What's a Fish?
Can you think of a definition that describes all fish?
What do all fish have in common? A few hints - A whale isn't a fish. Sharks are fish. A lungfish is a fish.
How is your definition holding up?
When it comes to the details, it's actually a tough question for scientists too, because the term fish is used "loosely" in an everyday sense. We tend to know a fish when we see one based on common and easy-to-observe characteristics, such as the presence of fins, gills and scales, but not all fishes possess these characteristics and some animals that are not fishes have fish-like features, such as the whales with fins.
Which of these fish have scales?
The great diversity of living fishes reflects the diversity of modern habitats, and the state of development of life on Earth. The design of each body type that exists today wasn't drawn from scratch to meet today's conditions in a custom way. They developed by stepwise change over countless generations over geologic time. It was more like morphing than building because the living things that developed had to be mostly based on what was there before. As the Earth changed, some of the changes became an advantage while others didn't. Species of living things came and went.
Fish morphed in this way from ancestors who weren't quite fish. In turn, when land became hospitable for life, land vertebrates morphed in this way from ancestors who were once fish. That's what we know from observing and analyzing animals and their genetic makeup, and studying the sequence of the fossilized remains of extinct animals.
Well known paleontologist Neil Shubin puts it this way - that the anatomy of his head and the seemingly illogical construction of its wiring, the major nerves, are like an old building that's been renovated by remodification of ancient infrastructure instead of being designed all new. He says, "Nobody in his right mind would have designed a building from scratch this way."
When we look at fishes as all the descendents of a single particular ancestor, one that has been designated as the first fish, it's easier to say what's a fish. Fishes are all the descendants of that first fish. Those that look like fish but aren't related don't count. Those that don't look like fish, but are actually related need to be included in the family tree. In human families, friends as well as relatives count, but in unraveling the development of features in living things, only relatives count because body characteristics are passed down by common genes. Whether those genes are successful depends on everything else - the environment, interactions with other living things including friends and predators.
How do fishes relate to other kinds of living things?
Since some fish that most people would clearly call a "fish" don't have even the most common physical traits of most fish, like catfish that lack scales,what characteristics do all fish possess? What are the key characteristics of fish?
The earliest fishes include the hagfish and lampreys. Sharks arrive on the scene early and the most common families of fishes that most of us would consider "normal" fishes are recent teleosts. Most major lines of fishes are still represented by at least a few living descendants today but the teleosts are the biggest group by far. A few lines go extinct before modern time.
Fishes as in inclusive group.
Because a genealogical tree often includes a lot of people before and after you, it becomes unwieldy to visualize after a few generations, particularly if you want to show a tree that's inclusive and not from a particular person's viewpoint. Overall, there are way more people now than even a hundred years ago, but family trees are messy because of intertwined families. The Tree-of-Life that shows the relationship of all life on Earth gets more branchy in the forward direction in time, but wider in reverse towards the trunk of the tree. It's simpler than family trees in general because the branches don't cross. Species are always subsets of existing species. New and multiple ones can be derived from existing speices, but species don't combine to form new ones like your family tree, where individuals are the product of two unrelated people.
How do you recognize a new species of fish?
What is a species? Individuals that interbreed and produce fertile offspring belong to units called species. Because they are related, these individual possess similar traits. While physical traits are important in identifying different species, it can be misleading to use only physical traits in identifying related species because some traits appear in unrelated families. For example, two unrelated animals that live in the same environment might have independently developed similar body parts to solve a common problem, such as the development of fins on fish and whales. The concept of species is too complex to go into detail here, particularly for plants.
Knowing a fish is like knowing someone, it involes knowing its name, being able to tell it apart from others, and knowing something about it, such as what it's like at different stages of life, who it's related to, what it likes to eat and how it makes its living.
Recognizing a new kid on the block requires knowing who already lives there. Since a newly discovered species doesn't come with a sticker that says so, it takes great expertise to recognize a new fish among many and to know what to call it. The name we give to a living thing isn't random. It is intended to have the correct "last name" that places it in the right family. The problem of naming goes hand-in-hand with sorting out family affiliation.
Although not entirely foolproof, DNA-based methods of sorting out family relationships have recently made great advances in sorting out the relationships between all living and extinct life. As more knowledge becomes available, there are bound to be some adjustments in the Tree-of-Life and how we name living things. On the other hand, that a tree that was developed from visual comparison of living and fossil specimens ...
How are fish named and identified?
• Genus and species • Types of type specimens • World of the first fishes