Biology

Ecological Functioning

There is relatively little published information on reproduction in Serpula, and most of what there is comes from non-reef situations.

Spawning seems to occur in the summer: Elmhirst (1922) reported that spawning occurs in June to August in the Clyde Sea area, and Allen (1915) found ripe specimens in the Plymouth area in August and September.

In Ardbear Lough, Bosence (1979) found dense settlement of larvae onto plates in August, but did not carry out investigations at any other times. Nelson-Smith (1967) states that no serpulids settle in winter in temperate areas, though settlement may extend throughout the summer. Length of the planktonic stage is unknown but comparison with other serpulids suggests it may be between six days and two months, although in other species the period has been shown to vary with season, salinity or food availability, and delayed settling may cause reduced discrimination of substrata during settling (see ten Hove, 1979 for additional references). Settlement preferences have been discussed in chapter III.

Growth and development of individuals seem to be relatively fast. Bosence (1979) found a mean tube growth of 9 mm in length over a one month period (August 1972), and described growth as periodic due to the presence of trumpet like enlargements on the tubes. It seems likely that these would be annual features, suggesting a lifespan of several years, but no published evidence to support this supposition has yet been found. Moore (unpublished) found that initially bare substrata could support dense aggregations of up to an estimated 15 cm in height after around three months.

Nothing has been published on the development rates of S. vermicularis reefs, which presumably take many years.

Given the rapid growth rates achieved, it seems likely that the worms would reach adult size, and probably maturity, within one year, as is normal within the serpulids. Orton (1914) observed that ten month old specimens in the south west of England could successfully reproduce.

The longevity of reefs is unknown, but it appears that they must take many years, and possibly decades, to develop to the extent of the larger reefs observed. Nothing is known about the proportion of small colonies which actually succeed in developing into large reefs. From the external growth structures on tubes it seems likely that individual worms seem to be capable of living for several years (see above).

S. vermicularis is a filter feeder and the arrangement of its tubes in reefs is such that interference between adjacent crowns is avoided. No details have been found regarding the mechanism of feeding or the type of food taken but the presence of laminarase, chitinase and cellulase in its digestive system (Michel & De Villez, 1978) suggests that quite large detrital particles may form an important part of the diet.

The boring sponge Cliona celata appears to significantly weaken the reef structure as the colony ages, but the main result of this seems to be that sections of older reefs fall away and form a nucleus for subsequent colonisation and growth, thus allowing reef development on areas where there may have previously been no suitable substratum (Bosence, 1979). Worms on fallen sections were reported to respond by growing away from the substratum.

S. vermicularis reefs act as a substratum for a wide variety of other organisms. These include numerous sessile organisms such as boring, encrusting sponges and massive sponges, ascidians and hydroids, the serpulid Pomatoceros triqueter, spirorbid and other tube worms, numerous encrusting bryozoans, the anemone Metridium senile, and numerous bivalves such as Monia patelliformis, Modiolus modiolus, Chlamys distorta, C. varia and Aequipecten opercularis (though many of the pectinids may only be temporary inhabitants) (Bosence, 1979; Howson et al., 1994; Connor et al., 1997). In shallow water dense growths of the red alga Phycodrys rubens may occur on the reefs. Mobile inhabitants which have been reported include numerous crab and other crustacean species, the urchins Echinus esculentus and Psammechinus miliaris, the brittle star Ophiothrix fragilis, the starfish Asterias rubens, and the whelk Buccinum undatum. The richness of the associated community is not surprising given the relatively open structure of the reefs and the lack of hard substrata in the areas where it is found. The MNCR marine biotope classification lists 34 species as occurring in at least 40% of records for S. vermicularis reefs. A further species, the tunicate Pyura microcosmus, reportedly occurred in only 20-40% of records but was described as highly faithful (i.e. found only in this or very closely related biotopes). Knowledge of associated biota is largely limited to macrofauna which can be observed on the outside of reefs. Investigations including more cryptic fauna are likely to reveal an even greater richness and diversity than presently known. Even very small ‘heads’ of serpulids (Pomatoceros spp), such as form on single shells in many sublittoral areas, have been reported to contain up to 68 taxa (Kaiser et al., in press); many of these were polychaetes, though isopods, amphipods and sipunculid worms were also important.

Bosence (1979) estimated that in Ardbear Lough, Ireland, about 5% of the Lough floor would have been hard substrate in the absence of reefs, but that the development of reefs had increased this to around 25%.

Other than the relatively local increase in hard substrate (see previous paragraph) no information on this subject was found. Given that reefs seem to develop only in enclosed areas with very limited water exchange, wider effects can only be expected within these limited areas, if at all. Since S. vermicularis is a filter feeder, extensive areas might have the potential to have effects on phytoplankton levels, but this is merely speculation.

Known predators of S. vermicularis on the reefs in Ardbear Lough were described by Bosence (1979), although their importance is not known. The wrasse Ctenolabrus rupestris and Crenilabrus melops were frequently seen biting open serpulid tubes and extracting the worms. Bosence described this as a trial and error activity which generates a considerable amount of gravel sized serpulid debris. A. rubens was frequently seen with its stomach everted down the worm tubes. Bosence also observed the urchins E. esculentus and P. miliaris feeding on serpulid tubes and found dissected stomachs to be full of tube debris, but thought they were unlikely to be able to eat the worms themselves, which can withdraw very rapidly deep into their tubes, and suggested they were more likely to be eating indiscriminately for the sake of the epifauna, flora and boring organisms. He noted that B. undatum, the edible crab Cancer pagurus and the squat lobster Galathea squamifera were commonly seen on the reefs but were not observed feeding directly on them, and thought that the latter was unlikely to be able to break open the serpulid tubes.

Although no competitors are known in relation to existing reefs, lack of competition for space was suggested by Bosence (1979) as one of the factors leading to reef development in enclosed sea lochs, along with limited water exchange leading to increased larval supply. The implication is that space occupiers such as algae, barnacles or mussels might, in some circumstances, prevent development of reefs.

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