Preliminary indications are that there are a range of variations in the
mix of associated species associated with the mound forming types of Modiolus beds.
Thus the appearance of samples dredged from off the south of the Isle of Man, before that
bed declined, was rather different from samples taken at the same time from beds only less
than 30 miles away in apparently similar situations NW of Anglesey. The most obvious
difference was that the Manx one had substantial numbers of the very large barnacle Balanus
hameri growing on the mussels, while off Anglesey they were absent.
Temporal changes are to be expected that are in no way related to the
long-term structural integrity of the persistent Modiolus beds. Ophiothrix fragilis
is often abundant in Modiolus beds at present, but examining long-term records from
the English Channel, Holme (1983) showed that there had been major changes to brittle star
beds off Plymouth apparently linked to changes in the population of the predatory starfish
An initial inventory of the associated epifauna of each bed will be
needed. Published and anecdotal evidence suggests that different beds have differences in
the degree of dominance by different epifaunal organisms. The reasons for such differences
are not clear. Some are dominated by turfs of hydroids and bryozoans, others have a
predominance of Alcyonium, others have more sponges, while in others there are more
barnacle and calcareous worm tube crusts on the mussels.
After the initial inventory, any major changes to the dominant epifauna
should be detectable using the video or camera sledge transects made to assess cover by
mussels. Sufficient images should be available to satisfy statistical criteria for the
reliability of detecting changes. Larger mobile epifauna such as whelks and starfish
should be detectable by the same means, but consideration might be given to deploying
appropriate traps for some of the smaller epifauna.
It is impossible to sample the infauna without destructive sampling.
The techniques for infaunal sampling are well reviewed elsewhere, though there are
complications in doing this quantitatively in mussel beds because of the abundance of
solid items liable to block the jaws of grabs. The infauna associated with beds where much
faecal matter accumulates will be rather different from the gravel binding type of bed. In
the former type there will be obvious parallels with the fauna known to be associated with
enrichment by anthropogenic organic wastes. It is reasonable to extrapolate from the
literature on Mytilus to suggest that in summer the faecal matter deposited by the
horse mussels will have a higher content of labile organic matter than sediments
depositing without the agency of mussel filtration. Responses to the organic enrichment
can be seen both at a gross scale and on the scale of a crevice fauna. Thus although horse
mussel beds may be in areas with moderately strong tidal currents, part of the associated
fauna is made up of a suite of organisms more often found in more sheltered conditions.
For example in the Lleyn Modiolus bed miniature versions of the Abra
community are to be found between the shells of very large dead horse mussels that have
remained in the life position but which have nearly filled with sediment. As far as is
known there have been no studies of the small scale heterogeneity likely to be found
amongst the infauna associated with the crests and troughs of horse mussel beds. It will
not be possible without further study to extrapolate directly from soft sediment sampling
protocols to determine how many replicates are needed to detect prescribed levels of