- Introduction to Reproductive System of Starfish
- How do Starfish reproduce? What is their life cycle?
- The Reproductive Life Cycle & Development of Starfish consists of the following Stages
- 1. Fertilization Stage: The fusion of egg and sperm takes place.
- 2. Embryogeny Stage: The formation of blastula and gastrula embryo takes place from the zygote.
- 3. Larval Development Stage – I: This is the first Dipleurula larva stage.
- 4. Larval Development Stage – II: This is the second Bipinnaria larva stage.
- 5. Larval Development Stage – III: This is the third Brachiolaria larva stage.
- 6. Metamorphosis Stage: The final stage during which the Bipinnaria larva will metamorphose by undergoing a metamorphosis
Here, in this post we will talk about the reproduction in starfish from the zoological point of view.
There are approximately 2,000 species of starfish, all of which live in marine waters of the ocean.
The following description of the reproduction in Starfish is generalized and based mainly on the starfish Asterias rubens.
Introduction to Reproductive System of Starfish
Asterias is unisexual in nature, that is the individuals are either male or female. Proper sexual dimorphism is not well-defined in starfish until and unless you study their internal anatomy.
While most animals are born as either male or female, the line separating genders gets a little blurry when it comes to starfish.
Some species of starfish can be hermaphrodite as well, meaning that the same individual have both male and female sex organs or other sexual characteristics.
In hermaphrodites, the same individual can produce both eggs and sperm at the same time and in a few of these, the same gonad, called an ovotestis, produces both eggs and sperm.
While some starfish species like Asterina gibbosa is protandric, meaning that the individuals remain male when they are young and turn to female as they become adults.
The reproductive organs of starfish don’t show the presence of copulatory organs, accessory glands, and reservoirs for storing sperms and ova.
Gonads (testes and ovary) are well-marked in the body of the individuals. There are five pairs of gonads present in starfish Asterias rubens.
Male Asterias rubens, have five testes (greyish in colour) that are located at the base of each arm, lying freely between the pyloric caeca and ampullae of the tube feet.
Female Asterias rubens, have five ovaries (pinkish-orange in colour) that are located at the base of each arm, lying freely between the pyloric caeca and ampullae of the tube feet.
It is to be noted that the gonads don’t produce sex cells (sperms or eggs). The aboral gland of starfish produce sex cells that later migrate to the gonads from where they are being released.
How do Starfish reproduce? What is their life cycle?
Starfish reproduce by means of the external fertilization process during their breeding season which for Asterias rubens is the spring season.
Reproduction is done when the female starfish starts to release her eggs directly into the seawater.
The male then as soon as possible will release almost billions of sperms into the open water.
Outside in the water, when the sperm cells come together with the eggs, the eggs are fertilized and begin to divide rapidly and as a result of which the life cycle of the starfish begins.
These fertilized eggs will result in the formation of numerous zygotes. Each of the zygote will then perform cleavage to form morula and blastula.
As soon as the blastula is formed the resulting embryos will now become a part of the animal-like community of plankton, known as zooplankton.
Eventually during their zooplankton phase they will enter into the larval stages of development and will eventually turn into Dipleurula larva, then into Bipinnaria larva, and then to Brachiolaria larva.
Eventually, as they develop into larvae and have completed their larval development, then they will soon metamorphose turning themselves to young starfish and then to reproductively active adult starfish.
As a result, in time, they will return to the seafloor and become starfishes.
The Reproductive Life Cycle & Development of Starfish consists of the following Stages
- Fertilization Stage: In this stage, the fusion of haploid male sperms (n) and haploid female eggs (n) takes place by external fertilization to produce diploid zygotes (2n).
- Embryogeny Stage: During this stage, the diploid zygote (2n) results in the formation of morula, then blastula, and then gastrula embryo by constant cleavage.
- Larval Development Stage-I: During this stage, the Dipleurula larva will be formed from the Gastrula.
- Larval Development Stage-II: During this stage, the Bipinnaria larva will be formed from the Dipleurula larva.
- Larval Development Stage-III: During this stage, the Brachiolaria larva will be formed from the Bipinnaria larva.
- Metamorphosis Stage: This is the final stage during which the Bipinnaria larva will metamorphose by undergoing metamorphosis, especially into the adult form.
Let’s know about each of these stages…
1. Fertilization Stage: The fusion of egg and sperm takes place.
In, this stage fertilization takes place, that is fusion of the male and female gametes leading to the formation of zygote takes place by external fertilization in the open water.
The female releases its eggs into the open water and the male releases its billions of sperms into the open water, where they both fertilize externally.
Female can release about 1 million eggs per hour, with a total release of about 200 million eggs in a particular breeding season.
Male releases about billions of sperms during the breeding season.
The sperm fuses by rapidly coming in contact with the egg, penetrating it by its chemical secretion, and getting inside while leaving its flagella outside.
A single sperm can fertilize a single egg and when the male haploid sperm (n) fuses with the female haploid egg (n) the resulting zygote thus formed will be diploid (2n).
The diploid (2n) zygote soon forms a delicate fertilization membrane around it.
2. Embryogeny Stage: The formation of blastula and gastrula embryo takes place from the zygote.
In this stage, the formation and the growth of embryo can be well-seen.
The diploid zygote that was formed during the fertilization stage will now turn into embryo to continue its further life cycle.
Each of the fertilized egg will have a small quantity of yolk. The yolk is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo.
Sooner the cleavage (a division of cells in the early embryo) starts which is very rapid, holoblastic, practically equal, and in radial symmetry.
The cleave of the early embryo is also indeterminate, meaning that the blastomeres (cells formed by cleavage of a fertilized eggs) do not have a fixed fate and if separated at a very early stage, will develop into separate larvae.
On the second day of cleavage, the blastula and coeloblastula stage is reached. The blastula embryo looks like a spherical, hollow, one-layered, and ciliated embryo which swims about freely.
Sooner, the blastula turns to gastrula which shows a two-layered cup-like appearance with outer ectoderm, and inner endoderm.
Other features like archenteron and blastopore can also be seen in the gastrula embryo. Later on, the gastrula forms mesoderm and left and right coelomic pouches.
The embryo after the gastrula phase can now be properly termed as a free-swimming larva.
3. Larval Development Stage – I: This is the first Dipleurula larva stage.
This is the first larval stage that can be seen during the life cycle of a starfish. It is believed that all echinoderms have been derived from a dipleura-like ancestor.
This Dipleura larva looks like a egg-shaped and bilaterally symmetrical organism.
The larva has a small mouth that is formed by the ectodermal invagination with the archenteron layer.
The archenteron differentiates into a digestive tract made of oesophagus, stomach, and intestine.
This larva can be easily recognized as the uniform ciliation of gastrula is replaced in the Dipleura larva by two ciliary bands i.e. a perioral band surrounding the mouth, and an adoral band lying inside the mouth.
This larva grows into the next larval stage by feeding on unicellular algae like diatoms, cyanobacteria collected by the beating of adoral band cilia.
The larva uses its perioral band cilia to swim forward rotating in a clockwise manner.
4. Larval Development Stage – II: This is the second Bipinnaria larva stage.
A Bipinnaria is the second larval stage in the larval development of most starfish and is usually followed by a Brachiolaria larval stage.
Those starfish that brood their young ones generally lack a Bipinnaria and Brachilaria larval stages, with the eggs developing directly into miniature adults.
In some books, you often find that the Dipleurula larva to be also denoted as early Bipinnaria larva. In both cases, it’s correct.
To form the Bipinnaria larva, the Dipleurula larva soon forms a large preoral lobe on its front side, which becomes bordered by the preoral loop of cilia.
Simultaneously, on each lateral side, it forms three lateral lobes which become bordered by a Postoral loop of cilia.
The preoral and postoral loops arise by the splitting of the perioral band of cilia.
The larva thus formed is the Bipinnaria larva. This is bilaterally symmetrical in nature.
Thus, in simple words, the bipinnaria is free-living, swimming as part of the zooplankton. When it initially forms, the entire body is covered by cilia, but as it grows, these become confined to a narrow band forming a number of loops over the body surface.
A pair of short, stubby arms soon develop on the body, with the ciliated bands extending into them.
5. Larval Development Stage – III: This is the third Brachiolaria larva stage.
The Bipinnaria larva swims and feeds freely and after some weeks transforms into the next larval stage, which is the Brachiolaria larva stage.
Brachiolaria larva has bilateral symmetry, unlike the adult starfish, which has a pentaradial symmetry.
It is also to be noted that, starfish of the order Paxillosida (Astropecten and Asterina) have no brachiolaria stage, with the bipinnaria larva developing directly into an adult by metamorphosis.
To form the brachiolaria larva, the lobes of the bipinnaria larva starts to develop into long, slender, ciliated, contractile structures, called larval arms.
Then from the preoral lobe arise three, short, and non-ciliated appendages, each ending in a sucker or adhesive disc.
The appendages are called brachiolarian arms, and the larva is now better called brachiolaria larva.
The larva soon sinks to the bottom, attaching itself to the substrate firstly with the tips of the arms, and then with the sucker, within about six to seven weeks time period.
During that time, the preoral lobe that represents the anterior end of the larva forms a sort of stalk for adhesion.
Once attached, it begins to metamorphose into the adult form in the ocean bed.
6. Metamorphosis Stage: The final stage during which the Bipinnaria larva will metamorphose by undergoing a metamorphosis
The Metamorphosis stage is the last stage of development. During this stage, the larva will metamorphose that is will turn into the adult stage by transforming its body parts.
The preoral lobe (representing the anterior end of the larva) that has formed a sort of stalk for adhesion will now degenerate and will become completely absorbed into the body.
Now the adult body will start to develop from the rounded posterior end of the larva.
The larval mouth, anus, and ciliated bands will disappear. And, new mouth and anus will be formed on the left and right side of the developing body.
Sooner or later, the five arm rudiments appear around the oral-aboral axis. Skeletal elements appear on the arm rudiments and radial canals will grow into them.
The general body cavity that is the coelomic cavity of the adult develops from the right and left coelomic sacs of the larva.
In each arm, two pairs of outgrowths form the first tube feet which serve for attachment. Further complex re-organizational changes will result in the formation of adult starfish Asterias rubens.