Environmental Requirements

Physical Attributes

No definitive published data on temperature requirements of S. vermicularis reefs was found. However, its broad geographical range as a species suggests that it is unlikely to be very sensitive to temperature changes in a British context.

As a species Serpula vermicularis has been reported to occur intertidally as scattered specimens and subtidally to depths of over 200 m (Clarke, 1960; Zibrowius, 1973). However, Nelson-Smith (1967) described its distribution as being restricted to shallow water and the lower shore. Moreover, in the only known locality in Britain where living reefs occur, (Loch Creran), these are restricted to depths of 0-14m, are extremely rare outwith the range 1-13 m, and the most well-developed reefs are within the range 6-10 m (Moore, 1996). Similarly in Ardbear Lough, Ireland, they have a restricted depth range of 2-20 m with maximum development between 2 and 15 m (Bosence, 1973; Bosence, 1979). It was suggested by Moore (1996) that the lower limit was restricted by depth, sediment type and substratum availability but that due to correlation between these factors it was not possible to determine their relative importance. There were certainly deep areas where suitable substrata appeared to exist but no reefs occurred. In Ardbear Lough, Bosence (1979) concluded from both observation and from transplant experiments that the lower depth limitation was probably determined by the occurrence of suspended mud and low oxygen levels in the deeper water, while the upper limit was probably due to lowered salinity, competition with algae, and high light levels. However, Moore (1996) quotes Anderson (1887) who described well developed serpulid reefs intertidally among Zostera beds in Loch Creran. Neither the Zostera nor the serpulids occur there now.

No detailed published information was found on the effect of salinity on S. vermicularis. Bosence’s work in Ardbear Lough suggested that lowered salinity is in part responsible for the lack of Serpula reefs above a depth of 2 m below extreme low water of spring tides, and for a lack of reefs at the head of the Lough where there is freshwater input. Although he produced salinity profiles of the Lough on one spring and one neap tide which suggested that salinities of less than 30 ‰ are probably unusual in the areas of extensive reefs, these are of limited value since such small enclosed loughs can be subject to extremely variable salinity conditions depending on local weather etc. Holt (pers. obs.) strongly suspected that a short-term low-salinity event during the spring killed off experimental cultures of Laminaria saccharina and Alaria esculenta in the top 5 metres a few hundred metres below the road bridge in Loch Creran, for example. However, S. vermicularis reefs were observed in intertidal areas of Loch Creran during the last century. The true effect of variations in salinity is therefore unclear, but some tolerance to short-term lowering of salinity seems to be implied.

As mentioned above, suspended mud and low oxygen levels prevented development of reefs in deeper areas of Ardbear Lough; conditions were clearly extreme, however, as there was no macroinvertebrate life at all in the deeper areas (Bosence, 1979).

Serpula vermicularis requires a hard substratum on which to form. It most commonly occurs on bivalve shells (Bosence, 1973; Nelson-Smith, 1967). In Loch Creran it was particularly common on shells of Pecten, Aequipecten and Modiolus, although reefs also formed on bedrock, boulders stones and man-made substrata such as fish farm weights, while large colonies were only rarely found growing on rock (Moore, 1996). Garwood (1982) reported that juveniles were abundant on colonies of the bryozoan Flustra foliacea off the north-east coast of England. It is likely that too much sediment on the surface of the rocks or shells would prevent settlement. Reefs form predominantly in areas where there is suitable substratum scattered throughout a muddy or muddy sand bottom. Reef development occurs by repeated subsequent settling of larvae on adult tubes but it is not known whether the larvae are actually attracted to or stimulated to settle on the adult tubes. Clearly there may be a preference for calcareous substrata in general, but this has not been investigated.

A limited turnover of water in order to facilitate larval retention within the system appears to be a prime requirement for reef development; thus reefs are unlikely to develop and endure except in sheltered sealochs where there are physical barriers at the mouth of the loch limiting tidal exchange of water (Bosence, 1979, Moore, 1996). Shelter from wave action which would be damaging to the reefs would presumably also be a benefit of such an environment.

Bosence (1979) found settlement was lower on the underside of experimental plates, and noted that a tendency for serpulids to settle in shady areas in preference to brightly lit areas has been reported by several other workers, though the precise causes and implications of this are not understood. Factors other than phototropism, such as temperature, siltation and algal growth, may be important.

 Physical Attributes

Reefs are clearly very fragile, being reported to spread partly by virtue of pieces falling away and then continuing to grow (Bosence, 1979) and to become more fragile with age (Bosence, 1973). This may in part be due to the activities of the boring sponge, Cliona celata (Bosence, 1979). In Loch Creran, Moore (1996) reported localised but severe damage to reefs by the scraping action of chain risers and by the movement of anchor weights. Although many of the individual worms survived and were continuing to feed normally, the value as a habitat was greatly diminished. Moore also suggested that overgrowth of other encrusting organisms could contribute to strengthening the colonies. The overall trend nevertheless seems to be of increasing fragility with increasing age and size.

Next Section                     References