Like the Christmas holly, Singapore's sea holly plants sometimes have spiny leaves.
Research by plant expert Jean Yong has shown that sea holly plants have variable leaf shapes in response to environmental factors, such as exposure to sunlight and salinity.
"In the harshest localities of any mangroves, where plants receive full sunlight and are often fully immersed in seawater, two species of sea hollies - the Acanthus ilicifolius and Acanthus ebracteatus - will develop very spiny leaves," said Dr Yong, an eco-physiologist at the Australian Research Centre for Mine Site Restoration.
He added that, in general, sea holly plants which receive greater shade, and/or freshwater, have rounder, less spiny leaves.
"We hope to find out which the predominant factor is - whether it is sunlight exposure or salinity - in determining the ultimate leaf shape and spine development."
Another unique trait about sea holly plants is their ability to deal with salt. While some mangrove plants keep salt from entering in the first place, sea hollies take salt in and discharge it through salt glands on their leaves, said botanist Shawn Lum, from Nanyang Technological University's (NTU's) Asian School of the Environment.
Here is more information about Singapore's sea holly plants.
This is a common mangrove plant that grows up to 3m in height. Its leaves are used to treat rheumatism and wounds and can be used to make a softening cream. In addition, a concoction of the leaves can be taken with the stems to promote longevity, or with the roots to improve hair conditions, and treat sores and snake bites, according to the National Parks Board (NParks).
This species is considered vulnerable. It can grow up to 2m in height. Its seeds are sometimes boiled to be used as ingredients in cough mixtures.
This species can grow up to 8m in height. This is considered the rarest among the three Acanthus species, said Dr Yong, and can be found only on Pulau Ubin and Pulau Tekong and in Sungei Buloh and the Western Catchment areas. This species also produces the least spines on its leaves.
•Source: NParks, Dr Jean Yong, Dr Shawn Lum