Reproduction, development and growth
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.
Larval development and settlement
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
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 Moore (unpublished) found that initially bare substrata could support dense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.
Longevity and stability
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.
Parasites and diseases
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%.
Wider effects on the environment
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.