The occurrence and development of 'Siamese twins' and other abnormalities in Oreochromis mossambicus (Pisces, Cichlidae)

ENGLISH ABSTRACT: Specimens from a clutch of Oreochromis mossambicus (Pisces, Cichlidae) exhibited a number of abnormalities such as enlarged pericardia, thin tube-like hearts, large depressions in the yolksac, spinal deformities and twinning. Descriptions of the differences in early ontogeny of two pairs of 'Siamese twins' over a period of at least eight days are given. The developmental pattern of the twins is similar to that of normal specimens but the developmental rate is slower in the twins. Possible explanations for these occurrences of polyembryony are given. *********** AFRIKAANSE OPSOMMING: Daar is gevind dat 'n broeisel van Oreochromis mossambicus embrio's abnormaliteite getoon het, soos byvoorbeeld vergrote hartsakke, dun pypvormige harte, groot holtes in die dooiersak, misvormings van die ruggraat en tweelingvorming. Beskrywings van die verskille in die vroe ontwikkeling van twee pare Siamese tweelinge gedurende 'n periode van nie minder nie as agt dae word verskaf. Die ontwikkelingspatroon van die tweelinge is gelyk aan die van 'n normale embrio maar die ontwikkelingstempo is stadiger in die tweelinge. Moontlike verklarings vir hierdie voorkoms van poli-embrionie word voorgestel.