Plate 2. Contracaecum within the pericardium of Sarotherodon alcalicus (S. grahami).
Photograph courtesy of Mr. P.W. Scott.
Numerous species of parasitic crustaceans have been reported from
tilapias. Of these the most serious pathogens are the branchiuran Argulus
spp. and the copepods Ergasilus spp. and Lernaea spp.. Sarig (1971) sum-
marizes the reports of Fryer and Paperna and describes the problems expe-
rienced with infestations by these parasites. Fryer (1961b, 1965a, 1968)
and Paperna (1969) found Lernaea to cause serious losses in ponds in Nigeria
and Israel and in Malawi (Roberts unpublished), they were found to be a
significant cause of mortality in T. rendalli ('T. melanopleum') culture in
ponds. The parasites are deeply embedded in the dermis or musculature
of the fish, and cause a severe necrotic ulcerative granulomatous response.
The lesions render the fish unmarketable, and fish with even a single parasite
feed poorly, and lose weight. Lernaea tilapiae is restricted to tilapias of Lake
Malawi and has been described by Fryer and Iles (1972) as inhabiting the
mouth and penetrating the tissues of the palate. In view of the finding of as
many as 9 parasites in this site, all causing severe inflammatory lesions, they
conclude that it must also interfere with feeding and mouthbrooding in
addition to the more general effects of "anchor" penetration. The infective
stage is the free swimming copepodid (the second larval instar) but a further
four moults, accompanied by a final sexual differentiation and mating are
necessary for the adult female to become pathogenic and insert her head into
the skin where the feeding activity and growth of the "anchors" causes the
severe reaction.
Argulus spp., the fish lice, are very mobile, unlike the sedentary Lernaea.
Several species have been found on tilapias (Paperna 1964; Fryer 1968 ; Sarig
1971). Its life history is more simple than that of Lernaea and although it is