Experts identify the parasite causing mass mortality in sea urchins

Long-spined sea urchins (Diadema antillarum) are vital herbivores that graze on algae. If left unchecked, algae often outcompete corals for space and resources, blanket them, block their light, and ultimately kill them. Thus, by feeding upon them, sea urchins are crucial for maintaining coral health and balance in marine ecosystems.

Unfortunately, in late January 2022, Diadema die-offs started to be reported in St. Thomas in the U.S. Virgin Islands. By late March, urchins from the Lesser Antilles, Jamaica, and the Mexican Caribbean began to be affected too, and by June mortalities were spiking in most of the Great Antilles, Florida, and Curaçao, leading to declines of 85 to 95 percent in sea urchin populations.

Recently, an international team of 42 scientists funded by the National Science Foundation (NSF) has managed to identify the culprit behind these severe die-offs: Philaster apodigitiformis, a unicellular eukaryote belonging to a group of 8,000 species called ciliates. Although P. apodigiformis has been known to infect fish, this is the first time that it was found to infect and cause mass mortality in invertebrates. 

“Rarely are we afforded the opportunity to understand marine disease events in this detail, where we can actually work out a cause of it,” said study lead author Ian Hewson, a marine ecologist at Cornell University.

Although scientists are not yet sure how to treat P. apodigitiformis infections, discovering the identity of this parasite could help them develop strategies for maintaining the health of sea urchins that are currently being raised for restocking efforts across these regions.

“There’s a long history of Diadema die-off; the big one happened in the 1980s, with the cause still unknown, and cascaded into a loss of 80-90 percent of the coral cover in the Caribbean,” said Dan Thornhill, a program director in NSF’s Division of Ocean Sciences. 

“There were a lot of factors involved, but the urchin die-off was the tipping point. It’s an ecological tragedy. It’s fairly rare and exciting to identify the parasite or pathogen behind one of these diseases in marine ecosystems, and to do it this quickly is impressive.”

The study is published in the journal Science Advances.

More about infectious parasites 

Infectious parasites are organisms that infect hosts, such as humans, animals, and plants, causing various diseases. These parasites can be categorized into three main types: protozoa, helminths, and ectoparasites.

Protozoa

These are microscopic, single-celled organisms that can multiply in humans. They are typically transmitted through the fecal-oral route (for example, through contaminated food or water or person-to-person contact). They can also be transmitted through vectors, such as mosquitoes. 

Some common diseases caused by protozoan parasites include malaria (caused by Plasmodium species), amoebic dysentery (caused by Entamoeba histolytica), and giardiasis (caused by Giardia lamblia).

Helminths

Also known as parasitic worms, these are larger and more complex organisms. They include flatworms (like tapeworms and flukes) and roundworms (like hookworms, pinworms, and trichinella). 

Helminths usually cannot multiply in humans. They are typically transmitted through soil-transmitted routes (i.e., eggs present in human feces are deposited on soil and can infect others if ingested or if they penetrate the skin).

Ectoparasites

These are parasites that live on the surface of the host. They include lice, fleas, ticks, and mites. Ectoparasites can transmit diseases from one host to another. Lyme disease (caused by Borrelia burgdorferi and transmitted by ticks) and plague (caused by Yersinia pestis and transmitted by fleas) are examples of diseases transmitted by ectoparasites.

Infectious parasites can cause a range of symptoms from mild discomfort to severe illness and death, depending on the type of parasite, the number of parasites, and the individual’s health status. They are more common in areas with poor sanitation and unsafe water sources. 

Many preventive strategies focus on improving sanitation, providing safe drinking water, and educating communities about personal hygiene. Treatment usually involves specific anti-parasitic drugs. In some cases, preventive treatments may be administered to at-risk populations.

More about sea urchins 

Sea urchins are marine invertebrates found in oceans worldwide. Their interactions with the environment, including their response to climate change, are complex and multifaceted. Here are a few key points to understand:

Role in Ecosystems

Sea urchins play a crucial role in marine ecosystems, particularly in kelp forests where they are a keystone species. They feed on kelp and help maintain balance in the ecosystem. 

However, in the absence of their natural predators (like sea otters or some species of fish and lobsters), their population can explode, leading to overgrazing and what’s known as an “urchin barren.”

Climate Change Impact on Sea Urchins

Climate change has been linked to changes in sea urchin populations and behavior. Warming waters may alter the habitats of sea urchins, pushing them towards the poles. 

Additionally, ocean acidification, another result of increased carbon dioxide in the atmosphere, can impact the ability of sea urchins to form and maintain their calcium carbonate shells.

Sea Urchins and Carbon Sequestration

Research has suggested that sea urchins may play a role in carbon sequestration. Kelp forests, which sea urchins help regulate, are excellent at capturing and storing carbon. 

However, if urchin populations become too large due to a loss of predators or other factors, they can overgraze and destroy these kelp forests, releasing stored carbon back into the atmosphere and negatively impacting this natural carbon sink.

Effect of Sea Urchin’s Diet

Some studies have shown that the diet of sea urchins might affect their resilience to climate change. For instance, urchins that feed on a certain type of algae seem to be more resilient to ocean acidification.

Sea Urchins as a Resource

In some regions, sea urchins are harvested for their roe, which is a delicacy in many cuisines. Changes in climate could potentially affect the availability of this resource.

The relationship between sea urchins and climate change is an active area of research, and new insights are likely to emerge as scientists learn more about these intriguing creatures and their role in the marine ecosystem.

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By Andrei Ionescu, Earth.com Staff Writer

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