|Researched by||Dr Harvey Tyler-Walters||Refereed by||This information is not refereed.|
|Other common names||-||Synonyms||Obelia flabellata (Pallas, 1766), Obelia plana (Pallas, 1766)|
A long, flexible hydroid colony with a prominent main stem and branches. Usually up to 20 cm in length but may reach 35 cm in British waters. Side branches of uniform length but shorter distally giving the colony a tapering outline. Main stem is long, dark and unforked but may become forked in older colonies. The main stem is reddish brown in colour, becoming dark brown to black with age. The segments of the stem, the internodes, are nearly straight, or slightly curved and perfectly tubular. Side branches usually divide into two just after the origin, occasionally into three, with subsequent branches arranged in a zigzag. In young branches the point where the internodes meet, the nodes, are dark, giving a characteristic alternating light and dark pattern. Side branches are usually lighter in colour than the main stem, and decrease in length along the length of the colony.
The polyps are borne in a thin chitinous cup, the hydrotheca. Hydrothecae are elongate (ca 320-500 µm), inverted conical or bell shaped, with a distinctly tapering low portion. The rim of the hydrotheca is either shallow castellate or shallow blunt-cusped but usually rubbed smooth. The base of the hydrothecae attach to the stem by a pedicel composed of up to 20 rings. The reproductive polyps (gonothecae) are elongate and flask shaped, ca 700-1050 µm in length, and release medusae in spring.
Obelia longissima may be confused with other Obelia species. For example, Obelia dichotoma may also be elongate but lacks the regular shape and extreme length of Obelia longissima (Cornelius, 1995b). Obelia bidentata has multiple branched stems even when young. Colonies of Obelia dichotoma may be distinguished from Obelia longissima growing in rockpools in spring by its long tubular, nearly straight and darkening internodes (Cornelius, 1995b). No reliable key is available to distinguish between the medusae of Obelia species (for discussion see Cornelius, 1995b).
- none -
|Phylum||Cnidaria||Sea anemones, corals, sea firs & jellyfish|
|Class||Hydrozoa||White weeds, sea firs, sea beard and siphonophores; hydroids|
|Recent Synonyms||Obelia flabellata (Pallas, 1766)Obelia plana (Pallas, 1766)|
|Typical abundance||High density|
|Male size range||2.5 -6mm|
|Male size at maturity|
|Female size range||Medium-large(21-50cm)|
|Female size at maturity|
|Growth form||Arborescent / Arbuscular|
|Growth rate||See additional information|
|Body flexibility||High (greater than 45 degrees)|
|Characteristic feeding method||Passive suspension feeder, See additional information|
|Typically feeds on||Small zooplankton, small crustaceans, oligochaetes, insect larvae and probably detritus.|
larval pycnogonids (see sensitivity to disease and parasites).
|Is the species harmful?||No|
Obelia longissima exhibits a typical leptolid life cycle consisting of a sessile colonial, vegetative hydroid stage, a free-living sexual medusoid stage, and a planula larval stage. For the sake of this review, the relatively long-lived and easily visible hydroid stage is regarded as the adult stage, while the medusa stage is considered to be a dispersive larval stage and the planula another larval stage specialized for settlement. The size range for males and females above relates to the medusa (see general biology larval). However, the definition of adult and larval stages in leptolids is a matter of debate (see Gili & Hughes, 1985).
The hydroid stage takes the form of a long, flexible colony with uniform side branches that shorten distally, arising from a basal stolon or hydrorhiza. However, the size and degree of branching vary with the environmental conditions and the availability of food.
In species of Obelia, a single basal stolon growing along the substratum may give rise to upright branches and feeding hydranths along its length. As it progresses the older hydranths regress proximally and new branches and hydranths develop distally, so that the stolon appears to migrate across the substratum. Branching increases as the colony receives more food than the stolons and stalks can use, and the colony turns from stolonic growth and occupation of its substratum, to upright growth and hydranth development to exploit the available resources (Berrill, 1949; Kosevich & Marfenin, 1986; Marfenin, 1997; Gili & Hughes, 1995; Stepanjants, 1998). The colony may be composed of several upright colonies of varying size and length interconnected by basal stolons (see Kosevich & Marfenin, 1986).
In Obelia longissima branching begins earliest behind the newest internodes of stolons at the periphery of the colony, in closest contact with the environment, and only if there is adequate food does branching continue in the central older parts of the colony (Marfenin, 1997). If food supply decreases then parts of the colony can be reabsorbed (Marfenin, 1997).
Many hydroids exhibit rapid growth, partially because the number of feeding hydranths, and hence the food catching potential, increases with size (Gili & Hughes, 1995). Growth rate is therefore, dependant on food supply (Marfenin, 1997). However, growth is also dependant on temperature. Berrill (1949) reported that stolons grew, under optimal nutritive conditions, at less than 1 mm in 24 hrs at 10-12 °C, 10 mm in 24 hrs at 16-17 °C, and as much as 15-20 mm in 24 hrs at 20 °C. Overall, growth is expected to be rapid, for example in experiments, Standing (1976) clipped the stems of Obelia back to the surface of his settlement plates every eight days since they grew back rapidly. Similarly, Cornelius (1992) stated that Obelia longissima and Obelia dichotoma could form large colonies within a matter of weeks.
The hydranths of the colony demonstrate a regular cycle of development and regression with, in general, older hydranths regressing before younger ones (Crowell, 1953). Each hydranth takes about 24 hrs to develop at 20 °C and lives for a few days before it regresses (less in unfavourable conditions) (Berrill, 1949; Crowell, 1953; Kosevich & Marfenin, 1986).
Hydroids are passive carnivores that capture prey that swim into, or are brought into contact with their tentacles by currents. Prey are then killed or stunned by the nematocysts born on the tentacles and swallowed. Diet varies but is likely to include small zooplankton (e.g. nauplii, copepods), small crustaceans, chironomid larvae, detritus and oligochaetes, but may include a wide variety of other organisms such as the larvae or small adults of numerous groups (see Gili & Hughes, 1995). In experiments, Hunter (1989) fed Obelia longissima on plankton consisting of larval crustaceans, eggs, veligers, echinoderm plutei, copepods and other invertebrate larvae between 50 -200 µm.
Seasonal changes in the composition of Obelia colonies (no species stated) was examined by Hammett & Hammett (1945) and Hammett (1951a,b,c,d,e) in the Massachusetts area . They reported that budding peaked in April, complete hydranths in August and free-living medusae in July. Hammett & Hammett (1945) suggested that seasonal decline was common, colonies declining in June in North Carolina and after July in Woods Hole. Berrill (1949) noted that rapid growth continued at temperatures as high as 25 °C but ceased at 27 °C. Brault & Bourget (1985) noted that Obelia longissima exhibited a annual cycle of biomass, measured as colony length, on settlement plates in the St Lawrence estuary. Colony length increased from settlement in June, reaching a maximum in November to March and then decreasing again until June, although the decline late in the year was attributed to predation, and data was only collected over a two year period.
|Physiographic preferences||Open coast, Strait / sound, Sea loch / Sea lough, Ria / Voe, Estuary, Enclosed coast / Embayment|
|Biological zone preferences||Lower circalittoral, Lower eulittoral, Lower infralittoral, Sublittoral fringe, Upper circalittoral, Upper infralittoral|
|Substratum / habitat preferences||Macroalgae, Artificial (man-made), Bedrock, Biogenic reef, Coarse clean sand, Cobbles, Large to very large boulders, Other species (see additional information), Pebbles, Rockpools, Small boulders|
|Tidal strength preferences||Moderately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Very Weak (negligible), Weak < 1 knot (<0.5 m/sec.)|
|Wave exposure preferences||Exposed, Extremely exposed, Moderately exposed, Sheltered, Very exposed, Very sheltered|
|Salinity preferences||Full (30-40 psu), Reduced (18-30 psu), Variable (18-40 psu)|
|Depth range||See additional information|
|Other preferences||No text entered|
|Migration Pattern||Non-migratory / resident|
Stepanjants (1998) reported that Obelia longissima was a cold water species, present in northern and southern hemispheres and the Black Sea but absent from tropical areas. Stepanjants (1998) therefore, regarded it as a bipolar species. However, Cornelius (1995b) suggested that numerous records from the Indo-Pacific probably referred to this species.
Obelia longissima occurs primarily in the subtidal but occurs occasionally in the littoral if washed up or in rockpools (Cornelius, 1995b). Zamponi et al. (1998) reported Obelia longissima in the sublittoral of Argentina between 36 and 70 m depth. Stepanjants (1998) noted that Obelia species were found in all oceans, preferentially no deeper than 200 m but cited a record of Obelia longissima between 300 and 510 m deep in Patagonian waters.Habitat preferences
|Reproductive type||See additional information|
|Reproductive frequency||Annual episodic|
|Fecundity (number of eggs)||See additional information|
|Generation time||<1 year|
|Age at maturity||See additional information|
|Season||See additional information|
|Life span||See additional information|
|Duration of larval stage||See additional information|
|Larval dispersal potential||Greater than 10 km|
|Larval settlement period||See additional information|
This MarLIN sensitivity assessment has been superseded by the MarESA approach to sensitivity assessment. MarLIN assessments used an approach that has now been modified to reflect the most recent conservation imperatives and terminology and are due to be updated by 2016/17.
|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).|
|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).|
|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.|
|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.|
|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).|
|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.|
|Tolerant*||Not relevant||Not sensitive*||Low|
|A decrease in emergence and hence increased immersion is likely to allow the hydroid to colonize new substrata. Therefore, tolerant* has been recorded.|
|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)
|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)|
|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).
|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.|
|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.
|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.|
|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.|
|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.|
|Tolerant||Not relevant||Not sensitive||High|
|Hydroids are unlikely to be sensitive to noise or vibration at the benchmark level.|
|Tolerant||Not relevant||Not sensitive||High|
|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 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).|
|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).|
|Intermediate||Very high||Low||Very low|
|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.
|Intermediate||Very high||Low||Very low|
|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).
|No information||Not relevant||No information||Not relevant|
|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.|
|No information||Not relevant||No information||Not relevant|
|No information found.|
|Tolerant*||Not relevant||Not sensitive*||Not relevant|
|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.|
|No information||Not relevant||No information||Not relevant|
|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.|
|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).|
|Low||Immediate||Not sensitive||Very low|
|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.|
|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.|
|No information||Not relevant||No information||Not relevant|
|No information found|
|Not relevant||Not relevant||Not relevant||Not relevant|
|Hydroids are not known to be subject to extraction.|
|Not relevant||Not relevant||Not relevant||Not relevant|
|Obelia longissima is not known to be closely associated with species subject to extraction.|
- no data -
|National (GB) importance||-||Global red list (IUCN) category||-|
Bault & Bourget (1985) reported that the upright branches of Obelia longissima were used as substratum by Mytilus edulis, algae, and the polychaetes Autolytus sp. and Spirorbis sp. Standing (1976) noted that Obelia dichotoma interfered with the settlement of barnacle cyprids on settlement plates but enhanced settlement by the ascidian Ascidia. Gili & Hughes (1995) cited data suggesting that Obelia species were important regulators of local populations of the copepod Acartia hudsonia.Obelia species are probably an important food source for epifaunal grazers such as some turbellarians, aplacophorans, gastropods including nudibranchs, polychaetes, pycnogonids, sea urchins, and fish (Salvini-Plawen, 1972; Sebens, 1985; Picton & Morrow, 1994; Gili & Hughes, 1995). Gili & Hughes (1995) suggested that hydroids probably play an important role in the marine food webs between the plankton and the benthos.
Stepanjants (1998) also suggested that the presence of Obelia medusae in the plankton may have a detrimental effect on herring larvae, so that artificial herring breeding grounds should avoid areas used for mussel culture, since mussel culture attracts abundant settlements of Obelia species.
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Last Updated: 23/07/2003