When young Israeli marine biologist Yisrael Schnytzer tells people he researches Lybia crabs for a living, many people, especially Israelis, say, “That’s interesting, but what’s the practical benefit?”
“Nothing,” he always replies, “they’re just really amazing.”
Apparently, Schnytzer is not the only one who thinks so. When Schnytzer, together with his fellow PhD student Yaniv Giman, as well as their professors Yair Achituv of Bar-Ilan University and Ilan Karplus at the Volcani Center, published a press release describing their latest research into Lybia crabs last January, they were inundated with media inquiries. Reports of their research have been featured in Science, National Geographic, The Washington Post, The Atlantic, Der Spiegel, The New Scientist, The Daily Mail, National Public Radio and dozens more publications. Next month, the BBC will arrive in Israel to film a show about the Lybia crab, the very first television show devoted to this particular crustacean, as part of a program about “animals behaving badly.”
Lybia crabs, also known as boxer crabs or pom pom crabs, are always found in nature holding another animal, a sea anemone, in each of their claws. In a series of experiments the four Israeli scientists showed that if the crabs’ sea anemones are taken away from them, they will fight another crab to get a sea anemone, then split it into two. Each of the two pieces of sea anemone will then regenerate itself, and the sea anemones in each claw will be identical clones. Sea anemones are sea-dwelling, predatory animals related to coral and jellyfish.
The sea anemones held by Lybia crabs in the Red Sea near the Israeli town of Eilat are of the genus Alicia and a species the researchers believe to be previously unknown to science. They could not find any examples of this species living by itself in the wild, although they acknowledge they did not do an exhaustive search.
Snorkeling for science
Schnytzer first learned of the crabs when he was looking for a PhD program in Israel, and met Professor Achituv of Bar-Ilan University.
“He told me about this little crab that holds a sea anemone in each of its claws and no one knows what they do, and the only paper published about them was in 1905.”
Schnytzer signed up. He and his colleagues were required to travel to Israel’s southern port city of Eilat every few months and go snorkeling in the Red Sea, where they would find the coin-sized crabs in shallow water, under craggy rocks.
“When you turn a rock over, many creatures suddenly go scurrying in every direction. You have to recognize what you’re looking for and grab him quickly.” (Schnytzer advises caution when turning over rocks at the Red Sea because one might accidentally turn over a highly poisonous stonefish, which resemble the encrusted rocks in their habitat.)
Schnytzer said he would typically lie in the water for four to five hours and on a lucky day come home with a few crabs. In total they collected more than 100 crabs over several years. They did not collect more due to restrictions by Israel’s Nature Reserves Authority as to how many animals can be removed from the wild.
In the lab, Schnytzer and his colleagues observed that every single crab was holding a sea anemone in each claw.
“It’s not every day that one animal controls another animal. The way I explain it to people is I say, ‘Imagine if you picked up a mouse in each hand and just never let them go. Everything you did would be using the mice.”
There are 9 or 10 species of Lybia crabs in the world, one in the Red Sea near Eilat and others down the African coast and the southeast Asian coasts. Schnytzer and his colleagues studied two species — the crabs from Eilat, which are light brown in color and resemble the rocks they live under, and a second, more colorful species from Indonesia that lives among coral. All species of Lybia crabs hold sea anemones in their claws although some species of crab hold an altogether different species of sea anemone.
Schnytzer speculates that the crabs use the anemones to sting predators who get too close as well as to catch food.
“That’s something we still need to study. A few times we put a fish in the tank with a crab. The crab waved the anemones at the fish in a boxing motion. On one occasion, the anemone touched the fish and the fish twitched as if it had been stung, swam away and did not come back.”
Schnytzer and his colleagues tried to figure out what the crabs would do if deprived of one or both sea anemones. They took tweezers and painstakingly pried the sea anemones out of the crabs’ claws.
If the researchers removed a single sea anemone, the crab would tear the remaining sea anemone into two. These two fragments would then regenerate over the course of several days into two identical clones.
If they removed both sea anemones, the crab would start to wrestle with another crab until it had grabbed a fragment of a sea anemone. It would then split the fragment into two, and both fragments would regenerate into whole sea anemones.
The researchers said that this seems to be a unique case of one animal getting another to reproduce asexually, although they are sure this species of sea anemone can produce sexually as well.
“While every crab is holding a pair of identical clones, if you look at the entire population, we saw three distinct genetic types. That can only happen if there is sexual reproduction.”
As for the fighting, Schnytzer notes that it was not particularly fierce.
“There was something ritualized about it. None of them died and none of them got wounded.”
Schnytzer pointed out that in nature, if a small animal attacks a large one and tries to steal a resource, the large one will kill it. But in the case of the pom pom crabs, the small one would always come away with a bit of anemone.
“That suggests to us that maybe the big one has to give, it’s as if there are rules. It’s as if it’s for the greater good, because if I don’t give him a sea anemone then I won’t have one later or something like that.”
The researchers also noticed it wasn’t necessarily the crab lacking the anemone that initiated the attack.
“In about 50 percent of cases, the crab with the anemones would initiate the fight, which is also really puzzling, and suggests there may be some kind of rules.”
Another puzzling aspect of the crab-anemone relationship is that on the surface, the crab seems to be getting a lot more out of it than the anemone. When the experimenters forcibly took sea anemones away from crabs and let them live on their own, they appeared to thrive.
“In nature crabs without anemones would probably not survive. We saw they could not actively collect the food because their claws are too small. In a small aquarium where you put food next to them they manage but we guessed in nature they would probably die.”
“The separated anemones, on the other hand — they grew and grew and grew. They appeared to be fine. They don’t need the crab and can gather food by themselves.”
Asked if this is an exploitative relationship, Schnytzer replied, “Yes, the crabs are stealing food that the anemone collects, keeping them small and making them reproduce when they want to. I guess that’s what you’d call exploitation.”
Schnytzer guesses the anemone must derive benefits from the relationship as well, but he is not sure what they are. He is also points out that what we see now is just a snapshot in the long process of evolution.
“Maybe in a few million years, they’ll both be benefiting or neither will. The anemones could evolve ways to sting the crabs. Maybe the reason we couldn’t find them in the wild is that they migrated away from the crabs. It’s hard to say, those are big questions.”
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