Protozoa, their role and impact on Aquaculture
Protozoa means “first animals,” and are described as unicellular (single cell, or acellular) eukaryotes [eukaryotes are distinguished from prokaryotes by the structural complexity of the cells - characterized by having many functions segregated into semi-autonomous regions of the cells (organelles), and by the cytoskeleton]. Protozoa exist either as individuals or live within a colony which comprise a significant percentage of marine plankton as a whole and may pose a threat as obligate parasites. More than 20,000 different species of Protozoa exist and are found in every conceivable niche in natural environments.
Methods of Deriving Nutrition and Reproduction
Protozoa obtain their food supply through three methods:
1. Holophytic protozoa obtain nutrients through photosynthesis.
2. Holozoic protozoa depend on plants and animals for food.
3. Saprophytic protozoa asorb organic matter through the cell wall.
Reproduction in protozoa varies. The amoeba for instance, divides into two cells through the process of fission.
(Illustration based on BBC's article on Cloning)
Other protozoa reproduce by budding, a process which consists of the cell swelling, and a bud pinches off. Spores are formed in some and repeated division may occur within a single spore.
Protozoans live at the protoplasmic level of construction. The protoplasm performs all necessary life functions in the absence of multicellular structures. The amoeba’s protoplasm is surrounded by a cell membrane. Protoplasm of the amoeba is differentiated into a nucleus which has no fixed position within surrounding cytoplasm. Amoebas feed by surrounding a food particle and engulfing it while a contractile vacuole pumps water out of the cell. Waste products are diffused from the membrane of the cell.
Protozoans are placed in one of four groups based on their mode of locomotion.
1) Pseudopodia are temporary projections of cytoplasm that allow individuals to creep.
2) Flagella are whip-like filaments that lash about creating movement. Flagella may number anywhere from one to eight though usually number between one to four.
Pseudopodia from Hunting for Hemocytes, Oysters in the Classroom, Maryland Sea Grant.
3) Cilia are fine hair-like structures surrounding the outer membrane. Locomotion is created in a rowing-like movement by the rapid beating of cilia. Some members in this group possess cilia throughout their life cycle and are true ciliates. Others, like suctorians, have cilia during early life stages but as adults become sessile with tentacles.
Flagella
4) Spores are produced by sporozoan protozoa. There are no locomotive structures, and movement is achieved passively from host to host, although pseudopodia may be present in early stages. All members of this last group are parasitic.
Cilia
Examples of Protozoa include both solitary and colonial organisms
Khawkinea
Amoeba
Vorticella, a colonial ciliate, Vorticella
Stentor, a solitary ciliate, Les Cilies
The term protozoa is a taxon of convenience, having no significance in classification schemes within the Kingdom Protista (or Protoctista) other than as a traditional functional grouping of all heterotrophic and motile species. Protozoa is comprised of a diverse assemblage of genera encompassing many unrelated groups. Modern taxonomic classification divides the kingdom Protista into 27 phyla give or take, two of which contain significant numbers of free-living, phagotrophic species.
Protozoa play an important, though often underestimated role in aquatic ecosystems. Recent studies show how abundant protozoans are in estuarine and coastal areas where they form an important connection within the microbial loop, linking bacterial and nonoplankton production to larger zooplankton and fishes. At the micro-level, grazing in the freshwater environment involves removal and consumption of material particles, organic debris and micro-organisms. This is carried out by the single-cell protozoans and multicellular biota (invertebrates). Grazing plays an important role in aquatic microbiology since it is a fundamental element in the control of microbial populations and also a major route to transfer of biomass within the food chain.
Protozoa have often been ignored in earlier studies of aquatic ecosystems, although awareness of their involvement in both pelagic (open ocean) and benthic (dwelling in the bottoms of sea or lakes) food webs has been increasing. The biology of such habitats cannot be properly described unless the protozoan community is taken into proper consideration. In pelagic commmunities, protozoa have frequently been ignored in studies focused on grazing patterns because rotifers and crustacean zooplankton take precedence in terms of population density, total biomass, productivity, rates of grazing, and nutrient regeneration. Methods of sampling protozoa differ from those required for macro-zooplankton. For instance, standard zooplankton nets are not suited for protozoa (mesh openings of 64 µ or larger) or nets which are recommended for rotifers (35 µm mesh size). As a result, information about planktonic protozoa has often been incomplete and fragmentary, with most studies limited to brief intervals in annual cycles or at particular depths and unrelated to the zooplankton community, as a whole.
Protozoa make a positive contribution because they consume organic debris, other micro-organisms in both flowing and standing water, as well as man-made aquatic systems of economic importance such as wastewater-treatment plants. Between Protozoa and multicellular grazing organisms competing for food, the relative impact of grazing by these two major groups can be considered in terms of biomass transfer and type of food supply.
Pelagic ecosystems are driven by the interaction of organisms that fall into four functional components: the autotrophic phytoplankton and the heterotrophic bacteria, protozoa and metazoa. The systematic study of the protozooplankton has traditionally lagged behind that of the other components because of their great heterogeneity, despite recognition of their important role in structuring pelgic ecosystems. The organisms representing this component are phylogenetically and functionally more diverse than either of the other components. Their study requires specialisation and dedication to specific size classes, taxonomic groupings or mineral-bearing taxa with the result that the component as a whole tends not to be regarded on an equal footing with the others of the plankton.At times, protozooplankton biomass may exceed that of mesozooplankton (Planktonic animals in the size range 0.2-20 mm. Examples: the copepod Calanus finmarchicus, and Rhincalanus gigas .
For example, flagellate protozoans are predominant predators of *picoplankton (primarily bacteria and cyanobacteria) and thus are essential components of the "microbial loop" within lakes and rivers.
* Picoplankton is a classification merely based on size: taxonomically, the group reaches across kingdoms, including bacteria (bacterioplankton), archaea, and eukaryotes. But what this group lacks in size they make up for in abundance: dip in a thimble, okay, a mini-thimble, haul in a milliliter. You'll find as many as a million organisms in surface water. Picoplankton are the most abundant living things in the world's oceans, and constitute 20-50% of marine primary productivity, making them the keystone of the oceanic food webs. Also, via photosynthesis or other chemical transformations (fixing of carbon, nitrogen, or sulfur), they have a key role in the global climate (via the carbon cycle) and the recycling of living matter in the seas.Many genera consume bacteria within the surficial layer of sediments and water column while larger taxa capture and eat algae or other protozoa. They, in turn, are preyed upon by some species of oligochaetes, chironomids, and rotifers.
Finfish, Crustaceans and Mollusks
Protozoa as a predatorial parasite
Monogenea are large protozoans such as "Ich", Epistylis, Chilodonella, and copepod larvae, and can be seen in as little as 10x magnification on a standard microscope.
Epistylis
Fish inflicted with Epistylis infection
"There are many factors that can cause a fish to get sick or develop sores. Fish are constantly exposed to various bacteria, fungi, viruses and parasites that occur naturally in the water. Generally the fish can deal with these with little or no problem. However, if fish are stressed or weakened by poor environmental conditions, they may not be able to fight off disease and may become sick. Some common diseases encountered in wild fish in North Carolina include white fuzzy patches on the skin caused by fungi or protozoans, red sores caused by bacteria, and black or yellow spots in the flesh of certain fish species caused by parasites. Some causes of stress include traumatic injury, spawning activity, rough handling and changes in water quality. When water temperature increases and/or fish are spawning, they are more likely to develop signs of disease. In most cases, the fish recover when conditions improve and stressors are reduced. Disease outbreaks typically don’t have a serious population level effect on natural fish populations. Usually only a small portion of a population will be seriously affected by a disease, and the population will rebound quickly."
Chilodonella
Even one parasite warrants immediate action
The parasite Chilodonella is a major threat to fish health and finding even one parasite warrants immediate treatment. Chilodonella is a potentially dangerous parasite for two reasons. First, unlike many parasites, it has a wide range of temperature tolerance and outbreaks often occur at low temperatures when fish are least able to resist. Secondly, despite is relative small size it, is potentially more dangerous than Ich, because in the initial stages there are no readily visible signs of its presence.
"Chilodonella is a single cell microscopic parasite that attacks a fish skin and gills. The fish will rub against objects and become inactive. If the gills are affected they will stay near the water surface and gasp for air. Cloudy spots develop on the skin. The skin patches turn white and begin to disintegrate (usually over a couple day period). This opens the door to secondary infections and/or fungus. This parasite can swim and can attack other fish in the same aquarium. In aquariums with more fish, it is likely to spread rapidly. Poor water conditions also accelerate this disease. It can be introduced with live foods or from ponds or even on a plant."
In aquaculture, understanding larval stages and life cycles of parasites is necessary. To verify a diagnosis, parasites can be killed and preserved in 10% formalin and sent to a specialist.
Life cycle of Ichthyophthirius multifiliis
Cause of "ick" in fish. Large macronucleus can be seen in histopath sections of fish tissues.
Ichthyophthirius multifiliis is a devastating parasite which affects channel catfish and sometimes destroys entire populations. It is not host specific and may affect a variety of cultured finfish species. It burrows under the skin of fish, causing white specks that can sometimes be seen with the unaided eye.
Ichthyophthirius multifiliis and is characterized by its relatively large size and horseshoe-shaped nucleus .
After maturing, the adult parasite called a trophozoite, leaves the fish and becomes free swimming for up to 6 hours, eventually attaching to substrate.
Icthyophthirius multifiliis trophozoite from freshwater fish
. Section through the surface of this ciliate, revealing the pocketed surface from which the cilia emerge. A number of basal bodies can be seen as well as numerous secretory bodies. 35,000x
From workforce.cup.edu
A membrane is secreted over the organism and cyst undergoes multiple fissions. As many as 1000 or more young, called tomites are produced. At temperature of 77 F, development into mature tomites can be completed within as little as 12 hours. When the cyst ruptures, tomites will begin the process of seeking a host, then penetrate the tissue of the fish host by cilary action and aid of an enzyme. Once penetrated, tomites mature into trophozoites and feed on the cell-tissue and fluid.
"A common sign of beginning stages of Ich infection are what is called "flashing", where fish will swipe against aquarium decorations or the gravel at the bottom of the tank in an attempt to seemingly scratch themselves. Only one or two small colonies will appear at first, and be very difficult to identify until this infection advances to near maturity." (fishtankguy.com)
More informaiton is available on Icthyophthirius multifiliis at an earlier entry onTrichodina spp. is a ciliate which occurs among virtually all species of cold and warmwater fish, and includes both fresh and saltwater fish.
Hybrid Striped Bass Culture.
Trichodina, a ciliate and parasite
This parasite may cause extensive mortalities especially in fingerlings and is sometimes associated with poor water quality and accumulation of waste. Epizootics (meaning epidemics in animals, root origin from the Greek, "epizoon" which refers to one animal living on the surface of another) may occur throughout the year though more common in spring and fall.
"Trichodina is a surface symbiont/parasite of fish. It has a broad oral disk surrounded by membranelles and an adhesive base."
Fish who are infected may begin showing white to whitish-gray blotches and copious amounts of mucus may be present, along with milky and opaque tail fins. Infected fish may congregate near incoming water and become lathargic. Heavy parasitic infestation can result in excessive mucus production which impairs respiration and results in possible suffocation, though sufficient levels of dissolved oxygen may be present in the water. Related genera affecting fish include Trichodinella and Tripartiella.
Ambiphrya
Ambiphrya (Scyphidia) and Apiosoma (Glossatella) are similar parasites. They are associated with high concentrations of organic matter and found attached to plants, rocks, and other substrate. They are found on all cultured warmwater fish and occasionally on salmonids, and are mostly a problem with fry and fingerlings. Infected fish may not eat and may swim lethargically near the surface. Because of hemorrhages on the gills (including possible hermorrhaging of the body) Ambiphrya causes increased sensitivity to low levels of dissolved oxygen.
Epistylis, is a stalked protozoan and has a row of cilia at the apical end used for drawing food into the main body. Under microscope the stalks contract periodically which aid in proper identification. In the western United States, Rainbow trout are common host to the Epistylis parasite, and may occasionally occur on channel catfish, and will affect other finfish species. Common symptoms in scaled fishes are erosion of skin, scales, and spines results in bloody lesions, hence the name "red sore" disease. Other symptoms include hemmorrhaging and excessive localized mucus production. Infestations in channel catfish involve spines and bones that underlie the skin on the pectoral girdle, head, and fins. Epistylis is associated with a high concentration of pollution and/or organic matter. Infection will cause infested fish to flash. fish eggs may appear "fuzzy".
Chilodonella
Chilodonella is found on the gills and skin of finfish and distinct in parallel rows of cilia along the body margin. Chilodonella is a ciliate with a round to heart-shaped, depressed body. It is colorless and flattened, creeping rapidly across the fins, gills, and body of its host. Chilodonella is typically a coolwater parasite although one species, C. hexatichus, causes mortality at water temperatures up to 70°F.
Ichthyobodo
Ichthyobodo is among the smaller protozoan parasites and often overlooked because it can only be detected under high magnification. Characteristically it is tear-drop shaped and has a "fluttering" appearance, like a tree leaf in a breeze when on gills or skin of the host fish. Ichthyobodo has one pair of flagella attached to the small rounded blepharoplast (small mass of chromatin embedded in cytoplasm at base of flagellum), but two smaller flagella appear before division.
Two species of Ichthyobodo have been documented, I. necatrix and I. pyriformis. I. necatrix is the most common of the two, is widely distributed throughout the United States and is common among young trout and salmon, where it is one of the most destructive ectoparasitic (parasites that live on the exterior of another organism) protozoans. Channel catfish fry and fingerlings are especially susceptible to this parasite.
Ichthyobodo necatrix
"Ichthyobodo necator (Costia necatrix) is often found on the gills of young juveniles in the hatcheries in cases of inefficient incoming water treatment (filtration)."
I. necatrix attaches to the host body using a flat disc, extending into the host cell. Portions of the host cell are engulfed and brought into the parasite as food vacuoles ("One type of vacuole is the food vacuole, which is a temporary vacuole containing food that is obtained through phagocytosis ["cell eating"]. In addition, lysosomes recognize these food vacuoles and fuse with them for digestion of food particles. Without food vacuoles, a cell would not be able to be sufficiently nourished."
Often called the blue-slime disease, I. necatrix causes a grayish-white to bluish film to form on the skin of its prey, which comes from excess mucus production.
I. pyriformis infests gills and body and is found primarily on trout. It is pear-shaped and moves in rapid spiral pattern that is differentiated from darting movements of I. necatrix.
Dinoflagellates are identified by a groove around the middle of the cell with a flagellum lying in the groove. Oodinium sp., a well-known dinoflagellate found in fresh, brackish, and saltwater fish. Disease caused by Oodinium is often referred to as "velvet disease" since the parasite may become so abundant that a fine yellowish sheen appears.
Oodinium
This parasite attacks the host' gills and skin and may be found attached to intestinal mucosa. The adult attaches to tissue with root-like appendages. Non-motile adults have a yellowish hue and frequently form clusters. Upon reaching maturity, they drop off and attaching themselves to a hard surface in the substrate, and begin multiplying, forming motile dinospores. Once a new host is found, their flagella disappear, and anchoring themselves to the host tissue with root-like appendages, until reaching maturity. Fish which are infested may scrape themselves showing signs of suffocation. A flashlight in a dark room may be used to inspect fish to check and see if light reflects from the Oodinium parasites.
Glenodinium Algae Dinoflagellates
Glenodinium, a dinoflagellate, has been associated with mortalities in channel catfish. It is normally a free-living algal cell which may develop into heavy bloom and give water a brown hue. Cells of Glenodinium may become entrapped in lamellae of fish gills causing a proliferation of tissue.
Trichophyra
"Trichophyra piscium belongs to a Phylum of protozoan organisms all of whom possess cilia (‘hairs”) during at least one stage of their lifecycle. Most species are free-living aquatic organisms that feed on bacteria, some are commensal organisms, living on but not harming the host, and cleaning bacteria off the host’s body. Several species of Trichophyra have been identified as commensal organisms living on the gills of freshwater fishes. Large numbers of Trichophyra may cause gill irritation, or may cause the fish to secrete excessive gill mucous decreasing it’s respiratory and osmoregulatory efficiency. Some Trichophyra have been found parasitizing blood from the fish’s gills. In April 2002, 14-month old lake trout (S. namaycush) at the Maine Department of Inland Fisheries and Wildlife Enfield fish hatchery were acting as though they had gill parasites. Fish with gill parasites will rub their heads, and bodies against hard surfaces presumably to dislodge the organisms. Upon examination of the affected fish, Trichophyra piscium was identified in large numbers infesting the fish’s gills. The fish were treated as directed by MDIF&W’s fish veterinarian, and the problem was resolved."
Trichophyra is a suctorian parasite infests gills of warmwater fish, and is distinguished by its round body and suctorial tentacles during the adult stage. It feeds on both passing protists as well as epithelial cells ('Epithelial' tissue works as a covering and lines organs in the body.) Gills may swell and become eroded, and anemia may follow. Affected fish become lethargic, may stop feeding, and may gather around inflowing water. Trichophyra populations may reach large numbers in an environment which contains high organic levels and low temperature.
Sporozoan protozoans are responsible for serious disease in fish. This class of protozoans is known by the spore morphology and by the number and location of polar capsules containing coiled filaments. Sporozoan parasites distributed in two basic groups, myxosporidians with two or more polar capsules and microsporidians with one polar capsule. Pathologists also recognize two other groups, coccidia and haemosporidea. Schäperclaus (1991), however pointed out that classification of sporozoa is difficult and that members of sporozoa have a single common feature, the formation of spores. Beyond that, classification is open to conjecture. Sporozoans tend to be host and tissue specific, having complex life cycles, and are untreatable. Life cycles of both myxosporidians and microsporidians begins at the spore level. When the host dies the spore drops to the bottom or accidentally eaten. Its polar filament(s) is used to attach to the gut wall. In miscosporidia, DNA nuclear material enters host cells through the everted polar filament. The parasite may eventually transfer to the definitive loci, or it may be carried there by the white blood cells. Once at the loci, sporozoan parasites (now termed trophozoites) divides (shizogony) and fuse (sprorogony), forming a mass of spores responsible for disease. (Fundamentals of Aquaculture, Avault)
Myxosoma cerebralis, a myxosporidian, causes "whirling disease" among salmonids, (diseased fish swim in circles). The parasite enters through any external opening, damaging the cartilage in the axial skeleton of young fish which intereferes with the function of neural structures and coordination. When fish are affected, they whirl as if chasing their tails. At one time whirling disease was the cause of catastophic losses in trout culture in central and north Europe, however today it is no longer considered a serious threat due to measures to control it. This disease has occurred in certain eastern states and Nevada.
Chain pickerel gill arch with Henneguya xenoma
When a xenoma ruptures, millions of Henneguya spores are released. The spores drift in the water and attach to a new host with a grappling hook-like organ called a polar filament. Once attached to a new host, the organism forms a new xenoma and begins to multiply. Fish veterinarians, culturists, biologists and others concerned with fish health may treat infected fish with chemotherapeutic agents or surgical removal. Unfortunately, many parasites, including Henneguya, are not easily controlled by any therapeutic procedure, thus prevention remains the best medicine. Henneguya is a fish parasite that seldom causes severe harm to the host. External examination may reveal cysts in the skin and gills; whereas, internal lesions may be found on the liver, heart, kidney, spleen or any other organ. Infections are usually not life threatening to the fish unless they impair the function of a vital organ. (
"Microscopic examination of the Henneguya sp. “xenomas” reveal tadpole shaped unicellular organisms with two eye-like polar capsules inside."
Several apparent site-specific forms of Henneguya have been noted on channel catfish, on the skin and three on gill filaments. On the skin, a papillomatous form creates large lesions on body and fins, and nearly half the body may be affected. The second form creates a pustule or blister. Both forms may disfigure fish, but dressed fish show no signs of the parasite. The third form is of minor importance, occuring as a white cyst on the adipose fin of channel catfish fingerlings. On gills, an interlamellar form may cause extensive damage to channel catfish fingerlings. The second form usually has a few intralamellar cysts per filament, but does not pose a major problem. The third form produces discrete visible cysts on the gills. Henneguya has also been found in channel catfish viscera. (Fundamentals of Aquaculture, Avault).
Protozoan parasites and commensals of freshwater crawfish, prawns, and shrimp may be grouped as gregarines, microsporidians, ectocommensals, body invaders, and asptome ciliates. (Fundamentals of Aquaculture, Avault)
Gregarine protozoans, such as Nematopsis sp. have been observed in digestive tracts of shrimp in either the form of trophozoite or gametocyst. Life cycles of this parasite involve marine snails and/or clams. Trophozoites attach to the intestinal wall to asorb food, though over-all harm to the host is generally minor. Penaeid shrimp with Microspordian infestation can suffer from a chronic disease known as "cotton" or "milk" shrimp. Infestation is found throughout the musculature, depending on the species of microsporidia, or may be observed in particular tissue and organs. Microsporidians are found present in spore form with an envelope enclosing the spores in some species such as Pleistophora and Thelohania, but not all do. Nosema is one particular genus that lacks such an enclosing envelope. Infected shrimp may be active and feed normally, however microsporidians are suspect of inhibiting egg production in shrimp. Both macrobrachium and crawfish are susceptible hosts to microsporidians, though generally it is not considered a major problem in the United States. Microsporidian infection of crawfish in Europe is known as "porcelain disease". Spores are found spread throughout the musculature in heavily diseased crawfish and prawns. Research in areas of life cycles of microsporidians and their relation to crustaceans is incomplete.
"Protozoans such as Vorticella, Zoothamnium, Epistylis, Acineta and Ephelota. Affected shrimps are restless and their locomotion and respiratory functions are hampered. In heavily infected juvenile and adult shrimps, one can observe fuzzy mat-like appearance due to ciliate fouling. Maintain good water quality. Reduce organic load and silt in water exchange with good quality water."
Zoothamnium
"Zoothamnium niveum colonies on wood debris in the Indian River Lagoon. Zoothamnium niveum is a colonial Protist that reaches 2 - 3 mm in height. Individual zooids have an inverted bell shape and measure approximately 120um in height. The contractile vacuole is located below the peristomial lip.
"Zoothamnium prefers the gills (Johnson, 1978). Only rare incidences of heavy fouling affected the prawns adversely. Prawns have an increased oxygen demand just prior to moulting and heavy fouling can be associated with mortality due to anoxia. (Fisher, 1977)." (Fundamentals of Aquaculture, Avault)
Lagenophrys
"Lagenophrys limnoriae attached to the pleopods of Limnoria quadripunctata from the UK."
Corthunia
"Hall (1979) found that Corthunia sp, Epistylis sp. and Vorticella sp. were the most common peritrichous ciliates in cultured prawns. Common sites of infestation are the body, eye stalk, antenna, uropods and egg masses. Thelohania, a microsporidian, has been reported in various species of marine shrimps but rarely in freshwater prawns. Areerat (1988) reported one case of microsporidia infection in the opaque muscular tissue of Macrobrachium."
Freshwater Corthunia sp. is found on crawfish and occasionally prawns.
Acineta
"The Suctorian Acineta: feeding."
Acineta sp. is commonly found in brackish water on prawns.
Epistylis, Zoothamnium and Lagenophrys are found in both fresh and brackish water on crawfish and prawns. Lagenophrys is more prevalent on prawns than crawfish.
Body invaders are protozoans that on occasion are discovered wandering about the body of their host, but if this is a case of "accidental parasitism", and whether the crustacean serves as a paratenic host, is still unclear.
Apostome ciliates (considered to be commensal) are commonly found on crawfish and prawns and have been noted infrequently in the resting stage of shrimp. Apostomes will encyst on crustaceans' exoskeleton. When crustacean molt, protozoa hatch and feed on fluids released on the shed exoskeleton. They give the appearance of tiny transparent bubbles. Following reproduction, apostomes begin the search for a host. Apostome genera commonly found in North America are Hyalophysa, Gymnodinioides and Terebrospira.
In the North American oyster culture, several serious protozoan diseases have been identified. Haplosporidium nelsoni, known as MSX before 1966, is the cause of "Delaware Bay disease" in the American oyster and likely affects other species. Oysters which are infected become emaciated and have weak shell enclosure and recession. This disease is particularly lethal for oysters, and influenced by high salinity. During dry years with high salinity (mid 60's), H. nelsoni caused high mortalities to oyster beds in Maryland, and rainy years (71-74) the disease remained confined to Mobjack Bay and the lower Chesapeake Bay. A salinity of 15 ppt or higher puts oysters at risk for the disease which may remain hidden up to 9 months. Some outbreaks encompassed mortalities up to as high as 50%-60%.
Haplosporidium costale, causes seaside disease which occurs in high salinity waters of over 25 ppt. Oysters affected by this disease characteristically fail to add new "bill" or shell, resulting in overall poor health. This disease occurs in Long Island Sound to Cape Charles and sometimes the Delaware and Chesapeake bays. H. costale has no tolerance for salinities below 25 ppt. Juveniles under two or three years may survive an outbreak, but killing up to as many as 20-50% of mature oysters. This parasite has been under study for more than thirty years because of its seriousness.
Perkinsus marinus known as "Dermo" is common to the Gulf of Mexico and lower Chesapeake Bay and Haplosporidium nelsoni, known as MSX before 1966, is the cause of "Delaware Bay disease" in the American oyster and likely affects other species.
The parasite invades the gut epithelium, possibly through the mantle. Once the epithelium is destroyed, the parasite is further distributed by blood throughout the remainder of the oyster. It inhibits normal gonadal development and severely emaciates affected oysters. Infections typically rise during warmer months and decline during colder months and salinities below 15 ppt, infections and resulting mortalities are thus reduced. Protozoans Marteilia refringens and Bonamia ostreae have contributed to serious problems in European waters with the oyster, Ostrea edulis.
Haplosporidian protozoans have been reported for moribund mussels in North America, but presently longterm implications remain unclear. Appearing to be an isolated event, extensive mortality of mussels on Prince Edward Island occurred because of infestation by a haplosporidian protozoan, identified as Labyrinthomyxa sp.
With increasing aquaculture of mussels (primarily Mytilus edulis), clams (various species), and scallops (several species), protozoan diseases are attracting more attention.
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