Intertidal Underboulder Communities

This habitat is found from the mid-shore down to the extreme lower shore, and encompasses areas of boulders (greater than 25 cm diameter) that support a diverse underboulder community. It can occur on a range of substrates, where there is a sufficient gap on the underside of the boulder to support an underboulder community. The richest examples of these communities are often found where there is running seawater, such as from tidal pools emptying.

intertidal boulders

Photo: Sarah Atherton/Sussex Wildlife Trust

Why are they important?

  • The underboulder habitat, along with fissures, crevices and any interstitial spaces between adjacent boulders, form a series of microhabitats that add greatly to the biodiversity of a shore.
  • The presence of boulders on a shore may lead to local modification to wave exposure, current strength and levels of trapped organic matter in the area surrounding the boulders.
  • The shade, moisture and shelter provided under boulders allow a diverse range of species to survive on otherwise hostile shores.
  • Underboulder habitat plays an important role in the life cycle of marine animals; providing refuges for eggs and juveniles of many species.

Intertidal Underboulder Communities in Sussex

Boulders beneath the chalk cliffs between Brighton and Eastbourne and the sandstone at Pett will support underboulder communities.

What are the threats?

  • Boulder turning for species such as winkles and crabs can disrupt underboulder conditions where boulders are not replaced in their original position
  • Sewage and nutrients can result in deoxygenation of underboulders, killing the species living there

Some associated species

  • Corkwing Wrasse Crenilabrus melops
  • a bristled chiton Acanthochitona fascicularis
  • Hairy Porcelain Crab Porcellana platycheles

Lightbulb Sea Squirt

Photo: Paul Naylor/Sussex Wildlife Trust

Links to more information

The Marine Life Information Network