Diving investigations on biodeterioration by sea-urchins in the rocky sublittoral of Helgoland

1. The Polydora ciliata-Echinus esculentus community of the rocky sublittoral of the island of Helgoland (North Sea) has been studied by SCUBA-diving methods and laboratory experiments.

2. The sublittoral zone of Helgoland at water depths between 8 and 20 m is settled mainly by the boring polychaete Polydora ciliata and associated organisms. P. ciliata bores all local types of rock, except Triassic gypsum beds and Cretaceous flint layers.

3. The main predator of P. ciliata is Echinus esculentus occurring in extremely high population densities (1–7 specimens/m2).

4.E. esculentus cleans surface areas from 100–800 cm2 of P. ciliata chimneys and subsequently rasps off the rock itself until it reaches the worms in depths from 0.5–1.5 cm.

5. At the location of vertical cliffs and well-stratified marls and limestones also deep furrows of up to 30 cm length and only 1.5–2.0 cm width are produced in the field and in laboratory experiments, within a few days.

6. The less frequently occurring sea-urchinPsammechinus miliaris, in contrast toE. esculentus, controls much smaller areas at a time and generally feeds on a surface area not larger than its body size.

7. Laboratory experiments and field measurements yielded erosion rates of up to 5 cm3 rock eroded/specimen/day forE. esculentus.