Rhodophyta, Red Algae

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Halymenia C. Agardh, 1817

Rhodophyta, Order: Gigartinales; Family: Halymeniaceae.

Etymology

The generic name Halymenia, means ‘sea membrane’, due to its flattened shape and transparent colour.

Description

The Halymenia is an erect, membranous, smooth alga, split at the edge. The thallus is gelatinous, shiny and flexible. The edges split into thin and wide lobes, sometimes such lobes are also found on the algal surface, rather than just on the sides. The algae connect to a substrate using a disk- like holdfast.

Size

The Halymenia floresia, which has been reported on Israeli

shores, grows to a height of 30 cm; in deeper water it is apparently possible to find larger individuals. Other species, described by Professor Alim from Egypt, are smaller and reach 10 cm.

Colour

Bright red, sometimes edging towards green.

Special features

This flattened, large and red alga looks like Ulva but is red rather then green.

Habitat

The Halymenia grows on rocks at a depth of 1-2 m, and sometimes more. It has also been reported at depths of 30 and even 50 m.

 

 

Halymenia floresia - general view of the thallus.

The algal red colouring was used by the Ancient Romans to paint pictures, even though it fades in time, especially upon exposure to air. The grazing habits of herbivores in relation to this alga are still unknown; however, in the Philippines, the alga is consumed by inhabitants.

Biology and reproduction

The sporangium - the cystocarp - are spread over the entire alga. They resemble dark corpuscles.

Seasonality and distribution

Nemlich and Danin found large quantities of this alga along the Israeli coast from May to November, but also in April and December. It has also been found in Egypt and Turkey. This impressive alga has been documented in many books and collections. At the Hebrew University Herbarium more than ten examples of different species are located. Nemlich and Danin reported on them both underwater, “We found it in abundance”, but I have not personally found any, ever. To the best of my knowledge, the alga has not been sighted along Israeli coasts for the past twenty years, and it may be extinct. Individuals have washed ashore in Tel Aviv and Jaffa, but these cannot be described as natural habitats any longer. It is not clear whether the disappearance is due to the destruction of coastal habitats or the appearance of herbivores (a species of which may have arrived from the Red Sea) or for some other reason. A small, unidentified red algae individual was found at one point by researchers from the Israeli Oceanographic and Limnological Research Institute at Haifa. It survived in the aquarium in which it has been preserved, to the surprise of all concerned, and became an impressive example of Halymenia, which has since deteriorated somewhat. I hope these lines are not a eulogy, but will serve for future reference. The Halymenia is widely distributed in warm seas around the world.

Additional species

Two species which are most widely reported, and were also found by Tikva Edelsten in the Haifa Bay, as well as in the deep sea, are: Halymenia floresia (Clemente) C. Agardh and Halymenia dichotoma (Clemente) C. Agardh.

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