Modiolus modiolus

As part of the UK Marine SACs project, development of appropriate survey and monitoring methods for Modiolus beds/reefs is already underway. CCW is the lead organisation in this work and one of the authors (E I Rees) is involved. For this reason this section is more detailed than other sections in this chapter.

Those beds of the horse mussel Modiolus modiolus that build up bioherms are likely to be persistent features in the long term. Evidence for this comes mainly from the scale of build up, the ages of the older animals (30 - 40+ years), the wide range of size individuals usually present and anecdotal history. As a working hypothesis, Modiolus beds are considered to be more naturally persistent than beds of Mytilus and less likely to be renewed following catastrophic events. In the absence of more specific information, sometimes inferences here have to be drawn from Mytilus monitoring for commercial stock assessment purposes. Monitoring strategies do however need to be related to anticipated types of change that might befall bioherm forming Modiolus beds, though with some capability to detect unexpected types of event.

Changes to the favourable conservation status of any particular Modiolus biogenic reef, whether brought about by natural or anthropogenic causes, are most likely to happen in the following types of ways:-

  1. change to the overall extent of the designated bed, through destruction, fragmentation or spread at the margins
  2. integrity of the bed and change to the percentage cover, through breaks in the bed, cuts across it, division into discrete patches and ultimately even into a scatter of isolated clumps
  3. major change to the vertical relief of the bed
  4. physico-chemical changes to the sedimentary matrix of the bed, including the organic content and redox potential of the faecal deposits
  5. recruitment failure over a very long period so that ultimately the bed comprises just a sparse relict population of very old animals
  6. major changes in the rates of mortality through predation or diseases
  7. significant changes to the associated fauna:
  1. changes to the biodiversity, species mix and abundance of associated sessile epifauna, including any species of special functional or conservation interest, and the associated motile epifauna
  2. changes to the biodiversity and abundance of the associated infauna

Recommendations for monitoring are made here for each of the above attributes which might change. Some are relatively straightforward, some will rely on standard techniques for marine environmental measurements and some will require more evaluation and refinement.

Extent of a Designated Bed

Integrity of the Bed

Bed Relief Changes

Physico-Chemical Attributes


Predation and Disease

Associated Fauna