Saturday, 12 November 2016

Baby Mole Crabs

My observations sometimes form the main on-line documentation of some species and the mole crab (Albunea symmysta) is one of those.  Today, I was standing on Holloways Beach near Cairns, North Queensland, Australia when I started seeing little split pea-sized creatures zipping at speed across the wet sand after each wave.  They would dive into the sand a moment before it dried.   Within a fraction of a second after the swash had retreated leaving only wet sand between waves, they were moving 15-20 cm directly across the face of the beach (sideways, not up or down) and always to the north which was against the direction of the littoral drift.  I would see one every few waves and occasionally I would see a few moving at a time, perhaps 50 in 5 minutes.  It took ages to catch one to see what it was as I would only just see then dive into the sand before the next wave raced in and over them.
Juvenile mole crab ~5 mm long, probably Hippa pacifica (click image to enlarge)

Ventral surface of same crab showing spear shaped tail tucked between claw-like legs
Mole crabs filter feed with their antennae so are very strange crabs. Not much is known about them.

I was seeing the baby mole crabs near the peak of a king tide, both in the morning at about 7-8:30 am and again about 5:30 pm.  I have not looked at other times.  This is perhaps only the second time I have seen them in 30 years of scanning the beach for interesting things.  The weather was almost still in the morning and a 10 knot breeze produced a small chop that was crashing on the beach in the evening.  Only one factor was unusual, the beach had a small erosion scarp that was being attacked by the waves and creatures of the swash zone may have been trying not to be buried by the sand washing down from the scarp.

These observations help to fill in part of the life cycle of this most elusive animal and if anyone can add to these observations, feel free to comment.   I am not sure of the species but Albunea symmysta is the only mole crab I know to be present in the local environment and Hippa pacifica occurs on Lizard Island to the north.  Mole crabs are also in Indonesia and in India, however most of the information from these countries is on how to catch them or cook them.  The crab in the photos was released alive.  A video taken in California of a different species provides an idea of what juvenile mole crabs are like in the field.