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Saturday, 4 November 2017

Dive IV in Puerto Profundo

Yesterday in another of our Puerto Profundo dives, we came upon an old friend who has perfected a most fascinating disguise. We seldom see this character - it's only when a patch of background material starts moving in a certain way, and I pick it up and replace it on a very different background that Larry's able to get a photo. We don't know whether the disguise has been adopted as protection, but we think he/she are quite probably the predators. The disguise is used to keep their prospective dinners from suspecting that there's Danger nearby.

It's a crab species known as 'eurypodus latreilli' in Patagonia and as 'oregonia gracilis' in northern waters. Its common name is the Graceful Decorator Crab. It doesn't hurt to move him to another location. In the book Marine Life of the Pacific Northwest Andy Lamb states: "using virtually any material, this fastidious crab rips off pieces of its environment and then attaches them to its body. In captivity, if a specimen is moved to a different background, it will remove its old wardrobe and re-festoon."

If you like to walk ocean beaches and have that opportunity, every summer you may come upon all sorts of crab shells. Don't worry - if they have no meat on them, these are just abandoned shells that have been outgrown by bigger crabs. Of course, hard shells get too small as crabs grow, and they cast them off through a process called 'molting'. As GJ Jensen has written in 'Pacific Coast Crabs and Shrimps' "molting is a remarkable process. A molting crab leaves behind an exact and usually intact replica of itself, down to the surface of its eyes and gills and even the lining of its stomach".

If you see an abandoned crab molt, just imagine the complex manoeuvres the animal went through to get out of his shell. These shells are often used like recycled clothing by other underwater crustacean species. The propagurus fusitriton - pictured here - is wearing (in this case) a recycled fusitriton cancellatus shell.

propagurus gaudichaudi hermit crab 
A miracle of intricate timing occurs when a parasitic little arthropod living under the shell of a crab needs to grow a larger shell of its own. All shrimps and crabs (crustaceans) go through the molting process as they grow. So the parasite has to go through exactly the same growth cycle - losing a lot of musculature in its claw, building up a new exoskeleton under the old one - and timing all this to coincide exactly with the host crab's time of molting. Two co-joined arthropodic inmates I knew at the Wet Lab in the Vancouver Aquarium must have gone through this process together.

Since the molt itself takes very little time, preparation and execution for both partners in this duet has to be as perfect as a sonata performance at Carnegie Hall.


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At 2017-11-04 10:00 (utc) our position was 52°40.92'S 073°46.30'W

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