Electronic Flora of South Australia Genus Fact Sheet

Genus GIGARTINA Stackhouse 1809: 55

Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Gigartinaceae

Thallus usually much branched subdichotomously, laterally or irregularly, complanately or radially, branches terete to compressed, usually relatively thick, somewhat mucilaginous and drying cartilaginous. Structure multiaxial, with a compact cortex of anticlinal, subdichotomous filaments of small cells, larger inwardly, and a medulla of anastomosing filaments orientated at random or with the central ones more or less longitudinal and interlinked.

Reproduction: Sexual thalli monoecious or dioecious. Carpogonial branches 3-celled, borne on inner cortical (supporting) cells of lesser branches. Procarpic, with the supporting cell acting as auxiliary cell following fertilization, producing numerous gonimoblast initials which develop mainly thallus inwardly, with the cells cutting off laterally small clusters of carposporangia; enveloping tissue of varying compactness, present around the carposporophyte, sometimes with transverse absorbing gonimoblast filaments. Cystocarps swollen, appearing stalked in lesser branches. Spermatangia in inconspicuous sori, cut off from outer cortical cells.

Tetrasporangial sori irregular, linear or annular, lying near the boundary between the inner cortex and the medulla, developing tetrasporangia in short chains of 2–4 cells by transformation of and lateral divisions of inner cortical and outer medullary cells. Tetraspores released by excision and gelatinous extrusion of the entire sorus, leaving behind an eroded surface.

Life history triphasic with isomorphic gametophytes and tetrasporophytes, some species with mixed phases.

Type species: G. pistillata (Gmelin) Stackhouse 1809: 74.

Taxonomic notes: Gigartina (including Chondracanthus Kützing) is a large genus, widely distributed on temperate coasts of Australia, New Zealand, South Africa and most N. Hemisphere coasts. It is characterised by the cystocarps appearing stalked in lesser branches, and by the location of the tetrasporangial sori near the boundary between cortex and medulla, with the tetrasporangia being transformed from, and cut off laterally from, existing vegetative filaments.

The type species of Gigartina has only recently been investigated in detail, by Hommersand et al. (1992). The gonimoblast filaments penetrate the enveloping tissue and gonimoblast cells unite with vegetative cells by fusion or by secondary pit-connections to form a heterokaryotic placenta. Carposporangial chains derived from both diploid gonimoblast and heterokaryotic placental cells differentiate in clusters, separated by sterile placental tissue and surrounded by the enveloping tissue. Tetrasporangial sori occur near the boundary of cortex and medulla. Both intercalary cells and cells of short lateral chains differentiate into tetrasporangia which divide cruciately. The sorus tends to be liberated as a unit on maturity, leaving behind a distinctly eroded surface, which is repaired by the proliferation of subcortical cells (Hommersand et al. 1992).

The southern Australian species described below agree as far as known with the development of the carposporophyte for the type species, though union of diploid gonimoblast cells with haploid vegetative cells has not been established. While an enveloping tissue with transverse filaments is present in the type species (Hommersand et al. 1992), the degree of development of the enveloping tissue varies considerably in southern Australian species, being very slight in G. recurva to very compact in G. brachiata, G. densa, G. sonderi and G. pinnata. Further, in contrast to the type species, transverse gonimoblast filaments were not observed in many species. However, G. recurva and to a lesser extent G. pinnata, do possess numerous tubular gonimoblast filaments which penetrate between the cells of the enveloping tissue. On this basis the nature of the enveloping tissue should be disregarded as a generic character in the family.

G. wehliae is similar to G. recurva in the slight to moderate development of an enveloping tissue, but lacks any transverse gonimoblast filaments. However, until tetrasporangial sori are observed, the generic status of this species remains uncertain.

Chondrus Stackhouse is separated from Gigartina by having immersed carposporophytes in the flat branches and by the absence of enveloping tissue. Since the cystocarps of G. recurva are swollen in branchlets and are more circumscribed than those in Chondrus, which extend through the whole medulla, G. recurva is retained in Gigartina though the enveloping tissue is slight.

Release of carpospores in southern Australian species of Gigartina is not through an ostiole. The cortex directly outside the carposporophyte is generally shallow or depressed, and disintegrates when carpospores are liberated. As in the type species, the breakdown of cortical tissue does not penetrate to the enveloping tissue or carposporophyte. While Hommersand et al. (1992) refer to this "gap" as an ostiole, it does not constitute an ostiole in a true sense.

The development of tetrasporangial sori in the southern Australian species of Gigartina falls into two groups, which form a gradation:

1.       ; Tetrasporangia in short chains of 2–4 cells derived primarily from the transformation of inner cortical or outer medullary filaments (G. brachiata, G. densa and G. sonderi).

2.    ; Tetrasporangial in short chains of 2–4 cells derived primarily from the lateral divisions of inner cortical or outer medullary cells. The tetrasporangial sori in this group, while located near the boundary of the cortex and medulla, tend to extend deeper into the medulla. Tetrasporangial development in these species is identical to the type species. The species in this group include G. muelleriana, G. disticha, G. pinnata and G. recurva.

The prevalence of lateral chains in these species is linked with the depth and extent of the tetrasporangial sori, with lateral divisions being more prevalent in species with deeper sori (ie. G. muelleriana, G. disticha, G. pinnata and G. recurva). At this stage the differences in these two groups are considered to be of no more than subgeneric status.

As in the type species, all southern Australian species appear to lose their tetrasporangia by disintegration or excision of the sorus, sometimes the entire sorus, leaving behind a distinctly eroded surface. No cases have been observed of the release of tetraspores through pores or an ostiole in the wall.

The southern Australian species are therefore kept in Gigartina and the segregate genus Chondracanthus (Hommersand et al. 1993) is considered doubtfully distinct.

References:

HOMMERSAND, M., FREDERICQ, S. & CABIOCH, J. (1992). Developmental morphology of Gigartina pistillata (Gigartinaceae, Rhodophyta). Phycologia 31, 300–325.

HOMMERSAND, M.H., GUIRY, M.D., FREDERICQ, S. & LEISTER, G.L. (1993). New perspectives in the taxonomy of the Gigartinaceae (Gigartinales, Rhodophyta). Hydrobiologia 260/261 (Proc. Intn. Seaweed Symp. 14), 105–120.

STACKHOUSE, J. (1809). Tentamen Marino - Cry ptogamicum. Mem. Soc. Imp. Nat. Moscou 2, 50–97.

The Marine Benthic Flora of Southern Australia Part IIIA complete list of references.

Author: K.S. Edyvane & H.B.S. Womersley

Publication: Womersley, H.B.S. (14 January, 1994)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIIA, Bangiophyceae and Florideophyceae (to Gigartinales)
Reproduced with permission from The Marine Benthic Flora of Southern Australia Part IIIA 1994, by H.B.S. Womersley. Australian Biological Resources Study, Canberra. Copyright Commonwealth of Australia.

KEY TO SPECIES OF GIGARTINA

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