The Biology and Culture of Tilapias

Trichodinella spp., Tripartiella spp., Trichodina spp. and Chilodonella spp.
Only the latter pair has been reported as being of significance to cultured
tilapias but it is likely that the others will also assume importance in the
future. They all have a direct life cycle and reproduce by binary fission on
the skin and gills. They are present on most fish in small numbers and
characteristically cause problems in fish stressed by handling, poor feeding
and especially where there has been a drop in temperature. As the temper-
ature rises again, as with Ichthyophthirius, the parasites multiply on the
stressed and debilitated fish.
Thus, they are a problem in Israel (Sarig 1971) and the Southern U.S.A.
(Avault et al. 1968) where it is necessary to provide heated water facilities to
overwinter fish and high fish densityflow flow rates and minimal temper-
atures are the norm from economic necessity.
Sommede (unpublished) has induced severe Chilodonella infection
in S. galilaeus subjected to severe nutritional deficiency in a static water
system. It was not possible to establish the infection by direct transfer to
healthy, well-fed fish or to fish in clean water with good exchange. Fryer and
Iles (1972) consider Trichodina to be a particular problem in mouthbrooders
as these ciliates can invade the mouth and transmit the infection to fry. Sarig
(1975) has also reported heavy infection from cultured tilapias in Ghana,
stating that in many cases such infections were highly pathogenic and caused
heavy losses, especially of small fish. Guerrero (pers. comm.) has also fre-
quently implicated trichodinids in heavy, and often lethal infections of S.
mossambicus fry undergoing hormone sex-reversal treatment.
c) Bodonid parasites. Ichthyobodo(=Costia) necatrix is a well recognized
and highly pathogenic parasite of young, or severely stressed, salmonids
(Robertson 1979). In tilapias, bodonids have been associated with disease in
Alabama (Plumb, cited by Scott 1977) and in Israel (Sarig 1971). Sarig
reported that they occurred infrequently in late autumn and winter and only
on the gills of the affected fish, accompanied by Trichodina and Glossatella.
In experimental populations of S. mossambicus held in recirculating systems
at Stirling, Mohd-Shaharom (unpublished) has found mortalities associated
with a bodinid parasite closely resembling Ichthyobodo necatrix in heavily
stocked populations held at 26"C, which is a much higher temperature than
previous reports.
d) Sporozoa. Myxosporidia occur commonly as cysts replete with spores
in the tissues of most wild tilapias. Generally they are of the Myxobolusl
Myxosoma group. Baker (1963), Fryer (1961a, 1961b) and Sommerville (un-
published) have all recorded myxosporidians from wild tilapias. They rarely
show any evidence of significant pathological effect but in view of their life
cycles, they are potentially highly significant for intensive earth pond
culture, as is the case with Myxobolus and Henneguya infections in cultured
carps in Bangladesh (Sommerville and Iqbal, unpublished). Earth pond
culture greatly facilitates the parasite's life cycle since spores are released
during decomposition of a fish carcass (or possibly from living fish) and for
all known species require a period of potentiation in mud prior to the
development of infectivity. These parasites are more likely to be a problem
in countries where the height of the water table precludes seasonal drainage
and desiccation of the ponds.