Climbing performance of sea urchin Strongylocentrotus nudus for upright, thin objects in wave-induced oscillatory flow

摘要

In order to examine the possibility of creating kelp beds with upright thin structures above sea urchin-dominated barrens, oscillating-flow-tank experiments were conducted on the climbing performance by the sea urchin Strongylocentrotus nudus using upright rods of two different diameters (3 and 10 mm) with five blade pieces of the kelp Eisenia bicyclis as algal food attached at 2.5-cm intervals in four levels of orbital velocities (0, 0.2, 0.3 and 0.4 m/s). In the absence of oscillatory flow urchins readily climbed to graze on food items in sequence from the bottom. The frequency of grazing up to the top food significantly decreased with velocity but no significant difference in the frequency was detected between the thin and thick rods. Fitting of a Markovian model with stationary transition probability to the observed frequency of the grazed height (number of pieces of food eaten from the bottom) suggested that, once climbing was initiated, an urchin continued to climb at a particular rate determined by the water motion. The resultant model predicted that, even with increasing height of upright rods, an orbital velocity higher than at least 0.1 m/s would be required to prevent urchin invasion in urchin-dominated barrens.