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Trophic Interactions: Similarity of Parasitic Castrators to Parasitoids
Abstract
An analysis of life history features of insect parasitoids and crustacean parasitic castrators suggests that these are similar trophic phenomena, distinct from parasitism and predation. A parasitoid consumes only one host during its lifetime; parasitic castrators cause the reproductive death of only one host. Since population densities of many insect species are regulated by parasitoids, parasitic castrators may also play an important role in host population regulation. Parasitoids of insects and parasitic castrators of crustaceans (1) in single infections always kill the host; whereas lone parasites do not affect host viability; and predators kill many prey; (2) do not cause increasing pathology or increasing likelihood of mortality in multiple infections; whereas parasites often have an additive impact; (3) do not cause increasing damage in mixed species infections; whereas mixed parasite infections often have interactive negative effect; (4) Usually have mechanisms to reduce or eliminate multiple infec...
... Specimens deposited: Rediae with cercariae and metacercariae (CNP-Par 226) and metacercariae (CNP-Par 227) from the Reserve (ethanol fixed specimens). (15)(16)(17)(18)(19)(20)(21)(22)(23)(24) wide, consisting of approximately 14 to 16 cells, bifurcates at 438 (336-480) from the anterior end of the body and caeca extending to the posterior end of the body. Numerous cystogenous cells are located between the pharynx and the posterior margin of the body. ...
... Excretory system stenostomate: The excretory vesicle was divided into two chambers located at the posterior end of the body. The primary excretory tubes originated in the anterior chamber of the excretory vesicle and dilated in the area between the ventral sucker and the pharynx to accommodate 21-30 spherical refractile granules 14 (11)(12)(13)(14)(15)(16)(17)(18) in diameter. Flame cells were difficult to see, with at least 15 pairs. ...
Simple Summary
The apple snail is one of the most dangerous invasive species in freshwater environments. Using molecular and morphological tools, we re-describe an echinostomatid digenean parasitizing snails from two sites in the Buenos Aires Province, Argentina. The two stages found (i.e., rediae and metacercariae) demonstrate that the apple snail acts as the first and second intermediate host in its life cycle. The prevalence of the parasite was higher at one of the sampling sites, probably because the birds bearing the adult stage are more abundant in that area. A histological study showed that this parasite quickly invades multiple organs of the snail, which is different from most digenean infections, which only infect the gonad and digestive glands. Heavy deterioration of female and male reproductive structures associated with the presence of the parasite was also observed, which indicates castration.
Abstract
Pomacea canaliculata is one of the most dangerous invasive species. Morphological and molecular analyses have revealed that a digenean species belonging to the family Echinostomatidae parasitizes this snail at two sites in Buenos Aires Province, Argentina, South America. Molecular results confirmed that the species belongs to a genus closely related to Patagifer. Analysis of the 28S rDNA showed that the sequences of the rediae and metacercariae are identical, indicating that the apple snail acts as the first and second intermediate host. The cercariae may encyst as metacercaria inside the redia and also emerge and re-infect the same snail or another snail. The prevalence of digeneans was higher in one of the sampling locations (15.1% vs. 0.72%), probably because the bird species that acts as the definitive host is more abundant in that area. Histopathological examination showed that the parasite quickly invades multiple host organs (gills, intestines, albumen gland, lung, kidney, and mantle border) besides the gonad and digestive gland, as is usual in digeneans. In addition, the partial or total castration of snails was observed in cases of moderate and high infection intensity. In males, there was loss of integrity in testicular tubules, while in females, the replacement of ovarian tissue by rediae was found.
... The parasite does not, in fact, occur on crangonids but this is irrelevant as to the correct name of the species. Giard & Bonnier (1890) recognized the priority of Monoculus crangorum as the type species of Bopyrus but further confused matters by abandoning both Fabricius' and Latreille's species names and replacing them with their own set of names, each linked to a single species of host (reflecting Giard's belief that each parasite species had only a single host; see Kuris 1974). They wrote: "The name B. squillarum given by Latreille in 1804 does not have priority: Fabricius had previously (1798) called the same crustacean Monoculus crangorum. ...
... This illustration shows that L. pollicipedis is nearly identical with H. balani (see Goudeau 1970) and as Giard considered L. pollicipedis and L. anatifae to be congeneric, it can be assumed that L. anatifae also belongs to Hemioniscus. Giard (1887) likely placed the parasites of pedunculate barnacles in a different genus than those of sessile barnacles following his theory that parasites found on different hosts must belong to different taxa (Kuris 1974). Despite the possibility that H. anatifae n. comb. is synonymous with H. balani, we conservatively retain the two species as distinct until topotypic specimens of the pedunculate barnacle parasite can be collected and examined. ...
Several long-standing taxonomic and nomenclatural problems in bopyroid and cryptoniscoid isopods are addressed. Within Bopyroidea we propose five new synonomies, three new combinations, a correction of the spelling of one species and a replacement name for another. In addition, the likely erroneous conclusion that a monogenean is an associate of bopyrids is discussed. Within Cryptoniscoidea, we determined the familial placement of three genera that were previously designated as incertae sedis (Capitoniscus Bourdon, 1972 and Carocryptus Schultz, 1977) or of doubtful affiliation (Cumoechus Hansen, 1916). In total, five species of cryptoniscoids are placed into two new families (Capitoniscidae n. fam. and Cumoechidae n. fam.), each containing at least one species known from both female and cryptoniscus larvae/paedomorphic male stages. Finally, the status of two cryptoniscoid genera is addressed: Gorgoniscus Grygier, 1981 is placed in Hemioniscidae, and Leponiscus Giard, 1887, is synonymized with Hemioniscus Buccholz, 1866.
... Parasites are significant components of marine ecosystems because parasitic infections can have a great impact on the reproduction and population dynamics of host species (e.g., Kuris, 1974;Kuris & Lafferty, 1992). The rhizocephalans, which are the sister taxon of the thoracican barnacles in Cirripedia (Høeg & Lützen, 1985, 1995Høeg, 1995a;Høeg et al., 2019Høeg et al., , 2020, are important parasites of crustaceans, especially crabs and hermit crabs (Hines et al., 1997). ...
Most individuals of the rhizocephalan Peltogasterella gracilis (Boschma, 1927) produce only male (larger) or female (smaller) offspring, but some mixed-sex broods appear in autumn. Such variability may be an adaptation to the seasonally changing availability of hosts (for female larvae) and conspecific females (for male larvae), but details of the seasonal variation in the sex ratio and its ultimate causes have not been fully studied in P. gracilis. We studied the life cycle, host availability, and seasonal sex-ratio variation in P. gracilis by samplings from spring to autumn in 2017–2020 in Otaru, and additionally from winter to spring of 2007 in Oshoro Bay, Hokkaido, northern Japan. We measured its prevalence on the hermit crab Pagurus lanuginosus De Haan, 1849, the number of males per externa, the proportion of externae brooding different embryonic stages, and embryo size that are linked to sex. The prevalence was about 20% in most of the sampling period. The number of males per externa was 0.01–0.74 in June 2018–2020, but increased in later months to nearly 1.0. Eyed embryos and nauplii before hatching were common from June or July to September, but they were rare in other seasons. Female embryos were observed in all sampling periods, but male embryos were not found in July or August, by which time most females already have males implanted within their receptacles. It is suggested that P. gracilis releases both female and male larvae according to the availability of unparasitized hosts and of virgin externae, respectively.
... Even highly lethal pathogens are predicted to be ineffective drivers of extinction, because killing the host decreases the duration of infection and leads to a diminishing impact on host-population size (Anderson & May, 1981). Diseases that cause sterility are widespread in plant pathogens (Clay, 1991;Wennström et al., 2003) and in animal parasites ("parasitic castration"; Baudoin, 1975;Jaenike, 1992;Kuris, 1974), and sterilization also characterizes many sexually transmitted diseases (Lockhart et al., 1996). Our simulation results show that multimodal transmission of a sterilizing disease, which could be attributed to simultaneous vector-borne and aerial spore deposition, can result in very fast population declines and limitation of host abundance and range, and may be a contributor to population declines in alpine carnations. ...
Theoretical models suggest that infectious diseases could play a substantial role in determining the spatial extent of host species, but few studies have collected the empirical data required to test this hypothesis. Pathogens that sterilize their hosts or spread through frequency‐dependent transmission could have especially strong effects on the limits of species' distributions because diseased hosts that are sterilized but not killed may continue to produce infectious stages and frequency‐dependent transmission mechanisms are effective even at very low population densities. We collected spatial pathogen prevalence data and population abundance data for alpine carnations infected by the sterilizing pathogen Microbotryum dianthorum, a parasite that is spread through both frequency‐dependent (vector‐borne) and density‐dependent (aerial spore transmission) mechanisms. Our 13‐year study reveals rapid declines in population abundance without a compensatory decrease in pathogen prevalence. We apply a stochastic, spatial model of parasite spread that accommodates spatial habitat heterogeneity to investigate how the population dynamics depend on multimodal (frequency‐dependent and density‐dependent) transmission. We found that the observed rate of population decline could plausibly be explained by multimodal transmission, but is unlikely to be explained by either frequency‐dependent or density‐dependent mechanisms alone. Multimodal pathogen transmission rates high enough to explain the observed decline predicted that eventual local extinction of the host species is highly likely. Our results add to a growing body of literature showing how multimodal transmission can constrain species distributions in nature.
... Trematode infection inevitably leads to snail host castration and drastically reduces host fitness, which, in evolutionary terms, is equivalent to death of the host (Lafferty 1993, Fredensborg et al. 2006). In this sense, being castrated by a parasite is similar to being eaten by a predator (Kuris 1974). This means that parasitic castrators can exert a strong selective force that could favor the expression of behavioral, physiological, and morphological adaptations to minimize the negative impact of parasitism on host fitness (Lafferty 1993). ...
Heterogeneous environments pose a particular challenge for organisms because a single phenotype is unlikely to perform best across the variety of encountered stressors. To understand how species meet this challenge, we investigated the extent to which contrasting environmental pressures induced ecological and phenotypic responses in a natural population of a wide-dispersing marine snail at a small spatial scale. We analyzed several traits of Heleobia australis (Rissooidea: Cochliopidae) collected from heterogeneous, but highly connected, habitats from the intertidal area of the Bahia Blanca estuary, Argentina. We also conducted molecular analyses by amplifying the COI gene in individuals sampled from each habitat. We found that sympatric subpopulations of H. australis exhibited a strong phenotypic divergence in shell characters and body weight in response to thermal, saline, and dehydration stress, crab predation risk, and parasitic castrators. We proved that this differentiation occurred even early in life, as most of the characters observed in juveniles mirrored those found in adults. We also found a divergence in penis size in snails collected from each habitat and raised in common garden laboratory conditions. Molecular analyses confirmed that the individuals studied constituted a single species, despite the strong phenotypic differences among subpopulations. The small-scale phenotypic differentiation suggests that H. australis experienced a fine-grained environment where conditions imposed by different sources of stress favored the expression of beneficial traits. We discuss the role of plasticity in shaping adaptive phenotypic responses that increase the likelihood of persistence of subpopulations facing environmental stress conditions.
... Parasitic castration is a phenomenon where the parasite blocks or reduces the host reproductive output [71][72][73][74][75]. Parasitic castration and gonadal regression due to parasites are well documented among invertebrates, especially in molluscs and crustaceans [32][33][34]. ...
Parasitic castration is a phenomenon where the parasite blocks or reduces the host reproductive output. This study explored the impact of Tetracotyle wayanadensis infection on the reproductive performance of the freshwater fish Aplocheilus lineatus. Heavily infected A. lineatus showed slow and sluggish movements with reduced food intake and feeding activities. Histopathological observations of the ovary revealed severe necrosis and degeneration of previtellogenic and vitellogenic oocytes and follicular atresia. The oogonial nests were absent; chromatin nucleolar oocytes irregularly shaped and vacuolated. Perinucleolar oocytes have a vacuolated appearance with the hypertrophied highly columnar follicular cells invading them. Vitellogenic oocytes had a shrunken appearance with folded zona radiata; yolk globules broken down into smaller granules together with vacuolar degeneration of the ooplasm. The hypertrophied highly columnar follicular cells invaded the early vitellogenic oocytes to phagocytize the degenerating material. Zona radiata was found broken and liquefaction of yolk globules was evident with advancement in follicular atresia. Seasonal variation of parasitic infection has also been studied. Metacercaria infecting the vital organs showed seasonal variation with the highest values for prevalence and abundance during monsoon and mean intensity during winter. The lowest values for these de-scriptors were documented during the post-monsoon period.
Invasive species significantly impact ecosystems, causing damage to plant species and billions of dollars in loss annually, hence requiring the development of inventive and sustainable methods of management. The effective control of invasive species has become a significant concern in the field of modern ecology and conservation. Biological control is a management approach for invasive species that involves using natural enemies such as predators, parasites, and pathogens to control the population of invasive species. BCAs (biological control agents) have led to unexpected outcomes due to global environmental change and trade. Studying plant invasions under normal conditions is insufficient. The impact is linked to environmental change and human disturbances. An integrated approach is needed to evaluate BCA impacts and involve ecosystem managers and conservationists in diverse efforts. The use of genetically modified organisms for the purpose of managing invasive species is a highly promising technique, but there are also risks associated with their use, such as unintended effects on nontarget species. The effective management of invasive species requires the use of an integrated strategy that includes prevention, early detection, and quick response. Eradication is the preferred strategy, but if it is not possible, control and management strategies are necessary. These strategies endeavor to mitigate the utilization of pesticides and prioritize the implementation of biological control as the primary modality of defense. The future prospects pertaining to the biological control of invasive species encompass the utilization of genetic biocontrol techniques; however, apprehensions arise regarding the potential risks associated with their use.
Some cancers have evolved the ability to spread from host to host by transmission of cancerous cells. These rare biological entities can be considered parasites with a host-related genome. Still, we know little about their specific adaptation to a parasitic lifestyle. MtrBTN2 is one of the few lineages of transmissible cancers known in the animal kingdom. Reported worldwide, MtrBTN2 infects marine mussels. We isolated MtrBTN2 cells circulating in the hemolymph of cancerous mussels and investigated their phenotypic traits. We found that MtrBTN2 cells had remarkable survival capacities in seawater, much higher than normal hemocytes. With almost 100% cell survival over three days, they increase significantly their chances to infect neighboring hosts. MtrBTN2 also triggered an aggressive cancerous process: proliferation in mussels was ~ 17 times higher than normal hemocytes (mean doubling time of ~ 3 days), thereby favoring a rapid increase of intra-host population size. MtrBTN2 appears to induce host castration, thereby favoring resources reallocation to the parasites and increasing the host carrying capacity. Altogether, our results highlight a series of traits of MtrBTN2 consistent with a marine parasitic lifestyle that may have contributed to the success of its persistence and dissemination in different mussel populations across the globe.
Tamarixia radiata is one of the established biocontrol pests against the major Asian citrus psyllid, Diaphorina citri, a vector of Candidatus Liberibacter that is a causal agent of citrus Huanglongbing (HLB) disease. Updated information and regional exploration on biocontrol pests are important elements for effective disease management strategies. In this study, the diversity and parasitism rate of T. radiata populations were evaluated. Due to the importance of the host–parasitoid relationship, the presence of Wolbachia as an endosymbiont was also investigated. The parasitism rate of various T. radiata populations from Ecuador and China ranged between 57.27% and 66.32%, respectively, with a non-significant emergence rate and a statistically similar sex ratio. Sequence analysis of ITS and COI from T. radiata populations was consistent with the morphological hypothesis that the collections represent a single species, whereas phylogeny of the wsp gene confirmed the presence of Wolbachia pipientis as an endosymbiont within T. radiata populations. Based on partial COI sequences, the maximum genetic diversity such as total haplotype diversity (Hd = 0.788), nucleotide, diversity (π = 0.2439), and average nucleotide difference (k = 171.844) was also estimated for different T. radiata populations. Furthermore, neutrality tests based on COI sequences indicated an overall contraction in T. radiata populations, whereas an expansion trend was observed in associated W. pipientis strains. This study clearly demonstrated the presence of genetically diverse T. radiata populations that were able to parasitize D. citri effectively, and these can be further explored as promising biocontrol candidates in integrated pest management strategies to solve citriculture economic loss caused by D. citri.
1. Peachia quinquecapitata kept in sea water tanks at the Friday Harbor Laboratories were induced to spawn. The egg is 120 µ in diameter and is covered with spines 20 µ in length.2. Cleavage leads to a hollow blastula which becomes ciliated. As the cilia appear the spines become shorter and disappear.3. Gastrulation is by invagination. The planula stage is reached when the gastrula develops an apical tuft of long cilia.4. A swimming planula can be ingested by the medusa Phialidium gregarium. After ingestion the larva begins to grow and differentiate. Planulae which did not get ingested would not develop in the laboratory.5. The larvae remained endoparasitic for an average of 11 days and probably were feeding on material in the gatrovascular cavity of the host medusa.6. After 11 days the larvae became ectoparasitic and fed on the gonads of the host medusa. After an average of 31 days of ectoparasitism anemones had acquired their adult characteristics and dropped off the host medusa to become free-living.7....
The size of the zooplankton consumed by rainbow trout and yellow perch was studied in two Michigan lakes and compared with the size of the limnetic zooplankton. One of the study lakes was managed for both rainbow trout and warmwater game fish (a “combination” lake), and the other was managed solely for rainbow trout after it was reclaimed with toxaphene. Plankton samples were collected in the summer both before and after the introduction of rainbow trout, and compared with plankton in the stomachs of fish taken at the same time.Daphnids were the only zooplankton consumed by rainbow trout in both lakes and by yellow perch in the combination lake even though there were many other genera of zooplankters. Both species were very size selective and usually consumed only Daphnia over 1.3 mm in size while ignoring the many and often more numerous smaller zooplankton. Despite the broad range in size of these trout (7.9 to 17.2 inches) and yellow perch (2.8 to 9.8 inches), there was no strong evidence of an association between their length and the size of the Daphnia they consumed.Introduction of rainbow trout in the combination lake had no apparent effect on the daphnid population. However, changes in the net plankton did occur in the trout lake after the introduction of rainbow trout, smelt, and fathead minnows. Most obvious changes were: (1) the complete elimination of D. pulex and subsequent replacement by two other smaller species within 4 years, (2) a decrease in the average size of the daphnids from 1.4 mm to 0.8 mm, (3) a decrease in the percentage of daphnids larger than 1.3 mm from an average of 53.8% to 4.7%, and (4) a reduction in the volume and percentage of daphnids comprising the net plankton even though there was no reduction in their numbers. The gap left by the elimination of the large Daphnia was filled by other smaller zooplankters.Effects of predation on Daphnia, as well as the interactions between daphnids and planktivorous fishes are discussed. This study emphasizes the importance of determining the abundance of zooplankton of the proper sizes in the plankton when the survival of planktivorous fishes in lakes is investigated.
Vor der Nordostküste Englands lebt Hemiartbrus abdominalis auf Pandalus montagui, Spirontocaris lilljeborgii, S. spinus und Eualus pusiolus. Die Endgrösse dieses Parasiten und die Grösse, bei welcher er sich fortzupflanzen beginnt, sind direkt abhängig von der Grösse des Abdominal-raumes der Wirt-Art. Hemiarthrus wurde nur auf weiblichen Tieren gefunden (Ausnahme: P. montagui). Im Falle des protandischen Zwitters P. montagui wurden Männchen, Weibchen und Übergangsstadien parasitiert; allerdings überwogen die parasitierten Weibchen zahlenmässig bei weitem. Der Parasit bewirkt eine Verlangsamung der Eientwicklung des Wirtes; mit gelegentlichen Ausnahmen im Falle von P. montagui wurden die Eier nicht reif. Sofern parasitierte P. montagui zur Eiablage schreiten, misslingt im allgemeinen die Anheftung der Eier an die Pleopoden. Parasitierte P. montagui legen das Brutkleid verfrüht an.
On the basis of studies of seasonal changes in the infection levels of Sacculina carcini in Carcinus maenas a life history for Sacculina at Millport, Scotland is proposed. This life history differs from that proposed by Foxon (1940) in that the externa is considered only to live for one year, the interna subsequently regenerating a new externa. This life cycle receives support from observations on regenerating externa.
The biology, economic importance, physiology, ecology, cytology, and genetics of the parasitic wasp, Mormoniella vitripennis (=Nasonia brevicornis), are reviewed, and its use and value in research are also considered. Normally Mormoniella is an ectoparasite, and uses the pupae of larger Diptera as hosts; at 28⚬ C a generation is completed in ten days. The species is world-wide in its distribution, and may be found wherever hosts are available. Females are inefficient flyers, and males cannot fly. In genetical research, largely using Sarcophaga pupae as hosts, both spontaneous and radiation-induced mutants have been studied. In the R locus, described in "orthodox" genetic terms as a chromonemal region marked by eye color mutants as well as many other genes, all of which show complete linkage and govern various deleterious traits, including male and female sterility and reduced viability at various stages of the life cycle. Incompatibility in reciprocal crosses is reported to be governed by cytoplasmic fact...
The foraging ecology of the asteroid Leptasterias hexactis was studied at five rocky intertidal areas in the San Juan Islands, Washington, for 2-2@3 years. Feeding activity during high tides reaches a high point in mid- to late summer (July and August) and a low point in the winter (January). Three probable factors selecting for this cycle are (1) reproduction, virtually immobilizing the females, during the winter; (2) low food availability in winter compared to summer; and (3) violent winter storms. Persistently reduced low tide feeding during all but autumnal night low tides indicates that some light-related factor(s) select for a tidal cycle of feeding also. Factors believed responsible are desiccation stress and possibly predation by visual predators. A graphic model of foraging behavior is presented. Analysis of the diet of Leptasterias by both numbers and calories of prey consumed indicates that this starfish is a food generalist. Moreover, numbers of prey consumed, when compared to prey available, indicate those prey chosen selectively and those which are avoided. Lastly, calories consumed compared to calories available indicate why particular prey are chosen. Many factors are involved in prey choice including ease of capture, relative sizes of predator and prey, and caloric yield of the prey. Actual availability is defined as that portion of available prey (i.e., prey physically present) which could be consumed by a predator. The factors affecting prey choice, coupled with prey and predator distribution, abundance, and behavior must be considered when determining actual availability of prey. Actual availability coefficients are computed and suggest that (1) hard-to-capture prey are least available; (2) Leptasterias is more selective in summer and autumn, when food is more abundant; and (3) prey are more available to large starfish. Calculations suggest that, under normal conditions, Leptasterias would be severely energy-limited in winter if it foraged at rates similar to those of spring or summer, or both. Thus, food limitation may be partially responsible for reduced feeding in winter. Comparison of numerical patterns of consumption of energy-rich and energy-poor prey indicates that energy-rich prey are most important except in summer when energy-poor prey become the most important prey, presumably because of their tremendous numerical increase and domination. Comparisons between the foraging strategy of this starfish and theoretical models previously proposed support some predictions but not others, indicating that complex natural systems require more comprehensive and precise models. Predictability of resources in time and space probably determines the degree of resource specialization or generalization in animals.
IN the present discussions on factors limiting the size of natural populations1, investigations in helminthology, though they have much to contribute, have received little attention. A rigorously tested case of mutual habitat exclusion, as has been called for2, was demonstrated experimentally with Hymenolepis (Cestoda) and Moniliformis (Acanthocephala) in white rats3. More common than such cases of direct competition are examples of niche diversification among two to three congeneric species that may be found in the same host individual4-7. Co-existence of numerous related nematode species in vertebrates has also been claimed8.
1. Lagenidium chthamalophilum is described as a parasite of the ova of Chthamalus fragilis denticulata. The pathogen is characterized by the formation of a vesicle before sporangial protoplasm migration, and by highly vacuolate, stout vegetative hyphae. In these features, L. chthamalophilum differs from the usual interpretation of members of the genus. The fungus is compared with other Phycomycetes known to parasitize crustacean ova.2. Artificial inoculation experiments show L. chthanzalophilum to be specific for C. fragilis denticulata. The associated barnacle, Balanus amphitrite, is resistant to the fungus.
For some years investigations on the ecology of the common shore crab, Carcinus maenas, have been carried out in inshore Danish waters (the Isefjord area, Zealand) and among other things the migrations of the species were studied. In contrast to the observations from British waters of Williamson1 and others, it was observed that during the breeding season—which in the Isefjord comprises only May and June—the berried female crabs leave the littoral areas and hide in the more saline and deeper water (maximum depth 8–10 metres) until the larvæ are hatched. After this period they return to moult and copulate with the male crabs which stay all summer in shallow water (0–1.5 metres). The annual mean salinity of the Isefjord is 19.4 per mille.
Turbellaria are primarily free-living predators but each of their five orders contains families with representatives living in association with other animals. The commonest partners with which these associations are established include (1) echinoderms, (2) crustaceans, and (3) mollusks. One striking feature of these associations is the great extent to which Turbellaria show “host-type” specificity, in that the members of one family tend to be associated with a single type of host organism. Generally, commensalism implies the sharing of the same food by the turbellarian and its partner. This concept of commensalism provides a useful background against which most turbellarian associations can be considered. On the other hand, the parasitic turbellarians represent a climax to the gradual intensification of the relationship among commensal species and their particular partners and the other types of relationship noticed represent stages in the evolution of the parasitic habit. This chapter systematically reviews the occurrence of parasitism and commensalism in Turbellaria throughout the class, with emphasis to modifications in structure, physiology, or life history, which appear to be related to the transition from the basic free-living habit.