The problem with diversity (Part 1)

Most venomous animals make use of very similar types of toxins. However, the composition and the individual toxins of a venom vary significantly between different species (the black mamba has a very different venom to the box jellyfish – though both are very toxic), but also within the same species. I personally find the latter particularly fascinating, as this intra-species variation can stem from a myriad of factors, such as different geographic distributions, age, and diet.

This variety has primarily been associated with different prey availability. Indeed, in certain regions a species will primarily feed on large invertebrates (millipedes or scorpions) while in others, vertebrates (rodents or birds) might be on the dinner menu. This is also true for the differences in toxin cocktails that we find in juveniles and adults from the same species (ontogenetic venom variation). Snakes will often hunt different prey at different life stages and, naturally, the physiologies of these types of prey differ, requiring venom adaptation to successfully subdue and kill the favoured prey.

Geographic venom variation in different populations of the Mojave rattlesnake. The figure shows that venom composition can vary significantly between populations of the same species, depending on where they are found.

However, in some cases such differences in venom composition are also associated with long-term isolation (I am talking about an evolutionary time scale here, not this negligible COVID-19 isolation). I was confronted with a perfect example of this, when I visited the Andaman Islands.

This little tropical paradise is located in the Bay of Bengal, close to the Thai coastline. Yet, the only way to reach these islands is from the Indian mainland, since they fall under the sovereignty of India. These islands have a lot to offer, especially in terms of flora and fauna. Indeed, the Andamans boast a fascinating mixture of non-endemic taxa from Indo-China, Indo-Malaya, and even subcontinental India, alongside a high diversity of endemic species.

These endemic species, One of the reasons I really wanted to visit this secluded paradise was to see the endemic snakes there. I stayed at ANET (Andaman Nicobar Environment Team) and went on some awesome nature walks with the volunteers there. We saw a whole plethora of exciting wildlife, but the highlight for me was experiencing two species of venomous snakes in their natural habitat.

The Andaman Islands – what a tropical paradise.

The first snake encounter was with the yellow-lipped sea krait (Laticauda colubrina) - one of the most venomous (but also most chilled out) snakes on this planet. These kraits spend most of their life in the ocean but aren’t a true sea snake and like to come back to land to sleep at night. They possess extremely effective neurotoxins, which is very handy since they hunt fish. If they would manage to catch a fish, and then the venom would take 15 minutes to act, the fish would be gone, and the snake would never find its prey. Instead, these kraits effectively nuke the fish with toxins and ensure that it is incapacitated on the spot. This would be super dangerous for us humans, if yellow-lipped kraits weren’t the serpent equivalent of a puppy. There are no recorded bites world-wide (these guys aren’t endemic and found across many tropical oceans) and when handled they don’t seem to ever lose their calm. Having said that, I still wouldn’t recommend handling them – you are dead if they bite you.

Yellow-lipped sea krait – the venomous sea puppy.

The second species of snake I was super excited to see was the Andaman pit viper (Trimeresurus andersonii). They are featured in my first blog as an example of how not handle snakes (I was wearing flip-flops), but let me shine some more light on the severity of my stupidity. Firstly, we found around 20 of them within a 10 m radius of the hut we were sleeping in. Yet, since it was dark, there were likely a lot more, and I could have easily stepped on one – not great when wearing flip-flops.

An Andaman pit viper in both images. Try to find the one to the left.

The second reason why being so careless was stupid, brings us back on track with the focus of this blog. Whilst these vipers are very closely related to some other species from the same genus, the antivenom for these species does not work for the Andaman pit viper. This is due to the fact that long-term geographic isolation has led these vipers to develop a rather different venom cocktail. Hence, if I had been bitten, I would have been in serious trouble.

Fortunately, my stupidity was not punished and I wasn’t bitten, but other people on the Andaman Islands aren’t as lucky. There are quite a few species of venomous snakes there, which are very similar to others on the Indian, Malayan, or Thai Mainland, but none of the antivenoms work sufficiently well to neutralise the venom of the Andaman species. This geographic venom variation followed by a lack of effective antivenoms, is one of the issues we are trying to help solve in the Tropical Pharmacology Lab.

Two fishermen readying up their boat not too far off from where we found the kraits.

It’s important to note that geographic venom variation (caused by isolation or prey differences) isn’t the only kind of variation that can prove problematic. In cases where the venoms are primarily defensive, the types of predators that the venomous species are faced with also play a significant role. However, this will have to wait for the next ToxBlog.