14.1: Lab 14 Background

Learning Objectives

• Identify vertebrates to the phylum level based on visual characteristics or a list of traits.
• Build a dichotomous key to describe differences in traits between animal phyla.

Introduction: Vertebrates

Thus far, we’ve learned about various phyla of invertebrate animals. The main trait that distinguishes invertebrates from vertebrates is the absence of a spinal column. So vertebrates are animals with a spinal cord protected by a spinal column. While we sometimes refer to the spinal column as a “backbone”, it is not always made of bone! Some vertebrates don’t have any bones and instead have skeletons made of cartilage. Many animals on land are vertebrates. However many aquatic animals like fish, whales, and sharks are vertebrates as well. These organisms are all grouped into the phylum Chordata.

Animals in the phylum Chordata all have a few traits in common, including a nerve cord, pharyngeal slits, a postanal tail, and a notochord that helps embryonic development can protect the nerve cord. Some chordates are actually invertebrates because their notochord is not a series of vertebrae - hard, hollow structures that surround and protect the nerve cord. But our focus in this lab will be specifically on vertebrates.

Figure 1. Phylogenetic tree of vertebrates, from Yamamoto et al. 2017.

Fish

As we can see from Figure 1, most vertebrates can be classified as fish. In fact, from a phylogenetic perspective, even the tetrapods (amphibians, reptiles, birds, and mammals) are fish! But we really only use the term “fish” to refer to the aquatic, non-tetrapod species. These animals are very versatile in size, color, shape, the depth they can survive in, and the type of aquatic biome they thrive in (salt vs fresh water). Despite all these differences, all fish share certain traits: they live in aquatic environments, have scales, gills, and a skeleton made up of bone, cartilage, or both.

Figure 2. Fish diversity (image source).

Most vertebrates, including fish, amphibians, and reptiles, are ectothermic (which we may also know as cold-blooded). Ectothermic animals generate little to no internal body heat and so rely on the environment for a source of heat. Their body temperature tends to match the environmental temperature. Review Figure 3 and think about how it depicts the difference between ectothermic and endothermic (warm-blooded) animals.

Figure 3. A line graph showing the relationship between body temperature (°C) and ambient temperature (°C) in endothermic organisms, such bobcats, and ectothermic animals, such as snakes.

Agnatha (jawless fish)

As their name (both scientific and common) suggests, these fish do not have hinged jaws. However, they do have teeth! The most common examples include lamprey and hagfish. Lamprey are known for their circular rows of teeth that help them grasp onto the body of other animals where they consume the body fluids of their host. Some lamprey also eat algae and other microorganisms. Hagfish are better known for secreting a thick, fibrous, slimy mucus used as a defense mechanism.

Figure 4. Lamprey (left; source) showing its rows of teeth and a hagfish (right; source) secreting its mucus.

Chondrichthyes (cartilaginous fish)

As their name suggests, these fish have skeletons made of cartilage, not bone. For these organisms, there is not a single bone in their body! Examples include sharks, rays, and skates. Sharks are known for their rows of sharp teeth in a hinged jaw. However, sharks like the whale shark filter feed on plankton and microorganisms and therefore have reduced teeth with little to no function. Rays and skates both have more circular bodies with large pectoral fins that are fused to the head.

Figure 5. Examples of Chondrichthyes species, including (A) shark, (B) ray, (C) chimera, and (D) skate, from Wilson et al. 2021.

Osteichthyes (bony fish)

As their name suggests, these fish have skeletons made of bone. There are two main groups within the bony fish: ray-finned fish and lobe-finned fish. Almost all of the commercially available fish for consumption are ray-finned fish. In general, ray-finned fish make up a majority of all extant fish. These fish are characterized by relatively thin fins with thin bony spines connected by skin. Lobe-finned fish have much thicker, muscular fins with limb buds. Hundreds of millions of years ago, a lineage of lobe-finned fish began evolving fins that could also be used for moving on land, eventually leading to the tetrapods (animals with four limbs): amphibians, reptiles, birds, and mammals.

Figure 6. Many examples of the diversity in Osteichthyes (source).

Amphibia (amphibians)

Amphibians spend part of their life on land and part of it in freshwater. They are considered the first terrestrial tetrapods to have evolved from lobe-finned fish and represent an incomplete transition onto land because of their reliance on water for their life cycle. Most amphibians lay eggs in water. Larvae begin life with gills but metamorphose into adults with lungs and other traits necessary for life on land. Amphibians are the first to have true limbs and keratinized skin instead of scales. Gas exchange can also occur across the skin, so the amphibians’ skin is typically thin and kept wet. Some species have glands that produce a mucus that help keep the skin moist, and some have a mucus that carries toxins. Examples of amphibians including frogs, toads, and salamanders. The image below provides examples of amphibians.

Figure 7. Different types of amphibians.

Reptilia (reptiles)

This class of animals is the first group to successfully survive on land independent of water. One of the reasons for their success on land is due to their scales that are highly keratinized, reducing dehydration. They are also the first taxa to evolve the amniotic egg. In fact, reptiles, birds, and mammals all inherited the amniotic egg from their common evolutionary ancestor. The amniotic egg has extra layers of membranes and fluids that help protect the developing embryo against the external environment. Reptiles lay this amniotic egg within a hard shell, enabling them to reproduce in drier habitats without a body of water. Examples of extant reptiles include turtles, crocodiles, snakes, and lizards.

Figure 8. Examples of reptiles.

Aves (birds)

Data now suggest that birds belong within the reptile group (clade is the evolutionary term; see Figure 1). In fact, modern birds evolved from a specific group of dinosaurs called theropods, so in a way, dinosaurs still exist! However, birds display a number of unique adaptations that set them apart from reptiles. Unlike the reptiles, birds are endothermic, meaning they generate their own body heat through metabolic processes (like the bobcat from Figure 3). One of the most distinctive characteristics of birds is their feathers, which are modified reptilian scales. Birds have several different types of feathers that are specialized for specific functions, like contour feathers that streamline the bird’s exterior and loosely structured down feathers that provide insulation. Feathers also often act as a signal for males to attract females for mating. Of course, birds also have wings, although not all wings enable flight. Flightless birds, like ostriches and emus, often have reduced wings with little or no function. But others, like penguins, may have their wings modified for other functions like swimming.

Figure 9. Bird diversity, from Tobias 2022.

Mammalia (mammals)

Mammals are vertebrates that have hair and mammary glands used to provide nutrition for their young. Fun fact: mammary glands are actually just modified sweat glands! Certain features of the jaw, skeleton, skin, and internal anatomy are also unique to mammals. The presence of hair is one of the key characteristics of a mammal. Although it is not very extensive in some groups, such as whales, hair has many important functions for mammals. Mammals are endothermic and hair provides insulation by trapping a layer of air close to the body to retain metabolic heat. Hair also serves as a sensory mechanism through specialized hairs called vibrissae, better known as whiskers. These attach to nerves that transmit touch stimuli, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide structural protection, as in porcupines and hedgehogs, or even in its color, as in skunks.

There are three groups of mammals, defined by their modes of reproduction. Monotremes, including the platypus and echidna, are the only oviparous mammals, meaning they lay eggs instead of giving live birth. Marsupials, including kangaroos and opossums, give birth to very underdeveloped offspring who then climb up into the mother’s pouch to continue developing. Eutherians, also known as placental mammals, nourish their embryos internally with an organ called the placenta. About 94% of extant mammals are Eutherians.

Figure 10. Mammal diversity.

Introduction: Food Webs

In our labs, we’ve grown microscopic bacteria and fungi species from all kinds of surfaces. We’ve looked at microscopic protist species in pond water. We even grew microscopic fungi species from within plant leaves! We also surveyed the plant life around us on campus and learned about the enormous diversity within the animal kingdom. We can conclude that life is all around us and all around each other. Thus, we know that organisms must have evolved to live in the presence of one another, interacting in many different ways.

Ecology is the study of how life interacts with other living organisms and the surrounding environment. Long term evolutionary and ecological interactions between species (interspecific) are called symbioses. For example, exploitation describes an interaction where one organism consumes another organism, such as predation or herbivory. Similarly, some species feed off others but not so much that it kills the other organism, which is called parasitism. Mutualisms are a very different kind of interaction where organisms have evolved to live in the presence of one another cooperatively. In this case, both species benefit from their interaction. Lastly, commensalism occurs when only one species benefits while the other is unaffected.

There is an amazing and diverse community of life that exists alongside humans in our own neighborhoods and on our own campus. Humans often don't see it, notice it, or recognize it while we go about our day-to-day activities and busy lives. This part of our lab challenges us to take notice of, and document, the community of organisms living here around us. We will also observe how various species are interacting, label their relationships, and define their roles in the ecosystem. Organisms that perform photosynthesis will be labeled producers. Any organisms seen in a predatory relationship will be labeled some level of consumer. If the organism is eating living plant material, they are called primary consumers. If the organism is eating something that would eat plant materials, they are called secondary consumers. Then building from there, an organism that eats a secondary consumer is called a tertiary consumer. Some species, such as bacteria and fungi often found in soils, are decomposers that use dead plant/animal materials as a food source.

A visual representation of how species within an ecosystem interact with one another is called a food web. Below is an example of a food web for the savanna ecosystem (Figure 11). Notice how there are multiple arrows between some of the organisms - this is important because it shows the interconnectedness of species within an environment and highlights the reality that organisms don’t eat only one food source. Today we will be observing the ecosystem around us and attempting to draw a representative food web.

Figure 11. Diagram of a simplified food web for the savanna (image source).