Dr Dazza: Soldier Crabs

Soldier crabs can be highly visible and colourful on sandflats. They are fascinating creatures with several unique features.

The main species of soldier crabs in Moreton Bay is Mictyris longicarpus. One of the stunning features of soldier crabs is their vivid blue colour.

|Blue is a relatively rare colour in nature and is often used to signal danger. A good example being the blue ringed octopus. However, soldier crabs are not venomous or poisonous. They are simply trying to pretend that they are to any potential predators.|

Another unusual feature of soldier crabs is how they move. While most crabs move sideways and can’t move forwards, the soldier crab walks forwards!

Around the bayside, common refrains from those that spend time on the intertidal flats are “Where have all the soldier crabs gone?” or “Good to see that the soldier crabs are back!” So, what is the actual story?

Soldier crabs spend most of their time buried in the sediments of sandflats, but as the tide falls, they emerge and form large groups. Being in a large group is a common adaptation to reducing the risk of predation – safety in numbers.

It is possible to be on a sandflat on a falling tide and not see any soldier crabs as they simply haven’t emerged from the sediment yet. As the tide starts to rise the soldier crabs bury themselves again in the sediment, and they won’t be seen.

This pattern of emergence and burial means that if you only go to sandflats occasionally, you can miss their presence unless you are there for the short period of time in the tide cycle that they are active and aggregated. Further, the number of soldier crabs that do emerge on any given day is dependent on temperature with more emerging in warmer sunny conditions.

How soldier crabs use their sandflat home

Soldier crabs have a specific series of behaviours when they emerge from the sediment. The first thing they do is spend a short period of timing cleaning themselves of all the sand grains that have stuck to them when they were buried.

For approximately the first 15 minutes after emerging they make only small movements on the sandflats and feed slowly. They are not tightly aggregated at this time and particularly wary and will quickly burrow if disturbed. Soldier crabs eat the thin coating of detritus on sand grains. They scoop up sand grains with their pincers and use their mouthparts to sift out the food particles. They then discard the uneaten sand grains as little balls of sand which you can see on the sandflats.

After about 15 minutes of emergence the adult soldier crabs aggregate and simultaneously begin walking toward the water. This spectacular migration is termed the "trek". They periodically stop to feed during this migration but continue a directional march towards the water.

When they reach an area near water, they increase their feeding rate and start to move parallel to the beach. After they have finished their feeding phase they then tightly pack together and wander around the sandflat until eventually moving in a directional way towards the high tide mark before spreading out and then burying themselves.

The next time you are out and about on the sand flats of Moreton Bay, keep an eye out for soldier crabs and see if you can guess what part of their daily migration pattern they are on. But remember, if you can’t see them, they are probably just sleeping below the sediment waiting for their time to shine.

By Dr Daryl McPhee