Fish are so abundant in the ocean (even given how humans overfish, and them often being prey), I kind of wonder
1. Why don’t marine animals converge to a more fish like form? (Aside from say land mammals like whales and dolphines who decided they want to go to the sea)
2. What benefits are there in having a fish-like form in the ocean? It must be evolutionarily advantageous for some reason.
3. Did the first non microscopic multicellular beings have fish-form?
In: Biology
Marine animals did converge to fish-like forms. That’s why there are so many species of fish.
There’s technically no proper scientific definition for exactly what constitutes a fish. What we call “fish” are just a bunch of entirely separate species which all evolved to be roughly the same shape, because that shape is good at swimming.
So first we have to define what we mine by “fish-form”. I going to with definition “streamlined body with short fin-like limbs” with this broad definition we have plenty of spieces that have developed more streamlined bodies similar to what fish have.
1. Invertebrates have different enough body structures that evolving into fish like form makes no sense, since fish take advantage of spine for their mobility. For vertebrates you could easily argue that they actually have as far as creating, streamlined bodies suited for swimming. This is definitely true for mammals (Whales and Seals) and birds (penguin). It has been true for dinosaurs as well and you could argue it has happened for example to sea turtles (they are streamlined and feet have developed into flippers though obviously shell isn’t very fishlike) and in amphibians axolotl is example of spieces which has taken more fish like form by not going through metamorphosis.
2. Well in short good mobility in water. There used to be fish with armor plating but they have died out.
Quite literally every fish species evolved into “fish form”. That’s how many animals found it useful.
Advantage: move really, really well in water
Disadvantage: move really bad anywhere else, no limbs to manipulate objects
Not everything takes that form because there are other viable forms that offer different advantages. Evolution isn’t about getting to a “best” answer, its about getting to something that works well enough to procreate.
Compare sharks, dolphins, icthyosaur and rockets. These are three distantly related species, two of which spent time on land earlier in their evolutionary history, and a man made object. They all came up with the same body plan for moving quickly through a fluid – rear propulsion, a point at the front, a long sleek body and fins for stability and steering. There are clear advantages to this body type.
Marine animals that aren’t fish form occupy other niches. They may be more passive filter feeders, or occupy the ocean floor, or leave the ocean to lay eggs, or have hundreds of other lifestyles which the fish shape just isn’t suitable for, but for creatures that wasn’t to move quickly through the water, some variation on fish shape is generally the way to go. Hence all the fish.
There is no such thing as a fish. So that is exactly what has happened.
From Wikipedia:
[after] a lifetime studying fish, the biologist Stephen Jay Gould concluded that there was no such thing as a fish. He reasoned that although there are many sea creatures, most of them are not closely related to each other. For example, a salmon is more closely related to a camel than it is to a hagfish.[8]
Fish come in many different shapes.
There is no single fish unified fish body plan.
However you will note the for example Dolphins and extinct Ichthyosaurs converged independently to a certain fishy bodyplan.
The best shape is always dependent on the environment the food you eat and the stuff that might eat you and what sort of bodyplan you already have to adapt from.
I think part of the issue is the craboform isn’t only a sea thing. Multiple different branches ended in craboform. Like public lice.
Evolution doesn’t have a goal.
Becoming the perfect and dominant sea creature only happens by accident with some species.
Other species just kinda get stuck as a mediocre design. Some stick around longer than others before they go extinct or improve over thousands of generations.
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