Monterey Bay Aquarium Research Institute
Marine Botany

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Gelidium photograph

  • Kingdom: Protista
  • Division: Rhodophyta
  • Class: Rhodophyceae
  • Subclass: Floridophycidae
  • Order: Gelidiales
  • Family: Gelidiaceae
  • Genus: Gelidium


Kingdom: Protista
Current taxonomic systems include all algae in the kingdom protista. Algae were previously grouped in the kingdom Plantae since they possess chlorophyll and photosynthesize. They were removed from this kingdom due primarily to the simplicity of their structures and the lack of certain structures characteristic to vascular plants.

Division: Rhodophyta
The red color that most of the Rhodophyta display is due to the accessory pigment phycoerythrin. The major distinguishing characteristic of the Rhodophyta is the lack of any flagellated stage in the life cycle. It has been suggested that the triphasic life cycle found in many species of the Rhodophyta evolved to compensate for the lack of flagellated cells.

Class: Rhodophyceae

Subclass: Floridophycidae
Distinguishing features of the Florideophycidae include the presence of pit plugs, the presence of multiple discoid chloroplasts in a cell, apical cell division, and all multicelluar species.

Order: Gelidiales
The Gelidiales were previously considered part of the order Nemaliales, but were removed and placed in a distinct order by Kylin in the 1920's. The characters of the Gelidiales that this reclassification was based on include the absence of an auxiliary cell, the receptive cell to which the diploid nucleii is transferred after fertilization, and the Polysiphonia-type life history. However, these characters have been critically revised since then and are no longer thought to be unique to the Gelidiales. This led some researches to return the Gelidiales to the Nemaliales, as in Abbott and Hollenberg's Marine Algae of California. Current thinking on this topic seems to be that despite the lack of major identifying characteristics found only in the Gelidiales, this group possesses enough distinct characters in combination to merit ordinal status. These characters include:

  • Polysiphonia-type life history (triphasic with two diploid stages and one haploid stage)
  • Lack of an auxiliary cell
  • Nutritive filaments that develop during carpogonial formation, before fertilization
  • Common growth and branching pattern
  • Single cap layer in pit plugs
  • Transverse division of spermetangia (rather than oblique division found in most other red algae)

Family: Gelidiaceae
The order Gelidiaceae is made up of only two families, the Gelidiaceae and the Gelidiellaceae. These families are very closely related; the Gelidiellaceae are separated only due to the lack of rhizoidal filaments. The Gelidiellaceae contains a single genus while the Gelidiaceae contains eight genera.

Genus: Gelidium The genera of the Gelidiaceae have morphological or anatomical characters defining that particular genus from the others with the exception of Gelidium and Pterocladia. These two genera are easily confused on the basis of external morphology. The only reliable way to separate the gerera is through examining cystocarp structure under a microscope. The cystocarp of Gelidium protrudes equally on both surfaces of the branch and usually has one opening on each surface of the frond. In contrast, the cystocarp of Pterocladia protrudes on only one surface of the branch and usually has one or more openings on only one surface of the frond. Depending on the reference, Gelidium contains between 50 to 100 species.


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© 1999 Sharon C. Komarow. All rights reserved.