Sertularia dichotoma Linnaeus, 1759, coast of southwest England.
OTHER COMMON NAMES
Hydroid: colonies extremely varied in size and shape. Stems erect, up to 13.7 in (35 cm) high, monosiphonic and branched, flexuous to straight, thickened in old colonies. Internodes have several annuli at their base. Hydrothecae alternate, lateral, borne on completely annulated pedicels at the upper part of the internodes. Hydrotheca bell shaped, usually not very deep, thin walled; rim to crenate, slightly flared; diaphragm transverse to oblique. Gonothecae usually inverted, conical on an-nulated pedicels, truncated at the distal end, with a short distal neck when mature, where the aperture is.
Medusa: umbrella 0.09-0.23 in (2.5-6 mm) wide, circular, flat; mesoglea very thin; without gastric peduncle; mouth with four simple lips; four radial canals; gonads sac like, hanging from middle to end of the radial canals; numerous short, stiff, solid, not extensile marginal tentacles; tentacles with short endodermal roots extending into bell mesoglea; eight statocysts situated on underside of basal bulbs of some marginal tentacles. The velum is absent.
Almost cosmopolitan, even though the identity of the many nominal species of Obelia is still matter of debate, as is their distribution. (Specific distribution map not available.)
The hydroid is very common on rocks and algae on shallow hard bottoms; the medusa is very common in coastal waters.
Both polyps and medusae can move their tentacles in a rhythmic way to create currents that bring food particles toward the mouth.
FEEDING ECOLOGY AND DIET
Both hydroids and medusae feed on plankton, usually crustaceans, but also possibly phytoplankton.
Both medusae and polyps are quite constant in presence in their respective environments and reproduction occurs continuously.
CONSERVATION STATUS Not listed by the IUCN.
Obelia is traditionally depicted in all zoology textbooks as a paradigmatic hydrozoan, so that many generations of students have learned its name. Unfortunately, the medusae of Obelia, being deprived of velum, are far from being the archetype of the medusae of the Hydrozoa. Furthermore, the figures of Obelia medusae present in many textbooks depict a campanulate medusa, and not a flat one, as is the medusa of this genus. ♦
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