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information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Capitella capitata and Thyasira spp. in organically-enriched offshore circalittoral mud and sandy mud

08-11-2016

Summary

UK and Ireland classification

UK and Ireland classification

Description

In circalittoral and deep offshore mud and sandy mud adjacent to oil or gas platforms, organic enrichment from drill cuttings leads to the development of communities dominated by Capitella capitata, an opportunist polychaete especially associated with organically enriched and polluted sediments as described for SS.SMu.ISaMu.Cap (Warren, 1977; Pearson & Rosenberg, 1978). The bivalves Thyasira flexuosa or Thyasira sarsi may also be found in moderate numbers at some sites. Other taxa may be present in low numbers in areas of less severe enrichment including Pholoe inornata, Lagis koreni, Hermania scabra (syn. Philine scabra), Phyllodoce groenlandica (syn. Anaitides groenlandica), Mediomastus fragilis and Paramphinome jeffreysii. (Information taken from Connor et al., 2004).

Depth range

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Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

The biotope is defined by the presence of large numbers of the polychaete Capitella capitata (agg.), supported by organic enrichment from drill cuttings occurring in circalittoral and deep offshore mud and sandy mud adjacent to oil and gas platforms (Connor et al., 2004). Thyasira spp. bivalves may also occur in moderate numbers at some sites.

In deeper, less polluted examples of the biotope, with sandier sediments, polychaete Ophryotrocha dubia may also become dominant, and the biotope may develop into the SS.SMu.OMu.CapThy.Odub sub-biotope, which tends to be less diverse than SS.SMu.OMu.CapThy. Both these biotopes are therefore here assessed as a group. However, little information was available on Ophryotrocha dubia so SS.SMu.OMu.CapThy.Odub is not here considered seperately, and Capitella capitata, Thyasira spp. and Ophryotrocha dubia are considered the key characterizing species of the biotopes, and hence the focus of this assessment. Other taxa present contribute to species richness and diversity but are not considered important characterizing, defining or structuring species and are not considered within the assessment.

Resilience and recovery rates of habitat

Capitella capitata is a classic opportunist species possessing life history traits of rapid development, many reproductions per year, high recruitment and high death rates (Grassle & Grassle, 1974; McCall, 1977). Experimental studies using defaunated sediments have shown that on small scales Capitella can recolonize to background densities within 12 days (Grassle & Grassle, 1974; McCall, 1977). In Burry Inlet, Wales, tractor towed cockle harvesting led to a reduction in density of some species but Capitella capitata had almost trebled its abundance within the 56 days in a clean sandy area (Ferns et al., 2000).

In favorable conditions, maturity can be reached in <3 months and growth rate is estimated to be 30 mm per year. Adult potential dispersal is up to 1 km. The species complex displays reproductive variability and planktonic larvae are able to colonize newly disturbed patches but after settlement the species can produce benthic larvae brooded within the adult tube to rapidly increase the population before displacement by more competitive species (Gray, 1979). Bolam & Fernandes (2002) and Shull (1997) noted that Capitella capitata can colonize azoic sediments rapidly in relatively high numbers. Shull (1997) also demonstrated that this occurs by larval settlement, bedload transport and by burrowing. Thus, when conditions are suitable, the time for the community to reach maturity is likely to be less than six months.

Other species within the biotopes may recolonize more slowly. Tubificid populations tend to be large and to be constant throughout the year, although some studies have noticed seasonal variations (Giere & Pfannkuche, 1982). Many species, including Tubificoides benedii and Baltidrilus costata have a two-year reproductive cycle and only part of the population reproduces each season (Giere & Pfannkuche, 1982). Populations of Tubificoides benedii in the Fourth Estuary have not demonstrated clear seasonality in recruitment (Bagheri & McLusky, 1982), although mature Tubificoides benedii (studied as Peloscolex benedeni) in the Thames Estuary were reported to occur in December with a maximum in late February (Hunter & Arthur, 1978), breeding worms increased from April and maximum cocoon deposition was observed in July (Hunter & Arthur, 1978). Tubificids exhibit many of the traits of opportunistic species; it is dominant, often reaching huge population densities in coastal areas that are enriched in organic matter and is often described as an ‘opportunist’ species adapted to rapid environmental fluctuations and stress (Giere, 2006; Bagheri & McLusky, 1982). However, unlike other opportunist species, it has a long-lifespan (a few years, Giere, 2006), a prolonged reproductive period from reaching maturity to maximum cocoon deposition, and exhibits internal fertilization with brooding rather than pelagic dispersal. These factors mean that recolonization is slower than for some opportunistic species such as Capitella capitata and nematodes which may be present in similar habitats.

Little information was available for Thyasira flexuosa. The larval development of the congener Thyasira equalis is lecithotrophic and the pelagic stage is very short or suppressed (Tillin & Tyler-Walters, 2014). This agrees with the reproduction of other Thyasira sp., and in some cases (e.g. Thyasira gouldi) no pelagic stage occurs at all (Thorson, 1946, 1950). This means that larval dispersal is limited. Sparks-McConkey & Watling (2001) found that a population of Thyasira flexuosa in Penobscot Bay, Maine recovered rapidly (within 3.5 months) following trawler disturbance that resulted in a decrease in the population. Benthic reproduction allows recolonization of nearby disturbed sediment and leads to rapid recovery where a large proportion of the population remains to repopulate the habitat. 

Litttle information was also available for Ophryotrocha dubia. Annelids of the genus Ophryotrocha are small opportunistic worms commonly found in polluted and nutrient-rich environments. Within this group there's great variety of reproductive strategies, ranging from gonochoristic broadcast spawners to sequential hermaphroditic brooders, but many of the species have short generation times (Dahlgren et al., 2001).

Resilience assessment. Capitella capitata dominated biotopes are likely to reach maturity very rapidly because the species of the complex are short lived, reaching maturity within about four months and reproducing throughout the year. However, other species within the biotope may colonize more slowly. For example, Thyasira spp. have fragile shells that are vulnerable to damage, are thought to be slow growing, with benthic reproduction and sporadic recruitment. So where the majority of the population remain (resistance is High, Medium or Low), and/or recruitment by adult mobility is possible, resilience is likely to be High. However, where recovery through juvenile recruitment is required, this may be low in places where complete extinction of Thyasira spp. occurred. Although polychaetes tend to have high recovery rates and Capitella capitata will recolonize the habitat quickly, the low energy environments where the biotopes occur are likely to slow the time for most species to re-established biomass and age structured populations, particularly characterizing species Thyasira. Therefore, where impacts remove a significant proportion of the population (resistance is None), recovery is likely to be Medium (2-10 years).

Hydrological Pressures

 ResistanceResilienceSensitivity
Medium High Low
Q: High
A: Medium
C: Medium
Q: High
A: Low
C: High
Q: High
A: Low
C: Medium

Capitella capitata is a cosmopolitan species in coastal marine and estuarine soft sediment systems. Grassle & Grassle (1976) used electrophoretic enzyme analysis to determine that the global population is actually made up of several genetically distinct (and apparently genetically isolated) sibling species whose distributions overlap such that local Capitella capitata populations actually consist of a number of co-occurring sibling species. Within the complex, tolerances may vary and local acclimation is possible. Capitella capitata has also been recorded in extreme environments around hydrothermal vents (Gamenick & Giere, 1997), which suggests that the species complex would be relatively tolerant to an increase in temperature.

Experimental evaluation of the effects of combinations of varying salinities and temperature on Capitella capitata were carried out by Redman (1985) and Warren (1977). Redman (1985) found that length of life decreased as follows: 59 weeks at mid-temperature and salinity (15°C, 25 ppt); 43 weeks at high temperature and high salinity (18°C, 30 ppt); 33 weeks at lower temperature and high salinity (12°C, 30 ppt); 17 weeks at high temperature and low salinity (18°C, 20 ppt). Redman (1985) also found that net reproduction (Ro: the mean number of offspring produced per female at the end of the cohort) decreased as follows: 41.75 control; 36.69 under high salinity, high temperature; 2.19 high temperature, low salinity; 2.16 low temperature, high salinity. Therefore, a combination of changes in temperature and salinity may decrease the viability of the population. Warren (1977) used individual worms collected from Warren Point (south west England) to test response to high and low temperatures. Worms were acclimated to 10°C for 10 days and subsequently heated in a water bath to experienced a rise  in temperature of 1°C per 5 min. When the temperature had reached 28°C worms were removed at 0.5°C intervals and returned to a constant temperature of 10°C. The percentage mortality after 24 h was calculated. Larvae were removed from the maternal tube and tested using the same method. The experiments indicated that temperatures above 30°C were most critical; the upper lethal temperature was 31.5°C for adult worms and a little higher for the larvae.

Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). Wilson (1981) investigated temperature tolerances of six bivalve species from Dublin Bay. The author concluded that species variations in tolerance to increased temperature varied seasonally and with distribution along tidal height. Lethal temperatures for all six bivalve species in the study varied greatly and were, in most cases, well above 20°C. The maximum sea surface temperatures around the British Isles rarely exceed 20°C (Hiscock, 1998).

Sensitivity assessment. Typical surface water temperatures around the UK coast vary seasonally from 4-19°C (Huthnance, 2010). The biotope, based on the characterizing species, is considered to tolerate a 2°C increase in temperature for a year. The experimental studies by Redman (1985) suggest that changes in temperature may reduce the life-span of Capitella capitata, however, this effect is not considered to alter the character of the biotope as the short life cycle of this species should lead to rapid replenishment of the population. The experiments by Warren (1977) suggest that both the chronic and acute increases in temperature would not exceed the thermal tolerance of Capitella capitata. However, Thyasira spp. may suffer some mortality as a result of an acute increase in temperature so resistance is therefore assessed as Medium (loss <25%). Resilience is likely to be High, so the biotopes are considered to have Low sensitivity to an increase in temperature at the pressure benchmark level.

Medium High Low
Q: Low
A: NR
C: NR
Q: High
A: Low
C: High
Q: Low
A: NR
C: NR

Capitella capitata is a cosmopolitan species in coastal marine and estuarine soft sediment systems. Grassle & Grassle (1976) used electrophoretic enzyme analysis to determine that the global population is actually made up of several genetically distinct (and apparently genetically isolated) sibling species whose distributions overlap such that local Capitella capitata populations actually consist of a number of co-occurring sibling species. Within the complex, tolerances may vary and local acclimation is possible. Wu et al. (1988) collected animals at seawater temperatures of -2°C that harboured mature oocytes indicating reproductive activity even under low temperatures.

Warren (1977) used individual worms collected from Warren Point (south west England) to test response to high and low temperatures. Worms were acclimated to 10°C for 10 days prior to testing. The worms were cooled in a water bath to experience a decrease in temperature of 1°C per 5 min. When the final temperature was reached, worms were removed at 0.5°C intervals and returned to a constant temperature of 10°C. The percentage mortality after 24 h was calculated. Each experiment was repeated once. Larval Capitella capitata were removed from the maternal tube and tested using the same method. Both adults and larvae were tolerant of low temperatures, 50% of the adults and 65% of the larvae surviving at -1°C.

Thyasira flexuosa does not occur in the southernmost part of the North Sea but is distributed from Norway to the Azores, and extends into the Mediterranean (Tillin & Tyler-Walters, 2014). However, Thyasira populations in the British Isles are restricted to areas where the bottom waters remain cool all year round (Jackson, 2007). Short term acute periods of extreme cold and icing conditions are likely to cause stress and some mortality in bivalve populations (Dame, 1996). However, no specific information on temperature tolerances of Thyasira spp. was found.

Sensitivity assessment. Typical surface water temperatures around the UK coast vary, seasonally from 4-19°C (Huthnance, 2010). The biotopes, based on the characterizing species, are considered to tolerate a 2°C decrease in temperature for a year. The experiments by Warren (1977) suggest that both the chronic and acute decreases in temperature would not exceed the thermal tolerance of Capitella capitata. However, characterizing species Thyasira spp. may suffer some mortality as a result of an acute decrease in temperature so resistance is therefore assessed as Medium (<25% loss), but with low confidence. Resilience is likely to be High, so the biotopes are considered to have Low sensitivity to a decrease in temperature at the pressure benchmark level.

Low High Low
Q: Low
A: NR
C: NR
Q: High
A: Low
C: High
Q: Low
A: NR
C: NR

The biotopes occur in full salinity (30-35 ppt) (Connor et al., 2004) and it is highly unlikely that they would experience conditions of hypersalinity and no evidence was found to assess an increase in salinity above full.

Sensitivity assessment. No direct evidence was found to assess the effects of changes in salinity and OBIS data (OBIS, 2014) was used as the basis of the assessment. The minimum and maximum range of salinities for the characterizing species are 10.0-39.1 psu for Capitella capitata and 31.8-39.1 psu for Thyasira flexuosa. This data suggests that the key components of the biotopes communities would not be resistant of an increase in salinity to >40 psu, resulting in mortality of the characterizing species. Resistance is therefore assessed as Low (loss of 25-75%) but with low confidence. Once normal conditions are resumed, resilience is probably High, so that sensitivity is therefore assessed as Low.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Warren (1977) used individual worms collected from Warren Point (south-west England) to test response to reduced salinity. Individual Capitella capitata were acclimated to 33 ‰ for 1 week prior to exposure to salinities of 1.5 ‰, 5.5 ‰, 18 ‰ and 33 ‰. Larvae removed from the maternal tube were also tested in groups of 10. The results of tolerance tests showed that adult Capitella capitata acclimated at 33 ‰ were intolerant of reduced salinities below 20 ‰, all exposed adults died within 4 days when exposed at 18 ‰ and within 1 day at 9 ‰. The larvae were more tolerant, living for 10 days at 15.5 ‰ with little apparent ill effect.

Thyasira spp. inhabit waters of reduced salinity with 25-30 psu being optimal. However, adults exposed to lower than optimal salinities produced non-viable or slow developing eggs (Jackson, 2007). There is insufficient information regarding the effects of salinity on adults.

Sensitivity assessment. The biotopes are found within fully marine subtidal locations (Connor et al., 2004). However, there are records of Capitella capitata dominated biotopes (e.g. SS.SMu.ISaMu.Cap and SS.SMu.SMuVS.CapTubi) occurring in variable (18-35 ppt) and low salinity (<18 ppt) (Connor et al., 2004). Furthermore, the evidence presented suggests that the characterizing species are likely to resist a decrease in salinity at the pressure benchmark level from full (30-35 ppt) to variable (18-35 ppt). The biotopes are, therefore, considered to have High resistance to this pressure and High resilience (by default), so the biotopes are considered to be Not Sensitive to a decrease in salinity at the pressure benchmark level.

Medium High Low
Q: High
A: High
C: High
Q: High
A: Low
C: High
Q: High
A: Low
C: High

Increases and decreases in water velocity may lead to increased erosion or deposition, respectively. The associated pressures alteration to sediment type and siltation are assessed separately. Experimental increases in near-bed current velocity were achieved over intertidal sandflats by placing flumes on the sediment to accelerate water flows (Zuhlke & Reise, 1994). The increased flow led to the erosion of up to 4 cm depth of surface sediments. No significant effect was observed on the abundance of Capitella capitata and numbers of Tubificoides benedii and Tubificoides pseudogaster were unaffected, as they probably avoided suspension by burrowing deeper into sediments. This was demonstrated by the decreased abundance of oligochaetes in the 0-1 cm depth layer and increased abundance of oligochaetes deeper in sediments (Zuhlke & Reise, 1994). 

In the turbid waters of estuaries, where many mud habitats develop, a reduction in water flow is likely to result in a significant increase in siltation increasing the silt and clay content of the substratum. Decreases in water flow with increased siltation of fine particles are considered unlikely to alter the physical character of this habitat type as it is already found in sheltered areas where siltation occurs and where particles are predominantly fine. Reductions in water flow occurring through the presence of trestles (for off-bottom oyster cultivation) arranged in parallel rows in the intertidal area (Goulletquer & Héral, 1997), reducing the strength of tidal currents (Nugues et al., 1996) has been observed to limit the dispersal of pseudofaeces and faeces in the water column and thus increase the natural sedimentation process by several orders of magnitude (Ottman & Sornin, 1985, summarised in Bouchet & Sauriau, 2008). As the characterizing Capitella capitata can live relatively deeply buried and in depositional environments with low water flows (based on habitat preferences) and low oxygenation, they are considered to be not sensitive to decreases in water flow.

Sensitivity assessment. The hydrographic regime, including flow rates, is an important structuring factor in sedimentary habitats. The low energetic environments where the biotopes occur are therefore likely to be important in allowing for the development of the sandy mud substrata which characterize both biotopes. Where increased or decreased water flows altered the sediment type, this could lead to sediment reclassification and thus change is assessed in the sedimentary change assessment. As muds tend to be cohesive and the surface tends to be smooth reducing turbulent flow, an increase at the pressure benchmark may not lead to increased erosion. The biotopes resistance is assessed as Medium as a precautionary assessment, resilience is assessed as High (following restoration of usual conditions) and sensitivity is assessed as Low. The biotopes are not considered to be sensitive to decreased flows due to its presence in sheltered habitats and the tolerance of Tubificoides benedii for low oxygen and sediment deposition.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not Relevant to subtidal biotopes.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: NR
C: NR

Potentially the most damaging effect of increased wave action would be the erosion of the fine sediment substratum as this could eventually lead to loss of the habitat that characterizes the biotopes. Decreased exposure will probably lead to increased siltation and reduced grain size (muddy sediment). Changes in wave exposure may therefore influence the supply of particulate matter for tube building and feeding activities of the characterizing species. Food supplies may also be reduced affecting growth and fecundity of the species.

Thyasira gouldi lives in rather wave sheltered areas at the heads of sea lochs (Jackson, 2007). Increases in wave exposure may disrupt the sediment in which they live, cause continual displacement and physical damage to the shells which are thin and fragile. Disturbance of sediment by waves may reduce oligochaete abundance (Giere, 1977) and oligochaetes may be absent from very wave exposed shores (Giere & Pfannkuche, 1982).

Sensitivity assessment. The biotopes occur offshore and in sheltered habitats where wave exposure is likely to be negligible (Connor et al., 2004), so the biotopes are probably not resistant to changes in wave exposure. However, as the effects of wave action are attenuated with depth, the factor is only likely to affect the biotopes where it occurs at depths of less than 60 m in a strong swell or force 8 gale (Hiscock, 1983). An increase or decrease in wave height at the pressure benchmark (3-5% of significant wave height) is, therefore, considered unlikely to be significant. Resistance and resilience are therefore assessed as High, and the biotopes are considered Not Sensitive at the benchmark level but with low confidence.

Chemical Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Experimental studies with various species suggest that polychaete worms are quite tolerant to heavy metals (Bryan, 1984). High numbers of Capitella capitata have been recorded in areas containing high metal concentrations (Petrich & Reish, 1979; Ward & Young, 1982; Olsgard, 1999), although abundance of Capitella capitata in Norway has also been noted to have a significant negative correlation between sediment content of Cu and abundance of the species, with an obvious reduction in abundance at approximately 900 ppm Cu (Olsgard, 1999). Some impacts on population size and reproduction of Capitella capitata as a result of metal pollution, both in the field and the laboratory, have been observed.

Tests of copper toxicity have been carried out on the characterizing species Capitella capitata.  Laboratory tests carried out in water may not reflect sediment conditions where, again, copper toxicity and exposure is determined by a number of parameters including the degree to which it is adsorbed on to particles selected as food for deposit feeders. A 2-year microcosm experiment was undertaken to investigate the impact of copper on the benthic fauna of the lower Tyne Estuary (UK) by Hall & Frid (1995). During a 1-year simulated contamination period, 1 mg/l copper was supplied at 2-weekly 30% water changes, at the end of which the sediment concentrations of copper in contaminated microcosms reached 411 μg/g. Toxicity effects reduced populations of the four dominant taxa, including Capitella capitata. When copper dosage was ceased and clean water supplied, sediment copper concentrations fell by 50% in less than 4 days, but faunal recovery took up to 1 year, with the pattern varying between taxa. Since the copper leach rate was so rapid, it is concluded that after remediation, contaminated sediments show rapid improvements in chemical concentrations, but faunal recovery may be delayed with experiments in microcosms showing faunal recovery taking up to a year.

Rygg (1985) classified Capitella capitata as a highly tolerant species, common at the most copper polluted stations (copper >200 mg/kg) in Norwegian fjords. 

Sensitivity assessment. Contamination at levels exceeding the pressure benchmark may have negative effects. Nevertheless, the biotopes are considered Not Sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Suchanek (1993) reviewed the effects of oil spills on marine invertebrates and concluded that, in general, on soft sediment habitats, infaunal polychaetes, bivalves and amphipods were particularly affected. However, high numbers of Capitella capitata have been recorded in hydrocarbon contaminated sediments (Ward & Young, 1982; Olsgard, 1999; Petrich & Reish, 1979) and colonization of areas defaunated by high hydrocarbon levels may be rapid (Le Moal, 1980). After a major spill of fuel oil in West Virginia, Capitella increased dramatically alongside large increases in Polydora ligni and Prionospio sp. (Sanders et al., 1972 cited in Gray, 1979). Experimental studies adding oil to sediments have found that Capitella capitata increased in abundance initially, although it was rarely found in samples prior to the experiment (Hyland, 1985).

Capitella capitata is able to withstand relatively high hydrocarbon concentrations and may even take advantage of any available space, caused by mortality of other species.

In Finland, in oligohaline inland waters near an oil refinery, Baltidrilus costata (as Tubifex costatus) appeared to be sensitive to oil pollution and had completely disappeared from sediments exposed to pollution and did not recolonize during a 4 year post-pollution period (Leppäkoski & Lindström, 1978). Tubificoides benedii appears to be more tolerant and was found in UK waters near oil refineries as the sole surviving member of the macrofauna. Populations were however apparently reduced and the worms were absent from areas of oil discharge and other studies indicate sensitivity to oiling (Giere & Pfannkuche, 1982 and references therein).

Sensitivity assessment. Contamination at levels exceeding the pressure benchmark may have negative effects. Nevertheless, the biotopes are considered Not Sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Méndez (2006) showed that the effects of exposing the deposit feeding polychaete Capitella to sediment spiked with environmentally relevant concentrations of teflubenzuron (a chemical used to control infestations of sea lice) caused mortality in one species of Capitella and reduced the egestion rate of another. 

Sensitivity assessment. Nevertheless, the biotopes are considered to be Not Sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No Evidence.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not Sensitive at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

Medium High Low
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Capitella capitata exhibits a relatively high tolerance for sediment hypoxia, hydrogen sulphide concentration, and other sediment conditions avoided by many infauna (Henriksson, 1969). Forbes & Lopez (1990) experimentally demonstrated that reduced oxygen concentrations (pO2 = 20 mm Hg or less) led to decreased Capitella capitata growth rates and cessation of burrowing and feeding activity even when an abundance of food was provided. The authors hypothesized that animals rely solely on anaerobic metabolism once this threshold is crossed. Magnum & Van Winkle (1973) similarly observed that Capitella capitata oxygen uptake ceased when pO2 fell to between 0-34 mm Hg. The fact that experimental worms lost body mass under these conditions supports the contention that full aerobic metabolism cannot be sustained at very low ambient oxygen conditions despite a very high affinity of Capitella capitata hemoglobin for oxygen.

López-Jamar et al. (1987) stated that Thyasira flexuosa is adapted to living in reduced sediments and also is found in organically enriched sediments. However, Dando & Spiro (1993) found that numbers of the congeners Thyasira equalis and Thyasira sarsi decreased rapidly following the deoxygenation of bottom water in the deep basin of the Gullmar fjord in 1979-80. Rosenberg et al. (1991) exposed benthic species from the NE Atlantic to oxygen concentrations of around 1 mg/l for several weeks, including species of small bivalves. After 11 days in hypoxic conditions, bivalve individuals were still alive, although individuals showed increased stretching of syphon out of the sediment. In a meta-analysis study of hypoxia, median sub-lethal oxygen concentrations reported in experimental assessments, although no specific data was reported for all the characterizing species of these biotopes, the thresholds of hypoxia for different benthic groups was LC50 1.42 mg/l for bivalves, and sub-lethal (SLC50) of 1.20 mg/l for annelids (Vaquer-Sunyer & Duarte, 2008). 

Sensitivity assessment. Cole et al. (1999) suggested possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Different species in the biotope will have varying responses to deoxygenation. Based on the evidence presented, the characterizing species are likely to only be affected by severe deoxygenation episodes. However, some mortality of Thyasira spp. might occur in near anoxic (0% oxygen) conditions. Resistance to deoxygenation is, therefore, assessed as Medium. Resilience of the biotopes is likely to be High and the biotopes are therefore considered to have Low sensitivity to exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for 1 week.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

This pressure relates to increased levels of nitrogen, phosphorus and silicon in the marine environment compared to background concentrations. The benchmark is set at compliance with WFD criteria for good status, based on nitrogen concentration (UKTAG, 2014). 

Sensitivity assessment. Not Sensitive at the pressure benchmark that assumes compliance with good status as defined by the WFD.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Benthic responses to organic enrichment have been described by Pearson & Rosenberg (1978) and Gray (1981). In general, moderate enrichment increases food supply and increases productivity and abundance. Dense Capitella capitata populations are frequently located in areas with greatly elevated organic content such as areas of sewage disposal and below fish farms and mussel long lines, even though eutrophic sediments are often anoxic and highly sulfidic (Gray, 1979; Tenore, 1977; Warren, 1977; Tenore & Chesney, 1985; Bridges et al., 1994; Haskoning, 2006; Callier et al., 2007).

Benthic fauna underneath floating salmon farm cages in a Scottish sea loch showed marked changes in species number, diversity, faunal abundance and biomass in the region of the fish farm (Brown et al., 1987). Four ‘zones’ of effect were identified: in zone 1 directly beneath and up to the edge of the cages there was an azoic zone; in zone 2, from the edge of the cages out to 8 m, the sediments were highly enriched and dominated by Capitella capitella and Scolelepis fuliginosa. Kutti et al. (2008) studied organic enrichment of sediments below a fish farm in a fjord system (Norway), during periods of high organic loading production was mostly by Capitella capitata. The threshold for increased infauna production in this deep benthic ecosystem had been reached at an annual flux of 500 g C/m2 and continuous loadings at this magnitude over time might cause organic overloading of fish farm sediments.

Thyasira spp. are characteristic of organically enriched offshore sediments with Capitella capitata (Connor et al., 2004) and have been identified as a ‘progressive’ species, i.e. one that shows increased abundance under slight organic enrichment (Leppakoski, 1975 cited in Gray, 1979).

Borja et al. (2000) and Gittenberger & Van Loon (2011) assigned Thyasira flexuosa to their Ecological Group III - ‘Species tolerant to excess organic matter enrichment; these species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’.

Sensitivity assessment. Above evidence indicates that increased organic matter levels associated with aquaculture can favour Capitella capitata, Thyasira spp. and Tubificoides spp., resistance is therefore considered to be High, resilience High (by default) and the biotope judged as Not Sensitive. It should be noted, however, that sensitivity is greater to gross organic enrichment levels within the spatial footprint of activities.

Physical Pressures

 ResistanceResilienceSensitivity
None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

All marine habitats and benthic species are considered to have a resistance of None to this pressure and to be unable to recover from a permanent loss of habitat (resilience is Very Low). Sensitivity within the direct spatial footprint of this pressure is therefore High. Although no specific evidence is described, confidence in this assessment is High due to the incontrovertible nature of this pressure.

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

The biotopes are characterized by the sedimentary habitat (Connor et al., 2004), and a change to an artificial or rock substratum would alter the character of the biotopes leading to reclassification and the loss of the sedimentary community including the characterizing Capitella capitata, other polychaetes and oligochaetes and Thyasira spp. that live buried within the sediment.

Sensitivity assessment. Based on the loss of the biotopes, resistance is assessed as None, recovery is assessed as Very Low (as the change at the pressure benchmark is permanent), and sensitivity is assessed as High

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Capitella capitata can survive in a range of habitats including fine sands and areas with boulders, a change in sediment type was not judged to completely reduce habitat suitability for this species. An increase of sediment coarseness to sand would not exclude this species, based on published habitat preferences, but may have population level effects as habitat suitability may be reduced. Recovery would depend on the return of previous habitat conditions.

The characterizing species Thyasira spp. have a range of sediment preferences including mud, muddy sand, sandy mud (Jackson, 2007).

Sensitivity assessment. A change in sediment type to mixed or coarser particles could lead to changes in the density of Capitella capitata, other burrowing polychaetes and oligochaetes depending on species specific responses. However, the loss of the muddy sediment  that characterizes this habitat would change the character of the biotopes, the characterizing species, with potentially an increase in bivalves or crustaceans and is likely to lead to reclassification. Based on a change in character, the biotopes are considered to have a resistance fo None to this pressure, and resilience is assessed as Very Low (as a change at the pressure benchmark is permanent), and biotopes sensitivity is assessed as High.

None High Medium
Q: High
A: High
C: High
Q: High
A: Low
C: High
Q: High
A: Low
C: High

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation, exposing underlying sediment which may be anoxic and/or of a different character or bedrock and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment/water interface, may be exposed to conditions to which they are not suited. Removal of 30 cm of surface sediment will remove the polychaete and oligochaete community and other important species present in the biotopes. Recovery of the biological assemblage may take place before the original topography is restored, if the exposed, underlying sediments are similar to those that were removed. Hydrodynamics and sedimentology (mobility and supply) influence the recovery of soft sediment habitats (Van Hoey et al., 2008).

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotopes. Resistance is assessed as None and biotopes resilience is assessed as High. Biotopes sensitivity is therefore Medium

Medium High Low
Q: High
A: Medium
C: Medium
Q: High
A: Medium
C: High
Q: High
A: Medium
C: Medium

Capitella capitata is a soft bodied, relatively fragile species inhabitaing mucus tubes close to the sediment surface. Abrasion and compaction of the surficial layer may damage individuals. Capitella capitata and Pygospio elegans have been categorized through literature and expert reviews as AMBI fisheries Group IV- 'A second-order opportunistic species, which are sensitive to fisheries in which the bottom is disturbed. Their populations recover relatively quickly however and benefit from the disturbance, causing their population sizes to increase significantly in areas with intense fisheries' (Gittenberger & Van Loon, 2011). Chandrasekara & Frid (1996) found that in intertidal muds, along a pathway heavily used for five summer months (ca 50 individuals a day), some species including Capitella capitata and Scoloplos armiger reduced in abundance. Bonsdorff & Pearson (1997) found that sediment disturbance forced Capitella capitata deeper into the sediment, although the species was able to burrow back through the sediment to the surface again. 

Thyasira spp. are small bivalves, the shells are thin and fragile and abrasion is likely to lead to damage and mortality within the population depending on the force (Jackson, 2007). Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population recovered after 3.5 months. 

Sensitivity assessment. Abrasion may damage or kill a proportion of the population of the characterizing Capitella capitata, Thyasira spp. and associated species. Biotopes resistance is assessed as Medium and resilience as High, so sensitivity is assessed as Low.

Low High Low
Q: High
A: High
C: Low
Q: High
A: Medium
C: High
Q: High
A: Medium
C: Low

Rabaut et al. (2008) found that beam trawling on intertidal Lanice conchilega reefs reduced the abundance of Capitella capitata. Ferns et al. (2000), however, found that tractor-towed cockle harvesting had little effect on Capitella capitata, but species that are present at the surface were more badly affected. The tractor dredging removed 83% of Pygospio elegans (initial density 1850/m2). These results are supported by work by Moore (1991) and Rostron (1995) who also found that cockle dredging can result in reduced densities of some polychaete species, including Pygospio elegans.

Bergman & Van Santbrink (2000) estimated the direct mortality of benthic macrofauna caused by the single pass of commercial beam and otter trawls. The results showed that a single pass of a 4 m or 12 m beam trawl or an otter trawl, in shallow sandy areas and deep silty sand areas (with 3-10% silt) in the North Sea caused a mortality of 20-65% of bivalves and 5-40% of gastropods, starfish, small-medium sized crustaceans and annelid worms. The delicate shells of Thyasira spp. are vulnerable to physical damage (e.g. by otter boards), but small size relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals which pass through (Rees & Dare, 1993).

Sensitivity assessment. Capitella capitata and other characterizing species of the biotopes are present in the surface layers of sediment and may be damaged, displaced or killed by penetration and disturbance of the sediment. Resistance is assessed as Low and resilience as High, so sensitivity is assessed as Low.

Medium High Low
Q: Low
A: NR
C: NR
Q: High
A: Low
C: High
Q: Low
A: Low
C: Low

Low energy environments such as those where these biotopes occur, can be naturally turbid systems. There may be possible clogging of feeding organs in suspension feeders, such as Thyasira, resulting in some energetic cost to clear their feeding and respiration organs. If the suspended sediment has a high organic content, some suspension feeding organisms may benefit, and increased siltation may benefit deposit feeders, such as characterizing Capitella capitata. An increase in turbidity resulting from an increase in suspended solids may reduce primary production in the water column and on sediment surfaces therefore reducing the availability of diatom food, both for suspension feeders and deposit feeders. Deposit feeders and tube builders rely on deposition of suspended sediment. A decrease in suspended sediment will reduce this supply and therefore may compromise growth and reproduction.

Sensitivity assessment. The biological assemblage characterizing the biotopes is infaunal and consists of sub-surface deposit feeders. Increased suspended solids are unlikely to have an impact and resistance is assessed as High and resilience as High, so the biotopes are considered to be Not Sensitive. A reduction in suspended solids may reduce deposition and supply of organic matter, resistance to a decrease is therefore assessed as Medium, as a shift between deposition and erosion could result in the net loss of surficial sediments. A reduction in organic matter as suspended solids could also reduce production within the biotopes. Resistance is assessed as High, as over a year the impact may be relatively small, following restoration of usual conditions. Biotopes sensitivity is therefore assessed as Low

Low High Low
Q: High
A: Medium
C: Medium
Q: High
A: Low
C: High
Q: High
A: Low
C: Medium

Subtidal mudflats occur in sheltered environments and, in general, are accreting environments meaning that deposition rather than erosion is the dominant process, this means that the assemblages present (primarily deposit feeders) are adapted to natural levels of siltation through life history traits and can withstand burial (by repositioning in sediment or similarly extending tubes or feeding and respiration structures above the sediment surface). Capitella capitata has been categorized through expert and literature review, as AMBI sedimentation Group IV – 'A second-order opportunistic species, insensitive to higher amounts of sedimentation. Although they are sensitive to strong fluctuations in sedimentation, their populations recover relatively quickly and even benefit. This causes their population sizes to increase significantly in areas after a strong fluctuation in sedimentation' (Gittenberger & Van Loon, 2011). The effects of siltation will depend on the amount and rate that particles are added. Capitella capitata is sedentary and adults are judged unlikely to have any mechanism to escape from large inputs. A deep covering of sediment will prevent feeding. Where inputs are at low rates and similar to background sediments then adults may be able to extend tubes to reach the surface to feed.

Powilleit et al. (2009) studied responses to smothering for three bivalves; Arctica islandica, Limecola balthica and Mya arenaria. These successfully burrowed to the surface of a 32-41 cm deposited sediment layer of till or sand/till mixture and restored contact with the overlying water. These high escape potentials could partly be explained by the heterogeneous texture of the till and sand/till mixture with ‘voids’. In comparison to a thick coverage, thin covering layers (i.e. 15-16 cm and 20 cm) increased the chance of the organisms to reach the sediment surface after burial. This suggests that characterizing bivalve species such as Thyasira spp. are likely to be able to burrow through similar overburdens, although sudden smothering with 5 cm of sediment would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Furthermore, Thyasira flexuosa have highly extensible feet (Dando & Southward, 1986) allowing them to construct channels within the sediment and to burrow to 8 cm depth.

Sensitivity assessment.  Biotopes resistance to siltation based on Capitella capitata is judged to be Low with regard to the rapid addition of silts to a depth of <5 cm. Resilience is assessed as High as recovery is predicted to be rapid. Sensitivity is therefore assessed as Low. At lower levels of siltation, sensitivity will be likely to be lower. 

Low High Low
Q: Low
A: NR
C: NR
Q: High
A: Low
C: High
Q: Low
A: Low
C: Low

The pressure benchmark (30 cm deposit) represents a significant burial event and the deposit may remain for some time in a sheltered mudflat. Capitella capitata populations are likely to be significantly impacted. Some impacts on Tubificoides benedii and other oligochaetes may occur and it is considered unlikely that signficiant numbers of the population could reposition, based on (Bolam, 2011). Placement of the deposit will, therefore, result in a defaunated habitat until the deposit is recolonized.

Sensitivity assessment. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). Bivalve and polychaete species have been reported to migrate through depositions of sediment greater that the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Bijkerk, 1988; Powilleit et al., 2009; Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Resistance is therefore assessed as Low (25-75% loss) and resilience as High and the biotopes are considered to have Low sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not assessed.

No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No Evidence is available on which to assess this pressure.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not Relevant.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

As the characterizing biological assemblage occurs within the sediment and can be deeply buried (to 10 cm or more), an increase in light or shading is considered Not Relevant. Furthermore, the biotopes are sublittoral (Connor et al., 2004), not characterized by the presence of primary producers and are, therefore, not directly dependent on sunlight.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not Relevant to biotopes restricted to open waters.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not Relevant to seabed habitats. NB. Collision by grounding vessels is addressed under ‘surface abrasion'.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

The characterizing species of the biotopes live infaunally, so are likely to have poor or no visual perception and unlikely to be affected by visual disturbance. Visual disturbance is therefore considered Not Relevant.

Biological Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Key characterizing species within the biotopes are not cultivated or translocated. This pressure is therefore considered Not Relevant.

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

Invasion by the slipper limpet Crepidula fornicata may lead to shallower examples of the biotopes to revert to  SS.SMx.SMxVS.CreMed suggesting high intolerance as the original biotopes would be lost. Species richness might decline as Crepidula may dominate the seabed. Experimental relaying of mussels on intertidal fine sand sediments increased fine sediment proportions and led to colonization by Capitella capitata (Ragnarsson & Rafaelli, 1999), so that sediment modification by bivalves may not render habitats unsuitable for Capitella capitata.

Sensitivity assessment. Reclassification of the biotopes following invasion would result in loss of the biotopes. However, Crepidula is typically found around the low water mark and the shallow sublittoral to 60 m (Rayment, 2007), so the depth at which these biotopes occur is likely to offer some protection against invasion. Resistance is therefore assessed as High, and resilience as High (by default) and the biotopes considered Not Sensitive to the introduction of INIS.

 

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: NR
C: NR

Marine oligochaetes host numerous protozoan parasites without apparent pathogenic effects even at high infestation levels (Giere & Pfannkuche, 1982 and references therein). Furthermore, more than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996). A viral infection of the mutualist bacterium living on the gills of Thyasira gouldi was suggested as the reason for a major decline in the Loch Etive population (Jackson, 2007), but no information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found.

Sensitivity assessment. Based on the lack of evidence for mass mortalities in the biotopes from microbial pathogens, resistance is assessed as High and resilience as High (by default), so that the biotopes are assessed as Not Sensitive.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No characterizing species within the biotopes are targeted by commercial or recreational fishers or harvesters. This pressure is therefore considered Not Relevant.

Low High Low
Q: Low
A: NR
C: NR
Q: High
A: Low
C: High
Q: Low
A: Low
C: Low

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in these biotopes, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Loss of these species would alter the character of the biotopes resulting in reclassification, and would alter the physical structure of the habitat resulting in the loss of the ecosystem functions such as secondary production performed by these species.

Sensitivity assessment. Removal of the characterizing species would result in the biotopes being lost or reclassified. Therefore, the biotopes are considered to have a resistance of Low to this pressure and to have High resilience, resulting in the sensitivity being judged as Low.

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Citation

This review can be cited as:

Tillin, H.M. & De-Bastos, E. 2016. [Capitella capitata] and [Thyasira] spp. in organically-enriched offshore circalittoral mud and sandy mud. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/1114

Last Updated: 30/06/2016