Serpula vermicularis reefs on very sheltered circalittoral muddy sand
Ecological and functional relationships
Serpula vermicularis normally occurs as individuals encrusted on hard surfaces. A tendency to form aggregations in certain conditions is reported but true reefs have an extremely limited distribution. It has been suggested that dense aggregations of Serpula vermicularis tubes only occur in enclosed and sheltered locations, where dispersal of larvae may be limited and where a suitable substratum is present. The hypothesis that reef formation occurs in Loch Creran due to limited larval dispersal is not currently backed up by any evidence. Loch Creran actually has quite a high flushing rate (Hughes pers. comm.) and there are many far more restricted sites in the area with no reef development. It is also questionable whether lack of suitable substratum is a factor leading to reef development. There are extensive areas of bedrock outcropping from the floor of Loch Creran but these typically support very limited and localised reef growth. Further, Chapman et al. (2007) suggest that the lower depth limit of reefs is not set by shortage of available substrata. The formation of reefs therefore is likely to be due to a complex interaction of many factors.
In Loch Creran individual reefs are reported to reach a height of about 75 cm and 1 m across, although adjacent reefs may coalesce to form larger reefs up to 3 m across (Moore, 1996). Bosence (1979b) described reefs up to 2m in height and 1m across from Ardbear Lough but suggested that aggregated reefs could extend for several hundred metres.
Serpula vermicularis requires a hard substratum on which to construct its tube. The most common substratum for settlement is bivalve shells. In Loch Creran it was particularly common on shells of Pecten, Aequipecten and Modiolus. Reefs form predominantly in areas where there is suitable substratum scattered throughout a muddy or muddy sand bottom. In this way, previously bare substratum can support dense aggregations of worms and a high diversity of associated species.
The structure of Serpula vermicularis reefs is quite open, increasing surface and space for colonization, as well as for food and refuge, for an abundant and varied animal community. The rich associated fauna of organisms includes sessile organisms such as ascidians, hydroids, bivalves and other polychaete worms such as Pomatoceros triqueter and Sabella pavonina. There is also a mobile component of the associated macrofauna which is rich in amphipods, and also includes fish, crabs, whelks and echinoderms that use the reefs for feeding, refuge and egg-laying (Moore et al., 1998b; Poloczanska et al., 2004). The open structure appears to be related to the regular spacing of the apertures of the tubes at 10-15 mm apart which gives enough space for the expansion of the branchial crowns during feeding (Bosence, 1979b).
Predation of Serpula vermicularis by several species has been described by Bosence (1979b) although the importance of the species as a food source is unknown. The wrasse Ctenolabrus rupestris and Crenilabrus melops were frequently seen biting serpulid tubes and extracting the worms. The starfish Asterias rubens was frequently seen with its stomach everted down the serpulid tubes. Bosence (1979b) also observed the urchins Echinus esculentus and Psammechinus miliaris feeding on serpulid tubes but thought it unlikely that they were feeding directly on the worms, which can withdraw into their tubes very rapidly, and were more likely to be eating the epifauna and epiflora on the tubes. Predation of Serpula vermicularis by Cancer pagurus, Carcinus maenas, Asterias rubens and Ctenolabrus rupestris was observed in Salt Lake, Ardbear Lough (Minchin, 1987). However, long term video monitoring of reefs in Loch Creran revealed very few instances of attempted predation on the worms (Poloczanska et al., 2004).
Seasonal and longer term change
The growth of Serpula vermicularis
reefs may take many years so the major change over time is likely to be an increase in size of the reef. However, as growth of the reef proceeds, the old base is weakened by biological erosion by boring sponges and algae, and biting by fish and echinoderms. Segments of the reef then break off and provide new areas for larval settlement and this is the main way in which a reef growing from an original rocky substratum can extend outwards to cover areas of soft substratum (Bosence, 1979b). There may be seasonal changes in abundance of other species, such as hydroids, which often have life spans less than a year, and due to periodic recruitment of larvae.
Habitat structure and complexity
The reef habitat created by aggregated Serpula vermicularis
tubes can be up to 2m high (Bosence, 1979b). Initial growth is encrusting but after that the worms grow away from the substratum in a sinuous fashion, sometimes becoming intertwined and the reefs develop as new worms are added to old tubes. The reef is a structurally complex habitat as the open form of the aggregated tubes provides a large surface area and many spaces which supports a high diversity of sessile and mobile macrofauna. In Loch Creran, Scotland reefs were found growing in bedrock, boulders, stones, shells and man-made substrata (Moore et al
., 1998b). Large reefs were only rarely found growing on rock.
The community is predominantly faunal so productivity in the biotope is largely secondary. Red algae such as Phycodrys rubens
and encrusting corallines are present in the biotope, although not in very high abundance and so levels of primary production are not likely to be high. Although no information was found regarding the diet of Serpula vermicularis
, analysis of digestive enzymes suggests that quite large detrital particles may form an important part of the diet (Michel & De Villez, 1978). Several of the other species in the biotope, such as the ascidians and other polychaetes, are also suspension feeders. Phytoplankton, supplemented by non-living detritus, is likely to be the main food source for all these species (Hughes, pers. comm.). Secondary production could be substantial in large reefs.
- In the habitats in which this biotope is found, where water movement and exchange with coastal waters is limited, recruitment from local populations is particularly important because of the reduced supply of planktonic larvae from outside the system.
- In Loch Creran, larval settlement occurs predominantly from mid-June to mid October, peaking in late August-early September (Chapman et al., 2007). The length of the planktonic stage is not known but comparison with other serpulids suggests it would probably be between 6 days and 2 months. However, settling time may vary depending on salinity or food availability, and delayed settling may reduce discrimination of substrata for settlement.
- Reef development occurs by repeated settlement of larvae on the tubes of adults however evidence suggests that the larvae are not attracted to settle on adult tubes in preference to substratum. Experiments in Loch Creran showed that Serpula vermicularis larvae settled on slate in preference to scallop shell. There was no evidence of enhanced recruitment to occupied of unoccupied adult tubes, suggesting that gregarious attraction is unlikely to be a factor causing reef formation (Chapman et al., 2007).
- Recruitment of sessile organisms in the biotope, such as sponges, ascidians and hydroids, is almost entirely from planktonic sources. Some species have larvae that can disperse widely and these may arrive from distant locations. Others, particularly the hydroids and some ascidians have short lived planktonic larvae so dispersal distances are short and recruitment will largely be from local populations.
- Recruitment of the mobile predators and grazers may be through immigration of adults or via a larval dispersal phase. Mobile species such as decapod crustaceans, echinoderms and fish will settle from planktonic stages or migrate into the biotope.
- Red algae have non flagellate, and non-motile spores that stick on contact with the substratum. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition. However, red algae produce large numbers of spores that may settle close to the adult especially where currents are reduced such as in sheltered locations.
Time for community to reach maturity
Although the growth rates of individual Serpula vermicularis worms are relatively high (Bosence, 1979b), showing an average linear rate of 33mm/year for adult tubes (Hughes et al., 2008), it is likely that reefs will take many years to develop. The reef develops upwards and outwards as larvae settle on to the tubes of existing worms and so it may take many periods of recruitment for reefs to become large. Many other species in the biotope, such as ascidians, hydroids and bryozoans exhibit annual recruitment and many are short lived so populations are likely to reach maturity rapidly. There are some slow growing species, such as encrusting coralline algae, which take longer to achieve significant coverage. Species diversity within the reef is likely to increase with time. However, the time to maturity of the biotope will depend on the time for reef development which is likely to be many years.
This review can be cited as follows:
Hill, J.M. & Wilding C.M.
Serpula vermicularis reefs on very sheltered circalittoral muddy sand.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=41&code=1997>