Uncovering reasons behind the cannibalistic behavior of Tiger Moon Snails.
Delving into the natural world, we come across a variety of prey-predator relationships. One such intriguing prey-predator relationship is cannibalism. This is a relationship in which both, the prey and the predator, belong to the same species. Surprisingly, this behavior is seen across a multitude of organisms. How extraordinarily bizarre!
This naturally leads us to question the Why, What and How of cannibalism: Why does a species eat another of its own kind? What kind of inherent biological or external environmental factors cause cannibalism to occur? How did the different forms of cannibalism develop?
Thanks to the efforts of scientists in studying cannibalism in a variety of insects, fish, amphibians, and mammals, we now know the answers to some of these questions. We know that this behaviour arises as a result of years and years of evolution - as a way to acquire nutrition in response to factors such as food shortage or crowding. We have also uncovered different forms of cannibalistic interactions - female eating male (black widow spiders), male eating female (wolf spiders such as Allocosa brasiliensis), sibling eating sibling (mormon crickets), parent eating child (some teleost fishes) and even child-eating parent (some caecilians). But since every species behaves differently - each with a unique set of environmental and biological requirements, after a lot of observation, of a lot of species, under an array of conditions, scientists have been able to untangle only sections of this complex knotted ball of the cannibal yarn. There are many more instances of cannibalism in other species that require further observation.
(Left to Right) Instances of cannibalism: Female and male Black Widow Spider; Wolf Spider - Allocosa brasiliensis; Caecilia - Boulengerula taitanus; Mormon crickets. (Photos by: James H. Robinson, Science Photo Library; Phillip King, iNaturalist; Alex Kupfer; Debra Reid, Associate Press)
One such species is the carnivorous sea snail Notochoclis tigrina. Commonly known as the Tiger Moon Snail, it carries a moon-shaped spotted shell on its back and uses a large muscular ‘foot’ to crawl across the seashore. The snails’ typical diet includes marine animals such as bivalves or clams, whose shells the snails drill into to get to the edible chunks. These snails are also known to drill into and eat their own species. These holes drilled by the snails are very characteristic of their species, hence could be identified easily by the researchers. But the reasons behind this cannibalistic behaviour were yet unknown.
(L-R) Tiger Moon Snail Notochoclis tigrina; Tiger Moon Snails exhibiting Cannibalism in the wild.
(Photos by Marcus Ng, Wild Singapore; Dr. Chattopadhyay, et. al)
Dr. Devapriya Chattopadhyay and a group of scientists from IISER Kolkata set out to find out just this - they wanted to understand why these snails turn cannibalistic and what factors control this behaviour. Is it because of food shortage? Could the difference in size between the snails be a factor? Does this behaviour start only at a certain age of their life or does it increase with age?
Cannibalism experimental “shell-ups"
To answer these questions, the researchers collected live tiger moon snails and their preferred prey - the clam Timoclea - in buckets full of seawater and sediments from the Odisha shoreline. They brought these items back to the lab and transferred them to aquarium glass tanks containing saltwater, to create the experimental setups (or shell-ups :P).
(L-R) Research site - Chandipur, Odisha, where the Tiger Moon Snails were picked up from; Experimental set-up with the aquarium tank for the snails. (Photo by Chattopadhyay, et.al)
To test one of the primary explanations for the evolution of cannibalism - shortage of food - the snails had to be placed in an environment with limited bivalves to feast on. For this, a glass tank was set up that contained sea snails and only 50 bivalves. Another identical tank was set up with an equal number of sea snails but with 200 bivalves.
The scientists also wanted to find out if the predator or prey size played a role in instigating cannibalistic behaviour. What were the chances of a sea-snail eating another snail if its own size was larger or smaller than the snails around it? To test this, three tanks containing only sea snails were set up. Tank 1 and Tank 2 had small and large snails respectively, whereas Tank 3 contained an equal number of small and large snails.
Lastly, to identify if snails of all ages showed cannibalistic behaviour, data was collected from Tanks 1 and 2 of the second experiment described above, which contained only small and large snails, respectively.
Information on the number and size of drill holes in snails and bivalves was collated from these three experiments, every five days for two months. Each drill hole found on the snails was taken to be an event of cannibalism and the results were then compiled and analyzed.
The experimental set up of Tank 1 containing only small snails, Tank 2 containing only large snails, and Tank 3 containing both small and large snails.
Did the snails “slime up” for cannibalism?
In the past, scientists have speculated that cannibalism arose as a result of predator ineptitude – a long time ago on the evolutionary timescale, predators must have been too primitive to be able to differentiate between their prey and members of their own species, leading them to eat members of their own kind. This theory has since been rejected and replaced with the concept that cannibalism occurs at certain stages of food shortage. But as you are about to find out from the results of the above experiments, every behaviour is a consequence of multiple factors.
Video depicting cannibalism in Tiger Moon Snails
The scientists found that although a lack of bivalve prey played a large role in creating conditions for these snails to turn cannibalistic, it certainly wasn’t the only factor. From the first experiment, it was found that the tank that had fewer bivalve prey had a larger number of drill holes (or cannibalistic attacks) in the snails, compared to the tank that had 200 bivalves. This is proof that cannibalism rarely occurs in the presence of the organism’s preferred prey. From the second experiment, the size difference between the snails was found to play a significant role: out of the total, 74% cannibalistic attacks were in the tank that had snails of different sizes (Tank 3) while only 24% of attacks were in the tanks that had similar sized snails (Tanks 1 and 2). Not only this, findings from the third experiment showed that the older snails displayed more inclination for cannibalism than their younger counterparts - there were fewer drill holes created by the younger snails (Tank 1) as compared to the older snails in Tank 2! The second and third experiments seem to be helping us determine what factors affect the intensity of cannibalism in the absence of the preferred prey.
So what exactly seems to be going on? In environments that had fewer food resources, the sea snails were eating other snails, instead of spending their energy searching for the “rare” Timoclea bivalves. This trade-off makes sense because although the shells of other snails are harder to drill into than those of the bivalves, the snails end up spending less energy in this activity as compared to foraging.
A similar cost-benefit analysis goes into preying on snails that are smaller in size. Targeting smaller snails not only increases the capture rate of the large-sized snails but also lowers their energy expenditure. Even older snails operate by this technique of maximizing energy gains - by being choosy about the size and species of their prey, they take advantage of their own development and maturity. The younger snails might find it harder to drill into their hard-shelled relatives and so chose to feed on bivalves! How cool is that?
A unique feature of this paper was that along with using experimental data from the tank set-ups, the researchers also performed ecological surveys of recently dead tiger moon snails and fossil data! While collecting samples for the three experiments, they also collected dead specimens of the tiger moon snails and the Timoclea bivalves from the shores of Odisha and inspected them for drill marks and their size. Additionally, they also looked at the fossil record of the same species of moon snails from Poland, that were more than 15 million years old! And voila, similar to the findings from the second and third experiments described above, the recently dead snails and the fossils show that the larger and older snails were more prone to cannibalistic behaviour than the smaller and younger snails. The rationale behind studying fossils in addition to the experimental work is the ability to track the behaviour of a species over evolutionary time and also in a natural setting, allowing for a more comprehensive examination of the species’ behaviour.
All this goes to show that hunger has quite a beastly way of infiltrating our relatives! :D
Depiction of a complete (left) and incomplete (right) drill hole as created by cannibalistic snails on other snails. These are the drill holes that help researchers identify acts of cannibalism. Photo: Chattopadhyay, D. 2017: Predation to climate change: What does fossil shells tell us? Current Science, Vol-112, p. 1489-1493.
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Source: Chattopadhyay, D., Sarkar, D., Dutta, S., & Prasanjit, S. R. (2014). What controls cannibalism in drilling gastropods? A case study on Natica tigrina. Palaeogeography, Palaeoclimatology, Palaeoecology, 410, 126-133.
Link to the article: https://www.sciencedirect.com/science/article/abs/pii/S0031018214002934
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Article written by: Nikita Gupta
A huge vote of thanks to Dr. Devapriya Chattopadhyay for fact-checking the article and providing us with all the required media.
Link to her website: https://www.iiserpune.ac.in/~devapriya/index.html