Most fish possess a single heart, typically a two-chambered organ designed to efficiently circulate blood throughout their bodies. This heart acts as a central pump, driving blood through the gills for oxygenation before it travels to the rest of the body. While this single-pump system is effective for the majority of aquatic vertebrates, some creatures in the marine environment have evolved a more complex arrangement.
The Fish with Multiple Pumping Structures
The hagfish, an ancient and unique marine animal, challenges the conventional understanding of fish anatomy by possessing multiple pumping structures that function as hearts. These creatures are jawless, eel-like fish known for their ability to produce copious amounts of protective slime when threatened. Hagfish typically inhabit deep-sea environments, where they are scavengers, feeding on carcasses that sink to the ocean floor. Their elongated, scaleless bodies can range significantly in size, with some species growing over a meter long. While they possess a primary systemic heart, hagfish also utilize several additional accessory pumps distributed throughout their bodies.
Beyond a Single Heart
The circulatory system of a hagfish includes a systemic heart, located near the gills, which serves as the main pump. This primary heart is three-chambered, consisting of a sinus venosus, an atrium, and a ventricle. Beyond this main organ, hagfish have evolved specialized accessory hearts to aid circulation in their low-pressure vascular system. One such pump is the portal heart, positioned on the hepatic portal vein, which handles blood flow to the liver.
In addition to the portal heart, hagfish possess a pair of caudal hearts located in their tail. These caudal hearts are distinct because they are not self-contractile but are driven by surrounding skeletal muscle contractions. Cardinal hearts also help propel blood from extensive head sinuses.
How These Hearts Function
The multiple hearts of the hagfish are integral to its survival in environments characterized by low oxygen levels and significant pressure. Hagfish have one of the lowest blood pressures among vertebrates, and their circulatory system includes extensive blood sinuses where blood can collect. The accessory hearts play a role in maintaining blood flow through these widespread sinuses and overcoming the challenges posed by their low-pressure system. For instance, the portal heart ensures that blood from the digestive system is moved to the liver for processing.
The caudal hearts in the tail contribute to returning blood from the posterior parts of the body back towards the main circulation. This multi-pump system provides a degree of redundancy and flexibility, which is particularly beneficial in an environment where oxygen availability can be unpredictable. Hagfish hearts can continue to beat for extended periods, even in the complete absence of oxygen. This anoxia tolerance is attributed partly to their unique metabolic capabilities, allowing them to utilize alternative fuel sources when oxygen is scarce.