Mangrove Crab Migration

Over 300 mm of rain around Christmas last year marked the opening of the wet season. It also acted as a build-up signal for the army on Roebuck Plains, the army of the mangrove crabs (Neosarmatium australiense).


They live in the rearward mangrove zone which is flooded only during spring tide. The ground looks so dry in most of the time in dry season. You would hardly associate these sesarmid crabs with mangroves.

However, the rain turned the plains into a big swamp (right). They came out from the holes (below left), aggregated on the car track, and marched towards the sea. It was never an easy journey. Some of them died in the hot weather during midday, and some of them died under the tyres of the passing vehicles.

Fortunately, some of them successfully arrived at the shore, becoming veritable "mangrove crabs". We saw lots of them hanging around in Little Crab Creek (below right). However, two weeks later, they are now disappeared from the creek.


Instead, they are spending most of the time around the holes in the mangroves. This crab in hand might tell us why- it is gravid!
(Below right: This aggressive crab was trying to pull the GoPro into its hole in the mangroves..........)