Abstract

Florida manatees (Trichechus manatus latirostris) are unique, endangered herbivorous marine mammals. Very little has been done to date examining manatee embryological development. Gestation in Florida manatees is approximately 14 months, and new born calves are approximately 122 cm in length, with an estimated weight of 22 kg. Embryological studies are important because knowledge of normal development will help researchers understand trends in neonatal mortality and allow them to differentiate between healthy and unhealthy states.

In this study, developmental events in humans, horses, and cetaceans were used to estimate the stages of development and ages of two manatee embryos and one manatee fetus. The smaller embryo was at Carnegie stage 10 of development or at stage 2 of the cetacean embryonic stages established by Sterba et al. (2000). Its age was estimated to be 17-22 days. The larger embryo was at Carnegie stage 23 or cetacean stage 6. Both of these represent the last stage of embryonic development. Its age was between 40 and 60 days. The degree of development of the fetus corresponded to stage 8 of cetacean development, and it was estimated to be 56-78 days old.

Initial studies of two manatee embryos included imaging with nuclear magnetic resonance (NMR) and histological processing with Masson's trichrome and hematoxylin and eosin stains. One embryo, ID#18-85, was 14 cm in length, and several organ systems were easily discernible. The limbs had begun to develop. Circulatory and urogenital systems were well developed. The diaphragm and lungs were well preserved, but the lungs did not appear fully developed. The digestive tract was discernable. The brain was not well preserved, but the spinal cord and ganglia could be defined. Some skeletal ossification was seen, especially in the vertebrae and ribs. The second embryo, ID#MSW-97, was approximately 9 mm long but less well preserved. No external features could be defined. The heart and dorsal aorta were visible, and umbilical tissue dominated the body. The neural tube, somites, and mesonephros were also identified. However, due to degradation of the tissue, little additional information could be gathered from this specimen. A small manatee fetus, ID#M-78-16-F, length 24.5 cm, diameter 7.6 cm, and girth 17.1 cm, was imaged with NMR and is currently being histologically processed. To date, the head, flippers and heart have been completed. These findings will be helpful in understanding normal manatee development, identifying abnormalities and in determining evolutionary relationships between Sirenia and related Orders.

Acknowledgements

The authors would like to thank the University of Florida, Marine Mammal Health Program for funding portions of this work.

References

1.  Sterba O, Klima M, Schildger B. 2000. Embryology of dolphins: Staging and ageing of embryos and fetuses of some cetaceans. Advances in Anatomy, Embryology, and Cell Biology. 157:1-133.