12. Parasites

Penaeids act as intermediate and final hosts for a wide variety of parasites. Parasites that can complete their life cycle without intermediate hosts have caused massive mortalities amongst cultured penaeids. These pathogens usually occur at low frequencies in the wild but their incidence rises rapidly when prawns are held in high densities because transmission and hence infection is facilitated. Parasites associated with culture conditions have been reviewed by Lightner (1983, 1985) and Overstreet (1986). The most comprehensive records of parasites on penaeids in the wild are from the Gulf of Mexico (Hutton et al., 1959; Kruse, 1959; Sinderman, 1970; Couch, 1978; Johnson, 1978; Overstreet, 1978; Fontaine, 1985). Owens (1987) has pointed out that the metazoan parasitic fauna of penaeids from the Americas is well known, whilst that from the Indo-West Pacific is poorly known and that from Africa is almost unknown.

Some parasites kill their hosts, others alter the appearance or behaviour of the host while others are asymptomatic. One of the common symptoms of parasitic infection is cotton or cotton tail in which the abdominal muscles become opaque and white. It can be caused by viruses, bacteria, protozoans or physical trauma (Momoyama and Matsuzato, 1987). The crustacean epidermis responds to injury by laying down melanin and melanization of tissues is a common symptom of many types of parasitic infection especially those that damage the cuticle. Behavioural changes induced by parasites may be the same as those caused by physical stress. Prawns in low oxygen tend to remain emerged and do not burrow. This behaviour is seen in prawns infected by parasites that interfere with respiration either by covering the gills or through necrosis of blood vessels. Not all pathogenic conditions are the result of parasitic infection. An example is “red disease” in which the body and especially the gills, take on a red coloration. This arises from microbial

379

380 BIOLOGY OF PENAEIDAE

toxins in food that cause the hepatopancreas to atrophy and release stored carotenoids - which are red or orange - into the tissues (Lightner and Redman, 1985). Nash et al. (1988a) have described a series of clinical abnormalities including brown or black coloration of the gills, soft shells and necrosis of the cuticle in P. monodon grown in ponds with acid sulphate soils.

1. Viruses

The number of viruses known from penaeids has risen rapidly in recent years. The first penaeid virus (Baculovirus penaei) was reported in 1974 (Couch, 1974). In 1983, Johnson reported two more and by 1985, six viruses had been found (Lightner, 1985; Paynter et a f . , 1985; Tsing and Bonami, 1987). Most of the new identifications were made during studies of diseases of cultured prawns and Doubrovsky et al. (1988) point out that only three of the six known viruses have been found in wild populations. The viruses known to occur in penaeids come from four groups: three are baculoviruses, one is a picornavirus, one a parvovirus and one a reo-like virus.

Baculoviruses infect endodermal tissues (midgut and digestive gland) and appear to be endemic in wild populations in many areas (Brock et a f , 1986; Lester et al, 1987). They have been spread by the introduction of cultured species (Colorni et al., 1987; Lightner, 1985). Baculoviruses have been found in P. aztecus, P. duorurum (Lightner, 1983); P. esculentus (Paynter et a f . , 1985); 2'. japonicus (Sano et a f . , 1981); P. marginatus (Brock et al., 1986); P. merguiensis (Doubrovsky et a f . , 1988); P. monodon (Lightner et a f . , 1983a; Nash et al., 1988b); P. plebejus (Lester et al., 1987); P. styfirostris and P. vannamei (Lightner, 1983).

A probable picornavirus, known as IHHN virus (Infectious Hypodermal and Hematopoietic Necrosis) was discovered in P. styfirostris imported into Hawaii from South America (Lightner et a f . , 1983b). It causes cell damage to ectodermally-derived tissues (cuticular epithelium of the body surface and the lining of the foregut and hindgut, antenna1 gland epithelium and nerve tissue) and of mesodermally derived tissue (muscle, connective tissue, vascular system and reproductive system). IHHN is highly lethal to P. stylirostris but has little effect on P. vannamei although it may cause neoplastic lesions in this species (Lightner and Brock, 1987).

A parvovirus (HPV) has been found in the hepatopancreas of P. merguiensis, P. monodon, P. chinensis [ = orientafis], and P. semisulcatus (Lightner, 1985) and P. esculentus (Paynter et al., 1985).

PARASITES 381

HPV has been found in penaeids throughout the Indo-West Pacific (Lightner, 1985).

Reo-like viruses have been reported from P. juponicus from the Mediterranean (Tsing and Bonami, 1987) and P. monodon from culture ponds in Malaysia (Nash et ul., 1988b). In both cases the virus was found in the hepatopancreas. It is not known whether reo-like viruses cause any disease in penaeids although they may be important in penaeids under stress in culture.

Prawns infected with viruses tend to show a similar suite of symptoms including cotton tail, poor growth, and reduced preening activity leading to a higher incidence of surface fouling by epicommensals (Couch, 1974; Nash et ul., 1988b). Secondary bacterial and fungal infections may also occur with extensive tissue damage. None of these symptoms is unique to viral infections and diagnosis is chiefly by histological examination, although immunosorbent assay techniques are being developed (Lewis, 1986). Mortality in infected stocks may be very high although all stages of prawns are not equally susceptible. Bell and Lightner (1987) found, for example, that large P. stylirostris showed fewer symptoms and were more resistant to IHHN than were smaller individuals.

Viruses can be transmitted vertically or horizontally. Vertical trans- mission involves the virus passing into the offspring via the ovary and eggs of an infected prawn. Lightner et ul. (1983b) found that penaeids that survived IHHN infections became carriers of the virus for the rest of their life, and could pass the virus on to their offspring. Horizontal transmission is mainly through ingestion of infected tissue (Couch, 1974, 1978). A reo-like virus has been transmitted to P. juponicus by feeding animals on pieces of digestive gland taken from infected prawns (Tsing and Bonami, 1984; Lightner, 1985). Horizontal transmission through feeding on diseased tissue is probably uncommon in the wild because moribund or dead prawns are more likely to be eaten by other predators. Under culture conditions, there is a greater probability of dead prawns being eaten by other prawns. Horizontal infection may also occur directly, Monoyama and Sano (1988) have shown that larvae of P. juponicus can become infected with baculoviral midgut gland necrosis (BMN) virus by keeping them in water containing inoculations of the virus. Momoyama (1988) found both vertical and horizontal infection of P. juponicus by BMN virus under culture conditions. All viruses do not use both pathways. Bonami et ul. (1986) found no horizontal infection of P. monodon by MBV (Midgut Baculo Virus) over 75 days under experimental conditions but the virus was transmitted vertically over three generations. Horizontal transmission is used in a bioassay technique in which tissue of suspect but asymptomatic animals is fed to juveniles of

382 BIOLOGY OF PENAEIDAE

P. stylirostris, a species that shows clear symptoms of viral infection (Lightner et al., 1987). Some aquaculturists consider that viruses are not a problem in healthy stocks and only cause mass mortalities in cultured stocks held in unsatisfactory conditions. Stress due to crowding enhances the rate and degree of infection in raceway-cultured P. monodon (Lightner et al., 1983a).

11. Bacteria

Although many bacteria cause diseases in marine crustaceans, they are not regarded as obligate parasites (Johnson, 1983). When present in large numbers, however, even normally innocuous bacteria can cause problems. The filamentous bacterium Leucothrix mucor, for example, attaches to the gill filaments of penaeids by a mucus layer. Occasionally this may cover the gills and interfere with respiration (Couch, 1978). A Leucothrix-like organism that killed P. californiensis was completely external; although it attached to the gill filaments, it did not damage the underlying tissues (Lightner et al. , 1975).

Cuticular lesions that appear as brown or black spots are sometimes found on prawns from the wild. The bacteria Vibrio, Alteromonas and Spirillum are commonly found in these lesions. The lesions start when minor damage to the epicuticle allows bacteria to colonize and break down the cuticle (Cipriani et al., 1980). The lesions can then be enlarged by chitinoclastic bacteria such as species of Beneckea. Pathogenic bacteria can then enter the body through the lesion and colonize tissues (Cook and Lofton, 1973). Although some bacteria such as V. parahuemolyticus can be transmitted to penaeids in their food, others like V. ulginolyticus and V. anguillarum can infect penaeids only through lesions in the body wall (Lightner and Lewis, 1975).

Fontaine (1985) isolated 17 species of bacteria from tissues of penaeids from the Gulf of Mexico; the most common genera were Vibrio, Aeromonas and Pseudomonas. Many of these bacteria are pathogenic to penaeids (Lightner and Lewis, 1975). Symptoms of bacterial infection include cotton tail, a reduction in swimming activity, disorientation while swimming, swimming on one side and resting motionless on the bottom. Infected prawns may also develop a pronounced dorsal flexure of the abdomen.

PARASITES

111. Fungi

383

Fungi are not common as pathogens on wild populations of penaeids. Fusarium spp. were found by Fontaine (1985) in only one prawn in a combined sample of 238 P . aztecus and P. setiferus. Fungi were found in under 5% of P. duorarum (Couch, 1978). Four per cent (n = 155) of P. monodon collected from the Cochin backwaters were infected with two species of fungi that caused necrotic brown lesions; the infected prawns were sluggish and moribund (Gopalan et al., 1980). Pathogenic species may be secondary invaders of existing lesions (Johnson, 1983). Fungal spores appear to settle preferentially on damaged crustacean cuticle and so lesions are very vulnerable to infection (Nyhlen and Unestam, 1980). Penaeid tissues can encapsulate hyphae of Fusarium solani with haemocytes and deposits of melanin and collagen (Bian and Egusa, 1981). Encapsulation by melanin appears to prevent secretion of chitinases and proteases by fungi, this slows or may stop fungal growth and the melanin causes brown or black areas (Kuo and Alexander, 1967). Lagenidium sp. has killed larvae in cultures (Johnson, 1983; Lightner and Fontaine, 1973). Species of Fusarium have killed P. japonicus (Egusa and Ueda, 1972) and P. californiensis (Lightner et al., 1975). In both penaeids, the fungus infected the gills and adjacent parts of the body. Melanin was deposited in some of the fungal lesions and so the condition is known as black gill disease.

IV. Protozoans

A. Sporozoans

The two main groups of sporozoans found in penaeids are the gregarines and the microsporidians. Gregarines appear to be benign; they are commonly found in the gut where they usually attach to the wall in the early stages. Their cysts are attached to the wall of the hindgut (Overstreet, 1973) These cysts burst to release gymnosperms that are probably expelled with the faeces. Gregarines are not highly pathogenic to their hosts. Kruse (1959) found gregarines in all of the wild P. aztecw, P. duorarum and P. setiferus that he examined. Fontaine (1985) found a 95% incidence of a gregarine (Nematopsis penaeus) in P. aztecus and P. setiferus. Generally they are more abundant in P. setiferus than in P. aztecus. The gregarine life cycle includes a second host, usually a mollusc (Johnson, 1978).

Microsporidians are highly pathogenic intracellular parasites. At least

384 BIOLOGY OF PENAEIDAE

three genera occur in penaeids: Ameson [= Nosema], Agmasoma [ = Thelohania] and Pleistophora (Lightner, 1985). The ultrastructure of Agmasoma duorara from P. duorarum has been described by Iversen and Kelly (1987). Some species infect muscles and cause cotton tail, they can also cause white patches or abnormal colour alterations under the cuticle. Other species are found in the gonads, gills, digestive gland and in muscles associated with the heart and blood vessels (Overstreet, 1973; Couch, 1978). Infestation rates in the wild are usually low although Kruse (1959) found 11% infection in a mixed sample of 4816 P. aztecus, P. duorarum and P . setiferus from the Gulf of Mexico. Fontaine (1985) found that less than 2% of the P. aztecus and P. setiferus that he examined were infected with microsporidians. Feigenbaum (1975) found no microsporidians in a sample of 130 juvenile P. vannamei from the Huizache-Caimanero lagoon. The prevalence of Ameson sp. and Thelohania sp. was below 0.1% in a large sample (200000) of prawns from northern Australia (Owens and Glazebrook, 1988). Infection rates can be much higher in prawns under culture conditions. Anderson et al . , (1989) found that five out of 15 specimens of P. monodon from ponds in Malaysia had marked infection of the hepatopancreas.

Microsporidians do not require an intermediate host to complete their life cycle. Some are transmitted transovarialy and in others the spores infect the host directly (Overstreet, 1986). Spores of Agmasoma duorara adhere to the inside of cast exuviae and prawns might become infected by eating exuviae (Roth and Iversen, 1971). Iversen and Kelly (1976) found that spores of the microsporidian Agmasoma penaei that had passed through a non-host fish could infect postlarval P. duorarum.

B . Ciliates, Flagellates and Suctorians

Many species of ciliates are found on penaeids. Several species of stalked ciliates can live on the appendages and gills and if present in large numbers can interfere with feeding, respiration or moulting (Lightner, 1985). Lagenophrys sp. lives on the gills of penaeids where it digests the cuticle and can become encysted in the underlying tissue (Fontaine, 1985). The damaged tissue forms melanin which colours the gills brown or black. The gill lamellae and often large parts of the gills become congested with haemocytes reducing the area for gas exchange. This may be fatal, especially in conditions of low oxygen. Two other ciliates, (Epistylis sp. and Zoothamnium sp.) found by Fontaine (1985) on penaeid gills, do not appear to cause damage although heavy infestations can give the gills a brown appearance. Lightner and Lewis (1975)

PARASITES 385

suggested that large numbers of Zoothamnium on the gills of penaeids could cause hypoxia. Overstreet (1973), however, found no difference in survival in low oxygen between P. aztecus that were heavily infected with Zoothamnium sp. and those with little or no infection.

Flagellates do not appear to be important parasites of penaeids and are seldom found on penaeids in the wild. A species of Leptomonas, found in the haemolymph of penaeids involved in a mass mortality of cultured animals, was probably a secondary invader of weakened hosts (Couch, 1978).

Suctorians attach to the gills of penaeids. About 22% of a sample ( n = 238) of P. aztecus and P. setiferus from the Gulf of Mexico carried suctorians (?Ephelota sp.) but they did not appear to cause any histological damage (Fontaine, 1985).

V. Platyhelminthes

Two major families of trematodes (Microphallidae and Opecoelidae) and two orders of Cestodes (Trypanorhyncha and Lecanicephalidea) commonly infect penaeids (Owens, 1987). Although cestodes and digenetic trematodes are found in all species of penaeids they do not kill their hosts or affect growth or fecundity. Owens (1985) speculated that cestode cysts could damage the nerve cord and thereby modify behaviour. Penaeids function mostly as intermediate hosts for helminth parasites; the final hosts are usually a vertebrate.

A. Cestodes

The final hosts of most cestode larvae found in penaeids are elasmo- branchs. Eggs shqd by adult cestodes in the final host are released into the sea. In many species, the eggs hatch into free swimming larvae that is eaten by an intermediate host - usually an harpacticoid copepod (Overstreet, 1983). It has not been established whether penaeids become infected by feeding on these copepods or by feeding on eggs (Overstreet, 1983). Inside penaeids, cestode larvae develop into plerocerci that encyst in the digestive gland where they appear as white spots. If the prawn is eaten by the final host - usually a ray - the plerocerci develop into adult cestodes. The final host is required for the parasite to complete its lifecycle; the absence of elasmobranchs from the Huizache-Caimanero lagoon system in Mexico is probably the reason for a lack of cestodes in juvenile P. vannamei from the lagoon (Feigenbaum, 1975).

386 BIOLOGY OF PENAEIDAE

Many species of cestodes found in penaeids are cosmopolitan. Parachristianella monomegacantha for example has been recorded from the Gulf of Mexico (Overstreet, 1973), Chile (Dailey and Carvajal, 1976) and the Gulf of Carpentaria (Owens, 1981). Owens (1987) suggests that this is the only penaeid parasite known to have a worldwide distribution. The prevalence of plerocerci in penaeids may be very high. Kruse (1959) found 94% of prawns in a mixed sample of Penaeus aztecus, P. duorarum and P. setiferus were infected with plerocerci of a single cestode species. On average each prawn was carrying 8.1 plerocerci of this species as well as plerocerci of other species. Encysted larvae of the cestode Pro- christianella hispida were found in the digestive gland, gut and heart of about half the P. setiferus and P. aztecus examined by Fontaine (1985). The level of infection with plerocerci varies between individuals. Penaeus setiferus commonly carry over 1000 plerocerci but it is rare for all the prawns in a sample to be infected (Overstreet, 1973).

In most penaeids, the prevalence and intensity of cestode parasitisation increases with size or age. Statistically significant size relationships were found for Parachristianella monomegacantha infecting Penaeus braziliensis and Parachristianella heteromegacantha infecting P. duorarum (Feigenbaum and Carnuccio, 1976), and for Prochristianella hispida infecting P . aztecus (Ragan and Aldrich, 1972). There was a size-related incidence of infection of estuarine stages of P. merguiensis by Parachristianella monomegacantha; prawns below 13 mm CL were not infected but above that size infection rose to peak (20% infection) at around 22 mm CL (Owens, 1981). This cestode has only two hosts and the intermediate host feeds directly on the eggs. Owens suggested that prawns became infected when they changed their behaviour at around 13 mm CL. At this size they begin feeding in the mangrove zone where they are more likely to pick up the cestode eggs.

Feigenbaum and Carnuccio (1976) consider that host responses are the main reason for differences in infestation intensities of penaeids. Aldrich (1965) found the overall level of infection by Prochristianella hispida was higher in P. aztecus than in P. setiferus. Penaeus setiferus respond to infections of Prochristianella hispida by enclosing them in a dense walled cyst that eventually kills the parasite (Sparks and Fontaine, 1973). This reduces the level of infestation in this species.

The level of infestation of P. merguiensis by larvae of Polypocephalus sp. is related to salinity (Owens, 1985). At salinities of 14-34%,,, around 27% of prawns were infected with a mean of 0.5 cestodes per prawn. At salinities above 34%,, both frequency of occurrence and level of infestation rose. At 38%0, 95% of the prawns were infected with a mean of 6 parasites per prawn. Infection of P. aztecus and P. setiferus by

PARASITES 387

Prochristiunellu hispidu occurred at salinities of 1-2%0 in Texas (Aldrich, 1965) and of P. uztecus at 3 to ll%o in Louisiana (Ragan and Aldrich, 1972).

Sediment may also affect parasite load although the mechanism is not clear. The heaviest infestation of P. uztecus by Prochristiunellu hispidu was of individuals found over the finest sediments (Ragan and Aldrich, 1972).

B . Trematodes

Nearly all digenetic trematodes require a mollusc as the first intermediate host. Penaeids can act as second intermediate hosts by carrying the metacercaria stage. This resembles the adult but lacks functional reproductive organs. After a feeding phase in the intermediate host, metacercariae encyst in tissues surrounding the gut or in the cephalothorax and wait to be eaten by the final host. This appears to be a teleost fish in the case of trematodes found in penaeids (Kruse, 1959).

Some trematodes are highly specific in their choice of host. In inshore waters in Florida for example, metacercaria of Opecoeloides fimbriutus were found in 100% of a sample of P. duorarum but in no P. setiferus or P. uztecus (Kruse, 1959).

VI. Nematoda

Nematodes parasitic in penaeids are found chiefly in the digestive gland and tissues surrounding the digestive gland and foregut of penaeids but some live in the lumen of the gut or in blood vessels in the gills. Juvenile Hysterothylucium sp. infect penaeids and other invertebrates; the adults are gut parasites of fish, fish-eating birds and mammals. At least two species of Hysterothylucium are found in penaeids in North America and, in summer, up to 31% of penaeids carry infections. Other than physical disruption of tissues, there is no apparent pathogenesis (Couch, 1978). The incidence of larval Hysterothylucium is highly variable in P. uztecus and P. setiferus, ranging between 10 and 100% (Fontaine, 1985).

VII. Crustacea

Bopyrid isopods and a single species of copepod are the only crustaceans known to parasitise penaeids. The copepod, Culigus epidemicus, was

388 BIOLOGY OF PENAEIDAE

found attached to the appendages and telson of 72% of P. monodon in a pond in Thailand (Ruangpan and Kabata, 1984). Isopod parasites have not been found on penaeids from the Gulf and Atlantic coasts of North America, but they are commtm on some species of penaeid in the Indo- Pacific region (Overstreet, 1973). The best known are orbionid bopyrid isopods; these have been recorded on penaeids from the Arabian Gulf (Dawson, 1958; El Musa et al., 198l), India (Thomas, 1977), Australia (Owens, 1983), Hong Kong (Bourdon, 1979) and the Philippines (Bourdon, 1981). Markham (1986) notes that bopyrids of the subfamily Orbioninae are found only on peneoideans and their geographical range is restricted to the warm Indo-West Pacific from the Red Sea to Japan between 40"N and 35"s.

Bopyrids live in the gill chamber of crustaceans where they attach to the gills and cause a conspicuous bulge of the branchiostegite. The female bopyrid is large, the dwarf male living on her body. The life cycle involves two hosts. After hatching, larvae search for and attack calanoid copepods on which they feed and grow for 10-20 days. The bopyrid then leaves the copepod, searches for the final host, and enters its gill chamber. Female bopyrids have piercing mandibles that allow them to suck fluids from the host. This withdrawal of energy is large; experimental studies on a carid (Palaemonetes pugio) showed that although the bopyrid was only 4% of the host's mass, it extracted 10% of the host's energy intake (Anderson, 1977). This energy is used by the bopyrid mostly for its own reproduction at the expense of the host's reproductive output. The host may become sterile and take on the secondary sexual characters of the opposite sex. Bopyrids frequently castrate their hosts (Reinhard, 1956) mainly through interfering with gonad development (Thomas, 1977; El Musa et al. 1981; Abu Hakima, 1984). They can also cause malformation of the petasma (Tuma, 1967; Thomas, 1977). Penaeids carrying bopyrids continue to grow but growth is affected. Female P. semisulcatus normally grow to a larger size than do males but Abu Hakima (1984) showed that male P. semisulcatus infected with the bopyrid Epipenaeon elegans grew to the same size as uninfected females.

Dawson (1958) reported that in the Arabian Gulf, only P. semisulcatus larger than 100 mm TL were infected with bopyrids. Mathews et al. (1988) however found low rates of infestation by Epipenaeon elegans of P. semisulcatus of 14-18 mm CL, infestation rates increased up to a prawn size of around 28 mm CL after which the rate fell apparently due to a loss of the parasite. In the Gulf of Carpentaria, P. merguiensis became infected with the bopyrid, Epipenaeon ingens as juveniles in estuaries (Owens, 1983). Female bopyrids mature only after the prawns

PARASITES 389

migrate offshore. Owens and Glazebrook (1985) showed that bopyrids on penaeids grow with their hosts and have a similar longevity although there is some loss of parasites as the prawns approach asymptotic length.

In a study of 180000 prawns from the Gulf of Carpentaria, Owens and Glazebrook (1985) divided the species into three categories of prawns with respect to the prevalence of bopyrids. The first category consisted of the most heavily infected prawns and included only one species, P. semisulcatus of which 2.9% were infected. In the second category were prawns with light infections (0.2-0.4%): Penaeus merguiensis, P. indicus, P. longistylus and Metapenaeus ensis. Prawns in the the third category were rarely infected: P. escufentus, P. monodon, P. latisulcatus and M. endeavouri. Owens and Glazebrook (1985) suggested that P. semisulcatus was the primary host for the most common species of bopyrid found (E. ingens) and that the parasite infected other species of prawns only when it could not find its primary host. This would explain why P. merguiensis was rarely infected when co-occurring with P. semisulcatus but was infected when alone. In the Arabian Gulf, the bopyrid Epipenaeon elegans also preferred P. semisulcatus and was not found on either P. japonicus or Metapenaeopsis novaeguineae although these were captured in the same area (Dawson, 1958).

VIII. Behavioural Changes Induced by Parasites

Some parasites induce behaviour that increases the probability of the final host finding the intermediate host (Holmes and Bethel, 1972). An example of this in penaeids was proposed by Overstreet (1973) who suggested that the high incidence of sporozoans in penaeids left in Mississippi estuaries, after most of the population had migrated offshore, may be due to a suppression of migratory behaviour by the parasites. The presence of a reservoir of infected animals could result in more prawns being infected when they recruit into the estuaries as juveniles.

IX. Parasites as Biological Markers

A study by Owens (1983) of the incidence of the bopyrid E. ingens on P. merguiensis in the Gulf of Carpentaria showed large differences in the level of infection of populations in different regions. The correspondence between the level of infection in particular estuaries with those in adjacent offshore populations indicated that there was little alongshore movement by P. merguiensis once they had left the estuaries. Owens

390 BIOLOGY OF PENAEIDAE

(1985) confirmed this lack of alongshore migration in P. rnerguiensis with a study of the incidence of larvae of the cestode Pdypocephu1u.s sp. These complementary but independent studies with two different parasites show that parasites can be used as biological markers for penaeids.