Phaeophyta, Brown Algae
Padina Adanson, 1763
Phaeophyta, Order: Dictyotales; Family: Dictyotaceae.
The scientific name of the genus, Padina, describes a bonnet, although its seems that this specific fashion is no longer prevalent. Two species mentioned here: Padina pavonia and Padina gymnospora. Pavonia, refers to the Peacock, due to the fan-like shape that resembles the tail of that bird. Gymnospora, refers to the manner in which the spores are usually found, and it means, ‘exposed spores’, since the spores of this species are not covered by indusia.
The mature plant resembles a bonnet or a fan, sometimes split into lobes that also resemble a fan. The grip upon the substrate is through a holdfast that contains a large number of rhizoids, which are root-like cells that emanate from the base of the fan. A microscope reveals concentric lines on the thallus that are comprised of lines of epidermal hairs; fertile plants exhibit rows of reproductive organs. The Padina is the only genus of the Dictyotales order that calcifies in its thallus.
The mature alga grows to a height of 5-15 cm, but smaller individuals may also be found.
The Padina appears in shades of brown to beige. Calcification lends the thallus lighter shades.
The Padina’s most notable distinction is its fan shape, upon which lightly shaded, usually calcium-rich concentric lines are evident. The edge of the fan is slightly condensed and seems cylindrical. Mature individuals split and are often covered with epiphytes. The genus is easy to identify, although it may sometimes be confused with Taonia or Stypopodum, neither of which, however, calcify. Identifying species of Padina requires microscopic examination.
The Padina grows in the intertidal and subtidal zones. It also develops in tidal pools and may be cut off from a regular water supply for extended periods. Padina species excel in their ability to withstand high temperatures and are an indicator of thermal pollution. It is usually the first macro- alga to appear in hot-water estuaries to the sea.
Biology and reproduction
The Padina proceeds through two generations, similar in shape. The haploid generation, the gametophyte, is generated from a spore and contains male and female reproductive cells (sometimes both on the same individual, and sometimes on separate individuals). The diploid generation, the sporophyte, is generated from a zygote and carries sporangia. The sporangium usually undergoes meiosis from which four static spores are generated, each capable of germinating into a new gametophyte. The reproductive organs develop in clusters parallel to the row of hairs. As mentioned, the Padina is one of the only species of brown algae that calcifies. The calcium consolidates in the light concentric lines. If dilute hydrochloric acid is trickled upon the thallus, the calcium deposits can be located according to the fermentation.
The two common species of Padina along Israeli shores are very similar and difficult to distinguish. The species most often mentioned in literature is Padina pavonica L.) Lamuouroux = Padina pavonia (Linnaeus) Gaillon, and the other is Padina gymnospora ( Kutzing ) Vickers.
In some instances, the two are wrongly described as Padina pavonica, which is the more common species. The Padina pavonica is common around the world and appears in nearly all flora, although due to the difficulty in identification, some individuals described as Padina pavonica may actually be other species. The morphological differences between Padina pavonica and Padina gymnospora are presented below. An additional species of Padina, larger in size, has been observed in Egypt, Padina boryana Thivy. It is difficult to distinguish between the two species, and a
microscope is necessary for definite identification. The Padina pavonica grows along the coast throughout the year, but mostly in spring and winter. It is reddish-brown and its fanned thallus is covered with a relatively thick layer of calcium. A microscopic cross-section reveals two layers of epidermis with small cells and a layer of elongated cells in between, which is sparse in hair (that is sometimes shed). The row of hair and sporangic cluster are covered by a membranous cover (indusium).
The Padina gymnospora is a summer plant and appears from the end of May until the beginning of December. Collected individuals are greenish-brown, and the layer of calcium is thinner than that of the Padina pavonica. The row of sterile hairs is exposed (no membraneous indusium), hence its name. A cross-section reveals that cells in the upper epidermal layer and the intermediary layer are equal in size, unlike the cells in the lower epidermal layer.
Research at the Israeli Oceanographic and Limnological Research Institute near Haifa by Friedlander and Ben- Amotz found that the Padina’s thallus contains vegetarian, hormonal-like substances that accelerate the sprouting of plants. A popular name given to the Padina is ‘Peacock’s tail’, due to its fan-like shape.