Sabellaria spinulosa on stable circalittoral mixed sediment
Image Ken Collins - Close up of Sabellaria spinulosa mound showing worm tubes composed of cemented sand grains and shell fragments. Image width ca XX cm.
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Ecological and functional relationships
As a result of the complex habitat created by the Sabellaria spinulosa tubes (see Habitat Complexity), there are a wealth of different species associated with SS.SBR.PoR.SspiMx. Sabellaria spinulosa crusts also occur amongst sediment so that a mixture of sessile or sedentary epifauna is mixed with burrowing fauna in the sediment. In the Thames estuary, Attrill et al. (1996) discovered that, in an area where Sabellaria spinulosa was among the most abundant fauna, species richness in this area was much higher than in surrounding areas due to the stability of the sediment and the high number of available niches. >200 species of invertebrates were recorded over a three year period in <5 m² (Attrill et al., 1996). The relationships between members of the associated community are not especially complex but the roles of various fauna have been elucidated below.
Aside from Sabellaria spinulosa, the community is dominated by various different polychaetes. These include deposit feeders such as Caulleriella zetlandica, Mediomastus fragilis, Scalibregma inflatum, Scoloplos armiger and Spiophanes bombyx. Carnivorous species may also be common including Eteone longa, Eumida sanguinea, Lumbrineris gracilis and Nephtys hombergi, the latter of which is also a scavenger.
Suspension feeders are diverse and may include dead man's fingers Alcyonium digitatum, the acorn barnacle Balanus crenatus, the tubeworm Pomatoceros triqueter and the baked bean ascidian Dendrodoa grossularia. Several suspension feeding bivalves may also be present, especially Abra alba, Hiatella arctica (a boring bivalve), Mysella bidentata, Modiolus modiolus and Sphenia binghami. Some of these are also deposit feeders as is the bivalve Nucula nitidosa. Sphenia binghami may be found nestled in crevices attached by a weak byssus. Other suspension feeders include the brittlestars Ophuira sp, especially Ophiura albida, amphipods such as Ampelisca sp. and bryozoans including Flustra foliacea and Alcyonidium diaphanum. Ampelisca tenuicornis is primarily a deposit feeder but is also capable of suspension feeding.
Mobile epibenthic predators include hermit crabs such as Pagurus bernhardus and pycnogonids. Pagurus bernhardus is an active omnivore that scavenges and preys upon various food items. It is also capable of suspension feeding. The pycnogonid Achelia echinata preys upon the bryozoan Flustra foliacea.
No macroalgae are associated with the biotope since it occurs below the compensation zone for photosynthesis for most algal species. Also, the turbid habitat within which the biotope is found may be detrimental to many algal species both in terms of increased light attenuation and physical abrasion caused by the scouring of the sand on the fronds.
Although Sabellaria spinulosa is, by its nature, an ephemeral species, the stable nature of the substratum associated with SS.SBR.PoR.SspiMx mean that the crusts of Sabellaria spinulosa may be well established, certainly more than one year old. George & Warwick (1985) found that most of the worms in the aggregation of Sabellaria spinulosa they studied in the Bristol Channel were more than one year old. Furthermore, the species associated with them were found to be slow growing. The associated community are likely to depend on the frequency of the disturbance to the habitat. Furthermore, areas of SS.SBR.PoR.SspiMx that have recently disturbed are likely to be characterised by a very different fauna to a well established and undisturbed variant.
Seasonal and longer term change
are known as 'r'-strategists and are adapted to live in frequently disturbed environments. Despite the stable nature of the sediment on which SS.SBR.PoR.SspiMx is found, winter storms may be expected to break up the Sabellaria spinulosa
matrix every few years although given the depth at which the biotope is found, it will be affected comparatively less than shallower mixed sediment communities. In areas where the biotope is periodically destroyed by storm events, a cyclical shift in biotopes from SspiMx to other biotopes e.g. SS.SCS.CCS.Pkef or SS.SSa.CMuSa.AalbNuc , with re-establishment of the Sabellaria
colonies in the following year, may occur (Connor et al.
, 2004). Crusts of Sabellaria spinulosa
are likely to reform within 1-3 years. George & Warwick (1985) found that most of the worms in the aggregation of Sabellaria spinulosa
they studied in the Bristol Channel were more than one year old. Furthermore, the species associated with them were found to be slow growing. Due to the lack of algal species, little change is to be expected in terms of floral growth. The most likely source of seasonal changes is species composition because some short lived species such as Chaetozone setosa
will die off over the winter months and, therefore, species diversity can be expected to decrease in winter. The time taken to develop, longevity and importance as a habitat of raised reefs of Sabellaria spinulosa
is not established but is now (2005) the subject of studies.
Habitat structure and complexity
At the Bristol Channel location studied by George & Warwick (1985), densities in excess of 4,000/m² for loosely aggregated Sabellaria spinulosa
were recorded. The Sabellaria
at their study site was loosely aggregated and not extended above the seabed in a 'reef' formation, that is, their study focused on a biotope more representative of SS.SBR.PoR.SspiMx.
In the UK SACs Biogenic Reef volume (Holt et al., 1998), CMX.SspiMx has been described as a biogenic reef and, although it may be destroyed by winter storms, will offer a stabilizing effect on the substratum. In addition to the stabilizing effect of the tubes, the physical structure of the mass of tubes themselves provides a matrix of burrows, nooks and crannies which are ideal for offering protection for nestling and cryptic species. Other tube building polychaetes include Lanice conchilega. Lanice conchilega tubes provide structure to the sediment, very much like a hollow rod stabilising the sediment (Jones & Jago, 1993). Tube building amphipods such as Ampelisca sp. will also contribute to the habitat complexity, as will the bryozoan Flustra foliacea. The matrix of various tubes and other erect structures will trap sediment providing food for deposit feeders. The trapped sediment also means that the biotope will be composed of habitats similar to both sedimentary and hard substratum environments, thereby increasing the number of potential niches. The aggregation provides shelter and protection for small species in an otherwise 'exposed' (in terms of nowhere to hide) sedimentary landscape.
'reefs' can support a highly diverse fauna. George and Warwick reported that the total production of extensive reefs of Sabellaria spinulosa
in the Bristol Channel was 34.1 g dry wt / m² / year. 96 % of production attributed to suspension feeders, of which Ophiothrix fragilis
dominated. This species is not though to be particularly common in CMX.SspiMx although Ophiura
sp., especially Ophiura albida
may be abundant. Sabellaria spinulosa
itself has a rather low rate of production (George & Warwick, 1985).
Recruitment processes are described for dominant and representative species.
Wilson (1970b) stated that the larvae of Sabellaria spinulosa spend between six weeks and two months in the plankton. Reproductive seasonality is unclear but George & Warwick (1985) and Wilson (1970a) have both reported larval settlement in March in the Bristol Channel and Plymouth areas respectively. George & Warwick (1985) also reported a secondary smaller settlement in November in the Bristol Channel. Wilson (1970a) found that the spawning period extended from January to March in Plymouth. Fecundity and recruitment may be variable (Holt et al., 1998) but may be similar to Sabellaria alveolata. Settlement of Sabellaria spinulosa is thought to be strongly influenced by the presence of existing Sabellaria spinulosa (Wilson, 1970a). The presence of Ophiothrix fragilis can greatly reduce recruitment (Holt et al., 1998). However, Ophiothrix fragilis is not commonly associated with SS.SBR.PoR.SspiMx although Ophuira sp. may have a similar effect.
- Hayward & Ryland (1995) and Segrove (1941) suggested that breeding of Pomatoceros triqueter probably takes place throughout the year. However, Hayward & Ryland (1995) noted a breeding peak in spring and summer and records from Port Erin by Moore (1937) indicated that breeding only took place in April in this location. Castric-Fey (1983) studied variations in settlement rate and concluded that, although the species settled all year round, very rare settlement was observed during winter and maximum settlement occurred in April, June, August and Sept-Oct. Studies in Bantry Bay (Cotter et al., 2003) revealed a single peak in recruitment during summer (especially July and August) with very little recruitment at other times of the year. Larvae are pelagic for about 2-3 weeks in the summer. However, in the winter this amount of time increases to about 2 months (Hayward & Ryland, 1995).
- Hornwrack Flustra foliacea is likely to be the most abundant of the bryozoan species associated with the biotope. Flustra foliacea bears both male and female zooids and is presumably hermaphrodite (see MarLIN review). Fertilization in brooding species such as Flustra foliacea is probably internal (Hayward & Ryland, 1998). Released sperm are entrained by the tentacles of feeding polypides and may not disperse far, resulting in self-fertilization. However, genetic cross-fertilization is assumed in oviparous and brooding bryozoans, although there is evidence of self fertilization (Hayward & Ryland, 1998). Dalyell (cited in Hincks, 1880) stated that ca 10,000 larvae were released from a specimen of Flustra foliacea within 3 hrs. Larvae are positively phototactic on release, and swim for only short periods. Daylength is an important cue for larval release in some species of bryozoa, and Flustra foliacea releases larvae in spring (February- April) (Eggleston, 1972a; Hayward & Ryland, 1998). The short larval life probably results in good local but poor long-range dispersal.
- Alcyonium digitatum spawns during December and January. Gametes are released into the water and fertilization occurs externally. The embryos are neutrally buoyant and float freely for 7 days. The embryos give rise to actively swimming lecithotrophic planulae which may have an extended pelagic life before they eventually settle (usually within one or two further days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975). In laboratory experiments, several larvae of Alcyonium digitatum failed to settle within 10 days, presumably finding the conditions unsuitable. These larvae proved to be able to survive 35 weeks as non-feeding planulae. After 14 weeks some were still swimming and after 24 weeks the surface ciliation was still active although they rested on the bottom of the tanks, by the end of the experiment at 35 weeks the larvae had shrunk to a diameter of 0.3 mm. This ability to survive for long periods in the plankton may favour the dispersal and eventual discovery of a site suitable for settlement (Hartnoll, 1975).
- Balanus crenatus is an obligate cross-fertilizing hermaphrodite. Nauplii larvae are released from the barnacle between February and September, with peaks in April and late summer when phytoplankton levels are highest. Nauplii larvae are planktotrophic and develop in the surface waters stages before eventually developing into a cyprid larva. Peak settlement occurs in April and declines until October. Metamorphosis usually takes place within 24 hours of settlement. April-settled individuals may release larvae the same July and reach full size before their first winter.
- Ampelisca spinipes is likely to be have direct development. Recruitment is therefore likely to be high in local areas and although the dispersal of juveniles is relatively low, adults are highly motile.
- The timing of reproduction and recruitment in the baked bean ascidian Dendrodoa grossularia depends partly on geographic location but the general patterns is of annual episodic reproduction with major periods of settlement occurring in spring and autumn (Millar, 1954). Fertilised eggs are brooded until an advanced larvae stage.
- Two types of development have been reported in Scoloplos armiger: a holobenthic type and a pelagic larvae. The holobenthic type crawls out from a cocoon fixed on the substratum and burrows immediately, usually associated with intertidal populations in North Sea region and adjacent waters and a pelagic larvae associated with subtidal populations (Kruse et al., 2003; Kruse et al., 2004). At the Isle of Sylt, North Sea, egg cocoons are found on intertidal flats between Feb-April (Kruse et al., 2004). Spawning varies with location. In North Sea, main spawn March, secondary (pelagic) spawn from offshore in Oct (Kruse et al., 2004). At Whitstable, spawned four times in one year, main late Feb-April (Gibbs, 1968). 600-1920 / m² Oosterschelde (Coosen et al., 1994), 800 / m² at Whitstable (Gibbs, 1968). Scoloplos armiger does not mature until 2 years of age. Many other polychaete species will be found in this biotope.
- The larvae of Lanice conchilega spend up to 60 days in the plankton, so that larvae could potentially disperse over a great distance, depending on the hydrographical regime. Heuers & Jaklin (1999) found that areas with adult worms or artificial tubes were settled and areas without these structures were not.
- Species such as Spiophanes bombyx are regarded as a typical 'r' selecting species with a short life span, high dispersal potential and high reproductive rate.
Time for community to reach maturity
is a fast growing annual species. Areas where Sabellaria spinulosa
had been lost due to winter storms appeared to recolonize up to a maximum of 2.4 cm during the following summer (R. Holt, pers. comm in Jones et al.
, 2000). However, George & Warwick (1985) found that, in the Bristol Channel, the reef was comprised mostly of worms over one year old. They also noted that the diverse small species found nestling within the reef were slow growing. Since SS.SBR.PoR.SspiMx is characterized only by the presence of Sabellaria spinulosa
, the biotope is likely to 'mature' into the recognisable biotope within one year. However, the epibiotic species typically associated with the special features of Sabellaria spinulosa
crusts are likely to take longer to develop. In stable conditions the community may continue to mature into a more diverse community over several years.
This review can be cited as follows:
Sabellaria spinulosa on stable circalittoral mixed sediment.
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=377&code=1997>