Comparative Parasitology 67(2) 2000 - Peru State College

228 COMPARATIVE PARASITOLOGY, 67(2), JULY 2000
Figures 17-19. Cephalic extremities of third-stage larvae of Spiroxys spp., lateral view (scale bars =
25 (Jim). 17, 18. Spiroxys hanzaki collected from experimentally infected copepod, Mesocyclops dissimilis
(17), and naturally infected sand loach, Cobitis biwae (18). 19. Spiroxys japonica collected from naturally
infected Rana rugosa (tadpole) in Akita, Akita Prefecture, Japan.
imentally infected copepods or naturally infected
fish. Measurements are stated in Table 1.
ADVANCED THIRD-STAGE LARVAE (Figs. 15, 16):
Morphology identical with that of the third-stage
larvae described above but with much larger
body (Table 1).
IMMATURE ADULTS: Morphology identical
with that of mature adults described in Hasegawa
et al. (1998), but much smaller in size: males
10.2-13.3 mm long (« = 5) and females 10.2-
15.5 mm long (n = 5).
Discussion
Although only a few third-stage larvae were
recovered from the experimentally infected copepods,
it is apparent that 5. hanzaki utilizes copepods
as its intermediate host, like most gnathostomatoids
for which life histories have been
elucidated (cf. Anderson, 1992). Compared with
Spiroxys contortus (Rudolphi, 1819) and S. japonica
(cf. Hedrick, 1935; Hasegawa and Otsuru,
1978), S. hanzaki shows some different
features in the life history. Hatched larvae are
much larger (175-294 long in S. contortus, and
148-207 long in S. japonica). The hatched larva
attaches at the bottom, like that of 5. contortus,
whereas the larva of S. japonica often swims in
the water. Moreover, the period to attain the third
stage in the copepods is much longer (10 to 14
days and 6 to 8 days in S. contortus and S. japonica,
respectively).
The large size of the hatched larva and the
slow development may be responsible for the
high mortality rate of the infected copepods.
Penetration of such a large larva through the alimentary
canal wall of the copepod may result
in perforation, through which pathogenic organisms
could easily invade the hemocoel, as shown
by the disseminated infection with flagellates in
Copyright © 2011, The Helminthological Society of Washington
the present experiment. Meanwhile, it is also
probable that the larva in the hemocoel would
stimulate some defense mechanism of the copepods
to eliminate the invader, because the larvae
often died without further development.
The worm size and morphology of the thirdstage
larvae from the copepods are similar to
those of the smallest third-stage larva found in
the salamander. This suggests that the third-stage
larva developed in copepods could be infective
to the final host. However, most of the gnathostomatoids
require the second intermediate or
paratenic hosts, often fish, in which the thirdstage
larva becomes the so-called advanced third
stage, that shows significant gain in body size
but without essential morphological change (cf.
Anderson, 1992). A similar pattern was also
postulated for S. hanzaki (Hasegawa et al.,
1998). Because the low tolerance of the copepods
to the infection in our experiments prevented
experimental infection of fish with raised
larva, it remains unknown whether the larvae
grow to the advanced third stage in fish.
In most parasitic nematodes, the fourth stage
exists between the infective third stage and fifth
(adult) stage. However, the presence of the
fourth-stage larva in Spiroxys is doubtful. Hedrick
(1935) stated that the third and fourth molts
of S. contortus were observed in the definitive
host turtles but did not present the morphology
of the fourth stage. In the life history study of
S. japonica, Hasegawa and Otsuru (1978) could
not find any larva that was distinguishable morphologically
from the third-stage larva in the experimentally
infected definitive host, frogs. Berry
(1985) described the larvae of Spiroxys chelodinae
Berry, 1985, collected from the stomach
ulcers of Australian chelonians, as fourth stage,
but the morphology resembles that of third-stage