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Analogies through the water surface

More than one year from now I remember myself fascinated by an inter-kingdom analogy in one of the smallest inhabited islands in the world, Gili Meno, not far from the island of Lombok (Indonesia).Floating synflorescence of Spinifex littoreus

During the days I was in Gili Meno, I spent part of my afternoons taking advantage of the low tide to watch starfishs and urchins while walking among the partly emerged corals. One of these afternoons I saw a strange urchin floating on the shallow water. Through a closer look I realised it was part of a grass belonging to the Poaceae family.

Floating synflorescence of Spinifex littoreus

The name of the genus is Spinifex and it is characterized by a stellate synflorescence, made of radiating racemes that closely resemble the shape of a sea urchin (see this page for an illustrated descritpion of different types of inflorescence and syninflorescence). This genus occurs in Temperate and  Tropical Asia, Australasia, and Pacific and is very important for the entire sand dune ecosystem as it helps stabilise the sand.

The species I had the chance to observe is Spinifex littoreus. This species in particular is characterized by female radiating racemes 8–15 cm long; with a scabrous rhachis terminating in a barren extension quill-like, spinous. Spinifex littoreus is the most spread species of the genus occurring from temperate Asia (China and eastern Asia) to tropical Asia (India, Indo-China, Malesia, and Papuasia), Australia and southwestern Pacific. You can find information on nearly 11000 species of grass of the world in an amazing section of the Kew garden website named GrassBase, while another blogpost on Spinifex littoreus can be found at this page.

Going back to Gili Meno, the similarity in shape among what was on and under the water surface was amazing. At a first glance I thought of an analogy, due to convergent evolution. In fact the pointed racemes of the Spinifex littoreus defend them against herbivory, as the spikes help the sea urchins defend them from predators, even if there is always a species that does not stop in front of such difensive strategies.

Big predator

I started thinking of other inter-kingdom convergences in shape between plants and animals. Many of them are developed by predators animal that mimic flowers or other parts of the plant to gain a short-distance attack; or by plants mimicking the female of an insect to be effectively impollinated; while a close similarity both in shape and function is not so common.

On the other hand the shape of Spinifex littoreus synflorescence has to be related not only to a difensive strategy, but also to the plant dispersal. Most of the species of the Poaceae family are dispersed by wind, but Spinifex special propagule is able to roll and tumble on the sandy beaches in fact its indonesian name is Rumput Lari, thet means running grass. This gives this species a special advantage with regards to other plants of the dunes. This type of movement is known for propagules of species belonging to other families, always developing in open habitats: Salsola iberica, Oenothera deltoides, Kochia scoparia. The unit blown by the wind may be an inflorescence, but also an entire (dead) plant. Usually in such species seeds fall off as they travel resulting in a dispersal along a long narrow band; indeed in open habitats these propagules can tumble for kilometers. For further information on this topic see this book.

So in the case of Spinifex a double analogy can be highlighted, the one with sea urchins concerning the protective function of a sphere of spiny ends. However sea urchins never move for kilometers in their adult shape, they disperse through floating eggs or swimming blastula; while Spinifex tumbles through open habitats as a way to disperse in analogy with other plants of different families.

s they help stabilise the sand, these grasses are an important part of the entire sand dune ecosystem.

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