Obelia longissima

Researched ByDr Harvey Tyler-WaltersData Supplied ByMarLIN
Refereed byThis information is not refereed.
Taxonomy
Scientific nameObelia longissimaCommon nameA hydroid
MCS CodeD521Recent SynonymsObelia flabellata, Obelia plana
PhylumCnidariaSubphylum
SuperclassHydrozoaClassLeptolida
SubclassLeptothecataeOrderProboscoida
SuborderCampanulariidaFamilyCampanulariidae
GenusObeliaSpecieslongissima
Subspecies  
Additional InformationObelia 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).
Taxonomy References Cornelius, 1995b, Howson & Picton, 1997, Stepanjants, 1998, Cornelius, 1990a, Cornelius, 1995a
General Biology
Growth formArborescent / Arbuscular, PenicillateFeeding methodPassive suspension feeder
Mobility/MovementPermanent attachmentEnvironmental positionEpibenthic, Epifaunal, Epilithic, Epiphytic, Epizoic
Typical food typesSmall zooplankton, small crustaceans, oligochaetes, insect larvae and probably detritus.HabitAttached
BioturbatorNot relevantFlexibilityHigh (>45 degrees)
FragilityFragileSizeMedium-large(21-50cm)
HeightUp to 35 cm.Growth RateRapid, see additional information
Adult dispersal potential10-100mDependencyIndependent
SociabilityColonial
Toxic/Poisonous?No
Additional InformationObelia 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).

Growth form
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).

Growth rates
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).

Feeding
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 change
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.
Biology References Cornelius, 1995b, Stepanjants, 1998, Berrill, 1949, Marfenin, 1997, Judge & Craig, 1997, Calder, 1990, Salvini-Plawen, 1972, Kosevich & Marfenin, 1986, Boero, 1984, Standing, 1976, Brault & Bourget, 1985, Crowell, 1953, Hammett, 1943, Hammett & Hammett, 1945, Hammett, 1951a, Hammett, 1951b, Hammett, 1951c, Hammett, 1951d, Hammett, 1951e, Cornelius, 1992, Cornelius, 1995a, Lauckner, 1980, King, 1974, Salvini-Plawen, 1972
Distribution and Habitat
Distribution in Britain & IrelandProbably occurs throughout the British Isles but may be confused with Obelia dichotoma so that its recorded distribution may be inaccurate.
Global distributionNearly cosmopolitan. Recorded north to the New Siberian Island and south to the South Orkney Isles in the Atlantic, penetrates the Baltic Sea and the Black Sea, with numerous records in the Indo-Pacific (Cornelius, 1995b; Stepanjants, 1998).
Biogeographic rangeNot researchedDepth rangeSee additional information
MigratoryNon-migratory / Resident
Distribution Additional InformationStepanjants (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.

Substratum preferencesLarge to very large boulders, Other species (see additional information), Bedrock, Small boulders, Cobbles, Pebbles, Coarse clean sand, Algae, Biogenic reef, Artificial (e.g. metal/wood/concrete), RockpoolsPhysiographic preferencesOpen coast, Strait / sound, Sealoch, Ria / Voe, Estuary, Enclosed coast / Embayment
Biological zoneLower Eulittoral, Sublittoral Fringe, Upper Infralittoral, Lower Infralittoral, Upper Circalittoral, Lower CircalittoralWave exposureExtremely Exposed, Very Exposed, Exposed, Moderately Exposed, Sheltered, Very Sheltered
Tidal stream strength/Water flowStrong (3-6 kn), Moderately Strong (1-3 kn), Weak (<1 kn), Very Weak (negligible)SalinityReduced (18-30 psu), Full (30-40 psu), Variable (18-40 psu)
Habitat Additional InformationSubstrata
Most hydroids do not show a high specificity of substrata (Gili & Hughes, 1995). Obelia longissima has been recorded from a wide variety of hard substrata including rocks, shells and artificial substrata (pilings, harbour installations, buoys, bridge supports), bivalve cultures (e.g. mussels and oysters), or floating debris, as epiphytes on kelp stipes or Halidrys siliquosa, and may occur in sandy areas where shells or other hard substrata provide attachment (Cornelius, 1992; Gili & Hughes, 1995; JNCC, 1999).

Habitat preferences
Water movement is important for hydroids to supply adequate food, gas exchange, remove waste products, prevent excessive siltation and provide suitable substratum. Hydroids tends to be abundant where water movement is sufficient to but not high enough to cause damage. Hydroids with long stems tend to occur in calmer waters (Riedl, 1971; Hiscock, 1983; Gili & Hughes, 1995). Hydroids tend to occur in low light conditions, possibly due reduced competition from algae and/or settlement preferences of their planulae larvae (Gili & Hughes, 1995). The majority of hydroid species are stenohaline, i.e. do not tolerate reduced salinities. However, Obelia longissima was reported from sites subject to reduced salinity such as the Taw and Fal estuaries (JNCC, 1999). Temperature is an important factor controlling growth and reproduction in hydroids, and many species have optimal temperature ranges for reproduction (Gili & Hughes, 1995). For example, Berrill (1949a) reported that growth in Obelia longissima ceased at 27 °C and that newly formed hydranths rapidly regressed at 25 °C.

Distribution References Cornelius, 1995b, NBN, 2002, JNCC, 1999, Picton & Costello, 1998, Stepanjants, 1998, Boero & Bouillon, 1993, Judge & Craig, 1997, Boero & Fresi, 1986, Hunter, 1989, Zamponi et al., 1998, Bourget et al., (in press), Boero, 1984, Cornelius, 1992, Riedl, 1971, Hiscock, 1983, Sommer, 1992, Berrill, 1948
Reproduction/Life History
Reproductive typeVegetative, See additional information, Gonochoristic, Budding Developmental mechanismPlanktotrophic, Lecithotrophic
Reproductive SeasonSpring - SummerReproductive LocationWater column
Reproductive frequencyAnnual episodic Regeneration potentialNo
Life span<1 yearAge at reproductive maturity<1 year
Generation time<1 yearFecundityup to 40,000 eggs in lifetime
Egg/propagule sizeUp to 200 µm in diameterFertilization typeExternal
Larvae/Juveniles
Larval/Juvenile dispersal potential>10kmLarval settlement periodSee additional information
Duration of larval stage11-30 days  
Additional InformationLife history
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. Therefore, age at maturity, longevity, and reproductive type vary with the stage in the life cycle. For the sake of this review, the relatively long-lived and easily visible hydroid stage is regarded as the adult stage, while the hydromedusa stage is considered to be a dispersive larval stage and the planula another larval stage specialized for settlement. However, the definition of adult and larval stages in leptolids is a matter of debate (see Gili & Hughes, 1985).
Asexual reproduction
Hydroids may reproduce asexually by budding to form another colony. Obelia longissima develops a system of basal stolons, branching to form a network across the substratum, that gives rise to one or more upright colonies (Berrill, 1949; Kosevich & Marfenin, 1986; Marfenin, 1997). A common form of asexual reproduction in hydroids is the formation of vertical stolons, which then adhere to adjacent substratum, detach and form another colony (Gili & Hughes, 1995). Hydroids exhibit remarkable powers of regeneration and Obelia longissima (as commissularis) rapidly heals cut ends of stolons or branches within 1-2 min, and new growth can rapidly occur from the cut end or both ends of an excised piece of stolon (Berrill, 1949). Asexual reproduction by fission or mechanical fragmentation of the colony may be an important factor in dispersal (Gili & Hughes, 1995).
Hydroids commonly form frustules or gemmules, which are thought to be resting stages, in response to stress (Gili & Hughes, 1995). In Obelia longissima short lengths of the hydrocladial coenosarc (the stems) are rounded off and detached from the colony (Billard, 1901a, b; Broch, 1927; Kosevich & Marfenin, 1986; Cornelius, 1992, 1995a). These frustules or gemmules are adhesive and stick to the substratum where they can form new colonies (Kosevich & Marfenin, 1986; Cornelius, 1995a). Frustule or gemmule production may be triggered by unfavourable conditions. For example, Cornelius (1992, 1995a) reported that placing a newly collected colony is sea water that was neither aerated or cooled prompted gemmule production. Kosevich & Marfenin (1986) reported that frustule formation was triggered by a acute temperature change of 4-6 °C or abundant food. Kosevich & Marfenin (1986) also noted that, in the laboratory, a frustule adhering to the substratum could develop its first hydranth within 24 hrs. Kosevich & Marfenin (1986) suggested that frustulation would enable the population to develop quickly in favourable conditions. However, most authors consider that frustules (gemmules) are probably resting stages formed to survive unfavourable conditions.
Reproductive structures, the gonothecae, develop in the older parts of the upright colony, at stem junctions (Berrill, 1949). Medusae develop within the gonotheca, budding from a central column of coenosarc, the blastostyle. As medusae develop distally within the gonothecae, they are liberated by the continued growth of the blastostyle through the opening at the top of the gonotheca, complete development from rudimentary bud to liberated medusa taking about 24 hrs at 18-20 °C (Berrill, 1949).
Sexual reproduction
Obelia longissima is dioecious, producing male and female medusae. The medusoid stage lasts between 7 -30 days (Stepanjants, 1998). At maturity the gonads migrate to the periphery of the radial canals. Fertilization is external with both eggs and sperm being released into the sea. Chemical attractants are believed to guide the sperm to the eggs (Cornelius, 1995a, b). Faulkner (1929) reported that Obelia geniculata had large eggs up to 200 µm in diameter. The eggs of other Obelia species may be similar. Fertilization results in an embryo that develops into a typical planula larva (Cornelius, 1995a, b; Gili & Hughes, 1995).
The planula larva is 1 -2 mm in size, ciliated and lecithotrophic. Longer-lived forms may contain a central cavity that may function in buoyancy (Cornelius, 1995a). Sommer (1992) suggested that the life span of planulae of Obelia species was 5 -21 days. The planula larva of some hydroids are released at dawn and are positively phototactic, becoming negatively phototactic prior to settlement and settle in shaded places, presumably to avoid adult competition with algae (Gili & Hughes, 1995). Stepanjants (1998) cites current evidence suggesting that the presence of microbial films may be important factors in the settlement of hydroid planulae.
Reproductive season
The medusae of Obelia longissima were reported from March to late April in southern England, May to June in southwest Norway and west Sweden and between the 28 March and 22 April in the Kiel Bight (Cornelius, 1995b). Obelia medusae were reported in the plankton in spring and summer in the Plymouth area (MBA, 1957) and from April to July around the Isle of Man (Bruce et al., 1963). Elmhirst (1925) reported that Obelia medusae were released over a 10 day period beginning on the last quarter of the moon in summer, suggesting a lunar periodicity. Hammett & Hammett (1945) reported that free-living medusae were present in the Massachusetts area in July. Hammett & Hammett (1945) and Hammett (1951a,b,c,d,e) concluded that while environmental factors influenced growth and differentiation of hydranths and gonangia, their development was primarily under endogenous control. However, Gili & Hughes (1995) noted that temperature was a critical factor controlling hydroid reproduction.
Fecundity
Fecundity will depend on the size of the colony and hence the number of gonothecae. Cornelius (1990b) suggested that an average colony of Obelia sp. might bear at least 100 gonotheca, each capable of releasing ca 20 medusae. Each female medusa could release about 20 eggs. Assuming that all the medusae survive to release gametes, Cornelius (1990b) estimated that an average colony could potentially produce about 20,000 planulae, although he also suggested that only one of these planulae was likely to survive to form a colony which itself might survive to reproduce.
Longevity
Unless destroyed by predators or physical damage, the colony may have a long life span (perhaps very long (Gili & Hughes, 1995)). No information concerning the life span of the resting stages (gemmules) was found. Gili & Hughes (1995) suggested that in situ studies indicated that hydroid colonies suffer significant mortality, leading to finite life spans. However, the ability to reproduce asexually and regenerate from damaged sections means that although any individual colony may have a finite life span the genetic individual (genet) may be considerably longer lived (Gili & Hughes, 1995).
Dispersal
Rapid growth, budding and the formation of stolons allows hydroids to colonize space rapidly. Hydroids are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). Fragmentation may also provide another route for short distance dispersal. However, it has been suggested that rafting on floating debris as dormant stages or reproductive adults (or attached to ships hulls or as medusae in ship ballast water), together with their potentially long life span, may have allowed hydroids to disperse over a wide area in the long term and explain the near cosmopolitan distributions of many hydroid species (Gili & Hughes, 1995). For example, Obelia longissima has been reported to raft, and Obelia species were included in the 'species club' of rafting species that occur on remote islands and have wide distributions (Cornelius, 1992). Obelia species, with their planktonic medusoid stage of 7-30 days, and a long-lived pelagic planula larvae of up to 21 days duration, have significant dispersal potential by larval stages alone (see Sommer, 1992; Cornelius, 1992; Boero & Bouillon, 1993; Gili & Hughes, 1995; Stepanjants, 1998). Boero & Bouillon (1993) note that with the ability of hydroids to raft on floating objects as colonies or resting stages, possibly on shipping, dispersal is potentially unlimited. However, Boero & Bouillon (1993) stated that the distribution of hydroids was not dependent purely on they ability to disperse but by their limits of environmental tolerance.
Reproduction References Cornelius, 1995b, Stepanjants, 1998, Berrill, 1949, Boero & Bouillon, 1993, Cornelius, 1990b, Kosevich & Marfenin, 1986, Brault & Bourget, 1985, Crowell, 1953, Hammett, 1943, Hammett, 1951a, Hammett, 1951b, Hammett, 1951c, Hammett, 1951d, Hammett, 1951e, Stepanjants et al., 1993, Cornelius, 1992, Bruce et al., 1963, Billard, 1901a, Billard, 1901b, Cornelius, 1995a, Broch, 1927, Faulkner, 1929, Sommer, 1992, Berrill, 1948, Elmhirst, 1925, Russell, 1953, MacGinitie and MacGinitie, 1949