, Algae , Fish , The zoobenthos
few biological particularities : The forms and reproduction
of the algae
size and form
size and shape of algae vary greatly and are results of diverse
modes of growth and anatomical organization.
we find shapes common to the different taxonomic groups and which
can be broadly classified into morphological categories.
Functional morphological groups have been validated by
physiological and ecological criteria, hence they can provide
useful information on the lifestyle of species. The order of
presentation of groups progresses from delicate to thicker forms
and attests to a growing complexity of the thalli.
forms : This is the case in the vegetative thallus of Acetabularia, consisting of a single giant cell with a
single basal nucleus. In some species, cells can attain
considerable sizes (over 5 cm) and are multinucleated (Valonia,
forms : These algae have filaments which can have internal
cross-walls, remain simple (Chaetomorpha) or become
branched (Cladophora, Griffithsia, Polysiphonia,
Filamentous axes may also lack internal cross-walls and
form more or less branched siphons (Bryopsis).
or mucilaginous forms : These multicellular algae have either a hollow tubular
(Enteromorpha, Rosenvingea) or more or less contorted
spherical thallus (Colpomenia, Hydroclathrus), bearing
internal cross-walls or internal filaments bathed in a
mucilaginous substance (Preadaea, Gibsmithia).
compressed, leafy and membranous forms : These
algae are all characterised by a very thin, often
translucent thallus which is one to a few cells thick and
is not (or only weakly) corticated. The surface is uniform
(Dictyota, Ulva) or displays fine veins or midribs
(Dictyopteris, Amansia). Texture is supple but
Thick , erect
: Algae in this group form erect thalli with intricate
branching and possess a coriaceous texture (Turbinaria,
forms : Algae in this group form thick corticated blades (Padina,
calcified and articulated forms : Calcified algae with articulated thalli are composed of
a regular succession of calcified segments (articles) and
supple segments (articulations) making for a robust but
flexible thallus structure (Amphiroa, Galaxaura,
encrusting forms : In
this group, calcification is very important, giving the
thallus a stony aspect. Thalli can be encrusting and
firmly adhere to the substratum (Hydrolithon), lamellate
or branched (Lithophyllum, Neogolithon).
lamellate, filamentous or hollow forms are
generally short-lived species with a rapid rate of
growth which enables in
most cases a rapid succession of generations. For
these forms, most of the cell mass is photosynthetically
active and the thallus surface area, which is relatively
greater than the volume, allows rapid uptake of
nutrients from the surroundings. These forms are
generally found among pioneer species, or in
opportunistic species that proliferate in a spectacular
manner if an enrichment in nutrients occurs. On the other hand, more compact and calcified forms
live longer. The high proportion of
non-photosynthetic tissues (95% calcium carbonate in
encrusting algae) and the weak relationship between the
thallus surface area and the volume, do not allow them
to metabolise as quickly as other simpler forms. Thus,
their growth is slower and they are generally perennial
algae. They constitute important populations that have a
strong effect on the physionomy of the vegetation, for
instance in Sargassum beds or in massive covers of
encrusting coralline red algae.
with conservation of the gene pool
can occur in two ways : vegetatively or
asexually with direct sporulation.
reproduction is brought about by a fragmentation of the thallus or
the production of specialised cells (propagules) ; in both cases
the detached parts of the thallus become fixed to the substratum
and quickly grow into a new individual.
reproduction involves specialized cells, where the spores derive
directly from a total transformation of the contents of a cell
called sporocyst. After their release, they directly germinate
into new individuals genetically semblable to their parents.
Sexual reproduction and life cycles
type of reproduction involves two kinds of specialised cells :
gametes and spores, with fundamentally different behaviours. Male
and female gametes, resulting from the transformation of the
fertile gametophyte cell contents, fuse after their release to
give rise to an egg or zygote which will grow into an individual
genetically distinct from its parents. These diploid individuals
called sporophytes will then produce, through meiosis, spores
called meiospores which will germinate into haploid individuals
called gametophytes which will in turn produce gametes. Hence the
reproduction cycle, involves an alternation of sporophyte and
gametophyte generations. These generations can be morphologically
similar (Ulva), or so dissimilar as to be sometimes wrongly
identified as separate species, as in the case of Padina (the
large sporophyte) and Vaughaniella (the tiny gametophyte)
Asparagopsis taxiformis (the
fan form lamella correspond to the large
sporophyte; the filaments carpet (at the left
bottom of the picture) correspond to the tiny
In the plant world, algae stand out by their extreme
diversity in terms of gametes and spores and the complexity of
their life cycles, the detailing of which is
beyond the scope of this book. These aspects of algal
biology are well documented in the following publications :
Lobban and Harrison (1994), Gorenflot and Guern (1989).
of taxonomic groups, male gametes are always released into the
environment and, except in red algae they are generally mobile
flagellated cells. Female gametes have a more variable behaviour.
They are at times dispersed, flagellated and of the same size as
the male gametes (green algae), or occasionally much larger as in
some brown algae ; at other times they may be non-flagellated but
dispersed as in brown algae (Fucophyceae). In the case of the
latter, female gametes attract the flagellated male gametes using
sexual attraction substances called pheromones.
female gametes are in some groups retained by the gametophyte and
the zygote resulting from fertilisation will grow on the parent
from the diversity in the type of gametes, algae show numerous
anomalies in their sexual reproduction behavour. The absence of
gametes of the opposite sex can lead, in certain extreme cases to
degradation of fertilisation and to the growth of new individuals
from one of the two kinds of gametes.
any case, reproduction in algae generally occurs discretely and
the specialised cells involved are difficult to see without a
microscope. However, in the green genera Halimeda and Caulerpa
reproductive activity involves the entire thallus which is fully
transformed into gametes (holocarpy) and disappears after their
release. The Halimeda case is remarkable as the thallus becomes
discolored and the cells containing the gametes emerge as green
balloons on the surface of the whitened segments.
Lobban, C.S. et Harrison, P.L. –1994 – Seaweed ecology and physiology.
Cambridge University Press, ix + 366pp.
R. et Guern, M. - 1989
– Organisation et biologie des Tahllophytes. Ed. Doin, 235pp.
C., N'Yeurt A.R. & Orempüller, J. - 2001 - Algae of french
Polynesia -Algues de Polynésie Française. Edition Au Vent des îles
- tahiti, 320pp.
C.E. & N'Yeurt, A.D.R. 1997. A revised Checklist of Polynesian
benthic Marine Algae, Australian Systematic Botany, 10: 867-910.