Evolutionary Anachronisms of the Ocean
Written by Roy Kittrell, September 2024
What are Evolutionary Anachronisms? Evolutionary anachronism, also known as “ecological anachronism”, is a term initially referring to attributes of native plant species (primarily fruit, but also thorns) that seemed best explained as having been favourably selected in the past due to their coevolution with plant-eating megafauna that are now extinct. I thought it would be best for me to give you a quick picture about this before we deep dive into its counterparts of the ocean.
The ring-tailed Lemurs of Madagascar are a fun and wild bunch of primates that inhabit the large island of Madagascar, thought to have been there since a small population was washed there from mainland Africa 50-80 million years ago.
Like many primates, these lemurs live in large groups known as Troops, so that they can groom, socialise and protect each other from dangerous predators, mostly by sounding loud alarm calls to tell other lemurs when they see predators.
But biologists that were observing them noticed something strange. Like many primates, the lemurs had a distinct call for land-based predators such as their main predator, the Fossa (Cryptoprocta ferox), and also a distinct call for flying predators, such as various birds of prey. The strange thing was, these birds of prey were seldom if ever seen attacking or even trying to attack ring tailed lemurs, although you would never have known this from how strongly the lemurs reacted to them. The more the biologists studied them, the less they understood why the lemurs reacted so strongly to something that was inherently not posing any threat to them.
It was only later discovered that there had previously been a much larger bird of prey living in Madagascar, the Madagascar crowned eagle (Stephanoaetus mahery) that went extinct as recently as the 16th century. Clearly, the ring-tailed lemurs never got this memo, and so continued to react extremely strongly to all birds of prey regardless of the actual threat posed by them.
This continued behaviour to a now extinct threat, a kind of ‘shadow’ left behind by evolution, is an example of a fascinating phenomenon in biology known as an Evolutionary Anachronism, and there are many equally as fascinating examples. Avocados, which were originally a food source for the now extinct giant Megatherium sloth of South America, is another.
Now that we understand Evolutionary Anachronisms, let’s look at some of the ones we can see in our reefs and oceans!
Giant Clams
Of all the invertebrates found in the ocean, the Giant Clams (in the Genus Tridacna) of the Coral Triangle are by far and away the largest. They can grow as large as 20m in length, and weigh as much as 230kg, and even live as long as 100 years if left undisturbed. Indeed, they are one of my highlights for diving and it is always a pleasure to see them up close.
Their size however is a bit of an anomaly. One of the next largest bivalves, the hilariously shaped Geoduck (family Hiatellidae) only grows to around 15-20cm long, making the Tidacna giant clams a true outlier for their group. So why do they grow so large?
One possibility is that the Giant Clam’s size is an evolutionary anachronism; some evolutionary pressure in it’s past caused it to grow as large as possible but now no longer exists, leaving us with a kind of biological mystery to solve. It’s possible that at some point in the past, these clams were preyed on by some animal that was thwarted by the clam growing larger. One possibility for this is the Cretaceous, when the oceans were dominated by large marine predators such as the Mosasaurus or perhaps Plesiosaurs. The extremely thick shell of the giant clam, far thicker than it’s thought would be needed for the organism, seems to be well-suited for the task of defending it from a direct attack.
Mantis Shrimp Eyes
You’ve probably already heard about how amazing the Mantis Shrimp eyes are; some of the most advanced eyes in the entire animal kingdom.
Some of their advanced features include:
- 16 different kinds of Photoreceptors (compared to 3 for humans)
- UV light detection
- Circular Polarised light vision (the only animal in the world known to do this)
- The ability to perceive depth with a single eye
For a small marine crustacean living on the bottom of the sea, this is, to put it very mildly, complete and total overkill as far as eyesight needs go, and evolution rarely wastes it’s efforts on needless things. So why do they have this vision system?
It is theorised that this is an example of an evolutionary anachronism. At some point in the mantis shrimp’s past, there was a significant benefit to having this level of eyesight. We know from the fossil record that Stromatopods (Mantis Shrimp) have been around for a very long time, approximately 340 million years ago. One possibility is that at some point the ocean was extremely clear, allowing more UV light to reach the sea bed, giving any animals able to see in this spectrum an advantage for avoiding predators and predating other animals themselves. It’s also possible that there was a predator that had extremely effective camouflage, leading to mantis shrimps needing to evolve the ability to use polarised light to be able to avoid them effectively.
Whatever the reason, the stromatopod eyes will forever and always remain a delightful mystery for biologists to ponder for many years to come.
The Mimic Octopus
Perhaps one of the wonders of the reef; the astonishing mimic octopus is one of my all-time favorite animals on the planet. One of the few animals that mimics not only one but several different others, including both plants and animals, it is in a category all on its own.
The mimic octopus can rapidly change the shape, colour and texture of its body to imitate different creatures depending on the type of danger it’s perceiving. For instance, a large diver appears nearby, it can squee
ze into a hole, and dangle two of its legs out with banded blacks and whites across them, looking just like a very threatening banded sea krait! Or maybe it will gently swim upwards with all of its legs pulsing at once, just like a big jellyfishmight.
But as scientists continued to observe them, they noticed that not all of the mimic octopus’s behaviours could easily be identified. Some of them were just downright strange; for instance, walking along the bottom of the reef with two tentacles out in front and one behind… or making itself into a traffic-cone shaped mound and standing very still… what could it be mimicking then?
There is a theory that some of these mimic behaviours are from animals or things in the reef that no longer exist… some genetic memory of them remained in the mimic octopus and now we are able to see strange hints and shadows of a long-lost creature… amazing!
Here is a video of a mimic octopus in action, along with some of the behaviours that are harder to understand… Do you think you know what creatures it’s playing as?
Other Anachronisms
The list of potential anachronisms in all of the different species in the ocean is nearly endless, but here is a short list of some other speculated evolutionary anachronisms in the ocean:
Leatherback Turtles: after laying eggs, the leatherbacks will go to great efforts to disturb the surrounding sand over their egg clutch, although it’s not clear why they would do this as all of the known predators for their eggs wait until they hatch and go after the hatchlings. It’s thought at some point in their past there was an oviparous predator that would dig up and eat their egg clutch, and this was a way to disguise where it was. The leatherback turtles’ size and ability to dive deep in the ocean also seems to be an evolution for a sea filled with large predators that no longer exist.
Pufferfish Venom: one of the few animals in the world that can produce Tetrodotoxin, the toxicity of this fish seems like complete overkill to the predators that would probably try to eat them. It’s possible this was an evolutionary response to a much more difficult and dangerous ocean at some point in the past. In fact, the excessive number of spines in many Tetraodontidae fish tend to indicate a much more dangerous environment full of predators that likely preyed on them. Now their most dangerous adversary is likely just the dolphins that chew on them to get high…!
Damselfish’s Extreme Territorial Behaviour: any diver will eventually run into a bite or two from this tiny but fiercely territorial fish, which is far in excess to anything that could be expected from such a tiny animal. It’s possible that this behaviour evolved from an environment in the past where competition was far higher and resources much scarcer, and no-one thought to tell the poor damselfish that it can take a break now, there’s plenty of sea floor to go around…!
Nudibranch and Flatworm Toxicity: the level and toxicity of many nudibranchs and flatworms, along with their very prominent aposematic warning colouration, seems to many biologists to be quite excessive to the amount of predation they experience, indicating that this may be a holdover from a previous time when they were predated on.
Parrotfish: These amazing and beautiful reef fish are known for their striking colouration, which can change from their juvenile stages through to adult and even throughout different times of the day. The reasons for this are not entirely clear, but it’s very common for various species of fish to mimic other animal’s warning colourations in order to avoid predators. Certainly, that’s the case for many parrotfish in their juvenile stages. It has been theorised that they are mimicking the colouration of the reef as it appeared in the distant past, when it was far fuller of life and colourful than it is now.
Pipefish: The sheer variety and colour variation in these fish seems to be too intentional not to be linked to plants and corals, likely some of which that now no longer exist.
Some of these anachronisms are quite common during dives. So, keep a look out for them while you are on your next drive trip with Flow, see how they fend of predators with their moves and colours.
Roy Kittrell is an avid naturalist diver and underwater photographer, his work can be found on instagram
@roythedivebro