BIOTIC Species Information for Obelia longissima
|Researched by||Dr Harvey Tyler-Walters||Data supplied by||MarLIN|
|Refereed by||This information is not refereed.|
See additional information
|Reproductive Season||Spring - Summer||Reproductive Location||Water column|
|Reproductive frequency||Annual episodic||Regeneration potential||No|
|Life span||<1 year||Age at reproductive maturity||<1 year|
|Generation time||<1 year||Fecundity||up to 40,000 eggs in lifetime|
|Egg/propagule size||Up to 200 µm in diameter||Fertilization type||External|
|Reproduction Preferences Additional Information||Life 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).
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).
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.
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 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.
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).
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,|