A few times in our Notebook wanderings of the Arctic so far, we have come across the creatures that live under water, but when we talk about narwhals, bowhead whales, salmon and chad, what really is the difference between these animals? Well, that’s what we’ll be finding out this week, when we look at what sets apart whales and fish, including a look at their ancestry, lifestyle and even the way they move. Enjoy!
Early underwater life
Life on our planet is thought to have started in the waters. It began around 3.7 billion years ago with cells, bacteria and eventually amoeba-like blobs. Over time these blobs developed into more complex and diverse shapes, and by 530 million years ago they had developed into the earliest fish. These were characterised by having a backbone and a skull, though they had not gotten round to developing jaws quite yet. These early fish were called agnathans, meaning jawless fish, and some of them are actually still around today in the form of the slime making hagfish and the horrifying looking lamprey.
485 million years ago onward, in periods known as the Ordovician and Silurian, fish started to develop jaws. The earliest lineages to have them were spiny sharks, and a group call placoderms whose head and body were covered by solid armour. It was at this point, in the late Silurian, that an important division took place. The fish lineage split into two directions. One led to the Chondrichthyes, meaning cartilaginous fish, which includes sharks, skates and rays. This group is characterised by having a body made mostly of cartilage, the same substance that makes up our ear lobes, and eventually led to all the sharks, skates and rays that we have today. The other group that split off at this point was the Osteichthyes, or bony fish. These fish, which had bones, split off into two more groups, the ray finned fish and the lobe finned fish.
The ray finned fish ended up producing species like tuna, cod, eels, sea horses, angler fish and many more. The lobe finned fish produced fish like the coelocanth, thought until recently to have been extinct for millions of years, until it popped up alive and happy off the east coast of Africa. However, they also led to another very interesting lineage… us! Lobe finned fish were one of the earliest creatures to crawl on to land, leading to the first amphibians, reptiles and eventually mammals. These are the fish that are the great great grandparents of much of life on land. This early group, the air breathing, land walking Tetrapods (meaning four-legged) developed around 385 million years ago.
Whales- return to the sea
So first we had jawless fish, of which only 2 types exist today, then we had the Chondrichthyes or cartilaginous fish which are our sharks, skates and rays of today, then we have ray finned and lobe finned fish and the resulting life on land (mammals, reptiles, birds etc…). Where do whales come into all of this? Are they one of the many different types of bony fish? A less toothy member of the shark family? Interestingly, no. Oddly enough the closest living relative of the modern whale is the hippopotamus. Around 50 million years ago whales and hippos shared an ancestor. The descendants that led on to create hippos were quite happy with a semi aquatic lifestyle, bobbing around in watering holes and then lumbering back on to land. The ancestor of whales on the other hand, committed to returning to the waters.
Starting off with four legs and a tail, this whale ancestor, one of which was called pakicetus, spent a lot of time swimming in water. It had fur and carnivorous teeth, and looked almost like a wild dog or giant rat. Over time its tail got stronger and its legs shorter, with the fingers becoming webbed and flipper like. The nostrils, through the slow process of evolution, started to move from the front of the face to the top of the head. Eventually the back legs disappeared completely (after all, why waste energy and resources growing back legs if you’re not going to use them) though a tiny remnant of the pelvis can still be seen in whales today, as it hasn’t totally evolved away yet.
This group of animals went on to create a diverse array of species, including dolphins, porpoises, blue whales, narwhals and belugas. Though they are ancestrally related to fish, they are as much of a fish as a cat, rat or bat, or indeed hippo is.
There is one little caveat which has to be mentioned here. Chondrichthyes is a scientific classification having particular rules and family traits which allow the sharks, skates and rays to be grouped together. ‘Fish’ however is a bit of a made up thing. It’s not a scientific term, and can actually be a bit of an annoying term for hardcore marine biologists.
What we mean when we say fish is creatures in the scientific Actinopterygii group (ray-finned), and in the Sarcopterygii (lobed finned) group, except not all of them because we don’t include the stuff on land, oh and even though they’re related we also don’t include the sharks (well, even that depends on who you ask..), so ‘fish’ isn’t an exact scientific classification.
If it’s just a broad rough group then, why don’t we include whales in it? Well, there are a few important differences between whales and fish which really do set them apart, aside from their ancestry.
The first is breathing. Fish living in the ocean for millions of years have developed to extract air from the water through gills, which filter the oxygen out. Some animals living between the water and land, like some crabs, still use gills, but for those creatures permanently on land this was an inefficient way of breathing as it depends on the gills staying wet. Mammals developed lungs through which we breathe, and it was mammals who ended up being the ancestors to whales. This meant that when the whales re-entered the water they didn’t have gills so had to continue breathing through their lungs, which is still true of whales today. Whilst they can hold their breath for long period of time, up to 90 mins in the case of sperm whales, they do still need to come to the water’s surface to breath. They adapted to this, as mentioned earlier, by their nostrils moving to the tops of their heads, forming blow holes which allow them to breathe easier when surfacing.
The second difference also comes from whales having mammal ancestors. Fish generally produce offspring through eggs. These are fertilised, grown and lain in a variety of ways. Whales however have the mammal reproductive system, where they carry young in their womb, have a placenta and suckle the young after birth, producing breast milk. Some sharks do also appear to give birth to live young, but in reality the eggs just hatch inside them instead of out, and they don’t produce breast milk, just leaving the young to fend for themselves. Whales also tend to have more extensive parental care compared to fish.
Another simple way to tell whales and fish apart is looking at their tail. Fish and sharks swim by moving their tail from side to side. When whales re-entered the ocean however, they had to learn to swim all over again and happened on an equally effective but slightly different method- they swim by moving their tails up and down. What is a whale shark then? Well, if you watch its tail you can see it moves side to side so it’s a shark, just called a whale shark because it’s big. Another difference between the groups is that all whales are warm-blooded, whilst (almost) all fish are cold blooded.
What’s the difference between whales and fish? Well, aside from fish actually technically not being a thing but being a word we’re going to use anyways, fish are the ancient animals of the sea. Whales on the other hand are a younger group who climbed on land first before returning to the oceans, a detour which has led to differences in physiology and lifestyle compared to their fishy ancestors. How they raise their young, move and even breath is more closely connected to their ancestors on land, but after returning to the oceans they have adapted valiantly into a diverse set of species spread all over the globe, from the tropics right up in to the Arctic.
Whale evolution: https://evolution.berkeley.edu/evolibrary/article/evograms_03
Evolution of the fish and beyond:
Cover Image: George Karbus