Scientists identify 2022 sea urchin killer

The search for the 2022 killer that decimated the long-spined sea urchin population in the Caribbean and along Florida's east coast is over. A team of researchers organized by Mya Breitbart, Distinguished University Professor at the University of South Florida's College of Marine Science, identified a single-celled organism called a ciliate as the cause of a massive die-off event to a marine animal vital to coral reef health.

Their findings were reported in Science Advances.

"We're beyond thrilled to get to the bottom of the 2022 mystery and a bit stunned we did it so quickly," said Breitbart, senior author on the Science Advances study and an expert in marine genomics. "We had a great team in place and the tools needed to do the ocean science equivalent of a forensic investigation."

Ciliates are microscopic organisms covered in hair-like structures called cilia that help them move and eat. They are found almost anywhere there is water and most are not disease-causing agents. However, this specific species of ciliate -- called a scuticociliate -- has been implicated in die-offs of other marine species, such as sharks, in the past.

Examining urchins collected from 23 sites in the Caribbean, the research team used a series of techniques to confirm the source of the die-off event.

After identifying the ciliate in every affected urchin specimen using genomic techniques, the team grew ciliates in the lab and performed infection experiments at the USF College of Marine Science. When the pathogen was introduced to otherwise healthy urchins in an aquarium tank, the urchins died within a few days -- replicating what was taking place in the ocean and confirming the ciliate as the disease source.

"We're excited to share this information with everyone, from reef managers to additional scientists so we can explore it further and try to stop its spread," Breitbart said.

The long-spined sea urchins inhabit shallow tropical waters and feed on algae that would otherwise destroy a reef. They began to lose their spines within days of contracting an unknown disease and died in droves starting in January 2022.

A similar die-off event took place in the early 1980s, which wiped out 98 percent of the long-spined sea urchin population. The culprit of that die-off remains a mystery.

Breitbart first got the call about the unfolding die-off at the end of March 2022. She immediately assembled a team consisting of Ian Hewson, lead author on the publication and a marine ecologist at Cornell University; Christina Kellogg, a microbiologist from the U.S. Geological Survey in St. Petersburg, Fla. who has worked extensively on coral reef diseases; and USF graduate student Isabella Ritchie.

"At the time, we didn't know if this die-off was caused by pollution, stress, something else -- we just didn't know," said Hewson, an expert in diseases that cause mass die-offs of sea stars, who flew from New York to the Caribbean Islands to observe the situation.

Even with the source of the mysterious die-off uncovered, questions still remain. For example:
 

• Is this ciliate new to the area, or was it there prior to the die-off?
• If it has been there, what environmental conditions favored its growth and why did it infect the urchins?
• Can it affect other species of urchins?

"One theory we have is that the ciliate grew well under high-productivity conditions that were observed in the Caribbean when the die-off first started," Kellogg said. "We're also curious about the fact that there is some overlap in some geographic areas where this die-off occurred and where corals are declining from stony coral tissue loss disease."

Read more at Science Daily

Reaching your life goals as a single-celled organism

How is it possible to move in the desired direction without a brain or nervous system? Single-celled organisms apparently manage this feat without any problems: for example, they can swim towards food with the help of small flagellar tails.

How these extremely simply built creatures manage to do this was not entirely clear until now. However, a research team at TU Wien (Vienna) has now been able to simulate this process on the computer: They calculated the physical interaction between a very simple model organism and its environment. This environment is a liquid with a non-uniform chemical composition, it contains food sources that are unevenly distributed.

The simulated organism was equipped with the ability to process information about food in its environment in a very simple way. With the help of a machine learning algorithm, the information processing of the virtual being was then modified and optimised in many evolutionary steps. The result was a computer organism that moves in its search for food in a very similar way to its biological counterparts.

Chemotaxis: Always going where the chemistry is right

"At first glance, it is surprising that such a simple model can solve such a difficult task," says Andras Zöttl, who led the research project, which was carried out in the "Theory of Soft Matter" group (led by Gerhard Kahl) at the Institute of Theoretical Physics at TU Wien. "Bacteria can use receptors to determine in which direction, for example, the oxygen or nutrient concentration is increasing, and this information then triggers a movement into the desired direction. This is called chemotaxis."

The behaviour of other, multicellular organisms can be explained by the interconnection of nerve cells. But a single-celled organism has no nerve cells -- in this case, only extremely simple processing steps are possible within the cell. Until now, it was not clear how such a low degree of complexity could be sufficient to connect simple sensory impressions -- for example from chemical sensors -- with targeted motor activity.

"To be able to explain this, you need a realistic, physical model for the movement of these unicellular organisms," says Andreas Zöttl. "We have chosen the simplest possible model that physically allows independent movement in a fluid in the first place. Our single-celled organism consists of three masses connected by simplified muscles. The question now arises: can these muscles be coordinated in such a way that the entire organism moves in the desired direction? And above all: can this process be realised in a simple way, or does it require complicated control?"

A small network of signals and commands

"Even if the unicellular organism does not have a network of nerve cells -- the logical steps that link its 'sensory impressions' with its movement can be described mathematically in a similar way to a neuronal network," says Benedikt Hartl, who used his expertise in artificial intelligence to implement the model on the computer. In the single-celled organism, too, there are logical connections between different elements of the cell. Chemical signals are triggered and ultimately lead to a certain movement of the organism.

"These elements and the way they influence each other were simulated on the computer and adjusted with a genetic algorithm: Generation after generation, the movement strategy of the virtual unicellular organisms was changed slightly," reports Maximilian Hübl, who did many of the calculations on this topic as part of his Master's thesis. Those unicellular organisms that succeeded best in directing their movement to where the desired chemicals were located were allowed to "reproduce," while the less successful variants "died out." In this way, after many generations, a control network emerged -- very similar to biological evolution -- that allows a virtual unicellular organism to convert chemical perceptions into targeted movement in an extremely simple way and with very basic circuits.

Random wobbling movement -- but with a concrete goal

"You shouldn't think of it as a highly developed animal that consciously perceives something and then runs towards it," says Andreas Zöttl. "It's more like a random wobbling movement. But one that ultimately leads in the right direction on average. And that's exactly what you observe with single-celled organisms in nature."

Read more at Science Daily