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Sunday, April 28, 2013

A long way to go

A few days ago Chris wrote to report extremely large numbers of crab larvae swimming in Tomales Bay.  Several weeks ago we heard that researchers were catching record high numbers of crab larvae in Bodega Harbor.  Today I saw drift lines of crab larvae molts on Salmon Creek Beach.  So, here we go.  

If you've eaten Dungeness Crab (Cancer magister), or have encountered adult crabs, but haven't yet seen one in the larval stage before, this is what a Dungeness Crab looks like when it's swimming around in the ocean before it undergoes metamorphosis and becomes a juvenile crab that lives on the bottom:


This is a Dungeness Crab megalopa.  It's quite large for a larval crab.  In the photo above, the length from the eyes to the base of the prominent rear spine is about 8-10 mm.  At this stage it still has a long, narrow abdomen.  In the photo above, the abdomen is tucked up underneath the body.  But when the crab is swimming, it's extended out behind the crab (see below).


And here's a nice close-up:


The megalopa is the final larval stage.  When the megalopa molts, it goes through an amazing transformation (like emerging from a magician's hat!) and becomes a juvenile crab:


The carapace (back) of this juvenile was only ~1 cm across.  It has a long way to go before becoming legal size.  In this area, a legal size Dungeness Crab is 5.75 inches (14.6 cm) carapace width.  It will take a crab approximately 3.5-4 years to reach that size.

The basic reproductive cycle for Dungeness Crabs looks something like this:

- adults mate during April-September
- females with eggs from November-February
- eggs start hatching in December, peak in March
- larvae spend 3-5 months in the plankton passing through several stages (1 protozoea, 5 zoeal, 1 megalopa)
- metamorphosis to juvenile benthic crabs during April-June
- approximately 1.5 years (11 molts) to sexual maturity (carapace width 10 cm or 4 inches)


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