BRILL ET AL.: INFECTION OF DORSAL AORTA IN YELLOWFIN TUNA 



fish captured at all other times of the year, and all 

 parasitized fish, based on the new regression 

 parameters. The two groups of unparasitized fish 

 show statistically significant differences in all 

 three. Fish caught during August and September 

 were on the average \1% heavier at a given body 

 size (i.e., relative condition factor = 1.11), had liv- 

 ers an average of 23% heavier, and had hearts an 

 average of 22% heavier than those unparasitized 

 fish captured at other times of the year. When 

 unparasitized fish, excluding those caught during 

 August and September, are compared with para- 

 sitized fish, there are now no statistically signifi- 

 cant differences in mean relative condition fac- 

 tors, mean relative liver weights, or mean 

 relative heart weights. The data from unpara- 

 sitized fish captured in August and September, 

 when included in the complete data set, are there- 

 fore responsible for the observed differences in 

 relative heart and liver weights between infected 

 and uninfected fish seen in Table 1. 



In Vitro Perfusion of the Dorsal Aorta 



Three parasitized and five unparasitized fish, 

 ranging in weight from 0.915 to 2.666 kg, were 

 used in this series of experiments. The intensity 

 of infection was subjectively classified as slight, 

 moderate, or heavy. Perfusion pressures were 

 normalized to a 1 kg fish weight by dividing the 

 observed pressures by the reciprocal of body 



300 



290 ■• 



en 



X 

 E 



e 



180 -■ 



I50-- 



90-- 



60 



30-- 



HEAVY 



MODERATE 



-^ SLIGHT 



-t- 



■+■ 



-+- 



-t- 



20 



30 iO 50 60 



Perfusion Flow ml/mm 



70 



80 



Figure 4. — In vitro pressure required to pump various rates of 

 saline down the dorsal aorta of parasitized and unparasitized 

 yellowfin tuna. The fish ranged in size from 0.915 to 2.666 kg. 

 Data have been normalized to a body weight of 1 kg. 



weight, in kilograms. Moderately and heavily in- 

 fected fish showed higher perfusion pressures at a 

 given flow rate than did unparasitized animals 

 (Fig. 4). While no data are available on the nor- 

 mal cardiac outputs and blood pressures in swim- 

 ming yellowfin tuna, restrained and lightly anes- 

 thetized yellowfin tuna have cardiac outputs of 

 approximately 40-60 mL/kg and dorsal aorta 

 blood pressures of 50-70 mm Hg (D. R. Jones and 

 R. W. Brill, unpubl. obs.). Figure 4 shows that at 

 normal cardiac outputs, the apparent occlusion of 

 the dorsal aorta caused by the parasites and host 

 inflammation is indeed real and should cause 

 moderately and heavily infected fish to have ex- 

 cessively high blood pressures, high cardiac en- 

 ergy demands and presumably reduced fitness. 



DISCUSSION 



Prevalence of Infection by 

 Species, Size, and Season 



Our data indicate that in Hawaiian waters D. 

 talismani is limited to yellowfin tuna. However, 

 D. talismani has been reported to occur in At- 

 lantic bigeye tuna (Bussieras and Aldrin 1965). 

 Since we examined relatively few individuals of 

 this species, we cannot rule out the occurrence of 

 this parasite in bigeye tuna in the central Pacific. 



Skipjack tuna, bigeye tuna, kawakawa, and 

 yellowfin tuna often occur simultaneously in the 

 same areas and show a great overlap in prey spe- 

 cies (King and Ikehara 1956; Waldron and King 

 1963). It is therefore unlikely that host specificity 

 is attributable to only yellowfin tuna ingesting 

 the intermediate host, which is not known but is 

 most likely a small crustacean (Deardorff et al. 

 1984). Host specificity could arise if the procer- 

 coid of/), talismani is not stimulated or is unable 

 to penetrate the gut wall of tuna species other 

 than yellowfin tuna, or that species of tuna other 

 than yellowfin are capable of immune rejection 

 (Orr et al. 1969). 



The reasons for the dramatic decrease in inci- 

 dence of dorsal aorta infection in large (>45 kg) 

 yellowfin tuna are unknown. Possible reasons in- 

 clude procercoids ingested less frequently by 

 larger animals, host destruction of the parasite, 

 increased mortality of parasitized fish, or move- 

 ment of the parasite out of the dorsal aorta into 

 other major arterial vessels. Of these alterna- 

 tives, increased mortality of infected fish seems 

 unlikely. 



While we did not demonstrate directly the pre- 



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