Egg microvilli may be involved in deciding sperm receptivity, and their number on the surface of aged mammalian oocytes is severely reduced Santella et al. In starfish, a decline in the proper functional interactions between sperm and aged eggs has been suggested Chambers ; Just Our recent studies on starfish eggs have indicated that the alteration of the structural organization in subplasmalemmal actin filaments renders the egg surface less reactive to the first sperm and more receptive to supernumerary sperm Puppo et al.
Eggs with hyperpolymerization of the subplasmalemmal actin displayed the formation of numerous irregularly shaped fertilization cones thus, polyspermic , yet also displayed unfunctional tethering of the sperm, suggesting a more direct role of actin in sperm interaction and incorporation Puppo et al.
Although belonging to the same phylum, sea urchin and starfish have evolved to use a fundamentally different temporal scheme at fertilization. Although fertilization of sea urchin awaits the completion of the egg meiosis, natural fertilization of starfish anticipates the meiotic program. To ensure monospermy, the starfish eggs must be fertilized when they are at the incomplete stage of meiosis; that is, during the interval between the GVDB and the extrusion of the first polar body.
The reason why starfish evolved to adopt this seemingly premature fertilization scheme has not been clarified, but it may have been inevitable because the same hormone 1-MA not only stimulates oocyte maturation but also promotes spawning of mature gametes Meijer and Guerrier On the other hand, although the fertilization envelope of the echinoderm egg may provide mechanical protection of the early embryo, the other functions that have been proposed may require careful examination in future studies.
The fully elevated fertilization membrane was not sufficient to ensure monospermic fertilization Fig. In view of the relative ease of applying imaging and electrophysiological methods following microinjection, starfish oocytes may provide opportunities to study the reciprocal regulation of cytoskeletal changes and ion channel activities.
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Skip to main content Skip to sections. This service is more advanced with JavaScript available. Advertisement Hide. Actin Cytoskeleton and Fertilization in Starfish Eggs.
Open Access. First Online: 04 January Download conference paper PDF. Although ovulation in mammals only releases a few oocytes, starfish and sea urchin provide many eggs that are highly accessible.
At the peak of the breeding season, sea urchin ovaries are filled with eggs that have completed meiosis and contain haploid DNA Vacquier At variance with the sea urchin, starfish oocytes in the female gonad at the peak of the breeding season are fully grown, yet they are arrested at prophase I of meiosis.
Only upon hormonal stimulation methyladenine, 1-MA from the adhering follicle cells, the oocytes re-enter the meiotic cycle and are spawned into seawater Kanatani ; Meijer and Guerrier Thus, even if belonging to the same phylum, sea urchin and starfish display a fundamental difference in the way in which their eggs are fertilized. Sea urchin eggs are fertilized after the completion of meiosis haploid stage, n , but starfish eggs are normally fertilized before the completion of the first meiotic division Fig.
In starfish, the period between germinal vesicle breakdown GVBD and the extrusion of the first polar body 2n represents the optimal period for successful fertilization. Indeed, insemination of the starfish oocytes before or after this period results in a high rate of polyspermic fertilization.
As an experimental model, starfish provides some advantages over sea urchin eggs in that a meiotic maturation can be induced in vitro, and b the large cell dimensions Fig. Starfish oocytes thus provide an optimal opportunity to examine the cytoplasmic changes that occur during meiotic maturation and fertilization.
Open image in new window. The idea that polyspermy is linked to the regional anatomy of egg surface and cortex can be examined in starfish oocytes at different meiotic maturation stages. As mentioned, the morphology of the starfish oocyte changes with meiotic progression, as judged by the structure of microvilli and cortical actin cytoskeleton. Meiotically immature oocytes are prone to polyspermy upon insemination and fail to elevate the fertilization envelope Fig.
They lack an effective fast or slow block to polyspermy Santella et al. However, most starfish eggs overmatured beyond the extrusion of second polar bodies display polyspermy at fertilization despite their ability to elevate a normal fertilization envelope Fujimori and Hirai The explanation for the polyspermy of these overmature eggs might be twofold.
Firstly, the tendency to produce polyspermy may be linked to the electrical properties of the plasma membrane. For instance, the resting potential of the mature eggs of A. Classical studies had indicated that the dispersal of nuclear contents into the cytoplasm and cortex is a prerequisite for cytoplasmic maturation Delage ; Chambers After the GVDB, a signal or a diffusible substance from the GV may interact with the egg cortex to induce the structural modifications that are essential for egg activation and monospermic sperm incorporation.
Both male and female sea stars hold their sperm and eggs in pouches at the base of their arms. They reproduce by free spawning, that means ideally the male and female release their eggs and sperm at the same time. The egg and sperm float until they meet up and the sperm is able to fertilize the egg. This is most likely to happen if a large number of starfish have gathered in the same area and spawn at the same time.
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Science Coronavirus Coverage U. Travel A road trip in Burgundy reveals far more than fine wine. Most species of starfish are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals. Some species are detritivores, eating decomposing organic material and fecal matter. Starfish occupy several significant ecological roles. Starfish are keystone species in their respective marine communities.
Their relatively large sizes, diverse diets and ability to adapt to different environments makes them ecologically important. The feeding activity of the omnivorous starfish Oreaster reticulatus on sandy and seagrass bottoms in the Virgin Islands appears to regulate the diversity, distribution and abundance of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles.
Organisms that dislike this disturbance are replaced by others better able to rapidly recolonize "clean" sediment. In addition, foraging by these migratory starfish creates diverse patches of organic matter, which may play a role in the distribution and abundance of organisms such as fish, crabs and sea urchins that feed on the sediment.
Several species of starfish sometimes suffer from a wasting condition caused by bacteria. Starfish are vulnerable to high temperatures. Starfish and other echinoderms are sensitive to marine pollution. The common starfish is considered to be a bioindicator for marine ecosystems. Starfish are inhumanely taken from their habitat and sold to tourists as souvenirs, ornaments, curios or for display in aquariums.
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