Journal of Undergraduate Research
Volume 5, Issue 3 - December 2003

A Descriptive Analysis of Florida Manatee (Trichechus Manatus Latirostris) Embryology

Jaleh Khorsandian-Fallah

INTRODUCTION

The Florida Manatee (Trichechus manatus latirostris) is an aquatic mammal of the order Sirenia1. Analysis of life history, population biology, morphology, and effects of anthropogenic causalities has revealed much about the Florida manatee. However, development of the manatee at the embryological and fetal stages is an area of study that has received little attention. Essentially, it is known that the manatee gestation period is approximately 12-14 months2, and that newborn calves are approximately 122 cm in length, with an estimated weight of 22 kg3,4,5. Understanding normal manatee embryological development may provide critical information for understanding trends in neonatal mortality allowing us to differentiate between healthy and unhealthy states. A standard description of normal manatee development is needed to provide comparative data. Investigating the development of this specialized group will allow scientists to better understand the place of Sirenia in evolutionary history as well as the relatedness of other groups that share Sirenian specializations.

During embryological development phylogenetic traits are often maintained and the order Sirenia is evolutionarily related to Hyracoidae (hyraxes) and Proboscidae (elephants), together composing the superorder Paenungulata6,7. This was originally supported by bone morphology and has been supported more recently with biochemical data linking these species8. Elephants, hyraxes and manatees all have internal testes, unique dentition, and lengthy gestations. These unique characteristics lend themselves to developmental inspections, as they are significant evolutionary adaptations shared between these three specialized groups. Specifically, the elephant’s unique development of nephrostomes in the mesonephric kidney, intra-abdominal testes, and development of a prehensile trunk could be indicators of an aquatic ancestory9. Proboscidae and Sirenia also share many uncommon characteristics, such as a perilymph foramen of the middle ear, pachyostotic bones, a prehensile lip, characteristics of tooth enamel, and a tooth replacement system that moves teeth forward from the rear in a conveyer belt fashion10,11,12. The order Hyracoidae shares with the Proboscidae a complete absence of gubernaculums and a long gestation period, approximately 2 years for elephants and 7.5 months for hyrax, which is unusually long for an animal of its size13. In all contemporary species of the superorder Paenungulata, testes of adults are found to lie close to the ventrocaudal surface of the kidneys6. The hyrax’s epididymis is long and stretched away from the testis, a characteristic reminiscent of the elephant and other testicond mammals13. However, elephants and Sirenia have a midline fusion of the ligamentum latum into a mammalian-like genital cord6.

The unique physiology of the manatee is the byproduct of an aquatic habitat and herbivorous diet (e.g. adaptations to facilitate diving, thermoregulation, navigation, osmoregulation, reproduction, and digestion)14. These morphological characteristics include dorsal positioning of the diaphram extending the length of the thoracic vertebral column and elongate, unlobed, and relatively flattened lungs that extend virtually the entire length of the body14. As mentioned above, Sirenia possess internal testes, which is also a typical adaptation seen in other marine mammals such as Pinnipeda and Delphinidae.

The current study has utilized MRI and histological techniques to visualize anatomical detail of two Florida manatee embryos. A description of general anatomical features and some elaboration on the skeletal, reproductive, and respiratory systems are included.

MATERIALS AND METHODS

Two embryos, which were opportunistically collected from the Florida Marine Research Institute (FMRI), were first imaged though MRI, a non-destructive method of studying animal tissue, based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. Although there are many uses of NMR, zeugmatography, or spin-imaging, was utilized to provide anatomical imaging of intact biological systems. The images reflect distribution of protons within water and fats, and provide discrimination between different tissues15. In manatee 85-18 tissue quality was excellent while embryo ID#M-395 showed signs of degradation.

The embryos, fixed in formalin, were then embedded in paraffin and histologically sectioned. Manatee ID#85-18 was sectioned at 8µm and every fifth section mounted. Slides were stained with H&E and Masson’s Trichrome in intervals of 40µm. The slides were then digitalized and key structures identified and labeled.

The second manatee ID# M-395 was sectioned sagittally at 40µm and every fifth section mounted. Two out of every five sections mounted were stained with either H&E or Masson’s Trichrome. The slides were then observed and distinct regions identified.

Table 1
Table of identified tissues and corresponding labeling of histological sections of manatee Embryo ID#18-85
Tissue Identification Number Tissue Identification Number
Sinus Cavity 1 Colon 12
Palate 2 Umbilical vein 13
Optic stalk 3 Umbilical artery 14
Optic cup 4 Umbilical stalk 15
Retina 5 Spinal ganglia 16
Optic lens 6 Diaphragm 17
Eyelid 7 Penis 18
Spinal Coard 8 Trachea 20
Kidney 9 Esophagus 21
Dorsal aorta 10 -- --
Small Intestine 11 -- --

RESULTS

Crown-rump length of Manatee ID# 85-18 was measured at 14cm. External morphology was well developed, and the manatee well preserved. No traces of hair were found on the body or head of the animal. Manatee ID#M-395 crown-rump length measured 9mm, with external features undefined and poor preservation of tissue. The exact age or phase of development of both embryos is unknown. Due do differences in stages of development both embryos were analyzed independently of one another.

The eyes, ears, mouth, and snout of manatee ID#85-18 were well defined as were the penis and anus. The embryo was a pale pink in color (probably due to preservation in formalin) and was preserved along with its placental tissue.

The Circulatory system of manatee #85-18 was well developed. Chambers of the heart were defined and the dorsal aorta as well as the umbilical artery and vein were identified through their paths in the body. The heart was located in the upper torso and, when cut coronally, was visualized before the lungs appeared. The unique characteristics of the Sirenian respiratory system were identified. The diaphram extended the length of the body and the dorsally positioned lungs were well preserved. The brochii of the animal could be clearly followed and seen to pervade a significant portion of the lungs. However, the lung tissue itself did not seem to be fully developed.

The nervous system was not as well preserved as some of the other systems. The brain had deteriorated beyond the point of regional identification, although the spinal cord and spinal ganglia were well documented. The optic nerves were preserved and detailed characteristics of the eye were observed (figure 1).

Figure 1:  Masson’s Trichrome stained coronal section of embryo ID#18-85
Figure 1:  Masson’s Trichrome stained coronal section of embryo ID#18-85 depicting the brain(19), sinus cavity(1), palate(2), optic cup(3), optic stalk(4), retina(5), optic lens(6), and eyelid(7).

The urogenital system was well developed and adequately preserved. Magnified observation of kidney tissue revealed well-developed glomeruli. The penis and its associated lumens were identified and the intrabdominal testes were clearly visualized (figure 2).


Figure 2:  H&E stained coronal section of embryo ID#18-85
Figure 2:  H&E stained coronal section of embryo ID#18-85 depicting the spinal cord(8) and spinal ganglia(16), kidney(9), dorsal aorta(10), small intestine(11), colon(12), umbilical vein(13), umbilical artery(14), umbilical stalk(15), and diaphragm(17).

The extensive digestive tract of the manatee was defined by the initial looping of the small intestine followed by the shorter large intestine and colon (figure 3).

Figure 3:  H&E stained coronal section of embryo ID#18-85 depicting the spinal cord(8), dorsal aorta(10), colon(12), umbilical vein(13), umbilical artery(14), and penis(18).
Figure 3:  H&E stained coronal section of embryo ID#18-85 depicting the spinal cord(8), dorsal aorta(10), colon(12), umbilical vein(13), umbilical artery(14), and penis(18).


The skeleton of the sectioned manatee was in a transitional phase. In most regions cartilaginous structures were found, and in some the beginning of ossification was evident. The spinal vertebrae and adjacent ribs were particularly well defined. The developing limbs of the manatee, however, were not quite as well defined or ossified. The bones around the skull were found in differing stages of development (figure 1 and figure 4 ).


Figure 4:  H&E stained coronal section of embryo ID#18-85 depicting the limbs, spinal cord(8), trachea(20), and esophagus(21).
Figure 4:  H&E stained coronal section of embryo ID#18-85 depicting the limbs, spinal cord(8), trachea(20), and esophagus(21).

Horizontal and sagittal MRI images of embryo ID#85-18 clearly revealed the developing ribs and vertebral column (figure 5). Heart and liver tissues were well identified, with the chambers of the heart discernable. Cephalic organs such as eyes, and brain are visualized, while formation and merging of the trachea and esophagus and other structures of the mouth are evident. Sagittal sections of the embryo reveal cavities and developing tissues of the head. Dorsally positioned lungs lay alongside the vertebral column. Organ positions and internal lumens of the upper torso of the animal are displayed. MRI photos present a compliment to the sectioned slides and reveal a composite description of the animal’s development.


Figure 5:  Sagittal section of MRI embryo ID#18-85 depicting the head, vertebral column and developing organs of the manatee
Figure 5:  Sagittal section of MRI embryo ID#18-85 depicting the head, vertebral column and developing organs of the manatee.

The smaller manatee, ID#M-395, was also preserved in formalin and embedded in paraffin. The sagittal sections of the embryo revealed poor preservation, as tissue integrity was not maintained. The external characteristics were undefined, and general facial characteristics were unclear. There were no discernable limb buds or visible distinction between the head and tail. The most obvious system in the manatee was the circulatory system where heart muscle was discernable and the dorsal aorta evident. Umbilical tissue, including the umbilical vein and artery, dominated the body. Neither lung tissue nor evidences of a diaphram could be seen in the animal. The brain, in the form of the neural tube, was seen and the somites of future vertebra were identified. The mesonephros was degraded but still recognizable. Although the tissues of the embryo ID#M-395 were identified, specific regions of interest or development could not be commented upon due to the degradation of tissue.

DISCUSSION

Unique, landmark features of the manatee were discernable in the larger embryo. The dorsally positioned, elongate lungs, a trademark of manatee physiology, were well defined and well developed, as were the associated respiratory tissues, such as the diaphragm and trachea. The respiratory system of a manatee, in its aquatic environment, functions in activities such as feeding, resting, and general buoyancy. The early presence of this system is perhaps indicative of the size and importance of the lungs. In comparison, the development of the pelvis, which is regressed in an adult animal, was insignificant and poorly defined. The internal gonads of the Order Sirenia are a unique trait, as they are primary testicond animals, which may be another adaptation of an aquatic environment. This adaptation could be helpful in streamlining the body, as well as for temperature regulation. Embryo ID#18-85’s histological sections, in conjunction with the MRI photos, reveals the picture of a well developed embryo which is complete save for the final stages of tissue maturation.

Many questions concerning the unique physiology of Sirenia and its evolutionary history can be answered through studies in manatee embryology. This first look into the manatee is key in clarifying the relationships that exist between the manatee, elephant, and hyrax, with similarities potentially leading to insightful hypothesis as to the aquatic adaptations of the manatee. Further studies in manatee embryology will help cement the evolutionary history of the order Sirenia as well as its relatives Proboscidae and Hyracoidae. The normal development of an embryonic manatee involves the specific phases of tissue development as well as growth in size. Through the knowledge of the differing phases of development, specimens collected in the future could be observed for possible developmental abnormalities.

REFERENCES

  1. Harlan, R. (1824) On a species of Lamantin resembling the Manatus senegalensis (Cuvier) inhabiting the coast of east Florida. J. Acad. Nat. Sci. Philadelphia, 3, 390-394.

  2. Reid, J.P., Bonde, R.K. & O'Shea, T.J. (1995) Reproduction and mortality of radio-tagged and recognizable manatees on the Atlantic coast of Florida In Population biology of the Florida manatee (O'Shea, T.J., et al. eds.), pp. 171-191. U.S. Dept. of the Interior, National Biological Service, Washington, D.C.

  3. Odell, D.K. (1977) Age determination and biology of the manatee, pp. 1-112. US Fish & Wildlife Service, Washington, D.C.

  4. O'Shea, T.J. & Reep, R.L. (1990) Encephalization quotients and life-history traits in the sirenia. Journal of Mammalogy, 71, 534-543.

  5. Marmontel, M. (1993) Age determination and population biology of the Florida manatee, Trichechus manatus latirostris, pp. iii-408. University of Florida, Gainesville.

  6. Van Der Schoot, P. (1996) Foetal Genital Development in Hyrax capensis, a species with primary testiconda: Proposal for the evolution of Hunter’s Gubernaculum. In The Anatomical Record: 244:386-401.

  7. Simpson, G.G. (1945) The principles of classification and a classification of mammals. Bull. Am. Mus. Nat. Hist., 85, 1-350.

  8. Kleinschmidt, T., Czelusniak, J., Goodman, M. & Braunitzer, G. (1986) Paenungulata: A comparison of the hemoglobin sequences from elephant, hyrax, and manatee. Molecular Biology and Evolution, 3, 427-435.

  9. Gaeth, A.P., Short, R.V., Renfree, M.B. (1999). The developing renal, reproductive, and respiratory systems of the African elephant suggest and aquatic ancestory. Proc.Natl. Acad. Sci.: 96:5555-5558 May 1999

  10. Husar, S.L. (1977) The West Indian manatee (Trichechus manatus) In Wildlife Research Report. U.S. Dept. of the Interior, Fish & Wildlife Serv., Washington D.C.

  11. Laursen, L. & Bekoff, M. (1978) Loxodonta africana. Mammalian Species, 92, 1-8.

  12. Shoshani, J. & Eisenberg, J.F. (1982) Elephas maximus. Mammalian Species, 182, 1-8.

  13. Maloiy, G.M.O. & Eley, R.M. (1992) The hyrax, Regal Press, Nairobi.

  14. Rommel, S., Reynolds, J.E.(2000) Diaphram structure and Function in the Florida Manatee (Trichechus manatus latirostris). In The Anatomical Record: 256:41-51.

  15. Gadian, David G. (1982). Nuclear magnetic resonance and its applications to living systems. Oxford University Press, New York.

--top--

Back to the Journal of Undergraduate Research