individual is low. Vincentiis and Renzoni 

 (1963) recognized what seems to be the same 

 organism in eggs of the Mediterranean edible 

 mussel, Mijfiliis galloproviiicialis L., from the 

 Gulf of Naples. A similar parasite, Chytridiop- 

 sis ovicola, has been reported from eggs of the 

 European oyster (Leger and Hollande, 1917). 



Taylor (1966) described a disease of Cali- 

 fornia mussels, M. califoruiaiius Conrad, caused 

 by another haplosporidan, Haplosporidiiini 

 tumefacieutis. The disease, characterized by 

 tumefactions of the digestive gland, was found 

 in 23 of 1,114 individuals examined. The gross 

 enlargement of the gland was apparently due 

 to Plasmodia of the parasite ; no necrosis was 

 reported. Samples of sea mussels collected si- 

 multaneously from the California coast by Tay- 

 lor were not parasitized by the haplosporidan. 



Sea mussels from the Baltic were found by 

 Raabe (1934, 1936, 1938, 1949) to harbor a 

 number of ciliates, including Ancistrocoma 

 pelseneeri Chatton and Lwoflf, Kidderia mytili 

 (DeMorgan) , Ancistruma mytili (Quenner- 

 stedt), and Hypocomides mytili (Chatton and 

 Lwoff ) . The same species of ciliates have also 

 been identified from Mytilus sampled in other 

 geographic areas (Kidder, 1933; Chatton and 

 Lwoflf, 1934; Kozloff, 1946). Although many 

 ciliates occur in bivalve Mollusca (Fenchel, 

 1965). a parasitic role has not been adequately 

 determined for most of them. 



Diseases Caused by Helminths 



Invasion of mussels by larval trematodes is 

 thought to be responsible in part for pearl for- 

 mation. Jameson (1902) believed that most 

 mussel pearls resulted from encystment of 

 metacercariae and encapsulation by the host. 

 Herdman (1904), who studied sea Mussels in 

 England, found pearls very common near Piel 

 and attributed them to invasion by larvae of 

 Distoniinn (Gymnophallus) somateriae. 



The literature on trematode-induced pearl 

 formation in mussels has been reviewed by 

 Stunkard and Uzmann (1958). The relation 

 between pearls in European sea mussels and 

 trematode parasites was first described by 

 Garner (1872), and later by Dubois (1901, 

 1903, 1907, 1909) who proposed the name 

 Distomum nmigaritarum (Gymnophallus mar- 

 garitarinn) for parasites found in reddish- 



brown spots which served as foci for pearl for- 

 mation in mussels from the French coast. 

 Jameson (1902) stated that the larval trema- 

 todes resembled D. somateriae which had been 

 described as an adult from the intestine of the 

 eider duck, Somateria moUissima, by Levinsen 

 (1881). Jameson referred the parasite to the 

 genus Lecithodendrium Looss and described 

 the process of pearl formation by the mantle 

 of the mussel around the metacercariae. Giard 

 (1903, 1907) confirmed these observations on 

 pearl formation. Odhner (1905) designated the 

 larvae causing pearl formation in mussels as 

 Gymnophallus bursicola. Similar metacercariae 

 were found by Staff"ord (1912) in mussels from 

 the Gulf of Saint Lawrence. Jameson and 

 Nicoll (1913) reviewed pearl formation in mus- 

 sels and concluded that several gymnophallid 

 larvae were involved. Since then other gymno- 

 phallid cercariae have been associated with 

 metacercariae in mussels (Palombi, 1924; Cole, 

 1935; Rees, 1939). Stunkard and Uzmann 

 (1958) fed mussels from Long Island to newly 

 hatched eider ducks and recovered adult gym- 

 nophallids, probably G. bursicola. 



Other larval trematodes have been described 

 from mussels. Cole (1935) reported "orange 

 sickness" of sea mussels at Conway in Wales. 

 The color was due to masses of orange pig- 

 mented trematode sporocysts in the mantle and 

 throughout the body ; the tailless cercariae they 

 contained were described as Cercaria te)iua)is. 

 A similar condition had been noted previously 

 by Atkins (1931a) . Cole also described a second 

 larval trematode infestation, caused by Buceph- 

 alus mytili. Uzmann (1953) found microcer- 

 cous trematode larvae in sea mussels from Long 

 Island and Connecticut that had a similar 

 orange coloration. Described as Cercaria mil- 

 fordensis, the larvae were primarily parasites 

 of the blood vascular system of the host and 

 had foci in the blood vessels of the mantle. 

 Sporocyst development precluded normal game- 

 togenesis in mussels ; infected mussels held in 

 aquaria had unusually high mortalities. Uz- 

 mann suggested that C. milfordensis infections 

 are probably lethal to the host under unfavor- 

 able environmental conditions. 



Diseases Caused b\ Parasitic (Crustaceans 



A well-documented example of the eflfects of 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



347 



