Fairy flies, known as Mymaridae, are the tiniest organisms with multi-chambered hearts.
Yesterday evening, while scrolling through my YouTube feed, I stumbled upon a fascinating video. It featured the Etruscan shrew, the smallest mammal on the planet in terms of weight, and its heart was simply astonishing, beating at an incredible rate of 1500 BPM! This made me wonder about the smallest organism known to have a multi-chambered heart.
Although the Etruscan shrew holds the title for the smallest mammal, it is by no means the tiniest organism. As I soon found out, answering this question is much more complex than I initially thought. To find the answer, we need to delve into the various circulatory systems that exist in the animal kingdom.
An Etruscan Shrew on a hand for scale (Photo Credit : Trebol-a/Wikimedia Commons)
Understanding Circulatory Systems
Organisms with hearts do not always possess circulatory systems that function like those of mammals, amphibians, or fish. In most multicellular organisms, the circulatory system serves as a means of transporting blood, hormones, nutrients, and so on.
It is worth noting that not all circulatory systems are responsible for oxygen transfer. For instance, in insects, oxygen is not transported by the hemolymph (the arthropod equivalent of blood, distinct from hemoglobin), but rather by the tracheal system (respiratory system).
Hemoglobin transferring oxygen (Photo Credit : ShadeDesign/Shutterstock)
Circulatory systems can be categorized into two types: open and closed. Mammals, amphibians, reptiles, and birds all have closed circulatory systems, whereas arthropods have an open circulatory system. These two systems differ in terms of how blood circulates within the organism’s body and the functions carried out by the blood.
Closed Circulatory Systems
In a closed circulatory system, similar to our own, blood is contained within veins and arteries, and the heart is a muscular organ. Mammals, birds, and crocodiles possess four chambers (2 atria and 2 ventricles) in their hearts. Fish have two chambers (an atrium and a ventricle), while amphibians and reptiles, excluding crocodiles, have three chambers (2 atria and a ventricle) (Source).
Here, the atrium is the upper chamber where blood is pumped in, and the ventricle is the lower chamber from which blood is pumped out. Once deoxygenated blood enters the heart, it is sent to the pulmonary circuit (lungs or gills). At that point, the blood becomes oxygenated and is then pumped back into the heart to be distributed throughout the body.
Flow of blood in a human heart (Photo Credit : cono0430/Shutterstock)
Open Circulatory Systems
In contrast, in an open circulatory system, the blood is not contained within blood vessels; instead, the blood (referred to as hemolymph in this case) flows freely within the body cavities and directly interacts with the organs and tissues of the organism. This means that the hearts of arthropods are not very similar to the hearts in closed circulatory systems.
In arthropods, the organ responsible for maintaining the flow of hemolymph within the body is called the dorsal vessel, which is commonly referred to as the heart. It extends from the abdomen to the head and is divided into multiple chambers by valves called ostia. These valves ensure that blood flows in only one direction. The contraction of the vessel is caused by the alary muscles, which are attached to each chamber laterally.
Circulatory system in an arthropod (Photo Credit : CNX OpenStax/Wikimedia Commons)
So, which organism has the smallest multi-chambered heart?
The answer lies in a tiny family of chalcid wasps known as Mymaridae, which are often called ‘fairyflies’ due to their wing shape resembling fairies, as well as their minuscule size. These wasps range in size from 0.2mm to 1.8mm (Source), making them almost invisible to the naked eye.
A microscope is necessary to observe them properly! These insects are so small that moving through the air is like moving through thick liquid for them, which is why they have wings fringed with hair that help them navigate through the atmosphere. The Mymaridae family consists of 103 genera and over 1400 species! (Source) They can be found worldwide in both temperate and tropical climates.
a specimen of a Richteria ara female measuring 1mm in size (Photo Credit : Huber, J.T./Wikimedia Commons)
The Mymaridae are also parasitic. They lay their eggs in the same location as the eggs of other insects. When the larvae hatch, they use the eggs of the other insects as a food source. As a result, they have been utilized in biological pest control programs and have shown significant positive outcomes. Notably, they have been effective in controlling pests in Eucalyptus trees in New Zealand, South Africa, South America, and Southern Europe.
Due to their small size, many Mymaridae do not develop hearts or tracheal systems during their larval stage since everything they need for growth is readily available where they hatch. Even as adults, their hearts are diminished due to their microscopic size. These unique characteristics enable life as a microscopic organism.
It is evident that this family of chalcid wasps is truly captivating and serves as a testament to how life always finds a way to adapt and thrive.