CRITTER: Crustose coralline algae: Lava reef cement

  • Crustose coralline algae

  • Crustose coralline algae

Giant waves have been crashing on the lava coastline in Kauai for millions of years. These waves break down the lava rock over time but we still have nice flat lava reefs we surf over. Why aren’t our reefs all ground up into sand? A million years of crashing waves should have reduced our porous lava reefs into rubble.

Hawaiian lava reefs have a natural cement that holds the reef together. When you go snorkeling or diving look close at the reef and you will see that in between the lava structure is a layer of purple colored material. This hard purple substance can look like a vein between the rocks but it can also look like a purple coating growing on the lava or even purple pebbles that roll around on the seafloor. When this purple substance gets washed up onto the beach by the surf it turns pure white and looks like a dead piece of coral with a twisted spaghetti like pattern. Every surfer here in Kauai has seen this substance on the beach but most people think it is dead coral.

The purple growth on the lava reef is crustose coralline algae. It is a hard plant like material related to seaweed. This algae species is rock hard and grows between the lava rocks gluing them together. When the waves break down the lava rock the crustose coralline algae grows and keeps the rocks from being washed away. This hard purple algae is the most common form of life on our Hawaiian coral reefs and most people do not even know it is there!

Kauai Island is much older than Hawaii Island so Kauai has a large amount of crustose coralline algae holding the reefs together. Over time when the waves start to break down the lava on Hawaii Island more crustose coralline algae will grow. Many of our best surf spots here in Hawaii are literally glued together with rock hard purple algae!

You can see what the crustose coralline algae looks like out on the reef in my underwater educational movie series up on my web at


Terry Lilley is a marine biologist living in Hanalei Kauai and co-founder of Reef Guardians Hawaii, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawaii go to


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