A scorpion on the defensive might break off its tail in order to escape -- sacrificing its stinger and even its anus as a result. The stump heals quickly, but the discarded bits never grow back, leaving the survivor unable to poop or inject venom into its prey. The findings were published in PLOS ONE this week.
Autotomy is the voluntary shedding of a body part, usually as part of an anti-predation tactic. The detachment occurs at a predetermined spot called the cleavage plane (indicated with dashed lines in the image above). Breaking off at these sites of weakness minimizes trauma and promotes the rapid sealing of vital fluids (called hemolymph). Some spiders do this with their legs, while some lizards might drop their tail. Only one species of scorpion has been seen detaching its abdomen, but the details are scanty. Now, a team led by Camilo Mattoni from Universidad Nacional de Córdoba in Argentina report the autotomy of the metasoma -- the hind part of the abdomen, or the tail -- in 14 Ananteris scorpion species from South America.
Out in the field, the team turned over lots of rocks during the day and used UV light to detect the scorpions at night, marking down when and where on the body the detachment occurred. Back in the lab, they examined museum specimens that had undergone autotomy and conducted a series of experiments with 35 Ananteris solimariae adults collected from Colombia. The researchers would hold onto the metasoma with forceps and pull backwards gently for no more than 30 seconds. Pictured below (left), an adult male with an arrow indicating the beginning of cleavage:
In 22 of the 25 males, the tail was severed at the joints between metasomal segments I and II, II and III, or III and IV -- which allowed the scorpion to escape. This happened as quickly 0.29 seconds after contact with the forceps. Only one out of five females did the same, and the process wasn’t observed in either of the two juveniles they studied.
Following detachment, the discarded bits kept vigorously writhing (pictured above, right) for up to 169 seconds. This automatic twitching might distract the predator while the animal makes a quick getaway. Furthermore, the detached tail reacts to contact, and in 47 percent of the cases, it even attempted to sting.
With minimal hemolymph loss, the severed surface healed rapidly and scar tissue formed in just five days. (You can see some great photos of healed stumps here.) However, the lost parts can’t regenerate, and the scorpion is permanently without the posterior part of its digestive system and the ability to inject venom by stinging.
As a result, it no longer defecates and can only capture small prey (like 5-milimeter crickets) using their mouthparts. After about 20 days or so, the abdomen of the scorpions were visibly swollen because of the excrement build-up; two of them even suffered a second autotomy due to the internal pressure. Extreme constipation and portion control sound pretty bad, but most of the males in the experiments survived and even continued to mate with females for up to eight months afterwards.
Images: C.I. Mattoni et al., 2015 PLOS ONE