International Journal of Marine Science, 2016, Vol.6, No.18, 1-14
9
al., 1984; Molnar et al., 2005). Sometimes, however, its location is very different from the site of gamonts or
subsequent stages. In
Eimeria brevoortia
, for example, merozoites and gamonts apparently occur in the intestine,
while sporogony occurs in the swim bladder (Hardcastle, 1944). In several instances the study of experimentally
infected hosts has done much to improve understanding of the sequential agametic development of coccidians.
Merogony in several fish coccidians is similar to that seen in homeotherm hosts. Meronts of
Goussiu iroquoina
,
Goussia sinensis
, and
Goussiu carpelli
develop within parasitophorous vacuoles (Paterson and Desser, 1981d;
Baska and Molnar, 1989; Steinhagen, 1991; Molnar et al., 2005), but no parasitophorous vacuole was evident
surrounding two meronts of
Eimeria variabilis
(Davies, 1990). In most coccidians the parasitophorous vacuole
membrane is produced by the host cell, but its origin varies. The host cell plasmalemma often forms the vacuole
membrane during the process of phagocytic entry by the sporozoite or the merozoite of the previous generation.
Merozoites appear to be produced by three methods: exogenesis (ectomerogony), in which merozoites are budded
from the surface of the meront; endomerogony, in which several merozoites are produced within the meront; and
endodyogeny, in which paired merozoites are produced internally. The ultrastructure of merozoites of piscine
coccidians resembles that of other closely related coccidians (Scholtyseck, 1973; Chobotar and Scholtyseck, 1982;
Ball and Pittilo, 1990). Merozoites of
Eimeriu vunusi
are bound by two unit membranes (Paperna, 1990); those of
Goussiu sinensis
are bound by a three-layered pellicle (Baska and Molnar, 1989). Young macrogamonts are
usually 5.0-26.0 pm in diameter. They are mostly spherical or ellipsoidal structures surrounded by one or more
delicate membranes and they commonly lie within a parasitophorous vacuole. Young macrogamonts of
Goussia
iroquoina
have elaborately shaped mitochondria, granular endoplasmic reticulum, and Golgi complexes (Paterson
and Desser, 1981a). Microgametogenesis of the present study follows a basic pattern similar to that described for
coccidians. A phase of nuclear division associated with growth of the microgamont preceded differentiation of
microgametes (Scholtyseck, 1973: Chobotar and Scholtyseck, 1982; Ball and Pittilo, 1990). Coccidians that have
been shown to conform to this pattern of development include, for example,
Goussia iroquoina
,
Goussia aculeati
,
and
Goussia zarnowskii
(Paterson and Desser, 1981b; Jastrzebski, 1989; Jastrzebski and Komorowski, 1990). This
observation contradicted to that of
Eimeria vanusi
, in which the microgamont nucleus does not subdivide before
microgamete formation (Paperna, 1990). Microgamonts develop within a parasitophorous vacuole, which in
Goussia iroquoina, Calyptospora funduli and Goussia sinensis
is bound by a single limiting membrane (Hawkins
et al., 1983; Paterson and Desser, 1984; Baska and Molnar, 1989). Oocysts tend to be spherical or ellipsoid
structures (generally 4.5-70 pm in diameter, depending on species), although some are cylindrical, as those of
Eimeria southwelli
Halawani, 1930 and
Eimeria quentini
Boulard, 1977. The oocyst walls are commonly thin, and
one species,
Goussia sinensis
, is reported to have a micropyle (Chen, 1956). Some thin oocyst walls tend to
collapse on to the sporocysts following sporulation. In coccidians of homeotherms the oocyst wall tends to be
thick, and is formed from two types of wall-forming bodies (WF1 and WF2) that are discharged around the
fertilized macrogamete, but in fish coccidians these bodies are not always recognized. Bodies resembling the
wall-forming bodies of coccidians of homeotherms have been described, but some of these may be involved in
sporocyst wall formation. Oocysts vary in their site of development. As for other stages of fish coccidians, the gut
is a favoured location but extraintestinal sites where oocysts may be found include liver, kidney, spleen, pancreas,
testes, ovary, peritoneum, swim bladder, gall bladder, adipose tissue and gill filaments. Within the same species,
oocysts may occur in the same location as merozoites and gamonts, or at different sites, suggesting that in some
cases migration occurs. Sporocyst walls of
Goussia auxidis
were found to consist of three layers. The outer layer
was a laminar envelope with 2-12 laminations parallel to the wall, an electron dense layer 14 nm thick, and a
transversely laminated wall 160-180 nm thick. Dispersal of oocysts from the gut of fish occurs presumably in the
faeces, where oocysts may be passed unsporulated, semisporulated, or fully sporulated. Tissue-inhabiting coccidia
such as
Eimeriu sardinae
from the testes of a number of fish coccidia apparently undergo “migration” during
development, but there is little to indicate how this occurs.
Sporozoites initiate infection probably when released in the lumen of the gut from an ingested sporulated oocyst
or free sporocyst (homoxenous), or from an ingested intermediate host (heteroxenous). Sporozoites then enter host
