|(Art by Echinoblog Art Department!)|
Basically the sea urchin T. niger was observed in surprisingly high abundance living on elevated rock surfaces.
|Meyenaster feeding on Tetrapygus. Image by Azulprofundo5170|
|Tetrapygus niger photo by Jabaezam|
In other words they JUMP For it!! Drop and ROLL!
(note that the Echinoblog Art Dept. reconstruction below is somewhat exaggerated..)
Here is the graph (their Figure 3A) that shows an experiment that showed how many sea urchin detatchments occurred when they artificially motivated the sea urchins to detatch in reaction to predators and parallel control actions.
Thus, the "jump" response happened only in reaction to direct contact with predators and not simply the "smell" of them approaching
Figure 3B (above) shows what happened after contact but without detachment. Their behavioral cue was to proceed higher up the boulder.
Its worth noting that Just because the urchins would "fall" did not guarantee their overall survival or even escape. Spines or other structures could break. And if an urchin falls upside down and a starfish happens to be there? Yikes!
Various experimental trials showed some further depth..
- Of the two sea star predators-Meyenaster gelatinosus was regarded as the greater threat to the sea urchins. Heliaster (the sun star) illicited an escape response (i.e., the sea urchin moves away VERY quickly once it senses the predator) but not as severe as the one that Meyenaster did.
- Meyenaster's preference is to be on lower ground (it moved there when placed on boulders) whereas the urchins preferred to live on elevated surfaces such as boulders and so forth..
- Sea urchin survival was considered higher on elevated surfaces and is considered to be primarily a defensive behavior since food is actually rare on these elevated surfaces (contrary to the picture above).
- It turns out that fish are ALSO predators of these sea urchins which at one time may have preyed upon the urchins on top of the rocks, boulders and elevated surfaces..