At first glance, adorable little boxer crabs don’t look like kidnappers, but a closer look reveals incriminating evidence—living anemones grasped tightly in each claw. As it turns out the crabs, which spend the day hiding beneath rocks along shorelines in the Indo-Pacific, commandeer hostages for gathering food as well as protection.
This novel form of symbiosis was labeled as kleptoparasitism—the theft of food—in a 2013 scientific paper. The authors studied the relationship using aquarium experimentations and observations. In an unexpected finding, their research also revealed that the crabs intentionally starve their captives to keep them manageably small.
To feed, the crabs use their tiny victims like mops to pick up bits of food from their surroundings. The crabs quickly remove the particles with their walking legs, leaving behind just enough scraps to keep their anemones alive, but not enough for them to continue to grow. The researchers likened the behavior to “Bonsai”—retarding a plant’s growth by manipulation. When the anemones were removed from the crabs and allowed to live on their own they grew rapidly not only increasing their size by as much as 250 per cent, but also changing their color and shape.
It generally takes a lot of tedious rock turning to find a boxer crab. With the help of our dive guide at Tulamben—a well-known dive destination in Bali—we were able to increase our odds. He knew exactly where a pair lived and was able to find us one in a matter of minutes. Exposed, the crab wields its pair of anemones like miniature boxing gloves, appearing to throw a series of jabs in our direction. Afterwards the guide carefully turned the rock back over, returning the little pugilist to its rightful place. ~ Originally published in our Critter Hunt Column in Scuba Diving magazine, 2012
On our 2012 dives in Bali, we also learned that if we dived in the rocky shallows at night, we didn’t have to turn rocks over to find boxer crabs because they were often walking around in the open. They didn’t appear disturbed by us, each other, or any predators, so it was a good opportunity to watch them. They moved slowly, waving their anemone pom-poms from side to side, in what certainly looked more like feeding than defensive maneuvers. Whether they carry anemones for defense or for feeding, it is an intriguing story: invertebrates using tools!
We’ve also photographed another species of Lybia in Lembeh Strait. This was much smaller and fairly nondescript. In fact I couldn’t tell it was a boxer crab until Ned enlarged his image, revealing the tiny anemones in its pincers:
2020: Down the Rabbit Hole
An 1880 taxonomic study first described the association of the crabs with anemones and suggested their use for both feeding and defense. One of the most interesting notes came from a detailed study of boxer crab behavior in a 1905 paper: a crab, deprived of one of its two anemones, would split the remaining into two pieces, so it would have one for each claw.
A 1998 study looked at the intraspecific fighting behavior of the Hawaiian boxer crab, L. edmonsoni, using 50 crabs collected from three locations in Oahu. In fighting matches, they observed that the crabs rarely touched each other with their anemones, and when they physically grappled, they fought back-to-back, holding their anemones away from each other. It would seem that the crabs were protecting their anemones from harm. Two of the crabs they collected only held a single anemone, and they experimented with removing a single anemone from two other crabs. All four were later observed to be holding partial anemones in each claw, and in one case, they actually observed the crab splitting its lone anemone by pulling it apart.
In 2017, the same scientists who wrote the 2013 paper about the “Bonsai” anemones and kleptoparasitism, ran experiments of sea anemone splitting and sea anemone theft by boxer crabs. When they staged encounters between crabs with anemones and crabs that had no anemones, 73% of the time intense fights took place, resulting in the crab without anemones managing to steal an entire or partial anemone. After the theft, the crabs separated, and were observed splitting their remaining whole or fragment, resulting in partial anemones in each chela (watch the video from that study). All split anemones eventually regenerated, resulting in whole, genetically identical anemones in each claw.
So how do newly settled boxer crabs obtain their “starter” anemone? The researchers collected three very small (2-3 mm) crabs, removed their anemones and put them up against fully grown crabs with anemones. Despite the big difference in size, the smaller crabs initiated a fight and succeeded in stealing a whole or partial anemone. These were all laboratory observations – wouldn’t that be something to see in the wild!
Wait…Crabs with nudibranchs?!
Years ago, when writing about the small dorid nudibranch that preys on the fins of shrimp gobies, we ran across Dr. Tune Sakai’s 1961 description of a new crab from Hanama Bay in Japan. Described from three specimens, he named it Lybia hatagumoana, placed in the genus with the boxer crabs, and named, presumably, for their research vessel, Hatagumo. He remarked, “The holotype was carrying a tiny sea-anemone in each hand, and the allotype had a tiny nudibranch in one hand.” What??? A nudibranch??? Was this a one-time observation? How often has this been seen? How can we see this?
Thus began our search for a nudibranch-wielding crab. There isn’t much published about them, but Sakai’s observation was more than a single, coincidental occurrence. In 1993 Kikutaro Baba described the nudibranchs found clutched in the fingers of two more specimens of Lybia hatagumoana. He describes them as Gymnodoris sp., most closely resembling G. citrina: “The ground colour of the dorsal surface of the body slightly yellowish white in Material No. 1 and tinted more pinkish (orange yellow) in Material No. 2. The overall dorsum and sides scattered with tiny orange spots. The club of the rhinophore orange, and the stalk whitish.”
We have yet to find any information about whether or not this nudibranch was ever given a definitive name.
There are a lot of things I still want to see underwater, and a crab holding a nudibranch is one of them. Unfortunately, my hopes were dashed when I read the only other paper I could find that mentions this unusual association: In their 2011 paper, Drs. Jose Christopher Mendoza and Peter Ng recorded the presence of the subfamily Polydectinae (the boxer crabs) in the Philippines. They also described a new genus for this little nudibranch-carrying crab, formerly known as Lybia hatagumoana: Tunebia, “named in honor of Japanese carcinologist, Professor Tune Sakai. It is an arbitrary combination of his first name and Lybia.” They note that all specimens taken to date were from depths “no less than 70 meters” – a little too deep for scuba – Darn!
Even though chances are slim that we’ll ever see this crab for ourselves, Dr. Mendoza was kind enough to grant permission to share an image from his paper, so we’ll all have the search image – just in case. Scientific studies have enlightened us about crabs wielding anemones, but how do they find nudibranchs helpful? So many mysteries; so much to learn!
Check out figure D and the nudibranch held in the left chela:
Mendoza, Jose Christopher E., Ng, Peter K. L. (2011): The Polydectinae Dana, 1851, of the Philippines, with description of a new genus for Lybia hatagumoana Sakai, 1961 (Crustacea: Decapoda: Brachyura: Xanthidae). Zootaxa 3052: 51-61
J. E. Duerden (1905). “On the habits and reactions of crabs bearing actinians in their claws”. Proceedings of the Zoological Society of London 2: 494–511
Sakai, T. (1961). New Species of Japanese Crabs from the Collection of His Majesty the Emperor of Japan. Crustaceana, 3(2), 131-150
Baba, K., Noda, H. (1993) A rare collection of a small species of Gymnodoris (Nudibranchia: Polyceridae) held alive by the chelipeds of the crab, Lybia hatagumoana (Brachyura: Xanthidae), from the bottom off Kanayama Bay, Kii, Japan. Venus, 52(4), 283–289