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Comb Jellies in the Irish Sea

July 31 2018

Hidden in plain sight, the almost completely transparent comb jellies can be found throughout the world's oceans. Despite this, they are often missed unless you know what to look for - a brilliant dancing rainbow iridescence as they move through the water. We want to document and understand the behaviour of these tiny animals in the wild.

July 31 2018


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Preparation Stage

A step back before we step forward.

No luck yet in getting footage from inside a comb jelly swarm, so we thought we’d go back to the beginning and explain a bit more about why we’re interested in comb jelly behaviour. Yes, this probably should have been our first post…

More brains, more problems!

As humans, we are particularly proud of our brains. All of the things that make us who we are, our thoughts, memories, and emotions, are all functions of our brain. Underpinning all of that is a constellation of communication; bursts of signals from one neuron to another. But at a biological level, all that brain activity ultimately serves a single purpose; to control our behaviour. So, if you want to understand how the brain works, all you need to do is work out what is happening in the brain when we are actually behaving. Simple!

Of course, our brains are big, and the span of possible behaviours is immeasurable. That’s what makes humans a bit messy to understand. Unpicking the complexity of every thought and memory, every subtle emotion and feeling, every spark of creativity or burst of anger that underlies everything we actually do as people, and relating that back to the flow of information through the billions upon billions of connections in our brains, is simply too difficult. So we must simplify the problem. We need something with simpler behaviours, and a simpler “brain”. Comb jellies are about a simple as we can get.

If you ask someone to just imagine an animal, it’s unlikely they would think of something shaped like a comb jelly. We are so used the idea that you find the brain in the head that the concept of a “brain” in a small round animal seems so alien. They don’t technically have a brain of course, but that do have everything you find in a brain. In ctenophores, the nervous system isn’t concentrated into one place in the body, but is found in a sparse network, spread all across the body, just beneath the surface. It would be as if our brains were spread in a thin layer across our entire bodies, just beneath the skin. We’ve been working on figuring out how this is organised, but the other part of the equation is the output of the nervous system – behaviour.

So what do comb jellies actually do?

When we bring comb jellies back to our lab we can't keep them in a regular aquarium, because they just sink to the bottom. To keep them happy they need to be in flowing water. So we keep them in a kreisel tank, a type of aquarium with a constant circulating flow. In our tanks, the jellies are pretty active, and a crowded tank can look pretty chaotic. Keep looking though, and some patterns begin to emerge. A lot of their behaviours are related to feeding. They fan out their tentacles, creating an elaborate sticky web. Then they mostly drift along doing nothing, until some food gets caught in their tentacles. The tentacles can’t be directed into the mouth like an arm, but simply extended or retracted like a fishing line. To get ensnared prey into their mouth, the comb jellies spin around, wrapping the tentacle around themselves and shortening them until their meal passes by their mouth. Feeding behaviours are great because we know why they do it and what triggers them to do it. Many of the other behaviours we see, on the other hand, are a bit more mysterious. Sometimes they suddenly change direction and quickly swim up, or down. Sometimes they swim with their tentacles bunched up uselessly next to their bodies, or retracted fully inside. Why do they do this, and what causes this? And what is going on in their “brain” when these behaviours are switched on or off?

Of course, the key question for us, especially when looking at these non-feeding behaviours, is whether any of these are actually “real” behaviours at all. It’s possible that this is just something comb jellies do when in a tank in a lab. Does a comb jelly in our lab “know” it’s not in in the sea any more? Is there some difference between our tanks and the natural habitat of these animals that changes what they do, that changes the connection between the nervous system and the behaviour that nervous system produces? The only realistic way to answer this question is to look at their behaviour where that behaviour evolved; in the sea. Our search continues this week, we’ll post updates as soon as we have them.

Testing Our Underwater Camera Platform

Our goal for this week was to test the buoyancy and stability of our underwater camera platform. We drilled holes in the lower portion of the frame while the upper part of the frame was filled with air and covered in foam. We attached as many waterproof cameras as we could get our hands on, and set out to put our design to the test.

Mark, myself (Amy) and another comb jelly scientist from our lab (George Merces) ventured to Dun Laoghaire Harbour, south of Dublin city. It was a beautiful day and we were excited to see what footage we could capture. Maybe a little too excited. We forgot to switch one camera on. We switched another camera on, but forgot to start it recording. On a third camera, we turned it on, started recording… but forgot it was in time lapse mode. We had two out of five cameras anyway!

We also attracted the attention of the harbour police, who were curious about what we were up to. This is not an unusual experience for us. While some people carry out field work in remote and isolated places, our work usually takes us to harbours filled with people fishing or out for a stroll or walking their dogs, and on one occasion swimming right up to us. They’re usually curious about what we’re doing and why we are doing it, and getting to talk to people about our work is often one of the highlights of a day in the field.

I put together a video (below) which shows how our platform performed. Spoiler alert - it floats! Our underwater footage came out very clear (at least on two of our cameras). Unfortunately we didn’t catch any jellies in our net or on film, although we didn’t really expect to. However, we usually have more luck in Howth Harbour, so we will try again there next week. And this time, we’ll make sure everything is turned on first!

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Every journey has a first step, and this is ours.

At the heart of the task we set ourselves is what seemed like a simple challenge; getting cameras into the sea. Our real first step is figuring out how to do this. Many questions emerged; what type of cameras do we need? What size should the platform be? How do we make it float? Where do you buy PVC pipes in Dublin? We spent most of this week carefully planning this project. We knew exactly what we wanted to achieve. In the end the construction of the platform took 20 minutes! Planning is key!

So now we have our first thing to show for our efforts - a frame with multiple cameras and lighting attached. It might not seem like much yet, but it gives us something to build on (literally). If it works, we'll try getting it moving on the water's surface. And then, hopefully, below the surface...

Next week, we'll be testing buoyancy and stability, and hopefully have our first underwater footage. Oh, and a new paint job. Now that is a hard decision!

Why are we even trying to make our DIY underwater camera system anyway?

We have never built anything like this before. We know that it would be possible to buy an underwater vehicle that would help us answer the questions we have, and that would certainly be easier than doing it ourselves. But we also know that maybe there are others out there with questions like ours, or maybe those who don't have their own questions yet, but just want to be curious explorers of the sea. If we can do this, we show that anyone can do this. Yes, we may be trained scientists, but this is far beyond our usual skill set. We are more comfortable with a microscope and a dish of cells, but we also love the challenge of learning to do and make something new. And making mistakes along the way, so you don't have to!

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Tracking down comb jellies is an unpredictable business. Sometimes a trickle, sometimes a flood. They appear in the seas around Ireland each year in late spring or early summer; the same time as the basking sharks arrive on their migration from the warmer waters to the south, and they disappear together too as the winter closes in. Despite the vast differences in size between these giant sharks and the tiny jellies, both hunt the same prey; tiny copepods. Unlike the sharks though, the comb jellies are planktonic, which means their movements are largely at the mercy of the currents and tides. As a result, their comings and goings are difficult to predict. If you want to find them and capture their behaviour on camera, this is a problem.

Howth harbour, to the north of Dublin, has been a lucky spot for us. This is the fourth year in a row where we have been collecting comb jellies, and this is a site where we regularly find animals from April through to November. But not every time. We can't predict exactly when they will be around, and in what numbers. There are so many factors at play here that we've been unable to find the patterns among the noise.

Even some of the basic questions about the ctenophores in the Irish sea remain unanswered. Where do they go in winter? Do they drift away, do they die off, or is it a bit of both? And where are they coming from in the spring? Do they drift in, or do they suddenly appear as larvae hatched the previous year reach maturity? But one of the biggest mysteries is the teeming swarms of comb jellies that sometimes appear. During these ctenophore blooms, the density of animals makes them much easier to spot in the water, even though they are easily mistaken for bubbles when they rise to the surface. In the most extreme cases, the receding tide leaves a carpet of comb jellies stranded on the harbour steps (like in the photo below). These blooms seem like our best chance to actually capture ctenophores on camera.

We don't know exactly what causes these blooms; tides, temperature, availability of food, or something else altogether. Unfortunately, this means we don't know exactly when we're going to find one of these swarms. However, the experience of the last few years leads us to believe that September is the most likely time to stumble across a ctenophore bloom. So that gives us a timeline for our goal. We need to have our camera rig built and tested by the start of next month.

The clock is ticking!

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Great Post. I'm really looking forward to following your progress.

Expedition Background

Why Comb Jellies?

Comb jellies (or ctenophores) are among the simplest animals with a nervous system. Watching comb jellies is the closest we can get to looking back in time to the beginnings of brains and all the rich behavioural complexity that brought to the animal world.

But in biology, simplicity is rarely truly simple. Even in a "simple" animal like a ctenophore, the hows and the whys of their behaviour in the wild is still unknown. But if we can start to understand that, perhaps we can start to understand the evolutionary origins of all complex behaviour.

So what do we already know?

We know where they are. We can find the most common species (Pleurobrachia pileus, or the "sea gooseberry") near the shore on the east coast of Ireland. But finding them means trawling with a plankton net. We've collected animals and maintained them in an specialised aquarium system. Comb jellies (much like jellyfish) need to be kept in continuously circulating tanks; but this environment is a far cry from the world they are adapted to live in. We can watch their behaviour as they swirl and tumble in these tanks; their response to light, to touch, and to the food we give them, but we don't know how this relates to how they would react in the sea.

What is the plan?

The first step is to construct a floating camera rig. We'll document the design, construction and testing process as we go. We'll then deploy the rig at a site where we regularly find high numbers of Pleurobrachia. We'll then work on a system for observing the animals in deeper water. We'll be sharing our progress and observations here. Wish us luck!