Crayfish Study

 

A study I was involved with, conducted at the University of Arizona in the 1980’s, is very useful for illustrating several concepts related to habitat and animal behavior.  Crayfish were chosen as the animal for this experiment.  We created several ponds out of small children’s swimming pools. Each pool also had a ramp extending from the bottom to the top edge of the pool where, connected to a 2x4 pathway with side railings, it emptied into a bucket (Fig 2.).

 

                                                                                                Fig 2.

 

The ponds, filled with fresh water, were used with several combinations of experimental trials. 

 

Experiment #1                                                                                    outcome

5 crayfish are placed in the pond.                                  Overnight – all 5 crayfish leave pond

repeated several times and seasons                                           end up in the bucket. 

 

(Crayfish will leave their ponds to look for resources but only at night when they are safer from predators

They don’t leave because they like buckets.  They leave because the pond provides water- which they also breathe – but does not provide what they need for habitat to survive.)

 

Experiment #2

Cat food (5 cans) placed in pool                                   Overnight – 5 crayfish leave pond

5 crayfish are placed in the pond.                                    9 out of 10 times  end up in

repeated several times and seasons                                 the bucket. Sometimes 1 crayfish

  remains until 2nd night

 

(Crayfish leave their ponds – they have food and water, but they do not have enough of the right resources to try to stay there and survive.  So they find and take the way out to look for the right combination of resources)

 

 

Experiment #3

Cat food (5 cans) placed in pool                                   Overnight – NO crayfish leave pond

5 huts (empty coffee cans cut in half) placed                  Rarely after days 1 may be in bucket

5 crayfish are placed in the pond.                                  All usually remain as long as food is  

repeated several times and seasons                               replenished daily.

 

(Crayfish stay in their ponds – they have food, water, and cover. As long as there is enough food those 5 crayfish will stay virtually indefinitely.  If there are no members of the opposite sex eventually they will leave – looking for the missing resource - mates)

 

 

Experiment #4

Cat food (5 cans) placed in pool                                   Overnight – 5 crayfish leave pond

5 huts (empty coffee cans cut in half) placed                  and 5 crayfish stay; only 5 will stay

10 crayfish are placed in the pond.                                No matter how much food is  

repeated several times and seasons                               replenished daily.

 

(5 Crayfish stay in their pond – they have food, water, and cover. However, 5 others leave since while there are resources for crayfish there, only sufficient huts exist for 5 of them.  If I add 5 more crayfish the next night, which crayfish will stay in the pond and still be there the next day?  The original 5 who already ‘own’ the huts)

 

So, the behavior of the crayfish themselves has told us several things:

 

  1. Crayfish recognize habitat.  They apparently take keys from specific environmental cues. What is adequate habitat for crayfish?  (e.g., what things are necessary for them to try to stay)
  2. What is the carrying capacity of the ‘environment’? (e.g., how many crayfish can be supported by each pond, cat food, coffee can hut system)
  3. Who gets the resources?  (e.g., which ones get resources and therefore likely survive to reproduce)