A NEW MYXOSPORIDIAN PARASITE, THE CAUSE OF 

 "WORMY" HALIBUT/ 



By H. S. Davis, Fi.^h rathologixt, U. S. Bureau of FisJieries. 



The extensive halibut fishery of the Pacific coast is subject to a. 

 material annual loss owing to the fact that in a considerable per- 

 centage of the fish the flesh is abnormal and unsalable. In the great 

 majorit}^ of the abnormal fish the flesh is soft and " mushy " and 

 readily falls apart when cut, but in a small percentage it has a very 

 different appearance. Instead of being soft the muscles are hard and 

 firm as in normal fish but contain large numbers of white, wormlike 

 structures, whence the name " wormy " halibut. A comparison of 

 the flesh of *' mushy " and '' wormy " fish has convinced the writer 

 that they are caused by very different organisms, the " wormy " con- 

 dition evidently being due to an unclescribed myxosporidian parasite 

 for which the name Unicavsula muscularis n. gen. et n. sp. is pro- 

 posed. 



The appearance of the flesh in the " wormy " fish is well described 

 by Thompson (1916), as follows: "There is no external evidence 

 of the infection by the parasite which causes Svorminess.' A cut 

 through the trunk muscles parallel to the grain exposes the para- 

 sites as silvery threads lying among the muscle fibers. These threads 

 lie with the fibers in a regular way and extend from septum to 

 septum of the myomeres but not through them. The muscles of the 

 head are apparently less liable to infection than those of the trunk, 

 and usually none are to be seen in the head of even a heavily infected 

 specimen. The density of the infection varies somewhat in different 

 individuals. '' 



The writer has had no opportunity to examine halibut on the 

 Pacific coast, but the appearance of the flesh of an infected fish that 

 was sent to "Washington in cold storage agreed in every respect with 

 Thompson's description. In this fish the white opaque " worms " 

 were very common and presented a sharp contrast to the smaller 

 translucent fibers that made up the great bulk of the muscles. A 

 closer examination confirmed Thompson's observation that the 

 " worms " are simply hypertrophied muscle fibers filled with a 

 whitish granular mass. This granular material, which is composed 

 of the spores of a myxosporidian, is surrounded by a relatively thin, 

 transparent envelope made up of muscle fibi-ils embedded in sarco- 

 plasm and surrounded l)y the sarcolemma. The diameter of the in- 

 fected fibers in material preserved in formalin was about 0.6 to 0.8 



^Appendix VIII to the Report of the U. S. Commissioner of Fisheries for 1923. B. F. 

 Doc. 057. 



7727-5°— 24 1 



