Echinoderms are some of the most beautiful marine creatures in the animal kingdom that includes a list of more than 7000 species known so far. They fall under the Phylum Echinodermata of the Animal Kingdom.
These sea creatures can be mostly identified due to the presence of prickly spines made up of calcium carbonate possessed by their test or skin.
They look like they have a rough hard exterior. And it is also to be noted that these spiny-skinned creatures exhibit an organ system level of body organization.
Some of the famous examples of Echinoderms include sea stars (also called starfish), sea urchins, brittle stars, sea cucumbers, sand dollars, and crinoids.
Here, in this post, we will learn dedicatedly about the breathing and respiration process of Echinoderms. So, let’s get started.
How do Echinoderms breathe and respire?
First of all, let’s define respiration in accordance to breathing.
Both respiration and breathing are interrelated to each other. Breathing is the process of inhaling and exhaling gases between cells and the external environment, while respiration is that particular biochemical process that helps the cell to produce energy from the inhaled oxygen.
So, Respiration can be defined as the biological process of breathing through which living organisms produce energy for themselves, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.
Echinoderms typically breathe and respire by the simple diffusion of gases like oxygen and carbon dioxide in and out of their body cell membranes. They use the various respiratory organs that include branchiae, tube feet, respiratory tree, and bursae for breathing and respiration.
The oxygen dissolved in the surrounding water diffuses into their respiratory organs’ cells and carbon dioxide diffuses out through these organs’ cells very easily.
You can simply say that they use simple gills and their tube feet for the intake of oxygen and the release of carbon dioxide.
Now, it is to be cleared that not all species of echinoderms will have all of the respiratory organs mentioned above.
Each type of respiratory organ of the echinoderms works a little differently, with the primary function being to aid in breathing and respiration.
Just like Echinoderms like Starfish only make use of their dermal branchiae and tube feet to perform respiration. While Echinoderms like Sea cucumbers use a pair of respiratory trees to breathe through their anuses.
It is to be noted that echinoderms do not have a very well-developed respiratory system. But, the species of Sea Cucumber have a somewhat well-developed respiratory system.
In fact, all of the echinoderms do have one of the simplest types of respiratory system amongst the higher invertebrates.
All echinoderms are characterized by the presence water vascular system which is their distinguishing feature. The water vascular system also allows for gas exchange for respiration.
This system exhibits radial symmetry from the beginning and is equally developed in all echinoderms.
Let’s Know How These Echinoderms Breathe
Respiration in Starfish
They use their Papulae (singular: papula), also known as dermal branchiae or skin gills, which are projections of the coelom to serve in respiration and waste removal.
They exchange the water dissolved gases present in the surrounding water by the simple diffusion method.
Oxygen dissolved in the surrounding water diffuses into their papulae and carbon dioxide diffuses out through the papulae.
They also have their water vascular system through which water enters their body. 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.
Almost 20% of the total 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.
Respiration in Sea Urchins
Sea Urchins make use of their external gills as well as tube feet for respiration. They mainly use their 5 pairs of external gills for respiration.
Their 5 pairs of external gills are present around the mouth attached to the peristomial membrane. That’s why these gills are also called peristomial gills.
They use their gills and tube feet to exchange gases i.e oxygen in and carbon dioxide out of the body using the simple diffusion method across the concentration gradient.
They also have a good water vascular system ending in the tubed feet. The water that enters the body through the water vascular system helps in the circulation of oxygen to all body cells and intaking out the unneeded carbon dioxide from the cells.
Respiration in Brittle Stars
Brittle stars use their respiratory organs named bursae (singular: bursa) for the exchange of gases between the external water environment and the body cells all by simple diffusion method.
These bursae are the cilia-lined sacs. Each bursa is present on the bottom of the central body disk just opening between the arm bases.
Each of the bursae fits between the two stomach digestive pouches. Typically 10 bursae or you can say 5 pairs of bursae are found in these species.
The water with dissolved oxygen in it gets inside these sacs and flows through the bursae by means of cilia or muscular contraction where the gaseous exchange takes place.
Oxygen gets transported throughout the body by means of the hemal system which is composed of a series of sinuses and vessels.
The water vascular system also contributes to the oxygen transportation mechanism but its contribution is far less as compared to the hemal system.
Respiration in Sea Cucumbers
Sea cucumbers use their respiratory organs called respiratory trees present in their anus for breathing. They use their anus both for breathing and excreting.
Respiratory trees are situated at both sides of the digestive tract and is actually composed of a series of narrow tubules branching from a common duct. The gaseous exchange usually occurs across the thin walls of the tubules.
Their respiratory trees branch in the cloaca just inside the anus, where they perform the exchange of gases by simple diffusion method.
They intake the oxygenated water through their anus and then expel the carbon dioxide dissolved water through it.
They by contracting and expanding their bodies, drive water through the anal opening and then into the respiratory trees.
Respiration in Sand Dollars
Amongst the echinoderms, sand dollars have the most simple respiratory system, and also they are very simple animals. Their body looks like petal-shaped.
The upper side (dorsal part) of the sand dollar is often termed as petaloid, because of its petal-like appearance.
This petaloid is characterized by the presence of a series of tiny holes through which the tube feet emerge. The sand dollars breathe through these holes.
Sand dollars usually have five petaloids (also called ambulacra). Each of the petaloids is equipped with two rows of respiratory podia (also called tube feet).
It is to be noted that sea dollars don’t use their tube feet for locomotion but, they do use their tube feet for breathing only. Their tube feet are short and flattened structurally.
Here, in the case of sea dollars, the water vascular system is the only way to aid in breathing and respiration. So, in the process, seawater gets pumped into their “tube-like” feet, and as a result, water enters the canal system of the body through the tube feet.
During the time of the entry of water through the tube feet to the canals, the oxygen from the surrounding seawater is absorbed through the walls of the canals, rather than through the gills by the simple diffusion process.
Respiration in Crinoids
Crinoids also use their tube feet to breathe. Their tube feet are very thin-walled that easily allows the exchange of gases between the seawater and the body.
As the seawater enters their body through a large number of pores reaching the coelom of the animal, gas exchange takes place all over the large surface area of the arms by simple diffusion method all through the thin lining of the coelomic cavity.
Also, gaseous exchange takes place by diffusion through the tube feet and this aids in bringing oxygen to the coelomic fluid of the water vascular system and to the cells.
Crinoids are also classified by the presence of a haemal system which is composed of a network of fluid-filled sinuses. These fluid-filled sinuses also act as respiration and excretory system.