 |
| Parasite(Taxon) | Recorded hosts | Infected tissue or site of infection | Transmission |
|---|---|---|---|
| Pasteuria ramosa (Bacteria) | D. magna, D. pulex, D. longispina, other Cladocera | Blood, extracellular | Horizontal, from dead host |
| White Fat Cell Disease (Bacteria) | D. magna, D. pulex, D. longispina | Fat body, intracellular | Horizontal, from dead host |
| Spirobacillus cienkowskii (Bacteria) | Many Daphnia species | Blood, extracellular | Horizontal, from dead host |
| Aphanomyces daphniae (Fungi) | D. hyalina, D. pulex | Body cavity, extracellular | Horizontal, from dead host |
| Metschnikowia bicuspidata (Fungi) | D. magna, D. pulex, D. longispina | Body cavity, extracellular | Horizontal, from dead host |
| Flabelliforma magnivora (Microsporidia) | D. magna | Fat body, ovaries, intracellular | Vertical |
| Octosporea bayeri (Microsporidia) | D. magna | Fat body, ovaries, intracellular | Vertical and horizontal, from dead host |
| Gurleya vavrai (Microsporidia) | D. pulex, D. longispina | Carapace, intracellular | ? |
| Glugoides intestinalis (formerly Pleistophora i.) (Microsporidia) | D. magna, D. pulex | Gut wall, intracellular | Horizontal, from living host |
| Ordospora colligata (Microsporidia) | D. magna | Gut wall, intracellular | Horizontal, from living host |
| Larssonia obtusa (= L. daphniae) (Microsporidia) | D. magna, D. pulex, D. longispina | Fat body, intracellular | ? |
| Pansporella perplexa (Amoeba) | D. magna, D. pulex, D. longispina, D. hyalina, D. obtusa | Gut wall, extracellular | Horizontal, from living host |
| Caullerya mesnili (unknown) | D. pulex, D. longispina, D. magna, D. galeata, D. obtusa, Daphnia hybrids | Gut wall, intracellular | Horizontal, from living host |
| Echinuria uncinata (Nematoda) | D. pulex, D. magna, D. obtusa, other Cladocera | Body cavity, extracellular | Horizontal, to second host |
| Cysticercus mirabilis (Cestoda) | D. magna | Body cavity, extracellular | Horizontal, to second host (?) |
The parasites described in this chapter are by no means more important than any other parasites of Daphnia, but they are those that happen to be the most studied, partly because they have been found to be at least locally abundant. Parasites of D. magna are predominant because parasites of this well-investigated and largest European Daphnia species are best known. Most of my own work on parasites has used D. magna as a host. Also, parasites for whose entire life cycle can be completed under laboratory conditions were more intensively studied than the numerous species that we do not currently know how to propagate. Despite this bias in representation, however, the species introduced in this chapter give a good impression of the diversity of parasites known to infect the genus Daphnia.
We have a good knowledge of the taxonomic position of only a few endoparasites of Daphnia. For some species, we do not even know the approximate position, e.g., Caullerya mesnili; therefore, I cannot use a strict taxon-based listing of the parasite species. Instead I provide information on other aspects of their biology, which allows us to categorize them into groups so that they can be easily found. When DNA sequence data are available for more species, taxonomic position will be easier to define (Ebert et al. 1996; Refardt et al. 2002).
Six species of bacteria have been described parasitizing Daphnia. Four of them infect the hemolymph, whereas two are intracellular infections of the fat cells and the eggs, respectively. Bacterial infections are generally harmful to their hosts, drastically reducing host reproductive success.
Bacteria have been observed to infect Daphnia either as endoparasites or epibionts. However, only the taxonomy for Pasteuria ramosa has been worked out and published thus far (Ebert et al. 1996). The taxonomy of Spirobacillus cienkowskii is in preparation (M. Duffy, personal communication). The other species are either described by their typical pathology or are collectively placed into a group with roughly similar characteristics. Most species do not yet have a scientific name.
The recorded bacteria infect either the hemolymph of the host or are intracellular parasites. Infections of the hemolymph of Daphnia make the entire host appear milkish-white, brownish, pinkish, or yellowish. These infections can be seen throughout the body and have been found in many Daphnia species. Here I introduce two of these species: P. ramosa and S. cienkowskii.
In contrast, intracellular parasitic bacteria infect either cells of specific host tissues or eggs of the host while they are in the brood pouch. Here I give a short description of a little-known bacterium known by the name of White Fat Cell Disease. Its categorization into two groups of parasitic bacteria is not a taxonomic classification but a functional grouping.
P. ramosa is a Gram-positive bacterium belonging to a distinct clade within the family of the Alicyclobacillaceae (Ebert et al. 1996; Anderson et al. 1999; Preston et al. 2003). Other endospore-forming bacteria, such as Bacillus and Clostridium, are closely related to it.
P. ramosa is most frequently found to infect D. magna, but it also infects D. pulex and D. longispina. It shows a high degree of clone specificity within species (Carius et al. 2001). A few other Cladocera have been described as hosts, but it is not clear whether the parasite was indeed P. ramosa. P. ramosa was recorded in Europe and North America.
Figure 3.1. D. magna with (right) and without (left) P. ramosa infection. The parasite can be seen as a dark cloudy mass filling the entire body. The brood pouch of the infected female is empty, whereas the healthy female carries a clutch of eggs. This photograph was taken with the light shining from below. The infected host is larger than the healthy female, which is typical for P. ramosa infections.
Figure 3.2. Developmental stages of P. ramosa. In the final stage of spore development, the host is filled with the round spores that serve as transmission stages. These spores are long-lasting. In hosts in the terminal stage of an infection, one often observes a few cauliflower stages, suggesting that some spores germinate to start another growth cycle. The cauliflower stage is the first stage of P. ramosa that is clearly visible after an infection.
) (Metchnikoff 1888). Infected hosts stop reproduction, grow large, and the body becomes darkish and nontransparent in light. “Squash” preparations reveal large numbers of large, nearly spherical spores (about 5-µm diameter) or grape seed-shaped pre-spores in the hemolymph (Figure 3.2).
This bacterium causes chronic infections. Infected hosts are totally castrated, i.e., they stop reproducing about 5 to 15 days after infection takes place. In contrast to most other Daphnia infections, the hosts can live for a long time after the parasite has reduced their fecundity. In the laboratory, death often occurs only 40 to 50 days after infection. At death, hosts are filled with transmission stages (normally around 10 to 20 million spores per host, but up to 80 million spores have been observed). Infected hosts are often larger than uninfected controls. This form of parasite-induced gigantism is believed to be adaptive for the parasite (Ebert et al. 2004).
) can be seen. Four days later, alongside the cauliflower type, microcolonies (fractions of these rosettes, with some cell associations consisting of only 2, 3, or 4 cells attached to each other at the pointed end) can be seen. These are branches of the microcolonies, which break away. Each branch eventually forms a single spore that resembles grape seeds. In the grape-seed stage the endospores increase in size until, fully developed, they have a diameter of about 5 µm. These endospores are the transmission stages. They are clearly visible with a light microscope. Details about the ultrastructure of P. ramosa can be found in Ebert et al. (1996).
Transmission is strictly horizontal (waterborne) through spores released from the remains of dead, formerly infected hosts. No vertical transmission has been observed. Mud samples from ponds with infected populations are infectious, indicating the role of pond sediments as a parasite spore bank. Samples from sediment cores can be infectious after several decades (Decaestecker et al. 2003). Experimental transmission was possible at 15°C, 20°C, and 25°C without any noticeable difference (Ebert et al. 1996). Transmission stages are released only after the death of infected hosts. Spores liberated from the host cadaver come in contact with uninfected Daphnia and cause infections. Thus, P. ramosa follows a sit-and-wait strategy. It is not clear whether infection results from ingestion of spores or whether the parasite penetrates the epidermis of the host. The latter has been shown to be the mechanism of infection of P. penetrans (note the name!) infecting soil nematodes. In the laboratory, infections can be produced by grinding up infected hosts and adding the resulting spore suspension to host cultures.
This bacterium has been recorded from a wide range of species including D. magna, D. pulex, D. longispina, D. hyaline, D. obtusa, D. ambigua, D. curvirostris, D. laevis, D. dentifera, and several genera of other Cladocera including Sida, Simocephalus, Chydorus, and Ceriodaphnia. The species has been described from sites in Europe, Africa, and North America.
Figure 3.3. D. magna with (left) and without (right) S. cienkowskii infection. The red color of the infected host is the best indicator of the bacterium. The females were collected from a natural rock pool population in southern Finland.
). Hosts with well-developed infections can be easily recognized by the bright scarlet red color of their hemolymph (Figure 3.3). This color is caused by carotenoids (Green 1959) and is much more opaque than the color of hemoglobin in the blood, which is sometimes seen in Daphnia from habitats with low oxygen (compare Figure 2.7). During early stages of infection, infected animals are more whitish-pale and resemble hosts infected by other blood parasites. The bacterium itself is hardly visible with standard light microscopy.
Metchnikoff (1889) described the length of the life cycle of the bacterium as about 5 days. The life cycle includes several morphological forms, including ovals, rods, spirillae, filaments, and round spores. Hosts collected from natural populations in the terminal stage (red color stage) survive only 1-3 days under laboratory conditions and usually carry no eggs (Duffy et al. 2005).
Transmission is strictly horizontal. Prevalence can reach 10 to 15% for short time periods (Duffy et al. 2005).
WFCD is caused by a small coccoid pathogen, most likely a bacterium. Infections with this bacterium have been recorded in D. magna, D. pulex, and D. longispina. Clones of D. magna have been found to differ in their susceptibility to WFCD (Decaestecker et al. 2003). The disease has been found only in Western and Northern Europe thus far.
Figure 3.4. D. magna with WFCD. The same animal is shown under three different light conditions, with light coming from the top (left), from the bottom (right), and from the top and bottom (center). Note that the infected fat cells become less visible with light shining through the animal.
Figure 3.5. WFCD in D. longispina. D. longispina from a natural rock pool population in southern Finland.
and 3.5). The infection does not show the fuzzy spread through the body cavity that is seen with other parasites infecting the fat cells and ovaries (e.g., Octosporea bayeri). Usually, the infected tissue is clearly distinguishable from other tissues.
WFCD is rather harmful. It usually kills the host within 2 weeks, often much more quickly. Less virulent infections have been observed as well. Fecundity drops strongly with disease progression, and infected hosts have stunted growth.
Transmission is strictly horizontal. Transmission stages are released from dead hosts. There seems to be no transmission from living infected hosts and no vertical transmission.
Several species of fungi have been observed parasitizing Daphnia and other Cladocera. Taxonomically, they are poorly understood. They vary strongly in their appearance and their effects on their hosts. Fungal infections are generally harmful to their hosts, drastically reducing host reproductive success and survival.
Some species may not be obligate parasites, opening the possibility to culture them on an artificial medium (Couch 1935; Prowse 1954; Whisler 1960). Indeed, it has been reported that the endoparasites Aphanomyces daphniae, Metschnikowia bicuspidata, and the epibiontic Amoebidium parasiticum can be cultured in vitro, which opens up tremendous possibilities for experiments. To my knowledge, no other parasite group can currently be cultured outside Daphnia.
Host specificity seems to be rather low in fungi infecting crustaceans. From my experience, the parasitic fungi of Daphnia are the most difficult to work with and to identify. On the other hand, parasitic fungi seem to be the most devastating diseases of Daphnia, often killing the hosts quickly or destroying the broods.
This yeast is better known by the names Monospora bicuspidata and Metschnikowiella bicuspidata. It has been recorded from D. magna, D. pulex, and D. longispina as well as from a number of other crustaceans. It appears, however, that under this name a complex of similar species has been described.
Figure 3.6. D. magna with an infection of M. bicuspidata. This female was infected with a suspension of spores. The host is in the terminal stage of infection. The needle-like ascospores of M. bicuspidata fill the entire body cavity of the host.
Figure 3.7. Spores of M. bicuspidata. These needle-like ascospores of M. bicuspidata fill the entire body cavity of the host.
). The fungus grows inside the host until the entire cavity is filled with the needle-like spores (Figure 3.7). Spores are found in every part of the body cavity, even in the antennae. Hosts in late stages of infections become opaquely white and look as if their bodies are filled with straw.
Successful M. bicuspidata infections kill the host within 2 to 3 weeks, sometimes earlier. Host fecundity is reduced, with this reduction becoming stronger as the infection develops (Ebert et al. 2000a, 2000b).
The fungus is transmitted only horizontally (Ebert et al. 2000a). The waterborne spores are ingested with the food and penetrate the gut wall (Metchnikoff 1884). Spores are only released from dead hosts. Grinding up dead hosts in water and adding this suspension to clean cultures allows efficient Transmission of the host (Ebert et al. 2000a).
M. bicuspidata produces local epidemics in Daphnia populations, reaching prevalences above 10%. Across a 1-year field study in three English ponds, the average prevalences in D. magna, D. pulex, and D. longispina were 1.8, 3.0, and 3.7%, respectively (Stirnadel and Ebert 1997). Interestingly, while one pond showed D. magna as the most heavily infected host, in another pond close by, D. pulex and D. longispina were much more predominately infected than D. magna, suggesting some degree of local differentiation of hosts and/or parasite.
Microsporidia are obligate intracellular parasites. As a group they are clearly distinguished from other eukaryotes, but their taxonomic position is still debated. In older phylogenetic trees, they are often shown to be at the root of the eukaryotes; however, the finding that they possessed mitochondria in their evolutionary past provoked a reconsideration of their taxonomic classification. Now it seems likely that they are a sister taxon to the fungi.
The Microsporidia are the largest group of parasites of Daphnia. They are easy to recognize once spores are formed. At 20°C, this takes about 3 to 12 days after infection (Ebert, personal observation). Spores of most species are only a few µm in length (2.5 to 16 µm in the known Daphnia parasites) and are usually rather uniform in size and shape. Microsporidians are usually found to be tissue specific (ovaries, fat cells, hypodermis, gut, and epithelium), and the infected tissue can give important clues on the species. Depending on the infected tissue, infections may be clearly visible from the outside (even without a microscope) or are seen only once the host is dissected (e.g., infections of the gut epithelium). Important traits for identification are the number of spores produced by each sporophorous vesicle, as well as the size and shape of the spores. Larsson (1981, 1988, 1999) gives excellent introductions to microsporidia identification. Note that spore size may vary according to culture conditions (e.g., smaller spores were observed at lower temperatures (Friedrich et al. 1996).
Although microsporidian parasites are highly variable in their mode of transmission, a few generalizations are possible. Gut infections are usually transmitted horizontally from the living host. Infections of ovaries are often vertically transmitted. Microsporidian parasites appear generally to be the most host-specific group of Daphnia parasites
A number of microsporidian parasites have been found to infect the gut cells of their hosts. These species are difficult to distinguish. Typically, they produce small spores (mostly less than 3 µm long), often in conspicuous sporophorous vesicles that are most easily seen when the gut is dissected. Sometimes only a few sporophorous vesicles are found in the entire gut, but in other cases the entire gut is densely infected. Infections may be localized, often in the posterior part of the gut, so that they are not visible without dissecting the host. Transmission of gut microsporidians is typically horizontal, with spores being released with the host feces and ingested by filter-feeding hosts. All species studied thus far were rather avirulent to their hosts. The fact that they are highly transmissable, difficult to see, and that they cause little harm to cultures explains the frequent observation that clones that have been kept in laboratories for many years or even decades often carry a microsporidian gut parasite (D. Ebert, personal observation). There must be a large number of publications on Daphnia biology that, without the knowledge of the authors, report on experiments with infected animals.
Figure 3.8. D. magna with an infection of F. magnivora. This female is in the terminal stage of infection with F. magnivora. Hosts infected with this parasite often carry eggs until close to their deaths. The whitish mass is spores.
Figure 3.9. Spores of F. magnivora.
). Infected hosts are easily recognized by the large spore masses visible in the central part of the body. Spores measure about 2.4 x 4.5 µm and are lightly pyriform, with both poles blunt, often with one surface slightly convex (Figure 3.9) (Larsson et al. 1998).
Infected hosts suffer to some degree from reduced fecundity and reduced longevity (Ebert et al. 2000a). Virulence is, however, comparatively low. Infected hosts may live more than 50 days, and fecundity reduction is between 30% and 50% compared with uninfected controls.
In the laboratory, the parasites are transmitted with nearly 100% fidelity from mother to offspring. It is likely that there is also horizontal transmission, but all attempts for horizontal transmission in the laboratory have failed (Mangin et al. 1995) (Note: In Mangin et al. (1995), F. magnivora is named Tuzetia.)
An ultrastructural study and description of F. magnivora (Microspora: Duboscqiidae) was done by Larsson and coworkers (1998).
Figure 3.10. D. magna with an infection of O. bayeri. The same animal is shown under two different light conditions, with light coming from the top (left) and from the bottom (right).
Figure 3.11. Spores of O. bayeri. Note the variability in spore shape and size, which is typical for O. bayeri.
). Spores are variable in shape and size but are usually 4 to 5.6 µm in length (Figure 3.11). Larger spores are seen frequently, but these may be abnormally formed. Spores of O. bayeri come in two (maybe even three) types (heterosporous), which may have different functions (Vizoso and Ebert 2004; Vizoso et al. 2005).
Infected hosts have reduced life expectancy and reduced fecundity, with the degree of damage depending on the route of transmission, the host and parasite genotype, and the presence of multiple strains within a host (Vizoso and Ebert 2004, 2005a, 2005b; Vizoso et al. 2005). Fecundity reduction is usually visible only once infections are intense, i.e., after about 15 days.
Figure 3.12. Life cycle of O. bayeri. Horizontal transmission occurs when infected hosts die and spores are released from the cadaver to the environment. Environmental spores can survive outside the host for several weeks to months (e.g., the entire winter) and can survive the temporary disappearance of their hosts. Infected females can transmit the parasite to their parthenogenetic sons and daughters through vertical (transovarial) transmission. Vertical transmission also occurs in the sexual cycle through the resting eggs. Finally, ephippia may serve as a vehicle for parasite dormancy and dispersal, with a new cycle of vertical and/or horizontal transmission starting after hatching. Red thick arrows, transmission of parasite; black broken arrows, growth of hosts; thin black arrow, interaction between two hosts (Vizoso et al. 2005). Drawing by Dita B. Vizoso.
. Infections of O. bayeri can be cured with a chemical drug (Zbinden et al. 2005), which allows one to obtain uninfected offspring from infected mothers.
In rock pool populations of D. magna in southern Finland, this parasite often reaches prevalences of 100%. Early in the season, however, prevalence is usually lower (S. Lass & D. Ebert, manuscript in preparation).
This gut parasite was formerly known as Pleistophora intestinalis (Larsson et al. 1996). It has been recorded in D. magna and D. pulex from Western Europe.
Figure 3.13. Gut cells of D. magna with G. intestinalis. The arrows point to spore clusters of G. intestinalis inside cells of the host gut epithelium.
Figure 3.14. Spore clusters of G. intestinalis. When infected hosts are dissected, spores and spore clusters are set free.
and 3.14). Individual spores are rather small and are oval-to-kidney shaped (about 2.6 x 1.3 µm in 20°C laboratory cultures) (Larsson et al. 1996). There are a number of rather similar species infecting the gut epithelium.
This parasite is rather avirulent, as compared with many other Daphnia endoparasites (Ebert et al. 2000a). Infected hosts may live up to 50 days, and fecundity is usually only slightly reduced. External signs of infections are not visible.
Transmission is horizontal from living hosts (Ebert 1995). Spores are shed from the living hosts with the feces and float in the water until the next host ingests them. Vertical transmission does not occur. This parasite is very easily transmitted from host to host. As a consequence, prevalences are often close to 100% among adult animals, and it may be found throughout the year. It is among the few Daphnia parasites that may be described as being endemic. The parasite can be kept in even very small cultures of the host, and its presence may escape the attention of the untrained observer.
Figure 3.15. Anterior section of the gut of D. magna with intense infection of O. colligata. The diverticuli are very strongly infected. The light structures are spore masses of the parasite.
Figure 3.16. Spores of O. colligata. Typical for this species is the chain-like arrangement of spores, which can be seen when spores are set free from host cells.
). The spores are best seen in dissected guts, where spore clusters are seen inside the gut epithelium cells. Individual spores are pyriform and slightly larger (2.9 x 1.5 µm in 20°C laboratory cultures; Figure 3.16) (Larsson et al. 1997) than spores of G. intestinalis.
Ordospora colligata is rather avirulent, as compared with many other endoparasites of Daphnia (Ebert et al. 2000a). Infected hosts may live up to 50 days, and fecundity is usually only slightly reduced. External signs of infections are not visible.
Transmission is horizontal (Ebert et al. 2000a). Spores are shed from the living hosts with the feces and float in the water until the next host ingests them. Vertical transmission does not occur. This parasite is very easily transmitted from host to host. As a consequence, prevalences are often close to 100% of all adult animals. The parasite can be kept in even very small cultures of the host, and its presence may escape the attention of the untrained observer.
Figure 3.17. D. galeata infected with C. mesnili. Clusters of C. mesnili can be seen on the gut as white, roundish spots.
Figure 3.18. Spore cluster of C. mesnili.
and 3.18). Infections have not been seen in the gonads.
Bittner et al. (2002) described this parasite as rather virulent. Laboratory-infected hosts have hardly any eggs, and survival is strongly reduced (fewer than 20 days on average). The parasite may drive experimental populations of D. galeata to extinction (Bittner et al. 2002). It also strongly influences competition among Daphnia species.
Transmission is horizontal from living hosts. Transmission stages leave the gut of the host and are ingested by other filter-feeding Daphnia. No vertical transmission was observed. A large-scale screen for this species in many pre-alpine lakes revealed that it is rather common, reaching prevalences of up to 50%.
The taxonomic position of this parasite remains unclear. C. mesnili was classified as a Haplosporidium (Chatton 1907; Green 1974), but this classification is certainly not correct. R. Larsson (personal communication) speculated that it may be related to Coelosporidium, a group of not-yet-classified parasites (see, for example, Lange 1993).
