To answer your comment, yes, you can say 'I ate octopus' on its own, but the past time is always understood. Perhaps it was mentioned earlier in the conversation, for example:
Did you have a good weekend?
Not bad. How about you?
Pretty good, actually. We went to that new seafood restaurant.
How was it?
Nice. I ate octopus.
In this conversation, it's clear that we're talking about a finished period of time, even though the phrase 'I ate octupus' comes a long time after the time period is mentioned.
This is different from:
1. Have you ever eaten octopus?
No. I've never eaten anything like that. I don't like seafood.
2. Have you eaten yet?
No, I'm really hungry actually.
OK, I'll make you a sandwich.
Conversation 1 is about an unfinished time period - in this case, the person's life up until now.
Conversation 2 is about a very recent time which is influencing the present. As Vivienne explained, the present perfect is a tense which links the past with the present - in this case, the person hasn't eaten and so he is hungry now.