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 Linke’s observations in this area during the 1940s).

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