Biodiversity & Conservation

A hydroid - Obelia longissima - General information

A hydroid

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Distribution map

Obelia longissima recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

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Obelia longissima is not listed under any importance categories.

General information

Key Icon Researched by: Dr Harvey Tyler-Walters Text page icon Refereed by: This information is not refereed.

Taxonomy icon Taxonomy

Phylum Cnidaria Sea anemones, corals, sea firs & jellyfish
Class Hydroidomedusa
Map icon Recorded distribution in Britain and Ireland Probably occurs throughout the British Isles but may be confused with Obelia dichotoma so that its recorded distribution may be inaccurate.
Habitat information icon Habitat information Found growing on algae and hard substrata in the subtidal. Occasionally found growing in intertidal rockpools and at extreme low water of spring tides. Detached subtidal colonies may continue to grow if washed up into rockpools and entangled with other species such as mussels.
Text page icon Description 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.

This review can be cited as follows:

Dr Harvey Tyler-Walters 2003. Obelia longissima. A hydroid. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19/06/2013]. Available from: <http://www.marlin.ac.uk/speciesfullreview.php?speciesID=3926>