How does a starfish breathe? In this lesson, we’ll learn about the structure and function of the echinoderm respiratory system, along with some variations between different echinoderms.
How Does A Starfish Breathe?
When you picture a starfish, crawling slowly across the ocean floor, it may not occur to you that these little animals breathe. Because they don’t have a respiratory system like yours or mine. There are no lungs, there’s no windpipe, and there’s certainly no chest rising and falling with the flow of breath. Animals in the Phylum Echinodermata, the echinoderms, include starfish (or sea stars), brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies.
They are an ancient group with strange, five-pointed bodies and a kinship with our own ancestors. They are not-too-distant cousins of ours. Let’s take a look at the structure and function of the respiratory system in Phylum Echinodermata, and figure out how these fascinating animals exchange gases with their environment.
Echinoderm Respiratory System Structure and Function
The structure of the echinoderm respiratory system is fairly simple, across the group, though each class of animals within the phylum has its own specific respiratory adaptations. In general, echinoderms typically respire by simple diffusion, using gills or specialized projections, like tube feet or pockets, to circulate water and oxygen through their bodies. Many echinoderms also use a simple hemal system, a series of pockets and tubes that serves almost like a net of veins and arteries. Now let’s examine the specialized systems of different types of echinoderms.
Sea stars have simple gills called dermal branchiae, really just tubular projections in the skin, which allow gas exchange to occur by simple diffusion. The internal, water-based fluid of the sea star, or starfish, circulates through the hollow body core.
This space is known as the coelom, from the Greek word for ‘hollow’. As water flows through the coelom, it circulates through these tubular skin gills, allowing oxygen to be taken in and carbon dioxide waste to be expelled directly from the tissues.
Brittle Stars, Sea Urchins & Sand Dollars
Brittle stars, relatives of sea stars, exchange gases in the base of each arm, at special pockets known as bursae. The bursae pockets are lined with tiny waving cilia, projections that help circulate water for gas exchange.Sea urchins and sand dollars have a small set of gills in the tissues around the mouth, known as peristomial gills. These gills function in much the same way as the dermal branchiae in sea stars.
The strange respiratory trees that branch from the body of sea cucumbers bear special mention. The sea cucumbers, among all the echinoderms, have what most closely resembles a true respiratory organ. This beautiful respiratory tree branches from the cloaca, or anal opening, at the anus of the animal. Sea cucumbers breathe by inhaling and exhaling water from their anus.
Like we said – it’s memorable! The respiratory tree is filled with tiny tubules that exchange gases and excrete waste.
Echinoderms have simple respiratory systems, yet the diversity of simple respiratory methods within this phylum is remarkable. Although the circulatory system is nothing like our own, the hemal system and water-based circulation in echinoderms allow direct gas exchange with tissues.From the simple dermal branchiae skin gills of sea stars and peristomial gills of sea urchins, to the bursae of brittle stars and anal respiratory trees of sea cucumbers, this group of related radial animals has developed several ways to take in the oxygen they need and to expel carbon dioxide waste.
In a marine environment, this adaptability has been essential to the survival of this group through time.