Caulerpa 
sertularioides

Halimeda 
opuntia

Padina
  boryana

Valonia 
aegagropila

The sandy bottoms of the lagoons are often in deeper waters, covered with a mucous film rich in bacteria or of a carpet of Cyanobacteria where tufts of filamentous red algae such as Polysiphonia, Ceramium mingle.

Sometimes, in shallow waters the Halophila ovalis seagrass forms sparse meadows.

The patch reefs’ hard substrata are always much richer in various green algae :

Caulerpes (Caulerpa seuratii, 
C. urvilliana, C. pickerengii, 
C. bikinensis), 

Halimedes (H. discoidea, 
H. gracilis, H. incrassata, 
H. lacunalis, H. micronesica, H.opuntia , H. taenicola) 

or 

Microdictyon okamurae, 
M. umbilicatum.

The calcareous red algae form purplishblue windings in the riggedness and the Galaxaura filamentosa thrive along the vertical walls as well as the close-cropped turf where small filamentous red algae such as Ceramium, Polysiphonia, Chondria, Herposiphonia, Griffithsia, Jania mingle.  

This area probably shelters the richest and most diversified flora of the reef complex. It is on the reef flats of atolls that Corallinaceae and Sporolithaceae formations are the most exuberant. The back reef is generally formed with a flush flagstone very much encrusted. A carpet of red filamentosous algae (Jania adhaerens) which trap the sediments, covers it. Small light brown fans of Lobophora variegata growth on this carpet. 

To the ocean, the beautiful brick-red colour algal crest form a calcareous algae platform with a dominance of Hydrolithon onkodes , H. gardineri and two species of Sporolithon, S. episoredion and S. ptychoides ; in very exposed areas, they even construct spectacular corbellings where the ocean waves break. Generally, a thin carpet of red filamentosous algae such as Ceramialae covers the calcareous algae, and feed the numerous herbivorous echinodermes and gasteropodes which are living in the crest anfractuosities. A few raising algae growth there forming bright green tufts of Chlorodesmis fastigiata, which mingle with greenish-yellow rosettes of Microdictyon umbilicatum and dark green blobs of the caulerpa such as Caulerpa pickerengii, C.urvilliana, and the Codium. We can notice locally hemispheric small pinkish cushions of  Amphiroa fragilissima, whitish blobs of Liagora ceranoides or  verdigris blobs of Halimeda micronesica.

This type of flora extends beyond 10 metres in depth. Red algae are here most abundant and diversified.

The calcareous red algae are abundant and the more common species is undoubtedly Pneophyllum conicum which form large patches, from pink-yellowish to dark red. They often cover the encrusting corals and can kill them.The brown-redish to dark brown thalli of the Sporolithon ptychoides mingle with the Corallinacea, particularly in the overhanged parts of the spurs and ridges zone. The outer slope also shelters the Halimedes Halimeda taenicola, H. discoidea and the Caulerpes. On certain atolls (Takapoto) Caulerpa bikinensis is locally abundant forming large, very dense meadows. In the interstices and the vertical walls 15 to 40 meters deep, we observe compact accumulations of dark red rosettes formed by the red alga Cryptonemia umbraticola.

The passes represent a specific environment, being generally traversed by violent currents.

The flora colonizes dropoffs and it primarily consists of calcified encrusting Corallinaceae species such as Pneophyllum conicum, Mesophyllum and Peyssonnelia. Halimeda minima and Cryptonemia umbraticola abound in anfractuosities.

Mesophyllum erubescens

Peyssonnelia
 rubra

Liagora ceranoides

The flora of the reef flat spillways or “hoa”, is generally very monotonous and varies little. The pavement which is often encrusted by the red alga Hydrolithon onkodes, is covered with a very close-cropped turf which traps the sediments. It is common to see the light brown thalli of Lobophora variegata in the shape of lightly attached lobes or fans and the pink clumps of Liagora ceranoides.