Fische als Feinde tropischer Seeigel

The techniques used by 9 species of fish (representing 3 families) to capture Diadema sea urchins were observed under water in the Red Sea at Eilat, Israel. Different species displayed different techniques. Fish belonging to the Lethrinidae and Labridae families devoured the entire urchin after capture; Labridae alone were also capable of breaking up large sea urchins by banging them on rocks. The fish did this by shaking its head while holding the urchin in its mouth. Under experimental conditions, fish can distinguish between large and small urchins, and show a preference for the small urchin. The fish also shun contact with some other species of urchins. The trigger fish Balistes fuscus exposed to a dummy sea urchin responds to optical stimuli (body shape and spines); when exposed to a living sea urchin it responds to both optical and behavioural stimuli (movements of the spines). Morphological and ethological preadaptation of the fish to alter its preference for other food is essential before it can develop differentiated capture techniques. The ecological significance of the behaviour of fish in the coral reef ecosystem and the alternating influences in the predator-prey-relationship are discussed.