Our kidneys are amazing organs that help regulate our bodies’ water and salt balance and excrete nitrogenous waste that would otherwise be toxic to us; they are our own personal clean-up crew. World Kidney Day (today!) aims to raise awareness of the importance of these organs in our health. Here at No Bones, we like to remind you that if a vertebrate can do something, chances are that invertebrates accomplish the same task in a myriad of different ways.
So, here is a bit of information on invertebrate osmoregulatory and excretion systems, which are just as important for those organisms! Overall, such systems, generally termed kidneys in vertebrates and nephridia or protonephridia in different invertebrate groups, are made up of tubular structures that employ osmoregulation to change the concentration of the solution (water, nitrogenous waste, and other solutes) that will be excreted from the body. Let’s take a look at some examples of metabolic waste excretion and osmoregulation systems in a few different taxa.
Sponges (Porifera) have a porous body made up of two cell layers surrounding the mesohyl (a more jelly-like layer), a convoluted body cavity called the spongocoel, and one or more openings from the spongocoel to the environment called oscula (singular is osculum). Sponges are filter-feeders and have flagellated cells on the inside called choanocytes that beat their flagella to generate the flow of water from the environment into the spongocoel through channels called ostia, as nutrients are filtered by microvillar collars on the choanocytes prior to the water traveling into the spongocoel, and out the osculum. Removal of metabolic waste happens through diffusion out of cells. This relatively simple mode of waste excretion is possible given that sponges only have 2 cell layers and a large surface area (they are covered in pores), so their cells are so close to the environment that waste does not need to travel far to be removed from the body.
Similarly, cnidarians do not have specialized organs, like our kidneys, for waste excretion. This is possible because like sponges, the vast majority of their cells are in contact with the environment. The general body plan of these organisms includes three layers: the epidermis (the outer layer of cells), the mesoglea (a soft jelly-like material sometimes containing cells), and the gastrodermis (a second layer of cells, lining the inside of the gastrovascular cavity that is open to the environment). So cnidarians can also simply diffuse their metabolic wastes out.
Flatworms (Platyhelminthes) possess a specialized system for waste excretion, albeit not a kidney. They have what are termed “flame cells” (or “protonephridia”) located all along two highly-branched tubules that run the length of the body. Each flame cell contains a group of cilia that look like a flame, hence the name of the structure, which serve to propel waste out from the tubules into the environment, through pores in the body surface called nephridiopores [1]. Other small organisms in the phyla Rotifera and Nemertea also use these specialized cells to filter and remove waste.
Segmented worms (Annelida) can have either protonephridia or another specialized system, called metanephridia. Metanephridia are tubes that are organized into pairs-- two nephridia are present in each segment of the worm. They have an opening with cilia, called the nephrostome, into the coelom (the fluid-filled body cavity) of the previous segment, where waste molecules that have filtered out of blood capillaries are taken in along with water and other small molecules. These molecules travel down the tubule of the metanephridium into the next segment, where further filtration occurs and any useful molecules are reabsorbed back into the capillaries before the rest is dumped out of the body through nephridiopores leading out of the body [2].
Crustaceans (Arthropoda) possess a pair of green glands (they are indeed green), also known as antennal glands or maxillary glands depending on where they are located, at the base of their antennae that serve as their osmoregulatory organs, while nitrogenous waste is excreted through the gills. Blood in the open circulatory system of crustaceans filters into one part of the green gland, called the sac, where it then moves down a convoluted tubule where further filtration and reabsorption occurs to save important molecules, and the final fluid ends up at the bladder. From there, it is finally excreted to the environment via pores [3].
So on World Kidney Day, let’s appreciate not only our own kidneys, but take on a worldly outlook and appreciate the diverse biology of our invertebrate cousins.
Learn more:
“About World Kidney Day.” World Kidney Day. International Society of Nephrology. 2015.
“Excretory Systems of Invertebrates.” Excretory System. University of California, Riverside.
“How do invertebrates excrete their wastes?” UCSB ScienceLine. UC Santa Barbara.
"Structure and development of the nephridia of Tomopteris helgolandica (Annelida)" by T. Bartolomaeus. 1997. In: Zoomorphology 117: 1–11.
[1] “Flatworms, Nematodes, and Arthropods.” Concepts of Biology. OpenStax College.
[2] Purves, William K., et al. Life: The Science of Biology: Plants and Animals. Vol. III. Google Books. Macmillan. 8 Dec. 2003.
[3] Alters, Sandra. Biology: Understanding Life. Third Edition. Google Books. Jones & Bartlett Learning. 2000.
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