[Toad asked,] “What did you write in the letter?” Frog said, “I wrote ‘Dear Toad, I am glad that you are my best friend. Your best friend, Frog.” “Oh,” said Toad, “that makes a very good letter.”
Frog and Toad are Friends, Arnold Lobel
Inside the phylum of vertebrates, there are number of classes or “states” that you would find in the atlas of the Kingdom of Animals. So far we have met the ones in the amphibian class, as well as fish, which are actually in three different classes: the bony fish (such as salmon, tuna, and most other common fish), the fish with cartilage (sharks and rays), and the jawless fish (including the strange hagfish). In the next couple of chapters, we will also meet the reptiles (snakes, turtles, and lizards), birds, and mammals (basically anything furry). By far the most numerous class is the bony fish, which has three times as many members as the next most populous (the reptiles and birds, which are pretty closely tied). The least populated class (also by far) is the jawless fish, with only about 50 species identified worldwide.
Animals come in an astonishing variety. Even animals that are relatively similar have some surprising differences. Fish and amphibians are both cold-blooded vertebrates that live at least part of their lives in the water. The differences between them abound, however, from the way that they breathe to their skeletons to their skin.
Let’s consider these two from the inside out. Fish have many of the same organs that we have, such as a heart and stomach, but they also have a special organ called a swim bladder, a fixture that allows a fish to stay at a particular depth of water without having to work for it. Fish are conspicuously missing one of our vital organs: lungs; instead, they have gills, slits made of thin flaps of skin near the fish’s head. Fish pass water from their mouths through their gills, which remove oxygen from the water much as our lungs remove oxygen from air. As we have already mentioned, some fish have skeletons made of cartilage, while most fish have skeletons made of bones like ours.
Amphibians’ most distinguishing characteristic is that they go through a metamorphosis, a change in the shape of their bodies. All other vertebrates are born as small versions of their adult selves, whereas young amphibians, such as tadpoles and efts, are born more like fish, with gills (although they are usually sticking out the sides of their heads and quite fluffy), and then grow to their adult form with lungs and breathable skin. Surprisingly, there are amphibians that have neither gills nor lungs: they breathe exclusively through their skin or sometimes also through the skin in their mouths. Otherwise, amphibians have organs similar to fish, except the swim bladder.
Looking at the outside of a fish, we see some distinct characteristics. Most fish are covered with scales, a special type of stiff skin that helps to protect the fish; it might also help the fish to swim faster or to blend into the environment. Fish do not have arms and legs like we do, but rather several different types of fins. The fins near the front, the pectoral fins, are used for steering right and left. Closer to the back, you will find the ventral fins, which are used for going up and down, as well as stopping. The dorsal fin – the classic shark’s fin sticking out of the water, is a stabilizer that helps to keep the fish from rolling. The tail fin, also known as the caudal fin, moves side to side or up and down to push the fish through the water. These fins, along with the streamlined body shape of most fish, allow such amazing abilities as salmon being able to jump up waterfalls or flying fish to glide above the water with modified pectoral fins or great white sharks to chase down their prey with great speed and skill.
On the outside, many amphibians look like lizards, with the four short legs of the newts and salamanders for scurrying and climbing; of course frogs and toads have longer back legs for hopping and jumping, while a few amphibians, the caecilians, do not have any legs at all. The amphibians are, of course, discernible from lizards by their smooth, moist skin which is permeable (air can move through it). Their skin has special adaptation to keep it moist, including special ducts along their sides, known as costal grooves, and moisturizers that keep them slippery and, in some species, makes them poisonous. Most also shed their skin regularly (sometimes daily), oftentimes eating it when finished.
Near my home in southern Indiana lives a simply terrific amphibian known as a hellbender (or a snot otter if you would rather). Despite its slightly off-putting name, these salamanders (the largest in North America) are harmless creatures vital to the shallow, rocky rivers they inhabit. They hide under one particular rock all day, every day like an old grump. When night falls, however, they become one of the top predators of the river, gliding among the rocks in search of its prey of small fish and crayfish.
Unfortunately, fewer and fewer hellbenders are found every year in rivers throughout the eastern United States. (This sad fate is befalling many amphibians – their special skin and peculiar habits make them especially susceptible to changes in their environment. Because of this, amphibians are considered an indicator species of an environment’s health, a gauge that shows when things are going well or less well.) Dirt and sand are running off of nearby roads and uncovered farm fields into the rivers, covering the rocks where these lithe amphibians hunt and reducing their ability to breathe in the water. Thanks to zoos across the country, however, hellbenders have a fighting chance. Zoos are raising these creatures from eggs (which look like large marbles with a teeny salamander inside) until they are large enough to be able to be more successful in the wild. Hopefully these “head start” programs will help keep this fascinating creature in our rivers for years to come.