The intertidal rocky shore 

identifying coralline algae and green seaweeds  

By Dr J Floor Anthoni (2007)
Coralline algae could well be the most amazing plants in the sea, as they are found from the shallowest rock pool to deeper than any plant can grow; from the cold temparate seas to the warm tropical coral reefs where they are perhaps the most important reef builders. Coralline algae are red seaweeds with red pigments. Green seaweeds are usually lush and edible. 
  • coralline algae: Coralline algae are like growing stones as a hard crust over the rock or as segmented leaves
  • green seaweeds: Green seaweeds are usually lush and edible
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coralline algae
The coralline algae are some of the weirdest creatures on Earth. They are red seaweeds that build limestone skeletons in which the plant is 'spread around'. They are living stones containing a thin soup of red seaweed. There are encrusting varieties that grow a stony leaf on the rock, and there are upright branching varieties consisting of beads of stone, joined by a very elastic band. These stony plants are amongst the most robust plants in the sea, found in the most exposed places. Their light sensitivity covers a vast range from right at the surface, being exposed to ultraviolet light, to the very deep where no other seaweed can grow for lack of light.

two radiate limpets in a fist-sized rockpool
0609107: two radiate limpets in a tiny rock pool. The pink patch shows where the pool is, as pink paint (Lithothamnion sp), a coralline encrusting alga, grows best under water. Note how the smallest limpet has left its homing spot, where it always returns to sleep. Because it shades the pink paint, a bare patch results. The pink paint is barren because it is grazed meticulously by the limpets who need someone else to do their backs.
pink paint and boring sponge
f019010: on this patch of rock one can clearly sea the various 'leaves' of pink paint, but each leaf is a separate plant. The biggest leaf is lumpy. Yellow boring sponge colonies (Clione cellata) have drilled their way through the calcareous pink paint.
lumpy pink paint (Corallina polymorphum)
f019009: a lumpy pink paint species (Corallina polymorphum).
turfing pink paint (Corallina officinalis)
0608237: turfing pink paint (Corallina officinalis) in a stable rock pool. These stony plants cannot be eaten, but as a stony extension to the rock face, they vastly enlarge its surface, allowing thread algae to grow and an army of almost invisible snails to feast on these. It also provides micro-protection for very small animals like slaters and snails and it provides a nursery for the young of others.
vertical wall of a stable and healthy rockpool
f032522: the vertical wall of a stable and healthy rockpool is densely covered in pink turf (Corallina officinalis), mixed with thin green alga (Enteromorpha spp). An army of small snails lives here.
f048936: some coralline algae grow a complicated leaf on top of long-lived seaweeds as on this flapjack

green seaweeds
Green seaweeds use a green pigment for trapping solar energy in photosynthesis. But some red seaweeds can look greenish too. Green seaweeds prefer shallow water, but some manage to live at considerable depths, to 30m.
slender sea lettuce (Ulva laetevirens)
0608232: slender sea lettuce (Ulva laetevirens) in a gutter of the rock. The bed of sand keeps it moist against drying out.
a grazed rosette of sea lettuce (Ulva rigida)
0608221: when tightly grazed to a rosette like this, the firm sea lettuce (Ulva rigida) can survive at mid tide amongst the surf barnacles (Chamaesipho brunnea).
sea lettuce (Ulva lactuca)
f051525: when left undisturbed, sea lettuce (Ulva lactuca) can grow very lush, but eventually dies and rots away when not grazed. A lush patch like this means that there are an insufficient number of grazers. Poor Knights marine reserve.
branched velvet weed (Codium fragile)
0609146: the branched velvet weed (Codium fragile) looks and feels like velvet. Because it stores a lot of water, it can at times be found above low tide. It is usually found in sheltered harbours. Here it mixes with the necklace weed (Hormosira banksii).
flat velvet weed (Codium dimorphum)
f051722: flat velvet weed (Codium dimorphum) forms rounded patches on the rock. It feels soft but lacks hairs.
intricate velvet weed (Codium convolutum)
f051216: the intricate velvet weed (Codium convolutum) grows rosettes of flat fingers on the rock. It feels soft and has hairs.
green grapeweed (Caulerpa geminata)
f051502: the green grapeweed (Caulerpa geminata) is sometimes found in healthy rock pools. Its roots form a growing network.
sea rimu (Caulerpa brownii)
f034130: the sea rimu (Caulerpa brownii) has feathered branches that look like the New Zealand rimu tree. It lives under water in medium shelter, where its running roots permanently occupy space. East Coast North Island.
f051712: a tuft of finely branched green algae (Cladophora crinalis?), feeling firm to the touch.