AbstractThis study examined the asexual and sexual reproduction in Holothuria /eucospilota inhabiting Nightcliff and East Point reefs, Darwin Harbour, Northern Territory. Asexual reproduction by binary division resulted in smaller anterior portions and larger posterior portions (A to P ca. 34.81 %). Individuals resulting from the posterior part (P and Pa) were observed more frequently on the reef and may be assumed to possess greater capacity for survival. Fission occurred throughout the year, and intensified in the East Point population from March to May, during maximum annual precipitation. Individual size in the population was between 50-150 gram in fresh condition (density 0.054 individual m-1 ) at Nightcliff, and 100-300 gram (density 0.077 - 0.29 individual m-2) at East Point. Small individuals are typical of a fissiparous population undergoing fission.
There was no indication of hermaphroditism in investigated populations. A single tuft of ovarian tubules developed simultaneously. Despite that the spawning season occurred yearly, ovarian tubules required less than 12 month to produce eggs. Even though the timing of spawning varied between years, gonads developed synchronously with maximum growth attained between January 1999 and April 1999. Spawning was suggested to occur some time during the second fortnight of April, between new moon and full moon. Gonads were rarely observed from July to August 1999, indicating the presence of resting period in which the post-spawning tubules were absorbed.
Histological and ultra sections revealed the ovarian tubule wall consisted of 4 layers: 1) ciliated and umbrella-like peritoneal cells; 2) a single layer of myoepithelia underlaying the peritoneal cells. The fibers were arranged in two directions, facilitating tubules in lengthening and shortening, as well as enabling the lumen to widen and narrow in order to evacuate full-grown oocytes; 3) connective tissue, which occupied most area of the ovarian wail, and became thinner as the tubule enlarged; and 4) inner epithelial cells bordering the tubule lumen, from which the follicle cells originated.
Corresponding with oocyte development was the growth of ovarian tubules, starting with early developing tubules containing oogonia and small previtellogenic primary oocytes (up to 20 11m in fixed condition) and terminating in fecund tubules carrying fullgrown postvitellogenic primary oocytes ( 140-150 µm in fresh condition and 90-100 µm in fixed condition). After discharging the oocytes, tubules deteriorated and were absorbed.
Multiplication of mitochondria in small previtellogenic oocytes may be the transition stage from oogonia to previtellogenic primary oocytes. Nuage became invisible as yolk granules appeared and became dominant in the cytoplasm. There is no indication of any particular layer arrangement of mitochondria and nuage. Vitellogenic oocytes were characterized by irregular membrane with abundant microvilli, which was likely to correspond with oocyte enlargement and material transfers. Microvilli decreased in number and became shorter in fullgrown oocytes. Fullgrown oocytes were covered by a thick jelly layer (17.5-20 µm in fresh condition), which isolated each oocyte. On the outer surface, follicle cells encircled each oocyte and may have remained until spawning. Signs of maturity, ovulation and germinal vesicle break down, were unlikely to occur within the tubules.
Even though there was distinct development of gonads and oocytes in the population, no juveniles in East Point and only 3 juveniles in Nightcliff were observed during the course of this study. This may indicate that sexual reproduction was either unsuccessful or the juveniles were cryptic and occupied different habitat to adults. Alternatively, fission was more significant in maintaining the population.
|Date of Award
|Jim Luong-Van (Supervisor) & Michael Guinea (Supervisor)