BIOTIC Species Information for Obelia longissima
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Researched byDr Harvey Tyler-Walters Data supplied byMarLIN
Refereed byThis information is not refereed.
General Biology
Growth formArborescent / Arbuscular
Feeding methodPassive suspension feeder
Mobility/MovementPermanent attachment
Environmental positionEpibenthic
Typical food typesSmall zooplankton, small crustaceans, oligochaetes, insect larvae and probably detritus. HabitAttached
BioturbatorNot relevant FlexibilityHigh (>45 degrees)
FragilityFragile SizeMedium-large(21-50cm)
HeightUp to 35 cm. Growth RateRapid, see additional information
Adult dispersal potential10-100m DependencyIndependent
General Biology 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).

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,
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