bivalve mollusc Gill and Gut Parasitic

I. Causative Agent and Disease Canada with seed from Japan. It Parasitic copepods are arthropods occurs in the European flat, Pacific and belonging to the subphylum Crustacea, native oysters and several other spe- class Maxillopoda and subclass cies of marine bivalves and has recently Copepoda. Copepods parasitizing been introduced into France. Mytilicola bivalve molluscs are mostly within porrecta occurs in ribbed and hooked the order and can be mussels and may also occur in other divided into two basic groups; obligate bivalves in the Gulf of Mexico. External endoparasites inhabiting the alimentary parasitic copepods have been reported in tract including the more common species the mantle and on the gills of various bi- of Mytilicola (intestinalis, orientalis, valve species from Europe and from the porrecta) and ectoparasites inhabiting Atlantic, Gulf and Pacific coasts of North the mantle and gill tissues including America, including British Columbia, several species within the genera Canada. In Alaska, unidentified parasitic Conchyliurus, Modiolicola, Myicola, copepods have occurred on the gills Myocheres, Ostrincola, Pseudomyicola and in the alimentary tracts of Pacific and Paranthessius. An exception to the oysters, basket cockles, blue mussels, latter group is Pseudomyicola spinosus rock scallops and native littleneck clams. that apparently is able to move back Many of these copepods have been and forth between the gill/mantle areas present both externally and internally ap- and the alimentary tract of bivalve pearing more like Pseudomyicola rather hosts. Mytilicola has been implicated in than Mytilicola. causing intestinal damage, poor growth and mortality in European mussels but III. Clinical Signs conclusive evidence is lacking regarding infestations are mostly its true pathogenicity. Most ectoparasitic found by routine histological examina- copepods appear to be commensals but tion. There are no clinical signs of their attachment to gill surfaces may external or internal copepod infestation cause localized tissue damage. When except in European reports attributing found within the GI tract, P. spinosus poor growth and mortality in mussels to may damage mucosal epithelial cells infestations by M. intestinalis. Infesta- followed by perforation, subsequent tions in the GI tract may cause meta- invasion into connective tissues and plasia (columnar epithelium reduced to encapsulation of the parasite by host low cuboidal or squamous) of mucosal granulomatous tissues. epithelium and in rare cases erosion and perforation from the appendages with or II. Host Species without host encapsulation by hemo- Mytilicola intestinalis is apparently cytes. limited to Europe occurring in mussels, clams, cockles and the flat in IV. Transmission waters from Denmark to Italy includ- Parasitic copepods of bivalve mol- ing the British Isles and Ireland but not luscs have a direct life cycle involving the Baltic. Mytilicola orientalis was copulation of two adult sexes and the introduced to the Pacific Northwest and production of eggs that hatch directly in

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ambient seawater or, as with endopara- the parasites are present in great num- sitic species, the egg cases are released bers. The pathogenicity of endoparasitic with feces to seawater. The hatched copepods in bivalve molluscs has been nauplii may undergo molts into other enigmatic with many reports indicating naupliar stages or into metanauplii and no pathogenicity. Other investigators in- finally to one or more copepodid stages. dicate poor growth and bivalve mortality One of the copepodid stages is usually caused by erosion of the mucosal epithe- the initial parasitic stage that either at- lium with perforation and encapsulation taches to the external target host tissue of the parasites in surrounding connec- or, in the case of endoparasitic forms, is tive tissues. Internal copepods may also drawn into the GI tract by filter feed- cause apparent occlusion of the large ing of the mollusc host. Other parasitic ducts connecting the stomach and diges- instars may occur until the copepod be- tive diverticulae. Mytilicola sp. occurs comes an adult. However, the number of more commonly in larger host molts and specific stages vary depending inhabiting the bottoms of sheltered areas on the species of parasitic copepod and and rarely occurs in bivalves less than 10 remain unknown for many. mm in length and in Pacific oysters less than or equal to 20 mm. In Alaska, there V. Diagnosis has been no bivalve mortality or poor Specific diagnosis is based on body condition associated with the pres- observing external copepods attached to ence of parasitic copepods. gills and mantle tissues or by dissect- ing reddish colored elongate worm-like VII. Human Health Significance copepods from the stomach and intestine There are no zoonotic human health of internally infested bivalves. Enzy- concerns with the occurrence of parasitic matic digestion also may be used for copepods in the tissues of bivalve mol- quantification. The body of Mytilicola luscs. sp. is about 5 to 12 mm long (depending on species), relatively dedifferentiated with reduced limbs and body segmenta- tion and overall is less recognizable as compared to other ectoparasitic genera that resemble free living copepods. Spe- cies is determined by various external morphological features (body length or size, mouth parts and antennae) of female copepods since the occurrence of males is generally transient. Histological examination of a parasitic copepod dem- onstrates general features of a complex body structure with carapace, segments, appendages, GI tract, gonads and striated musculature.

VI. Prognosis for Host The effects of ectoparasitic copepods on their hosts have been minimal and generally considered harmless, unless

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Gill and Gut Parasitic Copepods 

Histological section of with a Histological section of Pacific oyster parasitic copepod (arrow) attached to the with a parasitic copepod in the intestinal gill tissues causing localized inflammation lumen

Mytilicola sp. in gut (arrow and inset) of the clam Macoma balthica (Photo: Dorothy Howard, NOAA Cooperative Oxford, MD Laboratory)

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bivalve mollusc Arthropods 

Histological section of dead parasitic Similar dead parasitic copepod (arrow) copepod (arrow) surrounded by host surrounded by host inflammatory cells inflammatory cells after penetrating into in connective tissue of digestive gland of the connective tissues from digestive gland

tubules in Pacific oyster 

Histological section of a parasitic copepod (arrow) on the mantle tissues of weather- vane scallop

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