Range: Worldwide in aquatic habitats

Habitat: Fresh, brackish, and marine water (Tend to be benthic)

Size: Most are smaller than 1 millimeter

Diet: Bacteria, algae, detritus, and more

Threats: Cnidarians, flatworms, midges, amebas, and more

Lifespan: Less than a month

Transcript:

Today on Animal Fact Files we’re discussing hairybacks. Although you've probably never seen a hairyback, or maybe even heard of them, these microscopic animals live in all sorts of water sources. From lakes and rivers to sandy beaches and the bottom of the ocean, hairybacks can be found around the world, you just have to know where to look– and probably have a microscope because they're tiny. Hairybacks are classified in a phylum all their own, and their relationship to other animals remains somewhat of a mystery. Some scientists believe they're most closely related to nematodes while others argue they're more like rotifers, however, hairybacks aren't really closely related to either of these groups. There are over 800 classified hairyback species, but estimates suggest hundreds to thousands more undescribed species living the world over.

Hairybacks are teeny. Even the largest individuals top the charts at 3 millimeters in length, and most species are shorter than a millimeter! It's almost imperative to have a microscope if you want to see one for yourself. These small organisms are bowling pin shaped with a head and neck that attach to a round body. Their head sports whisker-like filaments that help them sense their surroundings, much like a cat's whiskers! At their rear, hairybacks have tubes that produce an adhesive that help anchor them to the substrate while they eat. Some even use these tubes to travel in a leech-like motion. While a majority of the group have these tubes, there are some species without them. These unique individuals often sport long tails and they spend their time swimming in open water as opposed to crawling along the water's bottom. 

They get the name hairyback because of hair-like structures present on their body which helps them move around. Oddly enough, however, these hair-like appendages are most dense on the hairyback's belly– not their back! It's for this reason they're sometimes listed as hairy bellies or hairy-bellied worms, which feels a far more appropriate common name! What do you think?

Hairybacks are so small they actually fit in between grains of sand and soil. They're described as solitary animals, but they can live in dense populations. In just a handful of water there can be hundreds if not thousands of individual hairybacks. In fact, they make up some of the most populous animals in the places they inhabit, sometimes beating out nematodes as the most abundant animals! While shifting through grains of sand, hairybacks search for food. They eat bacteria, algae, detritus, protozoans, and other microscopic organisms in the water column. In turn, hairybacks are eaten by flatworms, amebas, midges, and cnidarians such as comb jellyfish.

Hairybacks were likely some of the first animals studied under microscopes, however, there's still a lot we don't fully understand about them. They have a fully unique reproductive cycle in which females produce eggs through parthenogenesis. Parthenogenesis is the production of viable offspring without fertilization. These babies are also female and are genetically exactly the same as their mother. Hairybacks don't have a tube for pushing eggs out of the body, so they just burst through the mother's body wall. But don't worry this doesn't kill her! She'll lay up to four eggs before undergoing a transformation that makes the animal hermaphroditic. At this point, hairybacks are able to cross fertilize with other hermaphroditic adults and produce offspring sexually, though the exact process of how this happens is still a mystery.

Something worth noting about the four parthenogenic eggs produced is that three are quick developing, hatching within a day and already equipped with their own eggs to lay the following day. The final egg, however, is different. It has thicker walls and can withstand freezing and drying out. It's thought these eggs are laid as a backup measure in the event that something happens to the environment and the species needs to go dormant until conditions improve. It's also likely a way these animals move to new habitats with the more resilient eggs drifting to new areas before hatching. That being said, the entire life cycle of hairybacks occurs in less than a month!

For more facts on hairybacks, check out the links below. Give a thumbs up if you learned something new today. Thank you to our Patrons SpikeSpiegel93, Dad, and everyone else for their support of this channel! And thank you for watching Animal Fact Files!