Zimmermann et ai.: Parasitism by Naobranchia occidentalis 
379 
parasite larvae are apparently sufficiently small (Roubal 3 ). 
Both Cressey et al. (1983) and Roubal and Graham (1999) 
noted that the largest adult parasites were found on the 
largest fish — a finding that suggests that the gill filaments 
of smaller fish may be less suitable for infestation. 
Nectobrachia indivisa also selectively infested larger 
northern rock sole but rarely infested southern rock sole, 
suggesting an apparent species preference. Because N. in- 
divisa attaches by means of a bulla permanently embed- 
ded near the end of a gill filament, it seems less likely that 
this parasite would be limited by mechanical restrictions 
such as the diameter of the gill filament. The parasite Sal- 
mincola californiensis, which also attaches to its host by 
means of a bulla (Kabata and Cousens, 1972), seemed to 
be dependent on its sockeye salmon (Oncorhynchus nerka) 
host reaching a large size before the gill filaments became 
the preferred site of attachment (Kabata and Cousens, 
1977). Perhaps N. indivisa is able to infest northern rock 
sole successfully only after the fish reach a certain size. 
We did not perform histological analysis of gill filaments 
to determine differential damage caused by infestation of 
Naobranchia occidentalis and Nectobrachia indivisa. Rou- 
bal (1999) determined that damage caused by Naobran- 
chia variahilis was restricted to the infested gill filament 
and was minor. Kabata (1984) noted that naobranchids 
only partially compress the gill filament, do not completely 
restrict blood flow, and do not cause whitening of gill fila- 
ments. Roubal (1999) also determined that N. variahilis 
live by feeding on the blood supply and not by grazing the 
tissue, as do lernaeopodids. Kabata and Cousens (1977) 
reported macroscopic observations of significant atrophy 
and tissue reaction to as much as one-third of the gill fila- 
ment surface area of juvenile sockeye salmon due to the 
presence of S. californiensis. This type of damage is consis- 
tent with our macroscopic observations of the white gills 
of northern rock soles seen early in the Gulf of Alaska sur- 
vey. Kabata and Cousens (1977) also reported on micro- 
scopic observations of gill filament damage, sometimes in- 
cluding proliferation of the gill epithelium, hypertrophy 
of epithelial cells, fusion of adjacent filaments, thickening 
of filament walls, blood blisters, and erosion of epithelial 
cells as the oral apparatus of the S. californiensis scrapes 
them for ingestion. 
Margolis and Arthur ( 1979), Kabata and Whitaker ( 1984), 
and Kabata (1988) all reported that both Naobranchia 
occidentalis and Nectobrachia indivisa occurred on rock 
soles in Pacific Canadian waters, prior to the determina- 
tion that there are two species of rock sole in this area 
(Orr and Matarese, 2000). Neither Moles (1982) nor Love 
and Moser (1983) reported these parasites from rock soles 
from Alaskan waters and U.S. west coast waters, respec- 
tively. Our study reported new host records for Acantho- 
chondria vancouverensis and Haemobaphes diceraus on 
northern rock sole. 
The apparent preference of both parasites for the north- 
ern rock soles is supported by the conclusion of Orr and 
3 Roubal, F. R. 1999. Personal commun. Department of Para- 
sitology, The University of Queensland, Brisbane 4072, Queens- 
land, Australia. 
Matarese (2000) that northern and southern rock soles are 
two distinct species. Possible differences in the ecology of 
these newly described rock sole species, such as food hab- 
its, growth rates, habitats, spawning seasons and locations, 
nursery grounds, and seasonal and ontogenetic migrations 
might also account for the differential parasitism that we 
observed. It is not known if parasite prevalence and inten- 
sity is related to differences in the ecology and behavior 
of these closely related species, or related to differences 
in their anatomy or physiology. As more research is done 
on these species, based on the work of Orr and Matarese 
(2000), more potential differences will be determined. 
Acknowledgments 
The laboratory research of this project was performed by 
A. Jones, as an internship in partial fulfillment of a Bach- 
elor of Science degree from Western Washington Univer- 
sity. Steve Syrjala provided statistical advice. Jay Orr 
taught A. Jones the subtle differences between the rock 
sole species and how to identify the structures of inter- 
est. Discussions with Frank Morado improved our labora- 
tory methods and manuscript. Comments and suggestions 
from several coworkers greatly improved the quality of 
this manuscript and an anonymous reviewer provided 
helpful criticism of the manuscript. We thank Frank 
Roubal for sharing additional results from his studies 
and for hypothesizing on the infection dynamics of Nao- 
branchia. Teresa Turk, Steven Hochberg, Michelle Arm- 
strong, Dick Haight, and Ben Page examined fish out at 
sea for parasites. We are extremely grateful to Z. Kabata 
for examining samples of all four copepod parasites and 
confirming our species identifications. 
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