Zooplankton are heterotrophic invertebrate organisms found drifting in both fresh and marine water bodies. A group of gelatinous planktonic organisms have recently been discovered swimming by coordinating multiple water jets from separate but genetically identical units in a colony of Nanomia bijuga. Jet propulsion is common in marine animals, such as squid and octopi. However, the coordination of multiple jets moving a whole colony of organisms is rare.
The species Nanomia bijuga has a large geographic distribution and is seasonally numerous in the spring and summer. They are highly skilled swimmers and capable of altering their direction as well as making complex turns and reversing the direction of the whole colony. N. bijuga is a species of colonial organisms called physonect siphonophore that are related to jellyfish, sea anemones and corals in the phyla Cnidaria. A physonect siphonophore's colony arranges itself around a long stem, comes to the ocean surface to feed at night and then returns to the aphotic zone during the day to avoid predators.
"The coordination of multiple jets moving a whole colony of organisms is rare."
Water propulsion system
The members of N. bijuga colony that produce the water propulsion are called nectophores. Nectophores are genetically identical clones produced by asexual reproduction that are arranged in a propulsive unit called a nectosome. The nectosome is so strong that it is capable of towing its reproductive feeding units over distances that can reach up to 200 meters a day, which is the equivalent of a human body running a marathon while pulling the weight of their body mass.
Each member of the colony is a valuable asset to the whole. The older members of the colony can move large amounts of water and are found in the middle of the nectosome. The younger members move much less water and are found at the anterior of the nectosome, far from where the nectosome connects to the rest of the colony. The young N. bijuga have the ability to turn using small bursts. These small bursts of water allow the entire colony to quickly and effectively move the colony to catch prey or escape from predators.
"The young members have what we call a long lever arm; they are like the handle of a door. If you push on a door near its hinges—its axis of rotation—the door is hard to open. But if you push on the door handle, which is far from the axis of rotation, the door opens easily. A little force placed with a big lever arm has a big effect on turning." says lead author John H. Costello of Providence College, an adjunct scientist and Whitman Center investigator at the Marine Biological Laboratory.
When new nectophores are produced at the tip of the nectophore in a process called budding (a type of asexual reproduction) the older members of N. bijuga move to the middle of the nectosome. This continuous pattern allows all members of the colony to contribute to the propulsion and maneuvering technics that allow the species to be successful in their environment. "It's a quite sophisticated design, for what would seem like a simple arrangement," Costello said. Below is a YouTube video from the MBL that shows multi-jet propulsion in a colonial siphonophore:
The mechanics of water propulsion
The locomotive methods performed by N. bijuga are different than the current designs of underwater distributed-propulsion vehicles. Many engineers are now looking at the propulsion technics incorporated by this species in the hopes to make changes to enhance their designs. Currently human engineered multi-jet systems use thrust vectoring technics which allow the vehicle to manipulate the direction of the thrust from its engine or motor to control the angle of speed. Although N. bijuga are capable of altering jet orientation during forward and reverse movement, they ultimately do not vary direction orientation under normal observation.
Thrust vectoring is not a technique that N. bijuga use often. Instead, they have developed different locations for propulsion position and performance which works effectively when the colony coordinates their actions. Engineers are now looking at this technic of location propulsion for modern designs including underwater vehicles and rockets. This will depend on an increased understanding of propulsion coordination and opens the potential for biologically inspired solutions to contribute to human-engineered designs.
The evolutionary design of N. bijuga has had hundreds of thousands of generations to make changes and improve upon locomotive technics. As humans, it is important to remember that other species may have better designs for locomotion. Taxonomy laboratories are used to maintain records of the different species living in the biosphere. It is important for engineers to look at the biological examples around them that have been successful for hundreds of millions of years.