This one, I think, will be relatively straightforward. Last week I was at Mauritius. I spoke at a seminar, and I also had a chance to explore the island and to study the grasses growing near the ocean.

I saw this grass growing at nearly every beach, on all sides of the island. It clearly has a high salt tolerance, because at high tide, the ocean water completely inundated some of the plants. I didn't notice salt causing any damage to the grass.

This grass was also observed slightly up from the water line, where the sand and grass were not inundated daily.

It was especially impressive to see this grass, at high tide, completely covered by ocean water.

The mystery I would like to solve is this one: what species is this grass?

The answer to this mystery is Sporobolus virginicus, correctly solved by Mark Field:

@asianturfgrass Sporobolus virginicus?

— Mark Field (@cricketgman) December 24, 2013

There are a number of interesting characteristics of this grass that set it apart from two other common grasses (Paspalum vaginatum and Zoysia matrella) found in tropical coastal areas.

• S. virginicus is adapted to low rainfall and is very drought tolerant with low water requirements
• S. virginicus is believed to be pest-free
• Z. matrella is less widespread and one doesn't find it so much growing in sand; Z. matrella is found close to sea level and in saline conditions but usually is anchored onto rocks or other stable surfaces
• P. vaginatum will not tolerate drought and prefers moist to saturated sites

This fact sheet from the USDA provides details on S. virginicus.

@asianturfgrass @cricketgman plenty around the Coast of Australia, here's some more I saw yesterday growing well pic.twitter.com/cfqDAsOwp0

— Brett Morris (@brettmorris73) December 24, 2013

Dr. Brett Morris shared the above photo of S. virginicus growing in it's typical environment, right on the sand near the high tide line, where it will sometimes be inundated with saline water, and sometimes be exposed to extended periods of drought stress.

The leaves of S. virginicus and P. vaginatum appear very similar, and the best way to tell these grasses apart is to find plants with spikelets. There are profound differences in spikelet morphology between the two species. 

At right is S. virginicus found on the beach at Bel Ombre, Mauritius. Note the spiciform (spikelike) panicle. P. vaginatum, however, has paired racemes, as shown below.