About the Respiratory System of Starfish
Respiration is the process of breathing in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.
Starfish can only breathe in water and not outside of water, meaning that they are strictly water breathing aquatic animals.
Starfish have one of the simplest type of respiratory system amongst the higher invertebrates. Their respiratory system is very poorly developed.
Starfish belonging to the Phylum Echinodermata that includes some of the most beautiful and most familiar sea creatures.
Echinoderms normally use their respiratory organs like dermal branchiae (papulae), tube feet, respiratory tree, and bursae.
Echinoderms like Starfish only make use of their dermal branchiae and tube feet to perform their respiration.
Respiration is accomplished mainly by their numerous papulae present on the aboral surface.
These papulae are also known as the dermal gills or skin gills, that help to exchange gases (oxygen and carbon dioxide) and get rid of nitrogen.
These papulae are thin-walled, contractile outgrowths of the skin with their cavities continuing with the body coelom.
They can also exchange gases using their tube feet. The tube feet (more technically called podia) are small active tubular projections on the oral face of an echinoderm, also present under the arms of a starfish.
How do Starfish breathe? How respiration occurs in Starfish?
It is to be noted that starfish can’t survive out of the water as they can’t breathe. They will soon die if exposed to open air.
These are water breathing and can use their dermal gills present on the upper surface (aboral surface) of the body to breathe.
Their breathing mechanisms have a lot to explain why they can’t survive for a much longer time when out of water.
They use their Papulae (singular: papula), also known as dermal branchiae or skin gills, that are projections of the coelom to serve in respiration and waste removal.
Oxygen dissolved in the surrounding water diffuses into their papulae and carbon dioxide diffuses out through the papulae. That’s how they exchange the water dissolved gases inside the sea through their body.
The thin membranous tissue of the surface of the Papulae easily allows the respiratory gases to readily diffuse directly down their gradients.
It is also to be noted that Starfish also have a spacious coelom (an open, fluid-filled body cavity lined with tissue). The cavities of the papulae are continuous with the coelom.
Therefore a respiratory water current is set up during the continuous exchange of gases through the papulae to pass over the papulae by cilia lining their outer surface towards the coelomic cavity.
The cilia lining the coelomic cavities, cause the coelomic fluid to flow into them. As a result, the oxygen is provided to the coelomic fluid, and carbon dioxide is taken away to expel outside of the body.
So, that’s why it’s said that in starfish gaseous exchange takes place across branched projections of the body wall connected at the base to the coelomic cavity.
The coelomic fluid helps transports oxygen to the other parts and takes out the carbon dioxide while delivering oxygen by simple diffusion.
This gaseous exchange is done by the coelomic fluid because most echinoderms lack a distinct circulatory system.
Some gaseous exchange also takes place through the thin-walled tube feet. The oxygen travels through the tube feet into the water vascular system and carbon oxide gets exhaled out all by simple diffusion method.
In simple words, in starfish at least about 20% of the job of getting oxygen into the body and removing waste gases such as carbon dioxide out, is carried out by the tube-feet.
What type of respiratory system does Starfish have?
Starfish shows the presence of the open respiratory system because the oxygen that enters through the papulae and tube feet moves freely throughout the body and get taken up in the required cells by simple diffusion where there is a lack of oxygen.
In simple words, the starfish has an open respiratory system because the oxygen is allowed to move freely in the water vascular system.
And, also the carbon dioxide that is released from the cells when taking in oxygen is also allowed to move freely and get exhaled out of the body either via the tube feet or via the papulae.
As said, in the open circulatory system, the gases roam freely in the water vascular system. This doesn’t mean that the gases won’t roam freely in the other body cavities.
The concept is that part of the body cavity or coelom is the water-vascular system, consisting of fluid-filled vessels that are pushed out from the body surface as tube feet, papulae, and other structures that are used in locomotion, feeding, respiration, and sensory perception.
Meaning that the water vascular system is a type of coelomic cavity that is well connected with the tube feet, and so it’s not a different thing.
Now for better understanding, it is also to be noted that starfish don’t have a closed respiratory system because the gases (oxygen and carbon dioxide) don’t roam within only a common respiratory organ system and are not confined within a few organs only, as starfish don’t have any such organs.
So, starfish have an open respiratory system only, where the gases roam freely in the coelomic body cavity.
What is the function of the Water Vascular System during respiration?
The most peculiar and interesting role of the water vascular system of starfish is in bringing about locomotion through the tube feet by providing a hydraulic pressure mechanism.
The water vascular system is simply the body coelomic cavity with a network of water vessels in the body of the starfish that is composed of canals connecting numerous tube feet.
Starfish move by alternately contracting muscles that force water into the tube feet, causing them to extend and push against the ground, then relaxing to allow the feet to retract.
Such locomotion of the tube feet happens because of the water vascular system that circulates water through the canals carrying water to the ampullae and providing suction to the tube feet. This suction creates hydraulic pressure for the locomotory movement using tube feet.
Now as we have learned that the tube feet due to its thin wall may also serve for the respiratory exchange of gases.
That is, the dissolved oxygen in the water can be easily taken by the tube feet through a simple diffusion method, and that oxygen enters the open canal system of the water vascular system.
Thus, leading to the exchange of gases freely between the cells within the canals of the water vascular system connected to the tube feet.
So, that’s why it is also said that the starfish has a water vascular system in which the exchange between oxygen and carbon dioxide occurs, and also that the tube feet of the starfish acting like secondary gills.
What is the structure and function of the skin gills or papulae in a starfish?
Papulae or the skin gills are present on the aboral surface of the starfish that is on the dorsal surface of the starfish.
These papulae are very soft tissue patches, covered externally with the epidermis, and lined internally with peritoneum.
These are thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. In other words, the cavities of the papulae are continuous with the coelomic cavities.
The structures of the papulae and their cavities are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices.
This arrangement enables both easy flexions of the arms by the starfish and the rapid onset of stiffness and rigidity required for respiratory actions performed under stress.
The ultrastructural anatomy of the papulae shows an outer ectodermal epithelium and gland cells with proper connective tissue bundles maintaining the structure of the papulae underneath the epithelium.
The coelomic cavity of the papulae is lined with ciliated endothelial cells, which overlie an irregular layer of circular muscles.
A system of canals that are not lined by cells occurs at the base of the papulae with the circular muscles.
The papulae due to its thin-walled outer layer help in the proper diffusion and entry of dissolved oxygen from the water into the body cavities.
Thus, the papulae help in the exchange of gases in and out of the body properly by simple diffusion method as the coelomic body cavities are well-connected to these papulae.
Thus, helping to conduct proper respiration in starfish.