Coralline crusts in Atlantic infralittoral surge gullies and scoured rock

Description (English)

Scoured rock in wave-surged caves, tunnels or gullies often looks rather bare, and may be characterised by a limited scour-tolerant fauna of Balanus crenatus and/or Pomatoceros triqueter with spirorbid polychaetes. In areas where sufficient light is available and scour is severe, encrusting coralline algae and non-calcareous crusts cover the rock surface, giving a pink appearance. This biotope most commonly occurs at the bottom of walls in caves and gullies, where abrasion by cobbles and stones is severe, especially during winter. In some gullies, extreme scouring and abrasion produces a narrow band of bare coralline algal crust at the very bottom of the walls, with a band of P. triqueter and/or B. crenatus immediately above. Other scour-tolerant species, such as encrusting bryozoans may also be common. Crevices and cracks in the rock provide a refuge for sponge crusts such as Halichondria panicea and occasional anemones Urticina felina and Sagartia elegans. More mobile fauna is usually restricted to the echinoderms Asterias rubens and Echinus esculentus as well as the crab Cancer pagurus. Two variants have been identified: Wave-surged bedrock with coralline crust, B. crenatus and P. triqueter (MB1-2761) and coralline crusts on mobile boulders in severely scoured caves (MB1-2762).
Situation: Generally occurring at the base of walls in caves and gullies and on the floors of caves and gullies. Immediately above this zone a variety of biotopes may occur depending on the proximity to the cave/gully entrance. Typically sponge crusts and ascidians with a hydroid-bryozoan turf will occur in the outer to mid section (units MB1-272, MB1-273); sponge crusts and dense ascidians in the outer to rear section (MB1-274); and low-growing sponge crusts at the rear of caves (MB1-275). At some sites with extreme wave surge, MB1-2761 can form a zone towards the rear of the cave, beyond the sponge crust zone.

Source: EUNIS habitat classification