When we take a walk along the coast, it is very common to find starfish attached to the rocks. If we get close to them, they will try to flee, although their movement is so slow that they cannot do so. This fact, together with its strange morphology and its peculiar way of life, makes us very curious. Who are these animals? Where do they come from? How are starfish born?
There are about 2,000 known species that make up the class Asteroidea. They are echinoderms, that is, relatives of sea urchins and sea cucumbers. Like them, when they are born they do not have the shape that we all know, but are extraordinarily different and must go through a metamorphosis to become adults. You want to know more? Then don't miss this article on our site about the birth of a starfish
How do starfish reproduce?
To understand how starfish are born, we need to know what happens before they are born. When the breeding season arrives, a large number of males and females gather in one place. Once there, all together or in pairs, they hit each other, rubbing and intertwining their arms. This intimate contact and the release of certain substances causes the synchronized release of gametes.
Sperm from males and eggs from females unite in water, forming a zygote or egg. Therefore, fertilization is external and the eggs develop outside the mother. In very few species, the eggs form and develop inside their mother, but this is very rare.
For more information, we advise you to read this other article on How do starfish reproduce?
Birth of a Starfish
Starfish are oviparous animals. Their eggs are formed in the water and can float or settle on the seabed Normally, they do not receive any type of parental care, although some species are incubators. In this case, the eggs are housed in special structures that the parents have on their oral (inferior) side[1] or aboral (upper) [two]
So, how are starfish born? When the eggs are fully developed, they hatch into small larvae known as bipinnaria larvae They are tiny, elongated and bilateral, that is, their body is divided into two equal halves, like ours. They do not have the radial symmetry that characterizes adults. In addition, they can swim freely thanks to cilia that cover their larval arms.
In most species, starfish larvae have a planktonic life and swim in the ocean together with other organismsplants and animals. There, they are dedicated to eating to grow and develop properly. Their food, as we explained in the article What do starfish eat?, are other members of the plankton, such as algae, crustaceans or other types of invertebrates. In some species, however, the larvae retain the yolk or food provided by their parents, so they do not have to feed.
Next, we leave a video with the birth of starfish.
Development of a starfish
We already know how starfish are born, but how do they become adults? The life cycle of the starfish begins with the birth of a bipinnaria larva, a very simple being similar to the larvae of other marine animals. Little by little, it grows and becomes more and more complex. It grows sticky arms and a suction cup on its front part, where the mouth is. In this stage of development, it is called a brachiolar larva.
The brachylary larva returns to the bottom of the sea, where, thanks to the suction cup and a temporary stem, it attaches to a rock or coral. In this position, it remains motionless and undergoes a metamorphosis. The left side of the larva becomes the oral or lower side, while the right side becomes the aboral or upper side. The larval mouth and anus disappear, forming a new mouth on the oral side and a new anus on the aboral side.
When the metamorphosis is complete, the starfish already has its characteristic radial and pentameric symmetry. It has five arms and its characteristic ambulacral apparatus. Little by little, detaches from its stem and begins its new life at the bottom of the sea.
Life cycle of other starfish
Finally, it is important to note that, in very few species, the larvae develop on the body of the adults. This is the case of the "incubator" starfish. From the time they are eggs until they undergo metamorphosis they live embedded in the body of their parents This is the case of Ctenodiscus australis, which can carry up to 73 larvae on the surface of their body.[2] When they finally become independent, they are already juvenile starfish as we know them.
In other rarer cases, the larvae develop inside the mother's gonads and hatch as juveniles. This is the case of Patiriella vivipara, a hermaphroditic and viviparous starfish, whose young are born with 20-30% of the size of the adult. [3] So while most species have a lot in common, the answer to how starfish are born depends a lot on the species.
How are starfish born asexually?
In addition to reproducing sexually, starfish can make copies of themselves, that is, they also reproduce asexually. They carry it out through a process known as fission or fragmentation, which consists of dividing its central disc into several parts, taking with it the associated legs.
The central disk is made up of five equal portions, as if it were a pizza. From a single serving they can regenerate an entire body, including the missing legs. Normally, they split into two parts, and a single starfish gives rise to two stars. They both have identical genetic material and therefore are the same individual, so technically it's not a birth.
