Often marine scientists as well as fisherman want to test the murkiness (or turbidity) of the water. How can you do this quickly and accurately? Well, first you’ll need a Secchi Disk. With the cheap, easy-to-make Secchi Disk you can test water quality like a pro!

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The Secchi disk was invented in 1865 by Pietro Angelo Secchi, and it’s basically a circular disk hung from a rope and lowered slowly in the water until you can’t see the pattern on the disk anymore. Scientists call this depth the Secchi depth and is related to water transparency.

As you probably figured out, Secchi disk readings are not an exact measure of transparency, because it depends not only on a person’s eyesight but also the brightness of the day. But for most folks, the measurement is quick, inexpensive and relatively accurate for their needs.

The results are best when the disk is used between 10:00 am and 2:00 pm on the shady side of the boat or dock and the reading should be done by the same person. Both Minnesota and Indiana field scientists use this technique when assessing the quality of water in their lakes.

Here’s what you’ll need:

• A white plastic disk with an 8 inch diameter. A paint or bucket lid works well.
• A permanent marker.
• A broom handle (unless you’re measuring very clear water, and in that case use string).
• Graph paper.

Here’s what you do:

1. Divide the disk into four sections with the permanent marker. Color in two of the sections (such that if moving clockwise the sections alternate between being filled in and not filled in).
2. Attach the lid to the broom handle or string (if using the string, attach a weight to the other side of the lid).
3. Lower the disk into the water until you can’t distinguish between the dark and light sections. Mark that depth.
4. Pull the disk up and measure the first depth. Note that on the paper.
5. Lower the disk past the first mark and then pull it up until you can just make out the difference between the dark and light sections of the disk.
6. Pull the disk up, measure the second distance, and note it.
7. The average between the two depths is the Secchi depth.

With a name like hydrometer you might pause and say: “…a what?” You might have even gone a step further and added “why do I want one of those?”  Simply put, hydrometers test the density of liquids. Specifically, they compare the density of liquids to the density of water (a comparison called the specific gravity of a substance). A substance’s specific gravity is extremely useful. We use it to tell how creamy milk is, how much alcohol is in spirits, how salty the ocean is, and much more! In the following experiments we’ll test the salinity of several solutions.

(Note: This is different from a hygrometer, which measures the humidity of the air!)

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Here’s what you need:

• Drinking straw
• Modeling clay
• A drinking glass
• Salt
• Distilled water (or as filtered as you have on hand)
• Permanent marker
• Graph paper (optional)

Download Student Worksheet & Exercises

Here’s what you do:

1. Plug one end of the straw with a small marble-size ball of clay. This is your hydrometer.
2. Fill your glass with water (find a glass that holds about 2 to 2.5 cups of water).
3. Place the hydrometer in the glass. Add or remove clay until the straw floats  midway up your glass. Mark that level “0” with the permanent marker (because there is no salt).
4.  Remove the hydrometer.
5. Add 1 teaspoon of salt to the water.
6. Place the hydrometer in the glass. Mark the new level and label it “10” for 10 ppt (parts per thousand).
7. Add another teaspoon of salt to the solution.
8. Repeat step 6 (except this time mark the level “20” for 20ppt).
9. Repeat until you have marks up to 50 (or higher!).
10. Have a partner prepare unknown solutions of salt and water. Test them with the hydrometer. Graph your findings. Extra credit: try solutions at different temperatures!

How does it work?! The hydrometer works via the Archimedes Principle which states that an object will be buoyed up by the force equal to the weight of the fluid displaced. Thus, the more dense the fluid, the more force it exerts on objects floating in it. This is why the hydrometer moves higher as more salt is added. Cool, huh?

Troubleshooting: Is your hydrometer not working correctly? First, check the plug. If the plug is letting water the hydrometer may be getting heavier as you add salt—the opposite result you expect!

Exercises

1. What do hydrometers test?
2. What is specific gravity?
3. What is the Archimedes Principle?
4. Would a boat float better in water or honey? Why?

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Your brain is divided into two sides, called hemispheres. Through experiments, scientists have determined that each side is responsible for different things. We’ll talk more about this in Unit 19, but for now, just know that the left side of the brain is mainly responsible for language, while the right side is in charge of spatial perception. Also, each side of the brain controls the action of the opposite side of the body.

Humans are not the only animals with different hemispheres controlling different actions. This experiment will explore if feeding in lizards is controlled mainly by one side of the body, and if lizards feed mainly to one side.

Here’s how to experiment with lizard brain lateralization:

1. Obtain a lizard in a terrarium – can you find a friend that already has one and borrow it?

2. Set up a video camera to record the lizard for several days.

3. Place live crickets in the terrarium for the lizard to eat. Watch the lizard eat a few to get an idea of what the strike looks like, but the since you are capturing the action on video, you don’t need to sit there all day.

4. Watch the video of each cricket capture. If possible watch in slow motion.

5. Cover the monitor on which you are watching the strikes with clear plastic wrap, and draw a line from the center of the lizard prior to the strike (right between the eyes) to the point of capture. Be careful that nothing you are doing will damage your monitor.

6. Do this for all the strikes. (Twenty would be a good number to get some reliable data, but realize that will make this experiment take some time, as lizards take a while to eat 20 crickets.)

7. Once you have all your lines drawn, count the number of times the lizard struck to the left, struck to the right, or went straight ahead. Record your results.

What’s Happening: Remember that each side of the brain controls the opposite side of the body. You can determine with side of the lizard’s brain is more responsible for predation (eating) based on which side it strikes on more often. This will vary from lizard to lizard.

Mirror, Mirror, on The Wall… Did you know that a betta fish has a special relationship with a mirror? When you look in a mirror, you recognize that what you see is your reflection. Not all animals realize this. Many animals think the animal they see is another member of their species, and react with either fear or a show or bravado to defend to defend their territory.

Let’s examine the reaction of the standard betta. You’ll need a male betta in a bowl and a mirror… and a little bit of time.

1. Obtain a male betta, and place it alone in a bowl.
2. Place a mirror in the bowl.
3. When the betta sees his reflection, record his response.

What’s happening: Most male bettas will respond by showing all of their fins, a behavior known as flaring, designed to make the fish look large, and protect their territory. When mirrors have been left in bowls, males have become so obsessed with flaring they have even forgotten to eat and died, so take the mirror out as soon as the experiment is over!

In the animal kingdom, only primates have been shown to generally understand that they are looking at themselves in the mirror. Amongst humans, babies under 10 months of age generally don’t understand this concept. If your betta did not react by flaring, see if you can think of why. Is the fish used to living with others and not very territorial? This is the trouble with only testing one animal. There are usually other factors involved.

Lizards and snakes make up the largest order of the reptiles. Although we often think of them differently, snakes are basically legless lizards, from a biological perspective. Reptiles in this group are characterized by having scales or shields on their body and by having a lower jawbone that can be moved independently from the braincase. This allows snakes and lizards to open their mouths very wide, a trait that is especially noticeable when snakes, which also have a very flexible jaw, are eating relatively large food, as you can see below:

Besides the obvious lack of legs, snakes are distinguished from lizard by the lack of external ears. Snakes are all carnivorous, meaning they eat meat of other animals. Snakes frequently eat rodents, insects, eggs, and even other snakes. Almost all snakes lay eggs, and they generally abandon the eggs shortly after laying them.

The somewhat unusual body structure of snakes leads to some unusual characteristics. As was mentioned above, snakes have very flexible jaws. This helps make up for the fact that they cannot use limbs to grasp prey, like most animals do. Also, because their bodies are so long and narrow, if a snake has two of a certain organ, such as the kidney, they are found one in front of the other as opposed to side by side.

Only a small minority of snakes have venom. Of those that do, venom is usually used to immobilize and begin to digest prey, rather than as a means of self-defense. Nevertheless, snake bites can be both painful and dangerous, so it is always best to steer clear of snakes (as well as most other animals) in the wild. Snakes without venom usually kill prey by constricting, or wrapping their bodies around the prey and squeezing tightly.

About 6,000 species of animals belong to the class amphibia, commonly called the amphibians. A class is a group of living things in the same phylum or sub-phylum (in this case, vertebrata) that share certain characteristics. One of the most important characteristics they share is that they begin life in the water, but then spend most of their lives on land.

Although amphibians inhabit many environments, from tropical to arctic climates, they cannot live in saltwater, eliminating the oceans as a place to find these animals. Some amphibians do live in brackish water, which is slightly salty, but these animals generally live in or near freshwater. Amphibians are ectothermic and carnivorous, generally feeding on bugs and other arthropods.

The life cycle of amphibians is one of the most interesting of the vertebrates. Adult animals lay a shell-less egg, usually in a pond or some other freshwater location. A larva then hatches. The legless larva lives in the water, breathing through gills, as fish do. Slowly, over time, the larva undergoes a metamorphosis, or change in body structure. During this change, the larva takes on the adult form, losing its gills, growing four legs, and eventually becoming completely terrestrial, meaning that it lives only on land.

The lifecycle of the frog, in which the larva is called a tadpole, is typical of amphibians.
As part of becoming terrestrial, amphibians must undergo several changes. Their gills are replaced with another respiratory organ, like lungs, allowing them to breathe on land. Their skin also undergoes a change to keep them from losing water and becoming dehydrated. They develop eyelids to more effectively see in a terrestrial environment. Finally, an eardrum develops separating the exterior from the middle ear.

About 500 amphibian species are salamanders. These animals are generally characterized by tails, short legs, and moist skin. The moist skin of the salamander requires them to live in or near water more than many other amphibians. In fact, some salamanders live their whole lives in water. Others live outside water in the adult stage, but stay in swamps, where the ground is moist, and will not dehydrate their skin.

Salamanders are unique in both their respiration and feeding. Some salamanders have lungs and breathe in a way similar to mammals. Others keep their gills into adulthood, and remain in the water, breathing through their gills. Still others have neither gills nor lungs, and breathe through valerian respiration in which air is passed through the skin. In terms of hunting, a muscle called the hyoid muscle shoots out, along with the tongue. The tongue of the salamander is covered in mucus, and prey is captured in this sticky mucus. Salamanders are also the only vertebrate that can regenerate lost limbs.

Frogs and toads are members of the same order, which is a group of similar living things in the same class. People sometimes distinguish frogs and toads based on the fact that toads usually live in drier environments, and have leathery skin to help them in this environment.

However, there is really very little difference between animals referred to as “frogs” and “toads” in this reading, other than the fact that toads do not have any teeth and must swallow their food whole. To make things simpler in this reading, we will just call this group of animals “frogs.”

Frogs can be characterized by long legs and the absence of a tail. They spend their adulthood out of the water, breathing through lungs. Frogs enter the water in the adult stage only to reproduce. For this reason, the males of many species of frogs have mating calls to draw females into the water to reproduce.

Did you know that in order to catch a frog, all you need is a lure and a fishing pole?

How to Find Frogs

Once frogs lay eggs, they are generally fairly easy to spot in and around the swamps and marshes in which they live. Each frog egg starts out as a tiny dark spot surrounded by a thick layer of clear jelly-like stuff. The jelly acts kind of like a shell that protects the egg. Most frogs’ eggs form clumps. This activity will work around April, when frogs lay their eggs.
1. Visit a local pond or swamp and seek out some frogs. Listen for frog sounds and see if you can identify the type of frog.

2. Once you’ve found frogs, look for eggs.

3. Once you’ve found some eggs, make some observations. Are the eggs floating at the surface or under the water? Are they attached to plants or not? If they form a clump, is it small or large?

4. Come back again in a week or so. How do the eggs look different?
What’s Happening: Over time, the eggs will become larger and take on the shape of the larva (tadpole, that will eventually be hatched from it. If you simply can’t wait, you can grow your own frog farm using the materials here.

Watch the silent video below to see how to make your own frog farm!

The cartilaginous fishes are a group of about 1,000 species and share many things in common, including the presence of jaws, paired fins, a two-chambered heart, and bodies made of cartilage.

By far the largest group of fish are the bony fish. Eight species of bony fish make up a small group called lobe-finned fish, including the lungfish, a fish with the ability to breathe air, that can even drown if it is kept in water too long.

Another 27,000 species make up the ray-finned fish. Remember from above that there are a total of slightly less than 58,000 species of all vertebrates. It is clear that bony, ray-finned fish are the most common vertebrates.

The lungfish is one of only eight species of lobe-finned bony fish.

As with fish in general, bony fish vary greatly in size and weight, from the 3.3 meter (11 foot) ocean sunfish, topping the scales at over 5,000 pounds, to the tiny pygmy goby, a mere 1.5 cm (0.6 in).

In spite of the variation in size and weight, bony fish have several characteristics that group them together and make them unique amongst the fish.

First, these fish have the ability to regenerate bone from cartilage inside their body. Additionally, ray-finned fish are the only fish that can see in color. Finally, all members of this group have swim bladders, which they are able to add oxygen to or remove oxygen from. This allows the fish to control its density.

Why would a fish want to do this? As you may know, things that are more dense than the fluid they are in will sink, while things less dense than the fluid will float. By changing their density compared to the fluid they are in (water), a fish can cause itself to rise up higher or sink down lower as needed.

Here’s a short video of a puffer fish during its inflating and deflating stages:

There are a number of reasons why fish are important to humans. They provide a source of food, especially for people who live in areas near water. Fishing is also a popular recreational activity, and many people enjoy viewing these beautiful animals in aquariums every day. People have included fish, and legends of half-fish, half-human creatures in stories and legends since ancient times.

Fish are important to more than just humans however. The food web of the oceans and lakes of the world are some of the most diverse on the planet, and the wide variety of fish that live in these ecosystems play a crucial role in maintaining a balance. Humans have recognized this, and have begun to restrict fishing and recreational activities in areas where too much human activity could be harmful to the aquatic ecosystem.

There are 57,739 species of vertebrates. The majority of these vertebrates can be classified as fish. This includes jawless species of fish and cartilaginous fishes. (Those are fish with skeletons made of cartilage, the same material that makes up your nose.)

Fish are almost always ectothermic. This means that the body temperature of fish changes based on the outside temperature. This is different than other animals (including humans) who keep a constant body temperature no matter the temperature outside.

Additionally, fish generally lay eggs, have two paired fins, and have scales. Finally, fish typically have gills which allow them to get oxygen from water, allowing them to breathe while in their underwater habitat.

There are plenty of exceptions to these general characteristics of fish. Tuna, for example, have the ability to warm their bodies so that their body temperature is warmer than the cool water in which they live. Moray eels do not have scales. As you will read in the next section, not all fish have paired fins. Even what seems to be the most “fish-like” characteristic of all, living in water, is not something that all fish have in common.

Mudskippers are fish that spend a considerable amount of time on land, living for several days at a time on mudflats, where they absorb oxygen through their skin in order to breathe.

The group agnatha, also known as the jawless fish, make up one group of fishes. There are about 100 species of jawless fish, which can be placed into one of two groups – the lampreys and the hagfish. Interestingly, although these fish do belong to the vertebrate subphylum, they do not technically have vertebrae. In fact, this group of fish is so different than fish with jaws, it has led some scientists to wonder if they should be called “fish” at all.

Along with their lack of jaws, the jawless fish are notably different than other fish because they do not have paired fins. Agnatha do not have an identifiable stomach, and don’t have a true eye, instead having a light-sensitive eye-like structure. These fish have bodies made of cartilage and have a heart with only two chambers as opposed to the normal four.

Hagfish also produce a slimy substance which has led some people to call them “slime eels,” although they are not eels at all. The Pacific Hagfish is one example of a jawless fish.

What are the most important animals in the ocean? Whales? Sharks? Giant squids? Think smaller. Arguably, the most important animals in the ocean are found at the bottom of the food chain—the tiny creatures called plankton.

Plankton are microscopic animals that are found all over the world in large bodies of water. They have many different shapes and sizes because they’re not grouped together by how they look, but by their place in the food chain (the bottom). Becoming a planktologist—a scientist who studies plankton—is simple; all you need is a special net.

The plankton nets found in stores are very expensive, often more than $50! Luckily, though, they can be easily made from household items for less than $10.

This is a bonus experiment (the supplies for this project aren’t in the main shopping list), so you’ll find the the supply list of materials you’ll need (below).

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First, gather your materials. Here’s what you will need:

• A cheap pair of nylon stockings (if you use a used pair, be sure to ask the owner first).
• A wire hanger.
• Pliers.
• A plastic bottle.
• A strong rubber-band.
• Strong string (like fishing line, or kite string).
• A washer or plastic ring.
• A Stapler.
• A tray with a dark bottom.

Got all of that? Great! Here’s how to make it:

1. Unwind the hanger (ask for help) using the pliers.
2. Make a 6-10 inch (15-20 cm) ring with the hanger.
3. Cut stocking near the foot. Fit the open end over the wire ring and then staple it on.
4. Cut a hole the size of the mouth of the bottle in the toe, and fit it over the bottle.
5. Secure the stocking to the bottle with the rubber band. Make sure that it’s on tight.
6. Attach three pieces of string (2 feet (60 cm) each) to the wire ring. Space them evenly.
7. Attach the other ends of the string to a washer or plastic ring.
8. Attach a long piece of string to the washer or plastic ring. This is the towing string.

Let’s collect some plankton!

On a boat: Attach the towing string to the boat away from the propeller. Go slowly for 15 minutes. Pour the contents of the bottle into the tray.

On a dock: Throw the net into the water and drag it back a couple times. The more you do it the more plankton you will get.

Now, observe your plankton!

You should be able to see your plankton swimming around with your naked eye, but for a detailed look it’s best to look through a magnifying glass or a microscope. Sketch out a couple of them in a notebook and look at their different body types. Are there any that seem to be moving themselves? Are there any that seem to just be drifting? Although refrigeration helps preserve them, it’s best to observe the plankton as soon as possible.

Why this works: The net works by pushing plankton and water into the bottle and only letting water out. As the net moves through the water, it funnels water and plankton into the mouth of the bottle. However, when water and plankton go in, water and plankton also go out. The net makes sure that the plankton that comes out of the mouth of the bottle goes back in.

Troubleshooting: Not catching any plankton? The plankton may be escaping before they reach the bottle. Are there any large holes in the stocking? Is the bottle securely attached to the net? Remember, the plankton are tiny—any holes larger than those found in a normal nylon stocking will allow the critters to escape.

Another problem may be that the net is not facing forward. Make sure that the string is evenly spaced around the ring. If the ring isn’t facing forward when you pull it forward you may be filtering the plankton from the bottle! If you’re unsure, you can test it in a bathtub before taking it to the ocean, lake, or pond.

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The Hester-Dendy sampler makes it easy to collect marine samples. With some simple parts we can construct a device that we will leave in the water for a couple weeks. Then, presto! We have samples ready to study. It’s the no-muscle way to get specimens.

Some creatures to look for in your Hester-Dendy sampler include mussels,different species of algae, insect larva,and any organism that attaches itself to rocks.

To make your handy, you’ll need a set of materials and a workshop (so this is a bonus experiment!) Here are the supplies you’ll need to gather together – you’ll probably find them in the scrap bin in a corner of a workshop:

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• Five pieces of plywood (each 4 in²)
• One 12in eye bolt
• 2 nuts and 2 washers for each piece of wood
• Weights (fishing weights work well)
• Rope or cord to suspend the sampler

Here’s how you make it:

1. Drill a hole through the center of the wood squares.
2. Attach each wood square to the eye bolt with the washers and nuts. Make sure to leave about 2in of space between each piece of wood.
3. Attach the weight to the bottom of the sampler.
4. Attach the rope to the top of the sample.
5. Lower the sampler into the water where you want to gather samples (on the rocks near the muscles, for example), and leave it there for two weeks.
6. Pull it up in two weeks and marvel at your samples!

How does it work? The Hester-Dendy sampler works by creating new real estate for the organisms you’re sampling. Leaving it undisturbed for two weeks gives the organisms enough time to move in and get comfortable.

Troubleshooting: After two weeks there are still no samples… Well, the problem is either in the placement or the structure. If the sampler is too far removed from what you’re sampling, or in an area not conducive to them (too much movement, exposure to predators, etc) the organisms might now be able to move in. In that case, simply try several locations until one works. Or, the problem could be structural. If the space between the wood is too small, the organisms might not be able too fit. Alternatively, the wood may not be fastened tightly enough—meaning that it moves around while in the water, making it more difficult for the organisms to get on.

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