The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

Removal of the substratum would result in removal of the associated community and its component species, therefore an intolerance of high has been recorded. However, if suitable, the remaining substratum is likely to be recolonized and the population of Obelia longissima recover rapidly (see additional information below).

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

Hydroids usually colonize overhanging, vertical or steeply sloping surfaces presumably to avoid the possibility of siltation, smothering and/or competition from macroalgae. Smothering by 5cm of sediment (see benchmark) is likely to cover a large proportion of the colony, preventing feeding and hence reducing growth and reproduction. Although Obelia longissima forms long upright colonies up to 20 cm in length, the colonies are flexible so that smothering material is likely to bend the colony flat against the substratum. In addition, local hypoxic conditions are also likely to inhibit growth. Although, hydranths are likely to regress and portions of the colony or colonies are likely to die or be reabsorbed, parts of the colony is likely to become dormant, or otherwise survive for a period of at least a month, and recover rapidly once the sediment is removed. Therefore, an intolerance of intermediate has been recorded to represent loss of part of the colony or population. Recovery is likely to be rapid (see additional information below).

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Hydroids are suspension feeders and their feeding apparatus, i.e. the hydranth tentacles, are susceptible to physical clogging by suspended particulates. Epifaunal communities, including hydroid turfs, tend to dominate on vertical or steeply sloping surfaces where siltation is reduced and/or in areas of sufficient water movement to prevent suspended sediment accumulating. The accumulation of sediment can be detrimental, e.g. Round et al. (1961) reported that the hydroid Sertularia operculata died when covered with a layer of silt after being transplanted to sheltered conditions. Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. Obelia longissima forms long upright but flexible thin colonies and has been recorded in a variety of water flow and wave exposure regimes. It has also been recorded in estuaries, which are naturally high in suspended sediment. Overall an increase in suspended sediment is likely to clog the colonies feeding apparatus to some degree, depending on local water movement, and at a minimum is likely to interfere with feeding, resulting in a decrease in growth rate, and potentially a reduction in the biomass and cover of the hydroid. Therefore, an intolerance of intermediate has been recorded. Recoverability is probably rapid.

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

Hydroids are passive suspension feeders dependent on water currents to bring food particles within reach of their stinging tentacles. A reduction in suspended particulates may result in a decrease in food availability. An adequate food supply is required for rapid, upright growth of colonies. A reduction in food supply may result in regression of older colonies in the long term. Therefore, a reduction in suspended sediment may result in a decrease in growth and an intolerance of low has been recorded. Recovery is likely to be immediate once ambient conditions return.

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

Gili & Hughes (1995) note that few hydroids occur intertidally and fewer where they are exposed to the air. Cornelius (1995b) reported that Obelia longissima may occur in intertidal pools at extreme low water of spring tides or in pools in mussel beds, either washed up from deeper water of growing from settled planulae. Colonies growing in rockpools rarely reach the lengths of subtidal colonies (Cornelius, 1995b). Overall, Obelia longissima is likely to be highly intolerant of exposure to the air and hence desiccation. Therefore, an intolerance of high has been recorded. Recovery is likely to be rapid (see additional information below).

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Obelia longissima is a predominately subtidal species, intertidal representatives being restricted to low shore pools. However, an increase in emergence is likely to expose the most shallow proportion of the population to increased desiccation and extremes of temperature. Shallow water colonies may be lost and the upper extent of the resident population reduced, Therefore, an intolerance of intermediate has been recorded, although recovery is likely to be rapid.

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

A decrease in emergence and hence increased immersion is likely to allow the hydroid to colonize new substrata. Therefore, tolerant* has been recorded.

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Water movement is essential for hydroids to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). Flexibility of the otherwise rigid perisarc of hydroids is provided by annulations at the base of branches in many species including Obelia sp.
The biomass of Obelia longissima was reported to increase in direct proportion to mean free-stream water flow rate, in experiments in which the ambient water flow of between <2 and >50 cm/s were increased by 50% (Judge & Craig, 1997). They also noted that Obelia longissima colonies were bushier in increased flow. In experiments, Hunter (1989) exposed Obelia longissima colonies to oscillatory flow peaking at between 0.01 and 0.25 m/s, and unidirectional flow of 0.025, 0.05, and 0.1 m/s. He reported that feeding effectiveness varied with colony size and bushiness depending on the water flow regime. For example, increased colony bushiness decreased feeding effectiveness (the number of hydranths feeding) in unidirectional or low frequency oscillatory flow but increased feeding effectiveness in high frequency oscillatory flow (Hunter, 1989). Similarly, longer colonies had lower feeding effectiveness than short colonies, although the decreased feeding effectiveness was offset by the increased number of feeding hydranths in longer and bushier colonies. In unidirectional flow, the colony was orientated with the flow and hydranths at the base depleted food before it reach hydranths at the end of the colony (self-shading), while in oscillatory flow this effect was mitigated. Good mixing of the water in the vicinity of the colony increased feeding effectiveness by ensuring the water was replaced before the food was depleted (Hunter, 1989).

Obelia longissima has been recorded in a variety of water flow regimes, from very weak to strong tidal streams (JNCC, 1999), although in very weak tidal streams, wave action is probably a more important source of water movement. Kosevich & Marfenin (1986) suggested that the growth form of Obelia longissima was adapted to weak flow condition and susceptible to damage in strong flow. Hunter's study suggests that it can tolerant water flow of at least 0.1 m/s (Hunter, 1989). However, it is likely that an increase in water flow from moderately strong to very strong would be detrimental, due to the physical damage to large colonies. Therefore, the abundance or extent of the population may be decreased by an increase in water flow and an intolerance of intermediate has been recorded. Recovery is likely to be rapid (see additional information below)

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Water movement is essential for hydroids to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). The biomass of Obelia longissima was reported to increase with increasing water flow (see above; Judge & Craig, 1997). Obelia longissima was recorded in a variety of water flow regimes (JNCC, 1999) including very weak tidal streams. In conditions of weak water flow, wave action may be a more important source of water movement. However, where water flow is the main source of water movement, a decrease in water flow may be detrimental due to increased siltation, and loss of available hard substratum. For example, the accumulation of sediment can be detrimental, e.g. Round et al. (1961) reported that the hydroid Sertularia operculata died when covered with a layer of silt after being transplanted to sheltered conditions. Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. Therefore an intolerance of intermediate has been recorded. Recovery is likely to be rapid (see additional information below)

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Stepanjants (1998) regarded Obelia longissima as a cold water species, with a bipolar distribution, while other authors regarded this species as probably cosmopolitan in distribution (Boero & Bouillon, 1993; Cornelius, 1995b). Cornelius (1995b) suggested that numerous records in the Indo-Pacific were probably attributable to Obelia longissima. Given, this species wide distribution it is unlikely to be adversely affected by chronic temperature change at the benchmark level within the British Isles.
Berrill (1949) reported that growth in Obelia commissularis (syn. longissima) was temperature dependant but ceased at 27 °C. Hydranths did not start to develop unless the temperature was less than 20 °C and any hydranths under development would complete their development and rapidly regress at ca 25 °C. Berrill (1948) reported that Obelia species were absent from a buoy in July and August during excessively high summer temperatures in Booth Bay Harbour, Maine, USA. Berrill (1948) reported that the abundance Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20 °C during this period. The upwelling of cold water (8-10 °C colder than surface water) allowed colonies of Obelia sp. to form in large numbers. Berrill (1948) suggested that Obelia longissima grew vigorously in warm weather, although at temperatures above 20 °C, growth of terminal stolons and branches was promoted but the formation of hydranths inhibited. Therefore, it would appear that Obelia longissima is intolerant of acute temperature change above 20 °C.

Deep water colonies are probably buffered against the extremes of temperature potentially experienced by shallow or surface water colonies. However, thermal effluents may result in acute temperature change equivalent to the benchmark level. Therefore, an intolerance of high has been recorded, although recoverability is probably very high (see additional information below).

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Little information on the lower temperature limits of Obelia longissima was found. However, Kosevich & Marfenin (1986) reported that Obelia longissima was active all year round in the White Sea. Similarly, its northern limit lies in the Arctic Circle (Cornelius, 1995b; Stepanjants, 1998) suggesting that it probably tolerant of the lowest temperatures it is likely to encounter in Britain and Ireland. However, growth rates are reduced at low temperatures, and an intolerance of low has been recorded.

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Hydroids tend to shun well lit conditions, planulae becoming negatively phototactic prior to settlement, presumably to avoid competition with macroalgae (Gili & Hughes, 1995). Therefore, a decrease in light penetration may decrease competition for space with macroalgae. Bourget et al. (in press) noted that for any given water temperature on buoys in the Gulf of St Lawrence, water transparency and primary production influenced the biomass of fouling organisms, including Obelia longissima, most in many sample sites. Biomass was reported to increase with increasing transparency up to a transparency of 15 m after which it decreased again (see Figure 2, Bourget et al., in press). Increased transparency was presumably correlated with increased primary production and hence food availability.
An increase in turbidity (decreased turbidity) may reduce primary productivity and hence food availability in shallow water populations. Therefore, growth may be reduced and an intolerance of low has been recorded.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

Bourget et al. (in press) noted that for any given water temperature on buoys in the Gulf of St Lawrence, water transparency and primary production influenced the biomass of fouling organisms, including Obelia longissima, most in many sample sites. Biomass was reported to increase with increasing transparency up to a transparency of 15 m after which it decreased again (see Figure 2, Bourget et al., in press). Increased transparency was presumably correlated with increased primary production and hence food availability. Therefore, a decrease in turbidity may be beneficial and tolerant* has been recorded.

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Water movement is essential for hydroids to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). Obelia longissima was recorded from sites varying in wave exposure from very sheltered to extremely exposed (JNCC, 1999). The branches and stems are flexible and probably able to withstand oscillatory flow (see Hunter, 1989). This species probably occurs at greater depths in more wave exposed conditions, and probably does not reach the same lengths in wave exposed areas as in more sheltered areas. Therefore, an increase in wave exposure from e.g. exposed to extremely exposed is likely to physically damage long colonies. However, hydroids demonstrate a degree of phenotypic plasticity, so that the colonies would probably redistribute resources to form shorter colonies over a wider area. Therefore, an intolerance of low has been recorded.

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Water movement is essential for hydroids to supply adequate food, remove metabolic waste products, prevent accumulation of sediment and disperse larvae or medusae. Hydroids are expected to be abundant where water movement is sufficient to supply adequate food but not cause damage (Hiscock, 1983; Gili & Hughes, 1995). A decrease in wave action may allow the colonies to grow longer and more luxuriant. However, in areas of weak tidal streams, a decrease in wave action may significantly decrease net water movement, to the detriment of the colonies (see water flow above). Therefore, an intolerance of intermediate has been recorded. Recovery is probably rapid.

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Hydroids are unlikely to be sensitive to noise or vibration at the benchmark level.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Hydroid polyps may retract when shaded by potential predators, however hydroids are unlikely to be affected by visual presence as defined in the benchmark.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Abrasion by an anchor or fishing gear is likely to remove relatively delicate upright parts of the colony. However, the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies (see displacement). Populations on small hard substrata (e.g. cobbles, pebbles or stones) may be removed by fishing gear, constituting substratum loss (see above). Overall, a proportion of the colonies are likely to be destroyed and an intolerance of intermediate has been recorded. However, recovery from surviving hydrorhizae and occasional fragments is likely to be rapid (see additional information below).

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Fragmentation is thought to be a possible mode of asexual reproduction in hydroids (Gili & Hughes, 1995). Therefore, it is possible that a proportion of displaced colonies (or fragments thereof) may attach to new substrata and survive. Cornelius (1995b) noted that detached specimens of Obelia longissima sometimes continue to grow if entangled in the intertidal. Therefore an intolerance of intermediate has been recorded. Recovery is likely to be rapid (see additional information below).

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

The species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1981a) cited reports of growth stimulation in Obelia geniculata caused by methyl cholanthrene and dibenzanthrene. Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l.
No information concerning the intolerance of Obelia longissima was found. However, the above evidence suggests that several species of hydroid exhibit sublethal effects due to synthetic chemical contamination and lethal effects due to TBT contamination. Therefore, an intolerance of intermediate has been suggested, albeit with very low confidence. Recoverability is likely to be very high.

Heavy metal contamination

Evidence

Various heavy metals have been shown to have sublethal effects on growth in the few hydroids studied experimentally (Bryan, 1984). Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1976) reported that 1 µg/l Hg²⁺ was stimulatory, although the effect was transitory, exposure resulting in reduced growth towards the end of his 11 day experiments. Cadmium (Cd) was reported to cause irreversible retraction of 50% of hydranths in Laomedea loveni after 7 days exposure at concentrations between 3 µg/l (at 17.5 °C and 10 ppt salinity) and 80 µg/l (at 7.5 °C and 25 ppt salinity) (Theede et al., 1979). Laomedea loveni was more tolerant of Cd exposure at low temperatures and low salinities. Karbe (1972, summary only) examined the effects of heavy metals on the hydroid Eirene viridula (Campanulidae). He noted that Cd and Hg caused cumulative effects, and morphological changes. Mercury (Hg) caused irreversible damage at concentrations as low as 0.02 ppm. He reported threshold levels of heavy metals for acute effects in Eirene viridula of 1.5-3 ppm Zn, 1-3 ppm Pb, 0.1-0.3 ppm Cd, 0.03-0.06 ppm Cu and 0.001-0.003 ppm Hg. Karbe (1972, summary only) suggested that Eirene viridula was a sensitive test organism when compared to other organisms.
Although no information on the effects of heavy metals on Obelia species was found, the above evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination. Therefore, an intolerance of intermediate has been suggested, albeit with very low confidence. Recoverability will probably be very high (see additional information below).

Hydrocarbon contamination

Evidence

Little information of the effects of hydrocarbons on hydroids was found. Hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). The water soluble fractions of Monterey crude oil and drilling muds were reported to cause polyp shedding and other sublethal effects in the athecate Tubularia crocea in laboratory tests (Michel & Case, 1984; Michel et al., 1986; Holt et al., 1995). However, no information concerning the effects of hydrocarbons or oil spills on Obelia species was found, and no assessment of intolerance has been made.

Radionuclide contamination

Evidence

No information found.

Changes in nutrient levels

Evidence

A moderate increase in nutrients may increase food availability for suspension feeders, in the form of organic particulates. Marfenin (1997) noted that growth form and growth rates are dependant on food availability. Eutrophication may result in local hypoxic conditions (see below) and /or blooms of ephemeral algae. However, Obelia longissima was recorded from estuarine habitats (JNCC, 1999). Estuarine habitats are generally higher in nutrient levels than coastal waters. Therefore, Obelia longissima may benefit from an increase in nutrients at the benchmark level, and tolerant* has been recorded.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Little information concerning salinity tolerance in Obelia longissima was found. It has been recorded from estuarine sites in variable and reduced salinities (18-40 and 18-30 psu) (JNCC, 1999). It is a predominately subtidal species, unlikely to experience exposure to salinities greater than full seawater (ca 35 psu). However, the effects of exposure to hypersaline effluents are unknown and no assessment of intolerance has been made.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

Little information concerning salinity tolerance in Obelia longissima was found. It has been recorded from estuarine sites in variable and reduced salinities (18-40 and 18-30 psu) (JNCC, 1999). Therefore, it would probably survive a reduction in salinity from full to reduced. However, in estuarine areas a reduction in salinity from reduced to low may be detrimental. Therefore, an intolerance of intermediate has been recorded. Recovery is likely to be rapid (see additional information below).

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

Hydroids mainly inhabit environments in which the oxygen concentration exceeded 5 ml/l (Gili & Hughes, 1995). Temperature, salinity, food digestion and reproductive state have been shown to affect oxygen consumption rates in hydroids (Gili & Hughes, 1995). No specific data on oxygen consumption in Obelia longissima was found. Hydroids are dependant on water movement to provide oxygenated water. Cornelius (1995a) noted that placing a colony in still, unaerated water stimulated the production of resting stages (frustules), generally thought to be a response to unfavourable conditions (see reproduction). Sagasti et al. (2000) reported that epifaunal species (including several hydroids and Obelia bicuspidata) in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O₂/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition. Overall, an intolerance of low has been recorded to represent sublethal effects, albeit with a very low confidence.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

The medusae of Obelia species were reported to be parasitised by the flagellate Protoodinium chattoni in the Black /Sea and Mediterranean. Obelia sp. medusae can also act as secondary hosts for trematode parasites. For example, the metacercaria of Opechona bacillaris were reported to infest Obelia sp. medusae in summer in the Plymouth area (Lauckner, 1980). The larval stages of the pycnogonid (sea spider) Anoplodactylus pygmaeus parasitises the hydroid stage of Obelia species, occupying the gastric cavity, while the larvae of Anoplodactylus petiolatus parasitises the medusoid stage (King, 1974; Lauckner, 1980). Although no detrimental effects were reported, any parasite burden is likely to have subvital effects. Therefore, an intolerance of low has been recorded.

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

No information found

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Hydroids are not known to be subject to extraction.

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Obelia longissima is not known to be closely associated with species subject to extraction.