Rhodomelaceae

Introduction

The Rhodomelaceae is estimated to be the largest red algal family (Scagel, 1953; Dawson, 1966; Abbott, 1999) with about 125 genera and some 700 species recognized worldwide (Kraft, 1981). This family displays great diversity in vegetative structure but appears to be uniform in reproductive development (Scagel, 1953; Dawson, 1966). Rhodomelaceae are uniaxial and polysiphonious. The polysiphonious structure results from the cells of the central axial filament cutting off pericentral cells by longitudinal tangential divisions (Stegenga, et al. 1997). The Rhodomelaceae differ from other Ceramiales families by the formation of pericentral cells in an alternating fashion. Characteristics of the Rhodomelaceae based upon Scagel (1953), Hommersand (1963), Maggs & Hommersand (1993), Stegenga et al. (1997), Abbott (1999), and Womersley (2003) are listed below.

Characteristics

• Thalli range from having simple, radial symmetry to complex, bilateral symmetry or dorsiventral organization
  

• Apices are straight, strongly curved dorsally, or spirally inrolled (Fig. 1)
  
  

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  Fig. 1. Rhodomelaceae apices. A. Polysiphonia sp. B. Herposiphonia sp.. © 2005

• Specific formation of pericentral cells where they are cut off in an alternating sequence (Fig. 2)
  
  

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  Fig. 2. Sequential transverse section views of axial cell (A) divisions to form pericentral cells (labeled 1 thru 4) in an alternating fashion.  The developmental sequence goes from youngest segments on the left to older segments on the right. Drawing depicts a species with 4 pericentral cells. ©

• Number of pericentral cells ranges from 4 to 24 (Fig. 3)
  
  

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  Polysiphonia spp.. © 2005

  Fig. 3. Transverse sections showing variation in the number of pericentral cells in the Rhodomelaceae.

• Presence of trichoblast, either pigmented and persistent usually associated with vegetative plants or colorless and deciduous usually associated with reproductive plants (Fig. 4)
  
  

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  Fig. 4. Rhodomelaceae trichoblasts. Colorless, deciduous trichoblast in a Polysiphonia species (left) and pigmented, persistent trichoblasts in a Murrayella sp. (right). ©

• Four-celled carpogonial branch

• Cystocarps with distinct pericarp (Fig. 5)
  
  

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  Fig. 5. Polysiphonia sp. cystocarp. © 2005

• Spermatangia, in most cases, are associated with trichoblasts.
  

• Spermatangial heads can display an array of forms, such as, discoid (Fig. 6A) cylindro-conical (Fig. 6B), globose, or lobed.
  
  

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  Fig. 6. Comparison of spermatangia within the Rhodomelaceae. A. Plate-like spermatangia of Acanthophora spicifera. B. Cylindro-conical spermatangia of Polysiphonia sp. © 2005

•  Tetrasporangia are generally borne on pericentral cells (Fig. 7)
  
  

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  Fig. 7. Rhodomelaceae tetrasporangia. A. Polysiphonia sp. B&C. Laurencia sp., two different focal planes showing surface cortical cells (B) covering tetrasporangia (C). ©

References

Abbott, I.A. 1999. Marine red algae of the Hawaiian Islands. Bishop Museum Press, Honolulu. xv + 465 pp.

Dawson, E.Y. 1966. Marine Botany, An Introduction. Holt Rinehart & Winston, Inc., New York. 371 pp.

Hommersand, M.H. 1963. The morphology and classification of some Ceramiaceae and Rhodomelaceae. Univ. Calif. Publ. Bot. 35: 165-366.

Kraft, G.T. 1981. Rhodophyta, morphology and classification. In: The Biology of Seaweeds (Ed. by C.S. Lobban & M.J. Wynne), pp. 6-51. Blackwell Scientific Publications, Oxford.

Maggs, C.A. and M.H. Hommersand. 1993. Seaweeds of the British Isles, Volume 1 Rhodophyta, Part 3a Ceramiales. HMSO, London. 444 pp.

Scagel, R.F. 1953. A morphological study of some dorsiventral Rhodomelaceae. Univ. Calif. Publ. Bot. 27: 1-108.

Stegenga, H., J.J. Bolton and R.J. Anderson. 1997. Seaweeds of the South African West Coast.

Womersley, H.B.S. 2003. The marine benthic flora of Southern Australia Part IIID. Australian Biological Resources Study, Canberra. 533 pp.