Jellyfish Physics, Facts, and Simulation Tricks
Let's talk about Jellyfish. One of the first sims I ever made in Houdini was a jellyfish, and fondly remember rigging it in 3Dsmax as well. But what would happen if you'd like to build a jellyfish around physical theories? Let's find out.
This article is dedicated to an awesome professor named Paul at Humber College. The 3D jellyfish battle we had will always be one of my favorite memories.
An Introduction to Jellyfish
Jellyfish are probably one of the most interesting species of life on Earth. The bell shaped jellies have been around for over 500-700 million years. This makes them one of the oldest lifeforms on Earth. Other sea life such as fish only started to evolve around 370 million years ago. They are also distantly related to corals and anemones. A jellyfish's body is 95% water, and is usually made up out of three parts. The umbrella, the oral arms, and the stinging tentacles.
There are more than 2,000 different types of jellyfish in the world. About 70 of these species are harmful to humans. However, there are some estimates that the number of jellyfish species could reach over 300,000.
Jellyfish usually hang out in warm coastal water. However a few have been observed at depths of 30,000 feet (9,000 meters). There are a few that prefer to live in subarctic waters.
Types of Jellyfish
Let's talk about some unique types of jellyfish.
The one of the smallest types of jellies in the world is The Creeping Jellyfish. (HERE) It reproduces asexually by splitting itself in half. It is about 0.5mm in diameter. It's cousin The Australian Irukandji, (HERE) is about the size of a fingernail, and one of the deadliest jellies known to mankind.
The one of largest species of jellyfish is currently the Nomura’s jellyfish (HERE). They are taller than the average human being, and were first cataloged in 1921. A few other species come close to matching it's size, or are larger are the Lion's Mane Jellyfish (HERE), and the giant jellyfish Stygiomedusa gigantea (HERE). The Lion's Mane jellyfish can have tentacles as long as 15 meters, and a bell as big as 1.8 meters. While Stygiomedusa gigantea is one of the most endangered types of jellyfish. There have only just been a bit more than 17 sightings of it in the past 110 years.
One of the most famous types of jellyfish is the Portuguese Man-of-War. (HERE) However this creature is a bit deceiving. It's not actually a jellyfish, and it is not a single organism. It is a colony of deep sea organisms that work together to catch food. it's proper classification is siphonophore.
Some other types of jellies you can look into are:
Crystal Jellyfish. (HERE)
Bloody belly Comb Jellyfish. (HERE)
Cauliflower Jellyfish. (HERE)
White-spotted Jellyfish. (HERE)
Black Sea Nettle Jellyfish. (HERE)
Fried Egg Jellyfish. (HERE)
Flower Hat Jellyfish. (HERE)
Atolla Jellyfish. (HERE)
Research Simulations, and Swarm Behavior
Swarm behavior is the collective motion of a large number of self-propelled entities. An argument can be made that jellyfish follow a pattern of swam behavior.
jellyfish can sense the ocean's current and swim either with it or against it. They have the ability to sense when they are drifting, and correct their balance. There have been several studies on tracking the movement of jellyfish, and how jellyfish blooms are created. Jellyfish blooms are massive patches of growths in jellyfish populations. They are happening with increased regularity due to global warming, and are causing chaos in the ecosystem.
Blooms may be also caused by water pollution, and carbon dioxide build up.
When jellyfish cluster into these blooms, they cause disruption in fisheries and create unsafe swimming conditions. Therefore, the need to track them has become important. Researchers from Swansea University and Deakin University in Warrnambool, Australia, have been already trying to solve this problem. As of 2015, they have been able to create a realistic simulation of the movement of jellyfish blooms in the oceans.
it is still a mystery on how jellyfish migrate across the ocean. There is some debate that they might be using the Earth's magnetic fields to travel. Or purely rely on ocean currents. We also don't fully understand how jellyfish use their tentacles to swim. We know that they are mainly used for food consumption, but other than that they remain rather mysterious.
As few studies have been done to try and understand the physics of their tentacles. most of these studies analyzed the forward swimming performance jellyfish tentacles have. As well as how their density, length, placement, and number affects swimming speed.
In this process, scientists have been able to reverse engineer the system of jellies with a lot of precision.
It is important that we continue to map swam behavior of deep sea creatures. Now that scientists are able to map this behavior on 3D software, the process has become much easier. The first computer simulations of swarm behavior were simulated in 1986 with the simulation program Boids. This program created a set of agents that followed a set of rules of motion. The program was first designed to mimic flocking behavior in birds. However, this system is still applicable to other animals.
Modeling the Locomotion of a Jellyfish
The movement and locomotion of a jellyfish is primarily based around it's hunting behavior. Some jellyfish are more aggressive than others when it comes to catching prey. The Box jellyfish for example, is one of the most toxic jellies in the entire world, and makes full use of it's venom. They have developed more steering abilities than the average jellyfish, and have clusters of 6 eyes scattered across their bell to see food. Other jellies are more opportunistic predators, and will not encourage prey to fall into their tentacles. They will instead just wait for a fish to make an unfortunate mistake. Some of these creatures rely on photosynthesis for food, such as the upside down jellyfish. So this creature's swimming speed can range from very still, to averagely fast.
Most jellyfish move horizontally with the direction of the ocean currents. However, depending on the Earth's daily tidal changes, they can also drift vertically as well. Jellies have also been observed propelling themselves up to the surface of the ocean with great speeds, and then suddenly ceasing movement and drifting back down again.
A jellies' speed is not based on its size. However, the speed of a jelly is also dependent on the stiffness of it's bell. The stiffer the bell is, the faster and stronger the jelly will be in the ocean current.
More Hydrodynamics of the Jellyfish
Now to talk about some weird and interesting facts about these creatures.
Jellyfish are one of the few creatures in the world that have the ability to regrow and regenerate parts of their bodies. The moon jelly, for example, has regenerated it's entire form in under a few days. It does this in a process called symmetrization. This process requires mechanical forces generated by the muscle-based propulsion. It also requires the animal to recognize it's existing limbs, and then based on their form, copy and paste them over missing ligaments.
The younger a jelly is, the faster it's healing process will be as well. The baby jellies in the family Aurelia Aurita can rearrange their existing limbs, rebuild their muscular networks, and reenter themselves from an injury. They can do this all under 12 hours. As stated before jellies are closely related to coral and anemones, which are also creatures that contain self healing abilities.
Turritopsis dohrnii is considered the immortal jellyfish. It can alter the age and stage growth of it's cells. If it needs to become a younger version of itself it can do so within a number of days. Or if it needs to be a full grown jelly, it can do this as well. This process can go on indefinitely.
Jellyfish are very much based around scale-dependent hydrodynamics. This means their speed is based around their body mass, and life history. They have evolved to be more energetically economical with their movement. So they can move quickly when they want to, but also do so in a way that will not exhaust their entire body.
Some jellies are also bioluminescent. Most jellyfish bioluminescence is used for defense against predators. Jellyfish such as comb jellies produce bright flashes to startle a predator. It is estimated that 50% of all jellyfish species are bioluminescent. Some siphonophores can release thousands of small glowing particles into the water to mimic the appearance of algae and attract fish to feed on. Others use their tentacles as glowing decoys.
Replicating The Creature in Visual Effects Software
Over the past couple of years there have been a few great attempts to replicate jellyfish in 3D space. Pretty much anyone nowadays can build a jellyfish in a 3D software, however if you go the extra mile you can build a physically based one.
At Carleton University, University of Saskatchewan, and the PhaseSpace Motion Capture Inc, a few researchers have been busy. Dave Rudolf and David Mould, from their Departments of Computer Science have presented an automatic animation system for jellyfish based on physical simulations.
As of 2009 they have successfully modeled the thrust of an adult jellyfish, and it's overall locomotion. They then applied this animation to a 3D geometry, and let their jellyfish do it's magic. They studied the real creatures to make sure their model had an acceptable amount of accuracy when it came to the creature's movements and abilities. They even added a Perlin noise function on their jelly's bell to simulate the movement of it's head. The only detail they left out of their rendered simulation was the bioluminescence of the creature.
Now, if you were to replicate this creature in Houdini, you could have several different ways of approaching it.
- You could build a model of the jelly. Add some animation/rig it. Separate the different body parts. Then simulate the pieces one by one.
- You could you ripple solvers for the bell. As well as vellum cloth for the limbs.
- You could select poly groups on the model to choose where to add bioluminescence. Then add that glow with the default glow shader.
- Adding some Perlin noise for small displacement across the body would help create a sense of continuous movement.
- You could even use vellum hair for the libs, and point deform the geometry onto the moving hairs.
- Always have a bit of reference. :)
Naut your everyday jellyfish model: Exploring how tentacles and oral arms impact locomotion: https://arxiv.org/pdf/1908.04202.pdf
An Interactive Fluid Model of Jellyfish for Animation: http://gigl.scs.carleton.ca/sites/default/files/david_mould/jelly.pdf
Jellyfish food-finding strategy found to be more complex than thought: https://phys.org/news/2014-08-jellyfish-food-finding-strategy-complex-thought.html
Self-repairing symmetry in jellyfish through mechanically driven reorganization: https://www.pnas.org/content/112/26/E3365
The ontogenetic scaling of hydrodynamics and swimming performance in jellyfish (Aurelia aurita): https://jeb.biologists.org/content/206/22/4125
The surprising trick jellyfish use to swim: https://www.sciencedaily.com/releases/2015/11/151103134821.htm
Biologists Surprised by Unique Strategy of Self-Repair Discovered in Moon Jellyfish: http://www.sci-news.com/biology/science-strategy-self-repair-moon-jellyfish-02918.html
Jellyfish 'can sense ocean currents': https://www.bbc.com/news/science-environment-30936192
A tissue-engineered jellyfish with biomimetic propulsion: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4026938/
Naut Your Everyday Jellyfish Model: Exploring How Tentacles and Oral Arms Impact Locomotion: https://www.mdpi.com/2311-5521/4/3/169/htm
Modeling and simulation of fish-like swimming: https://www.math.u-bordeaux.fr/~aiollo/Papers_files/Journal%20of%20Computational%20Physics%202011%20Bergmann.pdf
Which types of jellyfish are there in the Arctic Ocean today – and which will still be there tomorrow?: https://www.awi.de/en/about-us/service/press/press-release/which-types-of-jellyfish-are-there-in-the-arctic-ocean-today-and-which-will-still-be-there-tomorro.html
Swarm behaviour: https://en.wikipedia.org/wiki/Swarm_behaviour
Jellyfish: The Next King of the Sea: https://www.smithsonianmag.com/science-nature/jellyfish-the-next-king-of-the-sea-679915/
JELLYFISH AND COMB JELLIES: https://ocean.si.edu/ocean-life/invertebrates/jellyfish-and-comb-jellies