This section is aimed principally at that part of the range of biotopes
containing Modiolus modiolus where the horse mussels build up substantial mounds or
bioherms, although some aspects may be applicable to biotopes with lower densities of Modiolus..
Spawning and recruitment may well be affected by physico-chemical
changes, for example due to resulting affects on feeding conditions (phytoplankton
levels), but we have found no detailed information regarding specific sensitivity on this
subject, other than basic information already given in chapter III.
It has often been established that the concentrations of suspension
feeders in Mytilus beds can deplete the seston available in the benthic boundary
layer downstream of them. Wildish & Kristmanson (1984, 1985) measured this effect in
flumes with Modiolus as well. Thus food limitation of growth might be a problem in
years with poor phytoplankton production. Further considerations in this respect might
include the effects of Ophiothrix fragilis, which often occurs in very dense beds
in the same areas as Modiolus. Given that Ophiothrix are extremely efficient
filter feeders, and that this species is prone to large fluctuations in population, it
might be speculated that long-term smothering by Ophiothrix beds could occur,
leading to an inability of the Modiolus to feed. George & Warwick (1985)
suggested that this was the reason behind very low recruitment and growth of Sabellaria
spinulosa in the Bristol Channel in 1976.
Predation of young Modiolus by crabs and starfish in particular
appears to be important, and it is known that at least intertidally predation by Asterias
particularly can be devastating to Mytilus beds. Factors affecting the proportion
of young Modiolus surviving through to the size at which predation appears no
longer to be a serious threat have not been studied, although in comparison with Mytilus
reefs, which are composed of much younger animals, the effect of one or two bad
years of recruitment would be far less serious. It is suspected that juveniles
living within the mass of adult byssus threads have greatly enhanced survival, in which
case infaunal Modiolus could be at a disadvantage since the byssus may be largely
Parasites and diseases
We have no knowledge of this subject regarding Modiolus,
although it is known that the boring sponge Cliona celata can badly damage the
shells of old Modiolus (Comely, 1978).