Rafting Sand Crabs

I was on the outer breakwater of the Cairns Marina when I observed that some of the floating mangrove leaves and twigs had tiny sand crabs riding on them. In about a ten minute period, I counted approximately a dozen small sand crabs and was able to photograph many of them. Being close to dark, it was at the limit of the cameras performance to focus on the jostling flotsam and only a few photos were really clear. I thought that possibly the presence of the floating marina fingers or the concrete breakwater may have been providing a novel habitat that the crabs were exploiting in a novel way, however went I visited a remote part of the outer harbour (near Second Beach) mangrove leaves with rafting crabs floated past my boat from a direction where no man-made structures were present. Clearly, this not an irregular behaviour. To my surprise, I even had photos of rafting crabs from almost the same day, one year before.

Sand crab rafting on a mangrove seed (Aug 2016)

Crab rafting on a mangrove leaf (Aug 2017)

Sand crabs (Portunus pelagicus) or blue swimmers as they are also known, have a planktonic larval stage so do not need to raft for dispersal. The one potential reason that I have found for small crabs dispersing using floating mangrove litter is to possibly to find more suitable habitat. The small crabs have a preference for intertidal habitat over subtidal habitat and prefer seagrass beds to open sandy or muddy substrates. Perhaps rafting provides sand crabs that settled in poor habitats a way to chance relocating to a better habitat. A search of scientific papers on the Internet reveals that whilst significant research has been undertaken on planktonic larval dispersal and that little is known about post-larval dispersal. It is known however that the crabs somehow actively select their preferred habitats and are in low densities outside these habitats.

A crab on fine seagrass leaves

The rafting crabs are usually quite small - this one was caught in seagrass - 21 July 2017

Larger crabs can also sometimes be seen swimming at the surface and on the day that the tiny crabs were seen rafting at the outer breakwater, one full size crab which spanned approximately 30 cm from nipper to nipper was cruising back and forth in a patch of light from a street light on the breakwater.   

The tiny crabs often swim from leaf to leaf