TOEFL Listening Practice: Lecture01
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MP3 – SoundCloud
- Triassic period :A
Silurian period :B
|Welcome to our first marine zoology lecture. Today we are going to look at fish – a very generic title, you probably think. So, I’ll begin by answering a very simple question. What is a fish? It sounds like an easy question to answer but actually, it’s not. Taxonomists – that’s biologists involved in the classification of living species, actually think in terms of what isn’t a fish. Basically, anything that is not, not a fish, is a fish. Specifically, the term fish describes any animal that is part of the Subphylum Vertebrata but is not a tetrapod.
|Subphylum Vertebrata and tetrapod? I’ve never heard those terms before.
|Well.. essentially animals that are part of Subphylum Vertebrata are vertebrates. And tetrapods are a group of vertebrates that includes amphibians, reptiles, birds, and mammals. In other words, a fish is not an amphibian, reptile, bird or mammal. However, many types of aquatic animals commonly referred to as “fish” are not fish in the sense given above; examples include shellfish, cuttlefish, and jellyfish, as they are not vertebrates.
|Now the classification of fishes has undergone a great deal of change over the last few decades. Taxonomists classify organisms according to their most recent ancestor. Each taxonomic unit thus will contain all the species that developed from that species. But since the 1980s, there has been a vast amount of work done in the field of genetics which has allowed us to recognise which species are in fact related, whereas in the past, all this was inferred from their morphology, so in the outdated system, we might see groups that do not contain all the descendants from an ancestral species.
|Hmm…Let me give you an example of what I’m talking about, this involves hagfishes and lampreys. In the old system, hagfish and lampreys would be placed together under a category of jawless fishes, separate from any fish with jaws. In a new system, hagfish would be separate from lampreys, in fact they would be separated from all vertebrates, while lampreys would be classed alongside vertebrates, because there is morphological evidence that, although lampreys are not vertebrates, they are descended from one. So as zoologists, I would advise you against the use of the word fish. Fish is a paraphyletic term, by which I mean that the term ‘fish’ includes some but not all of the descendants of a common ancestor. Terrestrial vertebrates are descended from a type of fish, so, in modern classification, that would make you and I and our pet cats and dogs all, essentially, fish..Ha. That’s why such terms have no place in modern taxonomy.
|Okay! Let’s go back to the first organisms that we now consider ‘fish’. Fish, like mammals, birds, reptiles, and amphibians, are all vertebrates, but the earliest ones, which first appeared during the Cambrian period of 550 million years ago, were actually soft-bodied animals that lacked true spine. They did, however, have a notochord, a special structure that is not considered a true spine, but it allows soft-bodied animals to be more agile than their invertebrate counterparts.
|Fish continued to evolve during the Paleozoic era, diversifying into a wide variety of forms. Many of these fish developed external armor that protected them from predators. In the Silurian period, we began to first see the fish with jaws, and many of these, sharks, for instance, became formidable marine predators. Fish in this period also began to develop paired fins. Many of the fish species of this period are now extinct. In the Triassic period, we began to see the fish with bones. Many bony fish probably evolved during the Triassic period, about 200 million years ago, and the most recent evolved orders of bony fishes include flatfishes, triggerfishes, pufferfishes, and molas. Now there are about 28 000 living species of fish that have been formally described and recognized.
|Okay. So, one important characteristic of fish is that it is ectothermic, which means it relies on outside heat sources in order to stay warm. Fish, like amphibians and reptiles, belong to a group called ectotherms meaning that these animals do not produce heat to maintain a constant body temperature so they rely on outside heat sources to stay warm. However, not all fish are ectothermic. Tuna, swordfish, and sharks are not entirely ectothermic – they can heat their bodies significantly above ambient water temperature.
|Alright! I think that is enough for today’s lecture. There are many other interesting things about fish we can talk about. We will continue our discussion about fish in the next lecture.