Yucatán’s karst caves are home to the marine crustacean Xibalbanus tulumensis, a rare and little-known creature living in a very remote habitat. New findings show that it paralyzes its prey with a neurotoxin that holds huge potential for the development of new drugs. A Frankfurt scientist specializing in animal venoms is taking a closer look at the crustacean.
When biologist Björn von Reumont travels to Mexico, he regularly undergoes a very thorough security check at the airport. “That’s because I have a whole load of suspicious equipment with me,” he says with a grin. Von Reumont has been researching a group of exotic-looking crustaceans that only occur on the Mexican peninsula Yucatán and a few other places in the world. His fascination with the subject began at the Zoological Research Museum Alexander Koenig (ZFMK) in Bonn, where he completed his doctoral thesis on the evolution of crustaceans. “Back then I collected and examined specimens of all the important groups of crustaceans,” says von Reumont, who is currently a visiting scientist at Goethe University Frankfurt.
One such group are the remipedes – crustaceans that today live exclusively in the marine waters of submerged caves in Mexico, Cuba, the Bahamas, Lanzarote (Canary Islands), and Western Australia. This pattern of habitats is puzzling to scientists, von Reumont explains: “It’s possible that these animals inhabit remote parts of the ocean depths, where they can move from one cave system to another” – a hypothesis that no one has been able to prove to date.
Beyond this, many other aspects of the remipede crustaceans’ biology also remain unknown for various reasons: Their living in remote places, for example, means researchers often have to carry their scientific equipment a long way on foot before they can start their investigations. Being able to dive is another prerequisite for finding the species. “Cave diving is not without risks and one should definitely learn how to do it properly,” says von Reumont, who completed a special diving course so that he could pursue his fascination for remipedes. For any dive, a guideline and a safety diver are further necessities.
Life in the underworld
Nearly 20 years have passed since von Reumont collected his first specimens of the remipede Xibalbanus tulumensis on the east coast of the Yucatán peninsula. The location has its own unique system of connected karst caves that are filled with seawater and a layer of freshwater on top. When the ceiling of a cave collapses, the result is a crater-like sinkhole, through which one can gain access to the cave system. Both the caves and the sinkholes are called “cenotes”, a term derived from the Mayan language. The caves were sacred to the Maya, who believed they housed the entrance to the underworld – partly because the cenotes are the Yucatán’s only natural reservoir of drinking water.
Today the cenotes are a tourist attraction, which poses a challenge for this delicate ecosystem. “Untreated wastewater is frequently discharged into the cenotes, where it rapidly spreads through the cave system,” von Reumont explains. Another problem, he says, is construction work on hotels and the railroad tracks for the “Tren Maya”, which encircles the peninsula and cuts through its interior. “If the cenotes’ sensitive ecosystem is destroyed, it will also mean the end of the crustaceans, who are adapted to that habitat,” the biologist is convinced. That is why it is important to protect the cenotes, and also to discover more about the animals and their lives, he adds.
First known poisonous crustacean
One remarkable aspect of the remipedes is their adaptations to life in the dark, saltwater-filled caves: In addition to pale skin, they have no eyes, for instance. “They don’t look anything like crustaceans – more like centipedes with a length of roughly 3 to 4 centimeters,” von Reumont explains. That visual resemblance notwithstanding, in his doctoral thesis he was not able to demonstrate a close relationship. Instead, it seems these creatures share a common ancestor with insects. “Unfortunately, we do not know yet when or how insects evolved from remipedes,” he says with regret. Nobody knows how the crustaceans develop and grow, how long they live or why they swim on their backs. Even their diet is still unknown. They likely use their senses of smell and touch to locate smaller crustaceans such as shrimps and bristle worms. “It’s technically challenging to watch remipedes hunting in the darkness,” says cave explorer von Reumont, who plans to examine the contents of the crustaceans’ stomachs. Using a technique called barcoding, through which certain genetic sequences can be analyzed, could determine which species of animals the remipedes feed on.
Remipedes have a unique way of preventing prey from escaping: They use their chelicera – the front claws near their jaws – to inject a venom that paralyzes it. Researching animal venoms has become a major area of Björn von Reumont’s work – one he came about by chance, or, to be more precise, through a tip from a colleague. After von Reumont started following this lead, he found proteins known to be toxins from other animal venoms, and in so doing discovered the first actively venomous crustacean – whereby “actively venomous” refers to the animal’s ability to actively inject the venom into its prey through a special venom apparatus.
Neurotoxins for drug development
In the years that followed, von Reumont, who was then a researcher at the Natural History Museum in London, studied the venom apparatus of X. tulumensis and the composition of its venom using cutting-edge methods, including computer tomography as well as transcriptomic and proteomic analyses. The latter reveal which venom components an animal produces. However, an exact characterization had to wait until he could travel to the Yucatán again to gather more X. tulumensis specimens from the caves. “Unfortunately, we don’t know enough about how these animals live to be able to keep them in the laboratory,” he says. Owing to their small size, collecting their venom means most are killed in the process. “I take from the natural habitat only the minimum number of animals necessary for this research purpose,” explains the biologist, who keeps an official permit issued by the Mexican authorities allowing him to collect these samples with him at all times. The required tissue is prepared on site, allowing it to be used in the laboratory later on.
In the case of X. tulumensis, von Reumont identified some new toxin families, which were named “xibalbins”, after the animal that produces them. One of them has a characteristic structural element known primarily from neurotoxins produced by spiders. Neurotoxins inhibit ion channels that also have important functions in mammals, which makes them interesting candidates for developing new drugs – for the treatment of pain and neurological diseases including epilepsy, for instance.
Deployment in aquacultures
To probe this potential, the researchers in Frankfurt tested the xibalbins’ effects on mammalian cells, which confirmed their neurotoxicity. However, von Reumont stresses that this is just the first step on the long path to a potential drug candidate. There are also other unanswered questions: “Of course we are interested in finding out whether the remipedes’ venom paralyzes their natural prey.” If the remipede venom works as expected on crustaceans, it could possibly be used to fight parasites such as fish lice in aquacultures. The presence of venom components that can dissolve part of crustaceans’ outer skin suggests that remipedes liquefy their prey and suck out the contents – similar to spiders.
As an evolutionary biologist, von Reumont is fascinated by the question of how and when the genes for the venom proteins evolved. This is unknown in most poisonous animals and requires analyses of their genetic material. “The genome of X. tulumensis is of particular importance for understanding the evolution of venoms in crustaceans, and above all that of their closest insect relatives,” von Reumont says. Together with his former research group on animal venomics in the LOEWE Centre for Translational Biodiversity Genomics at Justus Liebig University Giessen, and Senckenberg colleagues in Frankfurt, he has already decoded this genome. Von Reumont will carry out his next studies in Karlsruhe, where he became head of the State Museum of Natural History’s Entomology Section in January 2025. “I definitely want to go back to the Yucatán to collect remipedes as well as their potential prey. There are so many new methods we could use to investigate the toxins in more detail without killing a large number of animals.” Definitely a reason to link up with his old network and revive valuable contacts.
Larissa Tetsch
