How It Began
The origin of these new finds begins back on the early morning of the 7th of January 2002 while walking north of New Brighton beach in Christchurch, in the South Island of New Zealand.
Many very faint tracks were found in the sand, running east to west – (or vice versa) – between the sea’s edge and the high tide margin. These marks in the sand were very similar to those of a Sand Scarab Larva’s though obviously not caused by these grubs, as the width was twice as wide in comparison.
Always these tracks led to, or emanated from, tumbleweed type densely rolled swathes of matted plant roots, comprised of gorse, broom, lupin amongst others, which had been deposited at the full tide mark and left high and dry.
(Those bundles of plant matter that are commonly found washed up after rivers have been in flood, after being ripped out, tossed, rolled, and eventually pushed out to sea to be distributed by the ocean currents along beaches.)
The recent weather patterns were such that the Waimakariri River, about 20 kms ( 12 miles) north of Christchurch had been particularly full from heavy rains far inland to the west towards the Southern Alps, while at the same time blustery northerly winds had been blowing, forming a ‘pushy’ swell which drove south, down the east coast towards New Brighton, creating strong continuous currents at the shore that dragged all material in its path southwards.
Following many of these faint tracks in the sand soon revealed they originated at the ‘tumbleweeds’ and traveled down the beach to the sea, an observation arrived at when it was seen towards the end of one of the ‘runs’, many seagull footprints, meaning of course that a meal of some sort had been provided, and a gourmet one at that.
Walking along the water’s edge finally unveiled the culprits as being crabs. The first one found had reached the edge of the wet sand down at the low tide zone, that part of a beach where waves expend the last of their mysterious energy; (where a thin film of sea water lies on the surface) and digging down, there it was, residing a little below the surface awaiting the incoming tide to cover it, and then no doubt be upon its way once more
Whether the crabs would remain around Brighton where they eventually rafted to, or somehow detect river water and gradually head south towards the nearest available fresh water, (the Avon and Heathcote river’s outlet at the estuary a few kilometres south) is open to conjecture I guess, as long as they were not scoffed by some fish or other on the way that is. Rafting crabs, come to think of it, is quite an effective method of species distribution.
The crabs were formerly identified as Hemigrapsus crenulatus (H. Milne Edwards-1873) by Dr Colin McLay, Associate Professor, Deputy HOD, Zoology Dept. of the University of Canterbury, and are normally a river/estuary creature, living in that mix of salt and river water.