Parasitic isopods
(suborder Flabellifera) affecting the farmed
marine fish in Greece,
with special reference to Ceratothoa oestroides (family Cymothoidae)
|
Short review by Dr. PANOS
VARVARIGOS |
|
CONTENTS |
|
Copyright (c) Dr. Panos Varvarigos. Athens,
Greece. |
Taxonomy,
elements of biology and mode of transmission
Phylum: Arthropoda
(the species-richest phylum on Earth)
Subphylum: Mandibulata
Class: Crustacea
Subclass: Malacostraca
Hyperorder: Peracanida
Order: Isopoda
Fish parasites among
arthropods occur in the crustacean orders: Copepoda,
Branchiura, Isopoda.
The Branchiura are exclusively parasitic,
while most of the Copepoda and Isopoda are free-living. About 450 species of
isopods are parasites of marine and freshwater fish.
Suborder: Flabellifera
Family: Cymothoidae Family:
Anilocridae
In the isopod families Cymothoidae and Anilocridae,
which number about 200 species, are included parasitic species that remain on
the fish body throughout their life. The main genus/specie of interest to the
Greek marine fish farmers comprises:
Genus: Ceratothoa
Species: Ceratothoa
oestroides
Note that other isopod
species (Flabellifera- Cymothoidae,
Anilocridae) have been reported to infest net-pen
reared sea bass and/or sea bream, such as: Anilocra
physodes, Nerocila orbignyi, Emetha audouini, Ceratothoa
parallela. Although the name "Anilocra" is used widely by farmers for any type of
isopods infesting their fish, Anilocra and Nerocila adults are externally attaching to the skin
and fins of fish. Only their juveniles may be found occasionally in the buccal cavity of the fish.
Isopods are Malacostraca; their body is dorso
ventrally flattened and is lacking a carapace. The isopod thorax consists of 7
free segments with 7 pairs of thoracic legs. As a result of the well-sheltered
environment of the buccal cavity, species that
establish there have evolved a thinner cuticular mineralisation and the pleopods
of the three last pairs have transformed into respiratory organs. Paired eyes
consist of numerous eyelets. On its ventral side, between the swimming legs,
the female bears a brood pouch or "marsupium",
shielded by special plates, called "oostegites",
to carry the eggs and the larvae for some time after hatching.
A mature gravid female
isopod releases about 400-550 larvae at a time. Ceratothoa sp., are
constantly fertile, remaining in reproductive condition throughout the year.
There is little information as regards the generation time under different
water temperatures and the fluctuations in fecundity. Fecundity and hatching
rate increase in warmer temperatures, July being the period of optimum isopod
proliferation in the Mediterranean.
Members of these families (Cymothoidae, Anilocridae)
are protandrus hermaphrodites, i.e. an individual
develops and functions first as a male and then may become a female, the
presence of mature females being inhibitory for a further development of males
in their neighbourhood.
Eggs are laid into the marsupium, on the ventral side of the female, between its thoracopods. In the marsupium the
eggs hatch into "pullus larvae", which are
sexually non-differentiated and undergo a number of moults prior to reaching
the infective free swimming stage, when they abandon the marsupium
and actively search for a host. The entire larval development takes place in
the marsupium. Sexual differentiation takes place
after the pulli II larvae have left the marsupium.
The first pullus larva (pullus 1 stage, pullus primus, prehatch 1) is
only found inside the marsupium. It metamorphoses by
moulting into the second pullus (pullus
II larva, pullus 2 stage, pullus
secundus, prehatch 2), its thoracopods armed with hooks and with cuticle strongly
pigmented by numerous chromatophores. The second pullus has 6 pairs of legs and is also not sexually
differentiated. When released in search for a new host, the pulli
II can be termed "manca larvae". Manca larvae either in the plankton or attached to a fish
differ from juveniles since they have six pereonal
segments and sets of legs. After a subsequent moult, the 7th segment and pair
of legs appear and then the isopod becomes a "juvenile". They
function first as males and subsequently as females, according to
circumstances.
It is difficult to define
when a juvenile becomes an "adult" isopod. Stages of males,
transitional females and females (ovigerous and non-ovigerous) are referred to in the literature. Since they
are protandrous hermaphrodites, then technically,
only the females may be safely termed adults. The Flabellifera
isopod parasites adhere in pairs to different spots on the fish skin and fins
and may also live in the mouth and gill cavities.
Regarding the most
sensitive age of the fish hosts when the parasites attach in their buccal cavity, empirical observations agree with research
results that there is a significant correlation between the rate of infection
and the length of the host, fish fry being the evident target for isopod
attachment. For Ceratothoa oestroides pulli II, it had
been thought that younger fish seem to consider the juvenile isopods as
attractive prey, however, recent studies in aquaria
have shown that the pulli II larvae swim in search of
a host and that they actively attack fish. After attaching themselves on the
base of the tail fin or on the flank, the young isopods progress to the
anterior part of the body, go beneath the operculum and settle in the buccal cavity. The whole process from attachment onto a
host until settling in the buccal cavity takes up
about two hours.
There is strong competition
among the pulli seeking attachment in the mouth of
the host. Only two pulli may settle in the buccal cavity of the host comprising the pair of future
adults. Thus, although in the first phase of infection a fish may be attacked
and carry more than two pulli on its body surface and
gill cavity, eventually no super-infection is possible and mostly two isopods
may be hosted in the buccal cavity on any one fish.
Ceratothoa oestroides pulli II
larvae remain free swimming and capable of infecting a host for about 7 days at
22°C. During this period, even in case that after successful settlement the
host dies, the isopod larvae immediately abandon the dead fish and are still
capable of actively seeking another (not yet moulted to the sessile pre-adult
form). Later, after firm establishment in the buccal
cavity of the host, the parasites are incapable of migrating to another and
begin blood sucking (haematophagous). It is not
precisely known what is the future of adults or gravid female
isopods if forced to abandon their dead hosts.
Adult female Ceratothoa eostroides Adult ovigerous females (three on the left side) and ready to
release larvae (far right).
Eggs, pulli II
larvae, adult male (smaller) and adult female Ceratothoa
eostroides
Fish susceptibility
Parasitic isopods are fairly
common crustacean infestations of wild tropical marine fish. They are less
common in cold marine waters and not often found on freshwater fish. Parasitic
isopod fauna is rare in polar waters. The distribution of parasitic isopods, as
that of all other parasites, is closely related to the occurrence and ecology
of their hosts. Demersal fish in coastal waters are
infected most often, the parasites being rarer in bathy- and pelagic fish.
Members of the following families are most often isopod-infected: the Sparidae, Lutianidae, Serranidae, Trichiuridae,
Bramidae.
Intensive fish farming in
coastal waters in the Mediterranean provides a close to ideal environment for
isopod parasites, hence, farmed sea bream (Sparus auratus), but mostly sea bass (Dicentrarchus
labrax) infestations by isopods comprise a
frequent problem in the Mediterranean. Ceratothoa
oestroides is the most common among the isopod
parasites and inflicts major damage. The adults are found in pairs and occur
mainly in the buccal cavity of the sea bass, while
the infective larval stages (pulli II /manca larvae) and the juveniles are present in the buccal and opercular cavities, on
the fish head, behind the eye or behind the operculum, above the lateral line,
on the caudal fin and the caudal penducle of both sea
bass and sea bream. These comprise actual pathogens provoking acute tissue
inflammation and necrosis. It seems that the anatomy of the buccal
cavity and the dentition of sea bream inhibit the establishment of the isopods,
thus adults are rarely found on sea bream.
In the wild, the usual
hosts of parasitic isopods are mullets (Mugil
spp., Liza spp.), bogues (Boops boops), goldlines (Boops salpa), striped breams (Lithognathus
mormyrus), white breams (Diplodus sargus). These fish
species abound in the vicinity of sea bream (Sparus
auratus) and sea bass (Dicentrarchus
labrax) net pens feeding on waste feed and
comprise the vectors for the transmission of the parasites to the farmed
species. None of the cymothoid species reported on
farmed bass and bream are known to parasitise them in
the wild. It seems that these isopods are non host-specific parasites
transferred to cultured fish from the feral fish around the cages carrying
adult pairs of isopods, one of which is the ovigerous
female. The increase in the volume of farmed sea bass may have created a new
host-parasite association whereby Ceratothoa
oestroides has effected
a complete host shift. The parasites proliferate mostly when the sea-water
temperature increases during the summer peaking during July and August, when
the prevalence of infection in the cages may exceed 50%.
Pathogenesis and diagnosis
Ceratothoa oestroides
has become a major
pest, primarily for sea bass, but also for sea bream reared in net pens. Strong
regional pertinence is observed in the Aegean Sea (mainly the Eastern Aegean
Greek islands and along the Turkish coast). Other Greek regions are vulnerable
as well, among which the North and South Evian gulf are notable.
On young sea bass and
bream, pulli II larvae may be found in various
numbers on the skin, gills, gill and mouth cavities.
The adult parasites are found paired, attached to the buccal
epithelium mainly of larger sea bass.
Heavy infestations of
parasitic larvae may kill smaller fish when they first infect them seeking
permanent attachment. Pulli II larvae and juveniles
attack relatively younger fish, about 5g-20g of weight and cause considerable
damage to the skin around the head, the eyes and the gill epithelium by
injuring the gill lamellae. Their voracious haematophagy
and the mechanical damage of their hooks lead to severe inflammation and
necrosis of head, eye and gill tissues. The infested fish are usually apathetic
and anorexic and may show respiratory distress. The haemorrhagic and necrotic
head tissues are evident when observing the fish in their cage. When the sick
fish are removed from the water, several isopod larvae may be seen in their buccal and gill cavities and/or on the skin near the
opercula.
Injured tissues are
frequently invaded by secondary bacterial pathogens, such as Aeromonas spp., Tenacibaculum spp., Vibrio spp. and this may lead
to severe escalation of mortality. In young stocks, the cumulative mortality
due to parasitism by the pulli II larvae may run as
high as 15% even without any bacterial implications.
The adult isopods are haematophagus (feed on blood) and cause anaemia. The parasitised fish have significantly lower erythrocyte
counts as well as haematocrit and haemoglobin values.
The leukocyte counts are increased, obviating the host's immune response to the
presence of the isopods. In addition, the established adult isopods can cause
considerable damage to the mouth tissues with their biting and sucking mouth
parts, or their copulation activity. Their large size (up to 6 cm in length)
may cause atrophy of the tongue, dysplasia of teeth
and slackening of the cartilagenous tissues leading
to a "bag-shaped" lower jaw. Invariably, the presence of large adult
parasites in the buccal cavity interferes with
feeding, causes chronic stress and results in growth retardation and a
predisposition to bacterial and/or endo-parasitic
invasions.
Isopod infestation is
confirmed by gross observation of the parasites on the skin, mouth, or in the
gill chamber of the fish. In addition, they often produce the lesions described
above that characterise their presence.
|
|
Adult isopods in pairs
(large female, smaller male) in the buccal cavity
of sea bass. Slackening of the tissues underlying the attached parasites
often results in deformations -"bag-shape"- of the lower jaw area
(left). |
|
Pullus II stage Ceratothoa oestroides
larvae on the gills of a sea bass (seen on the pharyngeal teeth/gill rakers and among the gill lamellae). Gill inflammation
and focal necrosis is evident. The operculum has been removed to facilitate observation.
The fish must be examined fast after removal from the water, as the larvae
tend to abandon quickly their dead host. |
|
|
Inflamed and necrotic gills (left) and
necrotic head and eye tissues (right) caused on sea bass by Pullus II isopod larvae. |
Prevention and treatment
Recommended prevention
would be by means of stock management measures. Excessive fish densities in the
fry holding pens must be avoided. Often, in cases of heavy parasitism and
mortality, reducing the fish density is enough by itself to remedy the
situation. Additional preventive measures would be to: a) Avoid placing the
young fish in close proximity with the adult sea-bass, which are most likely to
harbour adult parasites in reproductive phase. b) Prefer deep sites with
sufficient currents, which disperse the juvenile parasites in a direction away
from the main body of the cage mooring.
It is worth noting that on
farms where injection vaccination of sea bass is routinely performed, manual
delousing of the anaesthetised fish, by means of small blunt forceps, prior to
injecting results in a sharp drop of fish retaining adult isopods. Hence, there
is a subsequent sharp reduction to the number of larval isopods and very little
damage on the fish fry in the next season. In addition, the anaesthetic used
prior to vaccination, has been seen to sedate the adult isopods, many of which
lose their grip and drop, still alive, in the tarpaulin.
Treatment of isopod larvae
infestations has been attempted with considerable success by means of hourly
formalin baths, at concentrations of about 150ppm, subsequent to enclosing the fish
in a tarpaulin and providing ample water oxygenation. Nonetheless,
re-infestation occurs soon after unless the stocking densities are reduced.
Bath
treatments by means of hydrogen peroxide, dichlorvos
(AquaguardTM)
or pyrethroids, such as deltamethrin
(AlphamaxTM)
or cypermethrin (ExcisTM,
BetamaxTM)
lack adequate experimental field data in the Mediterranean.
However, empirical commercial scale information exists as regards the
application methodology, the efficacy on adult isopods and larvae as well as
the toxicity to the fish. Field data on environmental implications or about the
potential acquisition of resistance by the isopods against these compounds is
lacking. Resistance has been demonstrated to develop in the case of salmon lice
Lepeophtheirus salmonis.
Laboratory experiments with
deltamethrin (pyrethroid)
have indicated that the minimal in vitro dose that kills Ceratothoa oestroides
adults in 2 hours is 0.05mg/litre. Experimental field data by the manufacturer
of AlphamaxTM
in offshore net pens of sea bass indicated that deltamethrin
at 3ppb for 30 minutes is efficacious and safe for treating small bass
(<10g) against the juvenile isopods. Larger sea bass parasitised
with adult Ceratothoa were effectively and safely
treated with 7.5ppb deltamethrin for 30 minutes.
Extensive
empirical/commercial data exists on the efficacy of cypermethrin:
5-10ppb for 60 min (ExcisTM) on all age classes of sea bass and
bream at many different locations. Cypermethrin has
been proven both effective against all stages of the isopods and safe for the
fish treated. Nevertheless, the effects on other marine arthropods in the
vicinity of the net pens have not been monitored.
In all cases a treatment tarpaulin is required as well as a continuous supply
of oxygen, especially during the warm season.
Environmental toxicity
studies for these organophosphate compounds in the warm Mediterranean waters
are necessary.
Against the salmon lice,
the usual doses for bath treatments using tarpaulins, at approximately 10oC water temperature are as follows:
> Hydrogen peroxide:1500ppm for 20min
> Dichlorvos: 1ppm for 60 min
> Cypermethrin: 5ppb for 60 min
Against the isopods, the
usual doses for bath treatments of bass and bream using tarpaulins, at 16-25oC water temperature are as follows:
> Cypermethrin: 10ppb for 60 min
> Deltamethrin: 10ppb for 30-60 min
Bath treatments have
serious drawbacks. Even assuming a safe for the fish and reasonably potent
chemical compound for use by no more than an hourly bath treatment, the
following practical problems would be encountered:
A. Cost, risk, labour, time:
A1. Expensive
product to buy in the large volumes required for repetitive use.
A2. Risk of accidental fish kills (mishandling, asphyxiation).
A3. Labour intensive and time-consuming operation, prohibitive on sites
with many large circular net pens.
A4. Re-infestations from wild fish are likely demanding repeat
treatments.
B. End product quality: (see also the
economic implications section)
B1. Withdrawal
periods should be adhered to hence, re-infestation may occur prior to harvest.
B2. Impossible to achieve 100% success rate, hence grading at harvest
would still be necessary.
B3. Killed parasites may remain in the buccal
cavity for some time (unknown) after treatment.
B4. The use of chemicals in aquaculture damages the perception of fish
as healthy and wholesome food.
C. Environmental compatibility:
C1. Adverse effects to the ecosystem.
The advent of a potent
in-feed treatment, such as emamectin benzoate (SliceTM), could alleviate drawbacks A2 and
A3 from the above lists. However, empirical observations with SliceTM applied
on sea bass in the field did not produce sustainable good results.
Economic
implications
The costs associated with
the isopod infestations of farmed sea bass may be categorised as follows:
1. Direct mortality of young stock due
to infestation by isopod larvae.
2. Accidental killing of fish during bath treatments (accidental
handling loss).
3. Veterinary and medication expenses (Vet. &
med.) plus labour (see previous section).
4. Rejections at harvest (degraded fish).
5. Extra labour for fish grading at the packing plant (quality).
6. Chronic stress and high propensity to other diseases (indirect
mortality).
7. Growth retardation.
Although there has been no
attempt to quantify these costs across fish farms, it is obvious that the
losses associated with the first four items on the list above could be found in
the farm records or diaries. The costs associated with chronic stress and propensity to other diseases are difficult to quantify (item
6).
At harvest, fish that have
survived parasitism are usually of inferior body condition, they may have
developed the "bag-shaped" jaw and the adult parasites may be found
attached in their mouth. Frequently, a number of larvae may also be found in
their mouth and gill cavity having been released from the adult female. These
repelling characteristics render such fish unsuitable for the market. The cost
of rejects may run high in cases of heavy infestations (usually 1%, but also
anything up to 25% of prevalence among harvest-size fish). Besides, there is
the considerable extra labour associated with grading, or manual delousing in
the packing plants performed by experienced operators (item 5 above).
A regards growth
retardation (item 7 above), research to-date in the Adriatic and the Aegean Sea
has shown that for fish of the same age class, parasitism by Cymothoids significantly stunts both body length and weight
when comparing parasitised with non-parasitised fish. Parasitism may result in fish that are 7%
shorter and 20% lighter on average.
Regulations
Isopod infestations do not
pose any risk to consumer health.
There are currently no
regulations in place.
Reference list
(1)
J. Grabda (1991) Marine
Fish Parasitology. An Outline.
Polish Scientific Publishers, Warszawa.
(2)
D.W. Bruno, D.J. Alderman, H.J. Schlotfeldt (1995) What should I do? A
Practical Guide for the Marine Fish Farmer. Published
by the European Association of Fish Pathologists.
(3)
E. J. Noga (1996) Fish
Disease Diagnosis and Treatment. Mosby -Year
Book, Inc.
(4)
E.P. Papapanagiotou, G. Photis,
G.A. Boxshall (1998) Incidence
of an Isopoda (Cructacea: Cymothoidea)
infection in cage farmed sea bass (Dicentrarchus labrax) in Northern Greece. 1st
Balkan Aquaculture Conference, Thessaloniki, Greece,
18-21 September. Greek Association of Aquaculture
Scientists.
(5)
E.P. Papapanagiotou, J.P. Trilles,
G. Photis (1999) First
record of Emetha audouini,
a cymothoid isopod parasite, from cultured sea
bass Dicentrarchus labrax
in Greece. Dis. Aquat. Org., 38: 235-237, 1999.
(6)
G. Sarusic (1999) Preliminary
report of infestation by isopod Ceratothoa oestroides (Risso, 1826), in marine cultured fish.
Bull. Eur. Ass. Fish Pathol., 19(3): 110-112,
1999.
(7)
J.P.G. Toovey, A.R. Lyndon (2000)
Effects of hydrogen peroxide, dichlorvos and cypermethrin on subsequent fecundity of sea lice, Lepeophtheirus salmonis,
under fish farm conditions. Bull. Eur. Ass. Fish Pathol.,
20(6): 224-228, 2000.
(8)
J. Treasurer, S. Wadsworth, A.Grant (2000) Research shows how lice build immunity to
treatment. Fish Farmer magazine, 23(5):12-13, 2000.
(9)
T. Horton (2000) Ceratothoa
steindachneri (Isopoda: Cymothoidae)
new to British waters with a key to north-east Atlantic and Mediterranean Ceratothoa. Journal of the Marine Biological
Association of the United Kingdom, 80, 1041-1052.
(10)
F. Athanassopoulou, D. Bouboulis,
B. Martinsen (2001) In
vitro treatments of deltamethrin against the
isopod parasite Ceratothoa oestroides, a pathogen of sea bass Dicentrarchus
labrax L. Bull. Eur.
Ass. Fish Pathol., 21(1): 26-29, 2001.
(11)
J. Hardwick (2001) Excis-Technical
presentation. Novartis Aquatic Animal Health Division.
(12)
E.P. Papapanagiotou, J.P. Trilles
(2001) Cymothoid
parasite Ceratothoa parallela
inflicts great losses on cultured gilthead sea bream Sparus
aurata in Greece. Dis. Aquat.
Org., 45: 237-239, 2001.
(13)
T. Horton, B. Okamura (2001) Cymothoid isopod parasitism: an emerging disease
of Mediterranean mariculture. 10th Int. Conf. of
the EAFP: "Diseases of Fish and Shellfish". Trinity College, Dublin,
9-14 September 2001.
(14)
B. Martinsen, S. Alexandersen,
B.H. Fossum (2001) Deltamethrin, an effective treatment against the
isopod sea lice Ceratothoa oestroides infecting farmed sea bass (Dicentrarchus labrax).
10th Int. Conf. of the EAFP: "Diseases of Fish and Shellfish".
Trinity College, Dublin, 9-14 September 2001.
(15)
S. Sevatdal, T.E. Horsberg (2001) Monitoring of sensitivity/resistance of
Norwegian salmon lice (Lepeophtheirus salmonis K.) strains to pyrethroids
(deltamethrin and cypermethrin)
with bioassays. 10th Int. Conf. of the EAFP: "Diseases of Fish and
Shellfish". Trinity College, Dublin, 9-14 September 2001.
(16)
S. Alexandersen, B.Martinsen,
B. Midttun (2001) Comparison
between Alpha Max (deltamethrin) and Beta Max (cypermethrin) with regard to clinical effect against sea
lice (Lepeophtheirus salmonis).
10th Int. Conf. of the EAFP: "Diseases of Fish and Shellfish".
Trinity College, Dublin, 9-14 September 2001.
(17)
B.H. Fossum, S. Alexandersen,
B.E. Grosvik, O.K. Andersen, B.Martinsen (2001) Sea lice treatment with Alpha Max (deltamethrin); investigation of environmental effects in
the field. 10th Int. Conf. of the EAFP: "Diseases of Fish and
Shellfish". Trinity College, Dublin, 9-14 September 2001.
(18)
A. Ramstad, D.J. Colquhoun, R. Nordmo, I.H.
Sutherland, R. Simmons (2001) Field trials
in Norway with SliceTM (0.2% emamectin
benzoate) for the oral treatment of sea lice infestation in farmed Atlantic
salmon (Salmo salar L).
10th Int. Conf. of the EAFP: "Diseases of Fish and Shellfish". Trinity
College, Dublin, 9-14 September 2001.
(19)
T. Horton, B. Okamura (2001) Cymothoid isopod parasites in aquaculture: a
review and case study of a Turkish sea bass (Dicentrarchus
labrax) and sea bream (Sparus
auratus) farm. Dis. Aquat.
Org., 46: 181-188, 2001.
(20)
I. Mladineo, D. Valic (2002) The mechanisms of infection of the buccal isopod Ceratothoa
oestroides (Risso, 1836), under experimental
conditions. Bull. Eur. Ass. Fish Pathol.,
22(5): 304-309, 2002.
(21)
T. Horton, B. Okamura (2003) Post-haemorrhagic
anaemia in sea bass, Dicentrarchus labrax (L.), caused by blood feeding of Ceratothoa oestroides
(Isopoda: Cymothoidae). Journal of Fish Diseases,
26: 401-406, 2003.
VETCARE Ô
VETERINARY SERVICES TO AQUACULTURE AND
DISTRIBUTION OF FISH HEALTH PRODUCTS
Copyright (c) Dr. Panos Varvarigos.