Epifaunal biodiversity

Infaunal biodiversity

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 Luidia ciliaris.

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 change.

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