Trematodes of Fishes — Iversen AND Hoven 
regard all Hirudinella reported from the wahoo 
as belonging to a single species, H . ventricosa 
Pallas. In an examination of 220 wahoo stom- 
achs from the Line Islands, Iversen and Yo- 
shida (1957) found a 98.2 per cent infection 
of these parasites. The hosts ranged in size 
from 108 to 172 cm. and as many as 17 of 
the worms were found in a single stomach, 
with 2 per host occurring most frequently. 
H. ventricosa may be present elsewhere in 
the Pacific, for Edmondson (1946) states in 
respect to the Hawaiian region, that "An un- 
determined species of trematode commonly 
occurs in the stomach of the Ono, Acantho- 
cyhium solandri (Cuvier).’’ These flukes were 
about 1 inch long when partially contracted. 
Kishinouye (1923) in speaking of wahoo 
from Japan says "A large distomum, about 
8 cm. in length, is almost always found in the 
stomach.’’ 
Family DIDYMOZOONIDAE 
Didymocystis acanthocybii 
Yamaguti, 1938 
HOST: Wahoo, Acanthocybium solandri (Cu- 
vier and Valenciennes). 
location: Base of gill arches and on 
operculum. 
locality: Christmas, Fanning, Washing- 
ton, Palmyra islands, and Kingman Reef. 
specimens: USNM Helm. Coll. No. 38171. 
PREVIOUSLY REPORTED HOSTS AND LOCALI- 
TIES: At base of gill arch of Acanthocybium sara 
(Lay et Bennett); Pacific, Japan, Yamaguti 
(1938). 
discussion: Of the 104 wahoo examined, 
53 revealed conspicuous cysts of this parasite 
(Fig. 1). The fish ranged in size from 86 to 
174 cm., and specimens of all sizes within this 
range were found to be infected. Figure 2 
shows the relationship of the paired worms 
within the cyst. 
Didymocystis sp. 
HOST: Yellowfin tuna, Neothunnus macrop- 
133 
Fig. 1. Photograph of encysted Didymocystis acan- 
thocybii on the side of the head of a wahoo (gill cover 
removed). 
terns (Temminck and Schlegel). 
location: On gill filaments. 
locality: 00° 28' N., 157° 48' W. 
specimens: USNM Helm. Coll. No. 38172. 
discussion: The condition of specimens 
does not permit identification, but they re- 
semble D. wedli Ariola, 1902, more closely 
than others. The infection rate of yellowfin is 
probably not high. 
discussion 
Concomitant studies of hosts and parasites 
from different geographic areas have been 
reviewed by Metcalf (1929) and Manter 
(1955), who have presented considerable evi- 
dence that parasites reflect the distribution 
and way of living of their hosts. A basic 
problem in the study of some pelagic fishes 
is to determine whether the population of 
each important species is continuous through- 
out its range or whether it is composed of a 
number of independent or semi-independent 
