Learning Objectives

• Define the basic traits of animals and the main features of invertebrates.

• Identify the three main types of body symmetry: asymmetry, radial, and bilateral.

• Describe the unique characteristics of the eight major invertebrate phyla.

• Explain how macroinvertebrates are used as indicators of water quality.

Key Vocabulary

Invertebrate

Animals that do not have or develop a vertebral column, also known as a backbone.

Ectothermic

An animal that relies on its external environment for its main source of body heat.

Radial Symmetry

The arrangement of body parts around a central axis, like spokes on a wheel.

Bilateral Symmetry

The property of being divisible into symmetrical halves on either side of a unique plane.

Sessile

An organism that is fixed in one place and is unable to move about.

Benthic

Relating to the bottom of a body of water, where these organisms are typically found.

Types of Body Symmetry in Animals

Asymmetrical

• ​These animals have no defined body shape or symmetry.

• ​​They cannot be divided into equal halves along any plane.

• ​A common example of an asymmetrical animal is a sponge.

Radial Symmetry

• ​The body parts are arranged around a single central point.

• ​​Drawing lines through the center creates multiple equal parts.

• ​This body plan is common in animals like jellyfish.

Bilateral Symmetry

• ​An animal can be divided into two equal mirror-image halves.

• ​​This creates a distinct left side and a right side.

• ​This leads to the development of a head, called cephalization.

What is an Invertebrate?

• Invertebrates are animals without a backbone, making up 97% of all animal species.

• They are ectothermic, relying on the environment to regulate their body heat.

• Many have an exoskeleton or shell for support and protection.

• Vertebrates, in contrast, have an internal skeleton made of bone or cartilage.

Solved Example 1
If scientists have identified 1.5 million animal species, approximately how many of those species are invertebrates?

Step 1: Analyze and Sketch the Problem

• Goal: Calculate the approximate number of invertebrate species.

• Knowns: Total animal species = 1.5 million; Percentage of invertebrates = 97%.

• Unknown: Number of invertebrate species.

• Formula: Number of Invertebrates = Total Species × (Percentage of Invertebrates / 100)

Solved Example 1
If scientists have identified 1.5 million animal species, approximately how many of those species are invertebrates?

Step 2: Solve for the Unknown

Solved Example 1
If scientists have identified 1.5 million animal species, approximately how many of those species are invertebrates?

Step 3: Evaluate the Answer

• The calculation correctly applies the percentage to the total number of species.

• The answer, 1,455,000, is 97% of 1,500,000, which is a reasonable and large portion of the total, as expected for invertebrates.

Invertebrate Phyla: Porifera & Cnidaria

Phylum Porifera

• These are the simplest animals, lacking true tissue layers, nerves, or muscles.

• They are sessile, meaning they are non-moving, and are also filter feeders.

• Most are hermaphrodites, meaning they are able to produce both eggs and sperm.

Phylum Cnidaria

• This phylum is defined by radial symmetry and the presence of stinging tentacles.

• Unlike sponges, members of this phylum possess true tissues in their body structure.

• They use specialized stinging cells called cnidocytes to help capture their prey.

Solved Example 2
A wildlife biologist surveys a stream and collects 100 macroinvertebrates. 75 are mayflies, stoneflies, or caddisflies. What percentage of the sample are pollution-intolerant species?

Step 1: Analyze and Sketch the Problem

Solved Example 2
A wildlife biologist surveys a stream and collects 100 macroinvertebrates. 75 are mayflies, stoneflies, or caddisflies. What percentage of the sample are pollution-intolerant species?

Step 2: Solve for the Unknown

Solved Example 2

A wildlife biologist surveys a stream and collects 100 macroinvertebrates. 75 are mayflies, stoneflies, or caddisflies. What percentage of the sample are pollution-intolerant species?

Step 3: Evaluate the Answer

• 75% of 100 is 75. This matches the number of intolerant species collected.

• The presence of a high percentage (75%) of intolerant species suggests the water quality is likely good.

Invertebrate Phyla: Worms

Phylum Platyhelminthes

• ​These worms feature a distinct characteristic known as bilateral symmetry.

• ​​Their bodies are flat and unsegmented, a key identifying feature.

• ​They exhibit cephalization, which means they have a defined head region.

Phylum Nematoda

• ​These worms are characterized by their round and unsegmented body shape.

• ​​Many species within this phylum are parasitic and live on hosts.

• ​Like flatworms, they also possess a distinct and developed head region.

Phylum Annelida

• ​This is the most complex phylum of worms with segmented bodies.

• ​​They have a true internal body cavity that is called a coelom.

• ​Like other worms, they also exhibit cephalization with a head region.

Invertebrate Phyla: Mollusca & Arthropoda

Phylum Mollusca

• Mollusks such as snails, clams, and squid are characterized by their soft and unsegmented bodies.

• Many are protected by a hard external shell composed of calcium carbonate for defense and support.

• This phylum is very diverse, with a wide variety of species living in different habitats.

Phylum Arthropoda

• Arthropods, including insects and spiders, are defined by their segmented bodies and jointed appendages.

• They possess a hard exoskeleton made of a substance called chitin, which provides protection.

• This is the largest and most diverse phylum within the entire animal kingdom.

Phylum Echinodermata: The Spiny-Skinned Animals

• The name Echinodermata means 'spiny skin,' like on starfish and sea urchins.

• ​These marine animals show radial symmetry in their adult form.

• Interestingly, their larvae begin life with bilateral symmetry.

• A water vascular system helps them move, and they have an internal skeleton.

Solved Example 3
A biologist counts 5 mayflies, 3 stoneflies, and 12 pouch snails in a stream sample. Using the pollution tolerance index (tolerant species = 1, intolerant = 5), what is the weighted average score for this sample?

Step 1: Analyze and Sketch the Problem

• Goal: Calculate the weighted average pollution tolerance score.

• Knowns: Mayflies = 5 (intolerant); Stoneflies = 3 (intolerant); Pouch Snails = 12 (tolerant). Index Values: Intolerant = 5 points; Tolerant = 1 point.

• Unknown: The weighted average score for the sample.

• Formula: Weighted Average = Total Score / Total Number of Organisms.

Solved Example 3
A biologist counts 5 mayflies, 3 stoneflies, and 12 pouch snails in a stream sample. Using the pollution tolerance index (tolerant species = 1, intolerant = 5), what is the weighted average score for this sample?

Step 2: Solve for the Unknown

Solved Example 3

A biologist counts 5 mayflies, 3 stoneflies, and 12 pouch snails in a stream sample. Using the pollution tolerance index (tolerant species = 1, intolerant = 5), what is the weighted average score for this sample?

Step 3: Evaluate the Answer

• The score of 2.6 is closer to 1 (tolerant) than 5 (intolerant).

• This indicates the water quality is likely stressed, as the higher number of tolerant pouch snails has lowered the overall score. The answer reflects the mix of organisms found.

Aquatic Macroinvertebrates

• Macroinvertebrates are animals without a backbone, visible to the naked eye.

• ​Aquatic ones are valuable indicators of water health as they live in water.

• Most are benthic, living at the bottom and unable to escape environmental stressors.

• Stressors include chemical pollution, sediment loading, and low dissolved oxygen.

Pollution Tolerance in Macroinvertebrates

• Intolerant species are very sensitive to pollution and environmental stress.

• ​Finding them, like Mayflies and Stoneflies, indicates good water quality.

• Tolerant species are organisms that can survive in polluted environments.

• Finding mostly leeches and tubifex worms indicates poor water quality.

Common Misconceptions About Invertebrates

Summary

• Animals are multicellular with varied symmetry (asymmetrical, radial, or bilateral).

• Invertebrates are ectothermic, lack a backbone, and include simple to complex phyla.

• Key phyla include Cnidaria, various worms, Mollusca, Arthropoda, and Echinodermata.

• Aquatic macroinvertebrates act as bioindicators, signaling water quality and pollution levels.