Parasites As a Limiting Factor in Exploitation of Pacific Whiting, Merluccius Productus

Parasites as a Limiting Factor in Exploitation of Pacific Whiting, Merluccius productus Z. KABATA and D. J. WHITAKER Introduction Conispora meridionalis (Ev valves of unequal sizes. The spores are dokimova, 1977) (Sankurathri et aL, subconical in lateral view, the The parasite fauna of Pacific In press). posterior corners of the valves form whiting, Merluccius productus, like More important, however, are the ing sharp processes. The surfaces of that of most commercial fishes in the parasites belonging to the genus the valves are smooth and the suture eastern North Pacific, have not been Kudoa Meglitsch, 1947, which, lines very fine. The polar capsules are adequately studied. The extent of our because of their histozoic habits, are pyriform, of unequal sizes, one cap knowledge is best exemplified by the of direct concern to the fishing in sule being distinctly larger than the fact that, while Margolis and Arthur dustry. Grabda and Grabda (1975), other three. (1979) listed 10 parasite species who examined whiting taken by the Kudoa panijormis (Fig. IB) has recorded for this fish in Canadian Polish fishing fleet off California, spores subrectangular in polar view, waters, a small-scale survey carried recorded two species of Kudoa whjch with rounded valves and smooth sur out fairly recently in the Strait of they found in the musculature: Kudoa faces. Suture lines are devoid of ridges Georgia (Sankurathri et al., In press) clupeidae (Hahn, 1917) and Kudoa and are often somewhat sinuous. augmented that list to 22 species. In rosenbuschi (Gelormini, 1944). The Polar capsules are pyriform. Detailed view of the strong impact that some former has never been previously descriptions and measurements of of these parasites exert on the Pacific found in the genus Merluccius, but spores are given by Kabata and whiting fishery, this lack of attention the latter is known to occur in the Whitaker (1981). is regrettable. muscle of Merluccius hubbsi on the The spores of Kudoa are important The parasites of Pacific whiting on Patagonian shelf. Kabata and not only as the best clues to the identi which attention has been focused Whitaker (1981) put the validity of ty of the species; they also play a recently belong to the protozoan these records in doubt, because of prominent role in the life cycle of phylum Myxosporea. Sankurathri morphological differences and these parasites. The spore is the stage (1977) described Conispora renalis dissimilarity in the host-parasite rela that serves as the means of dispersal from the renal tubules of the whiting tionships. However, further studies in of the parasite and as the source of in in Georgia Strait. This species was Californian waters are needed to fection for new host individuals. found later to be identical with Myx allow a defiilltive conclusion in this Released from the musculature of oproteus meridionalis Evdokimova, matter. According to Kabata and dead fish, it must float in water and 1977, discovered in the urinary blad Whitaker (1981), the two species of make contact with the fish it is to in der of Merluccius hubbsi Marini, Kudoa infecting Pacific whiting off fect. The release could occur as the 1933, on the Patagonian shelf. Ev North American shores are Kudoa result of natural death of the host and dokimova's (1977) description, thyrsitis (Gilchrist, 1924) and Kudoa subsequent decomposition. It is also chronologically before that of panijormis (Kabata and Whitaker, likely that moribund fish would be Sankurathri, was incorrect in placing 1981). This paper concerns these two disposed of by predators. The spores, that species in Myxoproteus. It was species. with the digested remains of their necessary to place it in Conispora, host, would then be voided by the Morphology and Life while retaining Evdokimova's original predator. Whatever the mode of Cycle of Kudoa specific name. It is now known as release, being heavier than water, they Only the spores have been de gradually sink and are likely to des scribed in detail for the two species of cend to levels deeper than the habitat The authors are with [he Pacific Biological Sta Kudoa parasitic in Pacific hake. of their potential host. Kovaleva et aL tion, Fisheries Research Branch, Department of Kudoa thyrsitis (Fig. IA) is distin (1979) pointed out that the distribu Fisheries and Oceans, anaimo, B.C. V9R 5K6, Canada. guished by stellate spores, with four tion of the various Kudoa species in 47(2),1985 55 infection is more likely to deliver a concentrated dose of spores to the new host. It is currently presumed that Kudoa reaches its target site within the host via the intestinal tract and the cir A culatory system, but evidence sup porting this presumption is still cir cumstantial, at least for this particular myxosporean genus. The earliest stage known in the life cycle of Kudoa parasitic in Pacific whiting is a large trophozoite filling a sometimes substantial portion of a muscle fiber. In some instances the entire fiber is filled by the parasite to the point of becoming visibly dis tended. Observation with the naked eye reveals these fibers, or pseudo cysts, as white or black streaks. A new generation of spores develops within sporoblasts contained inside these relatively gigantic trophozoites. The distribution of pseudocysts in an individual host is not uniform. There are no significant differences between the right and the left side of the fish, but the number of infected muscle fibers tends to diminish from the anterior end toward the tail of the fish. The area most heavily infected is the dorsal musculature immediately behind the head; the lightest infection is found in the caudal peduncle. In view of the life history of Kudoa, this type of distribution is not unexpected. The sporoplasms (the infective agents carried within the spore) are released in the intenstine, penetrate the gut B wall, and enter the bloodstream. They are then carried by blood via the hepatic portal system, through the liver, heart, and gills, to the dorsal aorta. Along the latter they are Figure 1. - Unfixed spores in lateral and anterior view: (A) Kudoa transported in the posterior direction. thyrsitis; (B) Kudoa panijormis. (From Kabata and Whitaker 1981). The blood vessels branching off from the aorta near its origin have the greatest chance of receiving the proto zoan intruder. The number of para sites dwindles with their transport the Atlantic Ocean coincided with species of fish. (Atheresthes stomias towards the tail. areas of upwelling, in which the provides an example of such a demer Effect of Infection spores can be maintained by the sal host along the Pacific coast of on Muscle Tissue ascending currents at the depths in North America.) The Pacific whiting habited by their host species. The ex is cannibalistic and is often taken with Both species of Kudoa mentioned ceptions to this rule are provided by the remains of its conspecifics in the above penetrate individual muscle Kudoa parasitizing bottom-dwelling alimentary canal. The latter way of fibers. They do not appear to undergo 56 Marine Fisheries Review any division, once within these fibers. Judging from some very high inten sities of infection, some division prob ably occurs after the sporoplasm enters the blood vessel. In conse quence, a single sporoplasm could provide the means for infecting many muscle fibers. Within the confines of a fiber, the feeding stage (trophozoite) expands and its surface membrane de velops countless irregular villosities. The normal structure of the fiber is disrupted, and its tissue gradually breaks up and is replaced by the parasite. The trophozoite gradually fills with spores of various stages of development. The parasite within the fiber is sufficiently isolated from the host to prevent its immediate defen sive reaction. At this stage the term "pseudocyst" is properly applied to the infected fiber, to distinguish this condition from the true cysts which Figure 2. - Transverse section through musculature of hake, showing uninfected fibers (N); young, white pseudocysts (W); and old, black are much more disruptive to the pseudocysts (B). tissues and are surrounded by connec tive tissue capsules deposited by the host. Each pseudocyst is a cylindrical, elongated structure, often apparent to the naked eye as a streak of color, distinguishing it from the surround gram of muscle tissue have been As demonstrated by Patashnik et al. ing, uninfected muscle fibers. The col found (Kabata and Whitaker, un (1982), the substances in question are or of pseudocysts varies with the age pub!. data). The infected fibers ob proteolytic enzymes. During the life of the infection. At the relatively early viously lose their contractility and can of the fish, the excretory system is ef stage, when the trophozoite is still no longer participate in the locomo ficient enough to prevent the ac growing and filling with spores, the tion of the fish. The fact that the fish, cumulation of the enzyme in quan color of the fiber is opaque white. As even when heavily infected, can still tities sufficient to result in serious the villosities of the trophozoites ap continue its locomotory activities, is damage to the muscle tissue. The proach the sarcolemma of the muscle understandable, when one takes into death of the host has a twofold effect: fiber, the enzymes produced by the account that the white muscle, which It terminates the functioning of the parasite leak out and are detected by constitutes the substrate of the excretory mechanisms, while, at the the host; the host's reaction becomes parasite, is not extensively involved in same time, the production of the en histologically evident. It consists of the normal cruising movements. zyme continues unabated. Facing surrounding the infected fiber with Kudoa has not been found in the dark biochemical changes induced by the histiocytes.

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