When is a clam not a clam?

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NikonUser
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When is a clam not a clam?

Post by NikonUser »

When it is an Ostracod!
These 2-shelled Crustacea, also known as seed-shrimps, were common in a recent FW sample.
They range in size, the largest in this sample is 0.60 mm long.
Under polarized light the shell can show several different colours.
Olympus BH2/BHS, SPlan 10x + 1.25x intermediate tube with 530 nm wave plate + 2.5x protection lens, D810; Zerene stacks
Attachments
5iv21 stracod plate.jpg
NU.
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.

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micro_pix
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Re: When is a clam not a clam?

Post by micro_pix »

Excellent! I hadn't ever considered looking at them under polarised light. I still have some empty shells so I'll have a look at those.

Dave

micro_pix
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Location: Southampton, Hampshire, UK

Re: When is a clam not a clam?

Post by micro_pix »

Interestingly the marine ostracod shells didn’t really show any colours under cross polarisation.

Dave

NikonUser
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Re: When is a clam not a clam?

Post by NikonUser »

Dave,
Perhaps your shells were too worn. My FW shells were from very fresh, some alive, specimens. My text says the shells are composed of a low-magnesium calcite exoskeleton (carapace) with an outer covering, cuticle, of chitin.
My guess is that it is the chitin that 'responds' to polarized light.
Perhaps marine species lack chitin.

Tony
NU.
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.

Nikon camera, lenses and objectives
Olympus microscope and objectives

Pau
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Location: Valencia, Spain

Re: When is a clam not a clam?

Post by Pau »

NikonUser wrote:
Mon Apr 12, 2021 3:03 am
My text says the shells are composed of a low-magnesium calcite exoskeleton (carapace) with an outer covering, cuticle, of chitin.
My guess is that it is the chitin that 'responds' to polarized light.
Perhaps marine species lack chitin
...
Calcium carbonate (both calcite and aragonite, being the later more usual in shells) are anisotropic, so both can produce interference colors, depending of the orientation and thickness.
Fibrous organic compounds like cellulose or chitin also can be optically anisotropic if there are differentiated orientation of the chains.
Both can contribute to interference colors under polarized light, IMO.
Pau

micro_pix
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Joined: Fri May 11, 2012 12:05 pm
Location: Southampton, Hampshire, UK

Re: When is a clam not a clam?

Post by micro_pix »

Maybe it’s the structure, even when broken the thin edges only showed the slightest hint of colour. The shells I had were closed and empty, I have no idea how old they are. I have photographed Ooids (calcite sand grain) under x polars and they where bright white against a black background - so anisotropic but no interference colours. I just found this on a geological website “ Minerals with very high birefringence -- such as calcite -- show such weak colors that they may appear "pearl" white”. So maybe that’s what’s happening and the chitin on the fresh specimens is producing the interference colours.

Dave

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