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Seaweeds > Division Chlorophyta
Sea lettuce
Ulva sp.*
Family Ulvaceae
updated Jan 13
if you learn only 3 things about them ...
It is only two cells thick so it grows rapidly when the water has lots of nutrients.
Avoid stepping on the green carpet of sea lettuce. It's slippery and animals live among the seaweeds.
Can eat or not? Yes you can if you're a pig. It used to be fed to pigs.

Where seen? Sea lettuce looks just like its namesake land plant. Seasonally, there is an explosive growth or 'bloom' of this seaweed. It is then so abundant that a thick soft 'green carpet' of washed up sea lettuce blankets the high shores. Be careful! The seaweeds can be slippery and conceal rocks and other things that might trip you or poke you. Also, many animals hide under this seaweed. Try not to step on this 'green carpet'.

Features: A thin blade which is only two cells thick! This allows sea lettuce to grow rapidly in nutrient-rich water as it has a high surface to volume ratio. Compared to most other seaweeds, sea lettuce species can better tolerate being exposed during low tide. So it grows near the shore. The blade is attached with a small holdfast, usually embedded in the sand or attached to a hard surface. During a 'bloom', large amounts can float freely and blanket a large stretch of the shore. Usually bright green, sometimes with a yellowish tint.

Which Ulva? According to AlgaeBase, there are more than 120 current Ulva species. Species used to be determined by the structure of the blade, cells and other tiny features. However, recent studies show that these can change with age, reproductive state and environmental factors including being chewed upon by predators. So it's really tricky trying to determine species of Ulva.

Some form sheets that are flat, thin and translucent, glossy and smooth. Usually entire without many holes. The edges are sometimes ruffled. Can grow to 10cm wide or more. Usually bright green.

Some form flat strips, thin and translucent, glossy and smooth with many holes. Often crinkled into twists, the strips are narrow (1- 2cm wide) and may be 20cm or longer.

Smell of the sea: When the shores are covered with sea lettuce, you can smell the distinctive aroma of seaweed gently toasting in the sun. This is truly the smell of the sea!

Sea lettuce babies: Sometimes, you might come across a sea lettuce blade that is white or transparent. This could be because the sea lettuce has become fertile and converted some of its cells into reproductive cells and released these cells. Often, this happens along the edge of the blade.

Role in the habitat: Sea lettuce is one of the seaweeds eaten by the Green turtle (Chelonia mydas), as well as other creatures. The dense tangle of seaweed also provides plenty of hiding places for the small animals that live in the seagrass lagoon.

Human uses: Sea lettuce is fed to pigs and livestock. In the past, they were collected in boatloads in the Straits of Johor, washed in freshwater then cooked and fed to pigs. Sea lettuce is cultivated for animal feed in some places.

In some places, it is also eaten by humans, as a salad or mixed with other vegetables. The species that are used commercially include U. lactuca, U. pertusa and U. reticulata.

It is also reported to have antibacterial properties, and to be used to treat goiter, gout, scrofula, burns and other irritants. Sea lettuce also makes good packing material to cover more valuable Caulerpa seaweeds during shipping and transport, or to cover fish for sale. As sea lettuce tends to grow well in polluted waters, it is also used as an indicator of water quality.

May sometimes form a thick
green carpet on shores.
Chek Jawa, Jan 09


Labrador, Mar 05



Transparent blade: reproducing?
Labrador, May 05


Tiny black sea urchin in sea lettuce.
Changi, Jul 08


Pipefish camouflaged on sea lettuce.
Changi, Apr 05

Some form sheets.

Others are ribbon-like.

Tiny shrimp on sea lettuce.
Changi, Jun 07

Sea lettuce on Singapore shores

Photos for free download from wildsingapore flickr

*Species are difficult to positively identify without close examination of internal parts. On this website, they are grouped by external features for convenience of display.

Ulva species recorded for Singapore
Pham, M. N., H. T. W. Tan, S. Mitrovic & H. H. T. Yeo, 2011. A Checklist of the Algae of Singapore.

  Ulva beytensis
Ulva clathrata
Ulva compressa
Ulva fasciata
Ulva flexuosa
Ulva flexuosa
subsp. paradoxa
Ulva intestinalis
Ulva lactuca
Ulva pertusa
Ulva reticulata

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