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In a shallow stream in the Valle dell’Anapo in southeastern Sicily, researchers saw something that had never been recorded for this species before: a large freshwater crab holding the tail of a live grass snake, almost one meter long, tightly between its claws. The discovery, reported in the journal Herpetozoa, gives scientists a clear look at an interaction they have long suspected but rarely witnessed in European waters.
The encounter lasted about ten minutes. The crab held on so tightly that the snake could not pull itself free. But the most interesting part of the discovery was not just this single event. On the same day, researchers found three more grass snakes in the area that had lost large parts of their tails. These injuries matched the kind of damage a crab might cause better than other known explanations. Later, researchers examined 33 Western Grass Snakes living in Sicilian streams where the freshwater crab Potamon fluviatile also lives. They found that nearly one-quarter of the snakes—24.4 percent, or eight animals—had injured tails. Many things can cause this kind of damage, including attacks by birds or mammals during the active season, rodents eating tissue, freezing or tissue death during hibernation, and snakes deliberately shedding their tails to escape predators. However, because the crabs and snakes live in the same places, freshwater crabs may be a possible cause of these injuries that scientists have overlooked until now.
Potamon fluviatile is an interesting part of this story. The species has recently spread to Sicily and across the island. This means that the relationship between these crabs and snakes may be fairly new rather than something that has existed for a long time. The crab is a large, powerful freshwater species native to parts of southern Europe. As it spreads into new waterways, it may meet more semi-aquatic snakes that hunt fish and amphibians in the same shallow streams. The researchers suggest that the crab may sometimes eat the muscle in a snake’s tail when the opportunity arises. This flexible feeding behavior fits with what scientists know about the varied diets of these crustaceans. A snake’s tail is especially vulnerable because it trails behind the body and is often underwater or dragged along the streambed. The snake also cannot protect its tail as well as its head. A crab with strong claws may only need to grab the tail and hold on. The observation in Valle dell’Anapo shows this behavior directly.
To find out how unusual this kind of encounter is, the research team carefully reviewed scientific studies, starting with the first review on the subject, published in 1995. They also searched online sources, social media platforms, and iNaturalist, a platform where people share wildlife observations. Altogether, they found just 21 records from around the world in which crabs were documented feeding on snakes or interacting with them as potential prey. These records included twelve crab genera from eight families and seventeen snake genera from eight families. Two records did not identify the animals in enough detail. The encounters happened on several continents wherever snakes and crabs share the same areas. The habitats included shallow streams, riverbanks, sandy beaches, rice fields, and even trees.
The range of crabs involved is striking. In Thailand, researchers recorded mud crabs from the genus Scylla. One study found snake remains in five percent of the crab stomachs examined. In 106 experiments involving the dog-faced water snake Cerberus rynchops, the crabs successfully caught and preyed on snakes in 24 percent of the trials. In Tobago, freshwater crabs from the genus Eudaniela were involved in six recorded encounters with snakes. Several involved the crabs actively attacking living snakes, usually eating the tail first. Marine and shore crabs have also been recorded interacting with snakes. On a beach in eastern India, a ghost crab from the genus Ocypode was filmed dragging a dead yellow-bellied sea snake into its sandy burrow. In Western Australia, a beach crab was photographed holding a black-ringed mangrove sea snake. Perhaps the most surprising example involves the coconut crab, Birgus latro. This land-dwelling crab can climb trees. In northern Guam, one was seen holding the tail of a large cat-eyed snake that was more than a meter long. The crab later released the snake. The encounter took place nearly a meter above the ground.
The snakes in these records also came from many different groups. They included sea snakes, homalopsid water snakes, natricine grass snakes and keelbacks, dipsadid colubrids, blindsnakes, and even a viper. This wide range suggests that crab-snake encounters are not limited to a special relationship between a few species. Instead, they may happen wherever crabs and snakes share the same habitats. Some records clearly show crabs attacking living snakes. Others probably show crabs feeding on dead snakes, while the details of several encounters remain unclear. This difference matters to scientists because hunting living prey and eating dead animals have different effects on an ecosystem. However, both types of encounters show that crabs can eat snake tissue.
The observation in Sicily was not the first recorded case of a Potamon crab attacking a grass snake. In 2020, researchers in Greece recorded a Peloponnesian freshwater crab, Potamon pelops, attacking and eating a young grass snake in a shallow stream. The crab ate the tail, the cloaca—the opening used to release waste and for reproduction—and part of the lower abdomen. These areas appear to be among the easiest parts of a snake’s body for a crab to grab. As with the Sicilian observation, researchers could not confirm whether the crab eventually ate the entire snake. A video that was probably filmed in Italy also shows a Potamon fluviatile eating a dead adult grass snake. However, the circumstances could not be confirmed, so the crab may have been scavenging rather than hunting the snake. Together, these European records suggest that the Mediterranean region may be a hotspot for such encounters because large freshwater crabs in the family Potamidae live alongside natricine snakes.
The study’s discussion of snake classification adds another important detail. Scientists recently changed the Western Grass Snake from a subspecies to a full species, based on detailed genetic studies. Further DNA research has helped explain how its populations are related and how they are spread across different regions. The Western Grass Snake includes five subspecies found from Great Britain through central and western Europe to Corsica, Sardinia, mainland Italy, and Sicily. This change in classification matters for ecological research because older accounts of grass snake behavior treated three species that are now recognized separately—the Western Grass Snake, the Grass Snake Natrix natrix, and the Iberian Grass Snake Natrix astreptophora—as if they were one species. As a result, much of the existing information about their predators actually described western European populations now classified as N. helvetica. Recognizing these species separately gives scientists new opportunities to compare their behavior, natural history, and ecological traits individually instead of treating them as one group.
The small number of recorded crab-snake encounters is itself useful information. Researchers found only 21 records after reviewing decades of scientific studies and searching widely online. This suggests that people rarely observe these events, even if they happen more often than the records indicate. In Sicily, however, nearly one-quarter of the snakes examined in streams where crabs live had damaged tails. This raises the possibility that crab-snake encounters are much more common than published records suggest. A snake may survive a crab attack after losing or injuring its tail. Unless someone witnesses the attack, the evidence may be limited to the injury itself. Citizen-science platforms and social media have helped researchers find more examples. Several records in the review came from these sources and might otherwise never have appeared in scientific publications. They included videos from Asia, Indonesia, Africa, and South America showing crabs eating snakes or feeding on their remains in places ranging from muddy rice fields to paved areas around homes.
One particularly interesting record comes from Brazil, where a freshwater crab from the family Trichodactylidae was seen feeding on a dead leopard keelback in a public pond. This finding shows that crab-snake interactions also occur in South America. Another record from Sri Lanka describes a common freshwater crab grabbing the tail of a bronzeback snake. The crab became distracted by people watching and let go. This example shows that these encounters do not always end with the crab catching its prey successfully. The outcome probably depends on the animals’ relative sizes, whether the snake can escape, and whether the crab is trying to catch prey or simply defending its territory or a stored food supply when a snake comes within reach.
The discovery in Sicily also matters for conservation and ecology. Freshwater crabs in the Mediterranean are often considered species of conservation concern because they face problems such as habitat damage, water pollution, and competition from introduced species. Learning more about their role as predators, even if they hunt only occasionally, helps scientists better understand how food webs work in these streams. Grass snakes also live and hunt in and around water, and they depend on healthy freshwater habitats. Any factor that regularly injures a noticeable share of their population deserves further study. Scientists do not yet know whether crab attacks cause a significant number of snake deaths or are simply an unusual part of natural history. However, the pattern of tail injuries found in Sicily suggests that the question deserves more research. The study’s authors believe these encounters are probably rare, but their review shows that they occur around the world and involve many different kinds of crabs and snakes. Until now, most of these interactions have gone unnoticed. The findings remind us that even in well-studied parts of Europe, important relationships between animals can still be discovered in a shallow stream.
Subject of Research: Grass snakes being preyed on by freshwater crabs and a worldwide review of crab-snake encounters
Article Title: Crabs as snake predators? An observation from southern Italy leading to a comprehensive review
Article References: Jablonski, D., Hegner, D., Faraone, F. P., & Mebert, K. (2024). Crabs as snake predators? An observation from southern Italy leading to a comprehensive review. Herpetozoa, 37, 299-303. https://doi.org/10.3897/herpetozoa.37.e128019
Image Credits: AI Generated
DOI: 10.3897/herpetozoa.37.e128019
Keywords: grass snake, freshwater crab, predation, Sicily, Natrix helvetica, Potamon fluviatile, herpetology, tail injury, decapods, ecology, citizen science, freshwater habitats