Oral incubation in Tilapia macrocephala. 1, Embryological studies. Bulletin of the AMNH ; v. 103, article 5

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New York : [American Museum of Natural History]
"Embryos of the mouthbreeding cichlid fish Tilapia macrocephala do not survive, extra-orally, in aquarium water taken from the tank in which the spawning occurred. Experiments were performed to determine the role of oral incubation in embryonic survival. The approach used was to find methods of raising the embryos extra-orally and to relate extra-oral survival with the factors of survival in the milieu of the oral cavity. To facilitate this study, a series of embryological stages were described and defined from fertilization to hatching. Certain of the more obvious microscopic changes were described in conjunction with the gross descriptions of the stages. The most effective experimental media were 40 per cent sea water and sterile aquarium water in which mean embryonic survival ranged between 80 and 85 per cent. The common factor found in these media was their capability of keeping the bacterial growth that killed the embryos to a minimum. The limited effectiveness of other experimental media, e.g., NaCl, artificial sea water, methylene blue, and lysozyme, was dependent on a balance between their toxic properties and their bactericidal (or bacteriostatic) properties. Fungus mycelia usually appeared on embryos raised extra-orally in aquarium water. However, it was demonstrated experimentally that the fungus was a secondary invader, since survival was high when embryos were raised in sterile aquarium water to which a fungus culture had been introduced. Conversely, no embryos survived when they were raised in sterile water to which a culture of bacteria had been introduced. The churning action of the mouth was found to be one of the survival factors in oral incubation. When embryos were placed in a small quantity of aquarium water in a mechanical shaker, survival rates were good. On the other hand, embryos that remained stationary while a large quantity of water flowed over them never survived. These experiments show that the slow mechanical churning of the embryos in the mouth is a factor in survival, whereas the quantity of water flowing over the embryos during respiration of the parent is not critical. The oral epithelium may secrete mucus which protects the embryos. However, experiments designed to test the protective nature of mucus were not conclusive. Special pharyngeal glands, described in this paper, may secrete a bactericidal agent. Similar glands were found in Tilapia ovalis and have been described in two other mouthbreeding cichlids, while we were unable to find them in two non-mouthbreeding species of the genus Tilapia"--P. 413.
p. 379-415, [13] p. of plates : ill. ; 27 cm.
Includes bibliographical references (p. 413-415).