Homologous trait Octopi and squid tentacles
Divergent evolution has the squid with two extra tentacles used for capturing prey
Cephelapod
Octopi and jellyfish have homologous traits. Octopi are cepholopods that have, on average, 8 tentacles used to grab their prey and drag it toward themselves. Jellyfish are slightly different in the way that they do not have a skeletal form and have an umbra shaped bell at the top of their bodies.
Both have tentacles but the octopus has an average of 8 while the jellyfish can have millions of tentacles depending on what kind. Each are used for food capture, but the Jellyfish uses these tentacles to inject toxins in its prey while the octopus normally uses it to crush its prey for consumption. These are different for the different ways of mobility as well as the different kinds of prey that each feast upon. While Octopi can sometimes chase their prey, which may be fast or even bigger than them, Jellyfish have slower mobility rates and need more tentacles to ensnare their food.
Jellyfish
Octopus
The acknowledged common ancestor of both of these species of animal is the Cephalopod. The structure of the Cephalopod contains tentacles in varying amounts and has an invertebrate structure simoilar to both Jellyfish and octopi alike.
The Whale is a large mammal that lives in the ocean and has a varying diet depending on the species. The Shark is a fish that primarily is a carnivorous animal within the sea.
Both Whale and Shark possess the analogous trait of fins for movement in water. There are pectoral fins (ones located on the side of the body) that help guide both whales and sharks alike. Dorsal fins are located on the top portion of the body to prevent rolling within the waters themselves, and Tail Fins help with propulsion in the swimming process.
The common ancestor could have possessed the trait of a fin (the Vertebrate). However, that was so long ago, the Mammal that the Whale evolved from needed to find sources of food and thus over thousands of years become more adapt to having fins and swimming, and the shark stayed the same within the vertebrate category
Shark
Whale
Hello Jeremy,
ReplyDeleteI found your post interesting an enjoyed reading it. However, I believe that cephalopod is a group of species, and not an actual organism.
Hello! I didn't even stop to think about jellyfish and octopus' tentacles and how they could be so similar but yet so different from one another. Its fascinating to know that jellyfish can posses millions of tentacles. I also did not realize that whales had a dorsal fins like the shark. I can now see how and why the whale does not need such a large dorsal fin like a shark because of the source of food they need to survive, compared to the shark which hunts for their prey.
ReplyDeleteOkay on your opening section, but this was your opportunity to more fully describe these organisms to offer a clearer picture to your readers about what might have driven the evolution of these traits. This applies to both the homologous and analogous sections.
ReplyDeleteGood description of the function/structure off the tentacles.
Ancestry is where you run into trouble. To be homologous traits, the tentacle structure needs to arise from a common ancestor. The problem is, while octopi are cephalopods, jellyfish are not. Octopi are from the phylum of Mollusks, and not all have "arms". That means they are a derived trait of octopi. Jellyfish are from the phylum Cnidaria, a group that includes sea anemomes. These are distantly related species at best and there is no evidence that the octopus and the jellyfish inherited their structures from a common ancestor.
This is actually a better fit as a pair of analogous traits: Similar structures arising from similar selection patterns due to similar environmental pressures.
Yes, fins in sharks and whales are indeed analogous traits and good description of those traits.
With regard to ancestry:
"The common ancestor could have possessed the trait of a fin (the Vertebrate)."
But wouldn't that mean these traits are homologous? Aren't we looking for evidence that these traits are NOT related genetically?
The common ancestor of the dolphin and shark is an archaic fish, who did possess these fin structures and also passed that trait onto extant shark species. So the question is, did the dolphin also inherit it's fin from that common ancestor? Dolphins "fins" evolved after dolphins split off from terrestrial mammals, i.e., long after the split with ancient shark. This provides us with the evidence we need to confirm that this trait did evolve independently in at least one of these organisms, making these traits analogous.
Good images.
I'd like to start by saying your titles are pretty spot-on. Who doesn't love a good pun? Anyways, great info and I've always found sea creatures to be so fascinating, they look as though they come from a completely different planet sometimes!
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