Thin crusts of Sabellaria spinulosa often act as a fast growing
annual and appear to be resilient phenomena. Sabellaria spinulosa is often referred
to as a pollution indicator (see chapter VI) and seems unlikely to be particularly
sensitive to changes in water quality except perhaps in the unlikely event of the supply
of sand with which to build its tubes being removed. No published evidence of any strong
sensitivity to natural events has been found.
Well developed, more stable reefs seem to be very unusual, and this
apparent rarity suggests that an unusual set of environmental factors and/or circumstances
is required for their formation. It might, therefore, be expected that they would display
sensitivity to some factor or factors, but there is little information from which to gain
any insight into this, and the following points are therefore rather speculative.
George & Warwick (1985) have suggested that growth and recruitment
of S. spinulosa can be inhibited or even prevented by dense populations of the
brittle star Ophiothrix fragilis, which can occur at very high densities thus
preventing adequate food particles from reaching the worms. This is thought to have been
the reason behind very low recruitment and growth of S. spinulosa in an area of the
Bristol Channel in 1976. Fecundity of the adults in the colony was also severely reduced,
possibly for the same reason. The possibility that the larvae themselves could be filtered
out by very dense O. fragilis (or other filter feeders such as Mytilus) was
not mentioned but should be considered. Using historical data, Holme (1983) found that
brittle star beds off Plymouth have undergone large scale changes in density due to the
changing fortunes of a predatory starfish, Luidia ciliaris, which in turn
might be explained in terms of changes in the penetration of oceanic Atlantic water into
the English Channel (the Russell cycle).
It is likely that stability of the reefs is to some degree a function
of stability of the substratum, which could be affected by frequency of storms, for
example. The more ephemeral reefs probably occur principally on relatively unstable
substrata, while longer lasting reefs might be limited to more stable substrata. Depth and
water movement as well as substratum might affect this. It is also possible that
regularity of recruitment might be a factor, but there is presently little knowledge of
influences on fecundity and recruitment, although they do seem to be variable, at least in
some instances (see chapter IV). (Given the long planktonic phase of six weeks to two
months, and the probable need for a dense swarm of larvae to be stimulated to settle by
the presence of existing tubes, some variability in recruitment would not be surprising).
Thicker, more established reefs, however initiated, would be more
likely to resist breaking up, and possibly more attractive as a settlement substratum, so
increasing their likelihood of long-term survival. Given that the worms may live for
several years, such reefs might be expected to survive despite occasional years of poor
recruitment, as suggested for the Bristol Channel populations studied by George &
Warwick (1984). No detailed studies on this topic have been carried out, however, and this
is therefore rather speculative.
There is little knowledge of predators of S. spinulosa on which
to assess likely sensitivity to changes in predator populations.
Michaelis (1978) mentions that during the 1950s S. spinulosa
"withdrew from the intertidal area of Niedersachsen" (the Waddensee), where
"Formerly the reefs of this worm were frequently found on the head of the
groins around the East Friesian Islands", but unfortunately offers no explanation.
However, the reefs were always apparently sporadic, and intertidal reefs of this worm have
not been reported elsewhere (See chapter IV for a summary of Linkes observations in
this area during the 1940s).