M. novaezealandensis-metacercariae achieved a smaller volume, on average, when infrapopulations of this parasite were large. Small metacercariae produced small in vitro-adult worms, which in turn produced fewer eggs. Crowding effects in the intermediate host thus were expressed at the adult stage in spite of the worms being cultured in a nutrient-rich medium. Furthermore, excystment success and egg-production in M. novaezealandensis in naturally infected crabs were influenced by the number of co-occurring Profilicollis cystacanths, indicating interspecific interactions between the two species. Our results thus indicate that the infracommunity of larval helminths in their intermediate host is interactive and that any density-dependent effect in the intermediate host may have lasting effects on individual parasite fitness.     

Sex ratios in field populations of two parasitoids (Hymenoptera: Chalcidoidea) of Coccus hesperidum L. (Homoptera: Coccidae)

We tested several assumptions and predictions of host-quality-dependent sex allocation theory (Charnov et al. 1981) with data obtained for the parasitoid Metaphycus stanleyi Compere on its host, brown soft scale (Coccus hesperidum L.), in a California citrus grove and in the laboratory. Scales ceased growing after parasitization by M.?stanleyi. Thus, M. stanleyi may gauge host quality (=size) at oviposition. Host size positively influenced adult parasitoid size, and parasitoid size in turn influenced adult longevity of M. stanleyi. However, parasitoid fitness gains with host size and adult size were similar in males versus females. Sex allocation to individual hosts by M. stanleyi depended on host size; females consistently emerged from larger hosts than males. Host size was important in a relative sense; the mean host sizes of females versus males, and of solitary versus gregarious parasitoids varied with the available host size distribution. The offspring sex ratio of M. stanleyi reflected the available host size distribution; the sex ratio of emerging parasitoids varied with the available host size distribution. We did not detect a “critical host size” below which males emerged, and above which females emerged; rather, only females emerged from hosts in the upper size range, and a variable ratio of males and females emerged from hosts in the lower size range. We conclude that the sex ratio of field populations of M.?stanleyi is driven largely by the available size distribution of C. hesperidum. In addition, we tested predictions resulting from theoretical analyses of sex allocation in autoparasitoids with data obtained on Coccophagus semicircularis (Förster) parasitizing brown soft scale in the field. The sex ratio of C. semicircularis was consistently and strongly female biased (ca. 90% females). Based on available theoretical analyses, we suggest that this sex ratio pattern may have resulted from a very low encounter rate of secondary hosts coupled with a strong time limitation in C. semicircularis females. This explanation was the most plausible given constraints stemming from the detection of secondary hosts, their variable location within primary hosts, and their handling times. Finally, the size of hosts which yielded single versus multiple parasitoids, and the sizes of these parasitoids, were compared. These comparisons suggested that: (1) M. stanleyi females gauge host sizes precisely, and in terms of female offspring; thus a fitness penalty is not incurred by females which share a host, while males benefit from sharing a host, and; (2) instances where multiple C. semicircularis emerged from a single host were probably the result of parasitism by different females, or during different encounters by a single female.     

Population structure and reproductive biology of the fiddler crab Uca inversa (Hoffman, 1874) (Brachyura: Ocypodidae)

The population structure and reproductive biology of Uca inversa (Hoffman) were studied in Mozambique for the first time. Crabs were randomly sampled on monthly basis during low tide periods from January to December 2002 at Costa do Sol mangrove, Maputo Bay, southern Mozambique. A total of 1131 crabs was obtained, of which 518 (45.8%) were males, 322 non-ovigerous females (29.3%) and 281 (24.8%) ovigerous females, respectively. The present population presents non-normal size frequency distributions, with males reaching greater size than females. The overall sex ratio (M:F) (1:0.84) was significantly different from the 1:1 ratio. Ovigerous females were present throughout the year and the embryonic development showed synchrony with the gonadosomatic index, in which females carrying eggs close to hatching were more abundant when the gonadosomatic index reached minimum values in the population. Egg number increases with female size. Juvenile recruitment was also continuous with high proportion of young recruits being recorded in winter, probably due to the high reproductive activity displayed in summer. U. inversa exhibits a rapid embryonic cycle accompanied by a rapid larval development and settlement in the study area.     

Parasite‐induced Changes in the Anti‐predator Behavior of a Cricket Intermediate Host

Many parasites with complex life cycles are known to modify their host phenotype to enhance transmission from the intermediate host to the definitive host. Several earlier studies explored these effects in acanthocephalan and trematode parasites, especially in aquatic ecosystems; however, much less is known about parasite‐mediated alterations of host behavior in terrestrial systems involving nematodes. Here, we address this gap by investigating a trophically transmitted nematode (Pterygodermatites peromysci) that uses a camel cricket (Ceuthophilus pallidipes) as the intermediate host before transmission to the final host, the white‐footed mouse (Peromyscus leucopus). In a laboratory experiment, we quantified the anti‐predatory responses of the cricket intermediate host using simulated predator cues. Results showed a decrease in jumping performance among infected crickets as compared with uninfected crickets, specifically in terms of frequency of jumps and jumping distance. Additionally, the relationship between parasite load and frequency of jumps is negatively correlated with the intensity of infection. These behavioral modifications are likely to increase vulnerability to predation by the definitive host. An analysis of the age‐intensity pattern of infection in natural cricket populations appears to support this hypothesis: parasites accumulate with age, peak at an intermediate age class before the intensity of infection decreases in older age groups. We suggest that older, heavily infected crickets are preferentially removed from the population by predators because of increased vulnerability. These results show that cricket intermediate hosts infected with P. peromysci have diminished jumping performance, which is likely to impair their anti‐predatory behavior and potentially facilitate parasite transmission.     

Apparent independence of the spawning and moulting cycles in female Antarctic krill (Euphausia superba Dana)

Summary Sixty female Antarctic krill (Euphausia superba Dana) spawned in shipboard experiments and the interval between egg-laying and ecdysis was noted. The number of eggs laid per female ranged from 263–3662, most females produced only one batch of eggs before moulting, and the post-spawn ovaries of all females contained few, if any, mature oocytes. As reported in other studies, the total number of eggs produced per female was not well correlated with body size. Females appeared to spawn at all times during the moulting cycle and although no diurnal rhythm in spawning was observed, moulting occurred mainly at night-time despite the animals being kept in near-constant darkness. No evidence of synchronous moutling was detected.     

Fouling community of the red king crab, <Emphasis Type="Italic">Paralithodes camtschaticus</Emphasis> (Tilesius 1815), in a subarctic fjord of the Barents sea

We examined the species composition of red king crab (Paralithodes camtschaticus) fouling communities in Dolgaya Bay, a small fjord of the Barents Sea, in August 2005 and 2006. In total, there were 13 species observed on 301 crabs collected from water depths of 5–90 m. Barnacles (Balanus crenatus; prevalence 42.9%) and blue mussels (Mytilus edulis; 11.6%) were the most common epibionts, while amphipods (Ischyrocerus commensalis) were the most common symbionts (28.6%). Infestation rates in Dolgaya Bay were different from those in an “open” area of the Barents Sea (Dalnezelenetskaya Bay), probably due to differences in hydrodynamic conditions. Differences in infestation prevalence and intensity were detected neither between male and female crabs nor between crabs collected at 5–35 m versus 90 m depths. Prevalence of common fouling species increased with host size. Amphipods I. commensalis colonized the carapace and limbs in Dolgaya Bay less frequently than in Dalnezelenetskaya Bay, probably due to interspecific competition with barnacles occupying the dorsal parts of the host. Juvenile barnacles and mussels dominated the fouling communities on the crabs. The age of barnacles did not exceed 2–4 months. However, the presence of 4-year-old mussels suggests that these older mollusks have been directly transferred from mussel beds to the hosts. Our results indicate that colonization by epibionts and symbionts is generally not disadvantageous for the crab hosts, except for some possible negative impacts of amphipods occupying the gills.     

Seasonal population dynamics of the nematode Cystidicoloides tenuissima (Zeder) from the River Swincombe, England

The seasonal population dynamics of adult and larval Cystidicoloides tenuissima were studied in its definitive hosts brown trout, Salmo trutta and juvenile Atlantic salmon, S. salar , and mayfly intermediate host, Leptophlebia marginata , from the River Swincombe, Dartmoor National Park, Devon, U.K. Infective larvae were present in each mayfly generation for almost its entire duration in the steam benthos. The infection parameters (prevalence and mean intensity) and maturation in the fish indicated C. tenuissima was an annual parasite exhibiting a seasonal periodicity and also systematic variation with the host age. Maturation was correlated to river water temperature. Infection parameters increased from September to May, then declined in June and July and remained relatively constant for the rest of the summer. Variation in the fish infection parameters over time, site, and host species appeared to be controlled by transmission related events; the availability of infective larvae, host feeding behaviour and water temperature. The availability of infective larvae and host diet controlled the rate at which parasites were added to the parasite population, but the pattern of gains and losses was determined by a temperature dependent rejection response.     

A trematode infection with no effect on reproductive success of a sand-bubbler crab

The infection effects of the parasitic digenean trematode on the body weight and reproductive success of the sand-bubbler crab were examined. Gynaecotyla squatarolae (Trematoda: Microphallidae) infects the body cavity of Scopimera globosa (Decapoda: Scopimeridae) and uses the crab as its second intermediate host. The parasites infected all reproductive crabs examined to varying degrees. Larger crabs of both sexes had more parasites than smaller ones, probably because body size reflects age, and older crabs had a longer period of exposure to infection. Males had more parasites than females, probably because of sexual difference in acting time on the surface. Ovigerous females stay in closed burrows and do not act on the surface during incubation, and so have less chance of infection than males. The quantity of infecting parasites did not explain variations in either body weight or reproductive success of individual crabs in a field experiment. The life history of this parasite, relative body size of the crabs, and cost and the possible benefit of manipulation for the parasite may explain these results.