sea lettuce

noun
any seaweed of the genus Ulva, having large leaflike blades.

Origin:
1660–70

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World English Dictionary
sea lettuce
 
n
any of various green seaweeds of the genus Ulva, which have edible wavy translucent fronds

Collins English Dictionary - Complete & Unabridged 10th Edition
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Encyclopedia Britannica
Encyclopedia

sea lettuce

(Ulva), a genus of green algae usually found growing on rocky shores of seas and oceans. Some species also grow in brackish water rich in organic matter or sewage. The thallus, which somewhat resembles a lettuce leaf, is a sheet of cells up to 30 cm (12 inches) long and two cells thick and is embedded in a tough gelatinous sheath. The life cycle consists of alternation of similar spore-producing (diploid) and gamete-producing (haploid) generations. Asexual reproduction is either by accidental fragmentation or by gametes that act like motile spores.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Sea lettuce is common on the lower portions of both the north and south beaches wherever there are cobbles for substrate.
The color may be an adaptation to living among bright green sea lettuce.
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