Name: Date:

Arthropods

Kingdom: Domain: Phylum

*Text pages 378-381. This is required reading.

-Arthropods belong to the Phylum ____________________, which means __________________

_______________ in Greek. This name refers to the many jointed appendages on the various

segments of the arthropod body.

-Below are some interesting facts about Arthropods:

-Arthropods are regarded as the world’s most successful Animal Phyla with an estimated

population of 108 (A billion billion)! This means that there are more than 160 million insects for

each person on Earth!

-Arthropods currently account for over 83% of all known species.

-Tiny crustaceans called copepods, less than a centimeter

long, are the most abundant animals in the ocean. There are

so many of them that they outweigh all the whales on Earth!

-In the following pages we will be exploring what has made

Phylum Arthopoda so successful.

-Circle the phylum to which are

Arthopods more most closely

related?

-Are Arthropods Protostomes or

Deuterostomes?

Biology 11: Phylum Arthropoda Page 1

-All modern Arthropods are descended from a single common ancestor. If an organism has not

descended from this ancestor, it is not an Arthropod. There are five branches in the Arthropod

phylogenetic tree.

1. In the same way that you have inherited particular characteristics from your parents, all

arthropods have inherited a set of five characteristics from their common ancestor. These

characteristics have allowed them to be so successful. We will explore these on the following

website: http://evolution.berkeley.edu/evolibrary/article/3_0_0/arthropods_03

Biology 11: Phylum Arthropoda Page 2

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2. Evolution works with the variation that exists in a lineage. The common characteristics of all

Arthropods discussed above have allowed this phylum to evolve in response to environmental

opportunities. We will explore their successful evolution by examining two Arthropoda

characteristics:

a) Exoskeleton

-Trilobites: Body made up of three segments, three lobes and one pair of antennae.

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-Just because an

organism has gone

extinct does not mean it

was not successful.

-95% of all marine

organisms have gone

extinct.

-Different species of Trilobites evolved their own variations on the exoskeleton:

i) Spines ______________________________________

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ii) Flexibility __________________________________

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b) Jointed Appendages

-Overtime the basic limb structure of the ancestral arthropod evolved into specialized forms to

suit different functions, allowing arthropods to invade new niches and habitats.

-Crustaceans: Have taken limb specialization to the extreme. For any tool that you can think of,

there is a crustacean limb that can do the same job.

Barnacle _____________________________________________________

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Mantis Shrimp________________________________________________

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________________________________________________-The diversification of Arthropoda limbs has been facilitated by three characteristics:

i) Exoskeleton _______________________________________________________________

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ii) Joints ___________________________________________________________________

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iii) Multiple Copies __________________________________________________________

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-Below are some more ways that limbs have been modified in other Arthropoda lineages.

-Did the

diversity of

Arthropod appendages evolve because they were needed?

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Evolutionary Constraints on Arthropods

3. What you inherit has limitations and even gets

in the way. We will take a look at some of the

constraints that evolution has put on Arthropods

based on their inherited characteristics and how

this has shaped this phylum.

-Largest Modern Marine Arthropod: Japanese Spider Crab,

Arm Span: 3.8 metres (12ft) Weight: 18.6 kg (41 lb)

-Largest Modern Terrestrial Arthropod: Coconut Crab,

Length: 40cm (1.3 ft) Arm Span: 1 metre (3.3 ft)

Weight: 4 kg (8.8 lb)

-Why are marine Arthropods capable of growing to a larger size?

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-Why don’t we see any Arthropods larger than this?

a) Arthropods have inherited an exoskeleton and jointed legs, traits that have opened up many

opportunities in Arthropod evolution. However, they have also blocked other possibilities.

We will explore three constraints on the size of Arthropods:

i) Molting

-Why do Arthropods have to molt?

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-Why is it a disadvantage to be a large organism if you have to

molt?

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-The Coconut Crab’s large size makes molting a very

dangerous and energetically expensive procedure.

These crabs spend an entire month in an underground

burrow shedding their exoskeleton and waiting for a

new one to harden.

-Praying Mantis under-go multiple molts (usually 7) before they

become adults.

ii) Exoskeleton Strength

-An exoskeleton provides structural support and points of

attachment for muscles that move the appendages.

But, despite all its benefits, the exoskeleton can

actually limit the size of Arthropods.

-To explore this idea, let’s think of an ant’s body as

cubes and its legs as tubes.

Biology 11: Phylum Arthropoda Page 9

-If we increase cube size 2x, does their weight

increase proportionately? ___________________

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-Tube strength is related to width and thickness of

the wall. When tube width and thickness increase

2x, tube strength increases by 4x. What does this mean for ant size?

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-What might be a possible solution to this problem? Would it work?

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iii) Respiration

-Crustacean/Arachnid Book

Lungs are the site of gas

exchange, with oxygen being

carried around the body in a

circulatory system.

-Insect Tracheal System is

an extensive network of branching tubes that relies on diffusion for the movement of gases

into, around and out of the body.

-If we increase the size of an

insect, the length of its tracheal

tubes must also increase. What

does this mean for the rate of

diffusion and, consequentially,

respiration?

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The Path Not Taken

-Evolution is an undirected process

constrained by the laws of physics

(such as gravity), genetics (which

might encode for building respiratory systems in a particular way) and environment (which

may not contain a niche for large, slow-moving and fragile insects).

-As seen in the case of Arthropods, useful characteristics can bring both evolutionary benefits

and constraints.

Characteristic Benefit Constraint

Exoskeleton

Jointed Limbs

Respiratory System

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-Evolution can only work on existing characteristics. Whatever direction future Arthropod

evolution takes, it will always be dependent on the characteristics they have inherited from

their common ancestors.

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