Crown of Thorns starfish II

Crown of Thorns starfish II

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This task is about how observations inform theories.
Over the past 20 years outbreaks of the Crown of Thorns starfish have caused large amounts of damage to coral reefs in the Pacific Islands. Crown of Thorns starfish feed on the fast growing corals of the coral reefs. 
 
illustration: crown of thorn starfish

Question 1Change answer

The Crown of Thorns starfish have certain adaptations/features that help them survive and
reproduce in the coral reef environment.
a)  Explain in the boxes below how each of the adaptations/features could help this starfish survive.
 
illustration: Crown of thorn starfish
How could the colour of the starfish help it survive?
How could the bristles/thorns of the starfish help it survive?
How could the many legs of the starfish help it survive? 
Scientists wanted to work out why Crown of Thorns starfish have become such a problem.
One theory suggests that when there are fewer predators starfish numbers increase. 

Question

Scientists made these four observations. Two of these observations show that when there are fewer predators the starfish numbers increase [the predator removal theory].
Choose the two observations that support this theory. 

    • No starfish remains have been found in the stomachs of large predator fish on coral reefs

    • Triton shells (which can eat starfish) are heavily harvested for Pacific Island ceremonies

    • Overall numbers of puffer fish (seen eating starfish) are low on coral reefs

    • Whole starfish can regenerate from small pieces left behind when they are eaten.

Question

Choose the two observations that would not support the theory.
 

    • No starfish remains have been found in the stomachs of large predator fish on coral reefs

    • Triton shells (which can eat starfish) are heavily harvested for Pacific Island ceremonies

    • Overall numbers of puffer fish (seen eating starfish) are low on coral reefs

    • Whole starfish can regenerate from small pieces left behind when they are eaten

Question

Do you think there is enough evidence to support the theory?  

  • No starfish remains have been found in the stomachs of large predator fish on coral reefs

  • Triton shells (which can eat starfish) are heavily harvested for Pacific Island ceremonies

  • Overall numbers of puffer fish (seen eating starfish) are low on coral reefs

  • Whole starfish can regenerate from small pieces left behind when they are eaten.

    •  Yes

    •  No

Explain why you think this.

Question

Scientists decided to do some further observations. 
Choose one of the following observations.

    • Count the number of triton trumpet shellfish on reefs in areas that are inaccessible to people.

    • Observe how much of each Crown of Thorns starfish a triton trumpet shellfish eats.

    • Examine the stomach contents of the triton trumpet shellfish.

Explain how this observation would help support or would not help support the theory.
Copyright: 

crown of thorn starfish

Crown of Thorns (Acanthaster planci). Koh Similan, Boulder City, Thailand.

Date Taken on November 25, 2005.
Source http://www.flickr.com/photos/jonhanson/89930167/
Author jon hanson on flickr.
Permission
(Reusing this file)
CC-BY-SA-2.0

Level:
5
Curriculum info: 
Science, Investigating in science, Living World, Understanding about science, Living World
Key Competencies: 
Thinking
Keywords: 
adaptation, starfish, predator removal theory, fish, observations, coral reefs, evidence
Description of task: 
Task: Identify how features/adaptations of a starfish help it survive, and decide whether the amount of evidence from scientists' observations supports or does not support their theory/inference. Assessment focus: using observations as evidence to inform theories.
Curriculum Links: 
Science capabilities
The capabilities focus is brought about by the conversations you have and the questions you ask.
 
Capability: Use evidence
This resource provides opportunities to discuss using evidence to support a theory. 

Capability: Critique evidence
This resource provides opportunities to discuss the validity of the evidence provided. 
Science capabilities: 
Critique evidence, Gather and interpret data, Use evidence
Answers/responses: 
 
Questions What to look for
How could the colour of the starfish help it survive? Bright colours warn predators it could be poisonous.
How could the bristles/thorns of the starfish help it survive? It has many spines that could be painful - a deterrent to predators.
How could the many arms of the starfish help it survive?
It has many arms so could be very large - too big for many predators to eat. 
It has many regenerative arms so can survive being attacked. Being able to renew its arms might mean it needs to be fully eaten to be killed.
Its many arms allow it to wrap around/attach easily to its prey. 
Choose the two observations that support the predator removal theory. 
  • Triton shells (which can eat starfish) are heavily harvested for Pacific Island ceremonies.
  • Overall numbers of puffer fish (seen eating starfish) are low in coral reefs. 
Choose the two observations that do not support the theory. 
  • No starfish remains have been found in the stomachs of large predator fish on coral reefs.
  • Whole starfish can regenerate from small pieces left behind when they are eaten. 
Do you think there is enough evidence to support the predator removal theory? 
Explain why you think this.
No. We can't be sure that it is just predators responsible for the increase in the starfish numbers because we don't know whether the quantity of starfish that the triton trumpet shellfish or puffer fish eat is enough to impact on starfish population numbers and/or if something else might be happening in this ecosystem.
Students might also note the evidence that points against the theory e.g., large predator fish on coral reefs do not seem to eat the starfish and that starfish would need to be wholly eaten as they can regenerate from pieces. These are possible reasons for increasing starfish numbers.  
 
Explain how one of the observations would help support or would not help support the theory? 
Explanations could include:
  • the shellfish count in undisturbed areas could give an indication of the normal numbers for the shellfish. High numbers here could indicate there is a problem in the disturbed/other areas. However it does not indicate what the problem might be. 
  • stomach contents of the triton trumpet shellfish would show what they eat and the proportion of each species they eat. 
  • observing the quantity of Crown of Thorns starfish eaten by the triton trumpet shellfish would help decide if the quantity eaten would impact on the total starfish population.                        
Results are based on responses from 40 Year 10 students.
Diagnostic and formative information: 
 Science capability 1: Gather and Interpret data (Observation and inference) and Science capability 2: Use evidence.
 
Question Students' response Next steps
How could the colour of the starfish help it survive? 
  • Most students associated colour with camouflage despite it being a very bright colour. 
If they could blend in with their surroundings like a chameleon and camouflage themselves to hide from predators. For example maybe they could move to a place thats full of bright purple and pink plants and/or rocks.  
  • Some students did begin to associate the bright colour as a way to be seen.
So that it can stand out.
Bigger fish might not like how bright the colours are. 
  • Ask students to consider how bright colour might help animals to survive in environments that are not the same colour as the animal. Can they find examples of when bright colours are not related to camouflage? Alternatively they could look for examples where bright colour is associated with being poisonous.  
How could the bristles/thorns of the starfish help it survive? 
  • Most students answered that the thorns were for protection.
  • Some students explained that the thorns would hurt predators e.g., sting. 
  • One student explained how the thorns could be poisonous. 
  
How could the many arms of the starfish help it survive? 
  • Most students associated the many arms to speed.  
So that the starfish can move around and away from predators.
 
Help them move faster from predators and so they can capture their prey faster. 
  • Some students used their prior knowledge e.g.,having suckers on the arms [these can't be seen in the photograph]. 
Help them cling onto things when there is a strong current, also help them move faster and catch food.
  • Ask students to think about speed in relation to many arms/legs e.g., Does having more limbs mean an animal moves faster?  Can students look for evidence that supports this statement? Can students look for evidence that does not support this statement? 
 
 
Science capability 3: Critique evidence
 
Question Student's response Next steps
Predator removal theory.
Observations that support or do not support the theory.
  • Most students could identify one of the observations that supported or did not support the theory. Very few students identified both observations for both questions. 
Is there enough evidence to support the theory?
  • Most students answered there was not enough evidence to support the theory. However very few gave an explanation of why they thought this. 
  • Most of the students who said there was enough evidence just explained the theory again.  
Explain how one of the further observations would help support or not help support the theory. 
  • If students chose the most directly linked evidence they could provide a suitable explanation e.g., most students who chose to examine the stomach contents of the triton trumpet shellfish said it could help decide the quantity of starfish each triton trumpet shellfish eats and what else it eats. Some students also said this could help decide if the triton trumpet shellfish eating the starfish was a problem or not.
  • If students chose one of the other 2 options they were not able to provide a suitable explanation. 

Students need to expand on their explanations. Any practise that you can give students, both for and against, will be valuable, e.g.,
  • looking at what the evidence shows and also what it does not show 
  • looking at other options, e.g., what else could be happening in the ecosystem 
  • asking questions, e.g., what else they would want to know to be sure of their explanation.   
Initially you could keep it simple but gradually increase the complexity of the thinking required.