The size and shape of algae vary greatly and are results of diverse modes of growth and anatomical organization.

Nevertheless, 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.

Vesicular 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, Ventricaria).

Ventricaria ventricosa 

Filamentous forms : These algae have filaments which can have internal cross-walls, remain simple (Chaetomorpha) or become branched (Cladophora, Griffithsia, Polysiphonia, Ceramium). Filamentous axes may also lack internal cross-walls and form more or less branched siphons (Bryopsis).

Bryopsis plumosa

Hollow 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).

Hydroclathrus clathratus

Thin 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 delicate.

Amansia rhodantha

Thick , erect coriaceous forms : Algae in this group form erect thalli with intricate branching and possess a coriaceous texture (Turbinaria, Sargassum, Gelidium)

Turbinaria ornata

Encrusting forms : Algae in this group form thick corticated blades (Padina, Lobophora, Peyssonnelia).  

Peyssonnelia sp.

Erect 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, Halimeda).

Halimeda taenicola

Calcified 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 (Mesophyllum) or branched (Lithophyllum, Neogolithon).

Lithophyllum spp.

Delicate, 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.  

Reproduction with conservation of the gene pool

Reproduction can occur in two ways : vegetatively or  asexually with direct sporulation.

Vegetative 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.

Asexual 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

This 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) or Asparagopsis taxiformis (the large gametophyte).

The fan form lamella correspond to the large sporophyte; the filaments carpet (at the left bottom of the picture) correspond to the tiny gametophyte. Padina boryana

The large gametophyte   Asparagopsis taxiformis

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).

Irrespective 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.

Finally, female gametes are in some groups retained by the gametophyte and the zygote resulting from fertilisation will grow on the parent gametophyte.

Independently 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.

In 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.

Halimeda reproduction