Starfish (sea star) are invertebrate animals that live in the sea. This marine biota usually has a central disc and five arms that are similar to star shapes. But there are also some species that have more than one arm.
(Also read: Vertebrate and Invertebrate Animals – Differences – Characteristics & Examples)
At present, there are around 1,500 species of sea stars that live in the oceans all over the world. Starting from the tropics to the polar regions that have cold climates. Sea stars are found on intertidal to the depths of abyssal, about 6,000 meters below sea level.
Star Sea Taxonomy
Sea stars are animals of the class and genus Asteroidea which is under the phylum Echinodermata, a group of invertebrate animals that are characterized by bilateral symmetrical bodies and have a spiny skin. Phylum Echinoderms including the invertebrate sub-kingdom in kingdom Animalia. (Also read: 9 Characteristics of Kingdom Animalia in General)
In the taxonomic system, the Echinodermata classification consists of several classes. One of them is Asteroidea class. The Asteroidea genus consists of many species. The number reaches 1,500 species. But there are generally 7 starfish classifications. Examples of sea star species include Asteroidea Sp, Pisaster ochraceus, Stichaster Australis, Achantaster Placci and many more.
Sea Star Morphology
The easiest morphological features to see in a starfish are the arms that surround the central disc. The aboral or upper surface can be smooth or spiked, usually covered by a membrane called overlapping plates. Many species are brightly colored, usually dominated by orange and pink. Some species also have dark body colors such as brown, black and gray.
Under his arm, there is an ambulacral gap in the shape of a long groove that reaches the mouth of a starfish in the middle of the central disc. In this ambiguous gap, there is a tube leg that functions as a means of motion. At the end of his arm, there are also eyespots that are very sensitive to light.
Starfish Physiology
In addition to the characteristics that appear on the surface of the body, starfish have distinctive characteristics in the digestive system, respiratory system, and reproductive system.
Digestive system
The digestive system of starfish is characterized by the following organs:
- The mouth is in the middle of the central disc, close to the esophagus and associated with the stomach;
- Digested food in the large stomach and absorbed by gastric sap, then sent to five arms through the intestine;
- Remaining digestion through the anus.
Respiratory system
Like the characteristics of echinoderms in general, starfish breathe using leather gills. Namely the protrusion of thin walls of the body cavity (seldom). Seldom is protected by cilia and pedicellaria.
Transportation System
The circulatory system in these animals is difficult to observe. It usually consists of several blood vessels that surround the mouth and are connected to radial vessels to each arm.
Reproductive and developmental systems
Sea stars reproduce in two ways, namely asexually and sexually. The reproductive system is closely related to the complex stages of development in a life cycle.
Sea Star Life Cycle
Sea stars undergo a complex life cycle, young starfish must undergo several stages of development before settling on the sea as an adult starfish. Although starfish are much sought after as ornamental animals to fill the aquarium, the proliferation of starfish in the aquarium is very rare.
Every now and then, we can see the starfish in the aquarium releasing their eggs or sperm to the surface of the water. But fertilization has never really happened. So far we have only watched adult starfish, without ever knowing their life cycle.
Though these starfish undergo a very interesting life cycle. Here are the
four stages of life of sea stars namely gamete fertilization, the initial stage of development, metamorphosis towards adult individuals, asexual fertilization and hermaphrodite species.
Conception of Gametes
Sea stars do external fertilization through a process called spawning similar to those happens to frogs and fish. To produce eggs, female starfish and male starfish each released egg and sperm to the surface of the water. Some of these sperm and egg cells meet, then fertilization occurs.
Successful fertilization will develop into eggs that wrap around the zygote. At the beginning of its growth, the bodies of starfish formed curved with the center of their bodies lifted away from the core. We can see gametes that have grown into this zygote with the naked eye. It usually looks like a lump of milk or white clouds that gather on the surface of the water.
Some starfish species do spawning seasonally, even though their breeding may last throughout the season.
Early Stage of Development
Some sea star species show different development patterns but overall follow the same cycle. After the gamete fertilizes and forms a zygote, the zygote develops into a type of bilateral larvae called bipinnaria. These small larvae eat smaller microorganisms around them. Bipinnaria floats in water for several days to several weeks. Before they finally transformed into non-feeding organisms that settled on the seabed. The result of this bipinnaria change is called brachiolaria.
Metamorphosis Towards Adult Individuals
After Brachiolaria settled in suitable habitat, its body developed into a radial symmetry. And gradually began to morph into adult individuals. There are several studies that prove that brachiolaria is looking for areas that contain adult starfish pheromones to settle which means a sign for Brachiolaria that the place is in accordance with the habitat of his life. After their morphological and physiological development was perfect, starfish began to grow to the size of an adult starfish as we often see.
Also Read: the Process of Metamorphosis in Animals
Asexual reproduction
In addition to sexual reproduction carried out externally, the fertilization of the egg by sperm occurs outside the uterus. Sea stars also breed asexually. This usually occurs due to unfavorable conditions for sea stars. In contrast to sexual reproduction that is intentional.
Just like other animals, starfish are not at the top of the food chain. Sea stars also have natural predators that can threaten their survival. In some battles defending against predators, starfish may lose some of their “arms”. But amazingly, starfish can re-regenerate the lost limb. In some cases, the separated limbs can also regenerated the form of new starfish species. This is what is meant by asexual breeding, namely the reproductive process without involving the egg and sperm cells.
Hermaphrodite species
Some starfish species have male and female genitals in one body, called hermaphrodites. Animals like this are able to do their own fertilization. Hermaphrodite starfish usually incubate their “children” in the form of bipinnaria. This is because their habitat does not allow larvae originating from their eggs to float freely on the surface of the water.
Now you know how the life cycle of starfish looks like even you don’t see it by yourselves. Hopefully this helps you on your Biology class!