ARTICLE 132    2015

COCKLE QUESTIONS

by Jenny Edwards

 

Last year ABC south-east radio’s Rural Report featured a segment on local fisherman, Allan Broadhurst who catches and sells three species of marine cockles.  At a talk organized by the Nature Coast Marine Group in May research student Penny Beaver of Southern Cross University, Marine Ecology Research Centre, presented her work on the age and growth of these three species. 


Just looking at a cockle you wouldn’t think there was much to find out about them.  Just two shells that close together around a bit of fleshy stuff which a growing number of people find very tasty.  However, as Ms Beaver explained, there are more questions than answers. 


Mr Broadhurst’s low scale operation has harvested these cockles for the past 14 years.  The Flamed (or Gray’s) Dog Cockle (Glycymeris grayana) grows to approximately 5.5cm and has a thick smooth shell with brown zig-zag colouration..  It is the shell most of us come across when walking along our beaches.  The Strawberry Cockle (Notocallista kingii) grows approximately to 4.4cm and has a smooth pink shiny shell with radiating darker rays.  Largest is the King Island Crassatella or Thick-shelled Clam (Eucrassatella kingicola) which grows to about 7.2 cm and is dark brown.  The shells of juveniles have pronounced ridges but they gradually disappear towards the outer edges of larger specimens.  These shells can be completely white in older individuals. 

 

The age of the cockles can be estimated from internal bands within the shell – a bit like tree rings - but the bands may or may not be formed annually.  The denser lines within the internal shell generally occur in winter when food is scarce and the water is at its coolest.  These slow down the metabolism and growth of individuals.  The study found individuals of the Flamed Cockle believed to be 30 years old, King Island clam 28 years, and theStrawberry Cockle 21 years.  A close relative of the Flamed Cockle (Glycymeris glycymeris) harvested around the Isle of Mann has been found to live to 107 years! 

Of the local species the Strawberry Cockle is the fastest growing, especially for the first four years, but all of them grow more slowly as they age. 
The cockles occur in sandy “beds” of about 100 square metres at depths between 30-40 metres.  Only where they are most numerous would it be viable to harvest them.  So far there are no answers to the questions of what makes the most suitable beds; how the cockle larvae know to settle there; or what microscopic algae they like to eat. 

 

From observation of the locally caught cockles they are spawning during the warmer months from April through to May.  The cockles are either male or female and release their eggs and sperm into the water simultaneously – what triggers this?  The fertilized eggs develop through a planktonic stage before settling to the bottom.  When they are small they can move around a little, much like a pipi does with its muscular “foot”.  But as they grow and the shells get heavier they are more confined and simply bury deeper when the conditions are not to their liking. 

 

So even when the fisherman returns to a known bed they can be buried too deep for the dredge to collect.  It seems even a cockle can be “the one that got away”! 

 

Cockles

L-R Flamed Cockle and young Strawberry and King Island cockles

 

Photo - Jenny Edwards