Jet propulsion in not new to marine animals; it is commonly see in jellyfish and squids. A new study found that a jellyfish-like species called Nanomia bijuga has taken jet propulsion to the next level by integrating multiple jets to form one synchronized swimming team.
What is multi-jet swimming?
Nanomia bijuga belongs to a group of physonect siphonophores that exhibit one of the most sophisticated colonial behaviors. Related to jellyfish and corals, physonects string together small individual family members to form an organized multi-jet propulsion system, wherein each member chips in and works in tandem with others to propel the whole unit forward. The jet-producing members of the physonects are called the nectophores. These are genetically identical clones that form a unit called the nectosome. During whole-colony swimming, the nectosome pulls along with it a group of other family members⎯called the siphosome⎯ that are in the feeding and reproductive stages (watch the video here).
These jellyfish-like physonects can run marathons
Nanomia bijuga is the most widely known physonect; it is a voracious predator in planktonic communities. It is a deft swimmer performing complex maneuvers and is capable to reversing the direction of the entire swimming colony. The organization of the swimming unit may look simple, but together, they are capable of migrating hundreds of meters daily between the deeper layers of the ocean during daytime and near-surface at night. This is equivalent to a human running a marathon daily while lugging around a weight equal or greater than his or her mass.
Division of labor among physonects to perform multi-jet swimming
The researchers filmed the swimming behavior of N.bijuga species and analyzed the images for their body size and motion, and then calculated their jet velocities based on the movement of the particles surrounding them. They found that the multi-jet propulsion was a result of ingenious division of labor among family members. Physonects at all developmental stages⎯even the smallest members⎯have a crucial role to play in the concerted movement. The smallest members have the lowest jet force and are key for turning, whereas the largest members are useful for forward and backward movement as they have powerful jets. The small members are strategically placed at the apex of the nectosome, far away from the point where the nectosome connects the siphosome. This ensures that even a small force can effectively steer the entire team, very much like a doorknob steering a door because it is far away from the hinges.
Physonects have thus evolved to elicit complex multi-jet propulsion maneuvers by recruiting the entire colony. This form of colonial swimming has helped them migrate great distances and survive predation. Furthermore, it could also inspire the future design of underwater distributed-propulsion vehicles.
Featured image courtesy of MMC-News.
