Every September since 2003, students in my senior class in freshwater ecology take net plankton samples from deep, soft-water Lochaber Lake, Nova Scotia, Canada. The lake is undergoing a transition to a more productive condition, characterized by intense blooms of Microcystis aeruginosa.
This year, students observed oblong, red inclusions within the bodies of copepod nauplii from the lake. I have not seen these before and I have no idea what they are. There were as many as 6 within the body of one copepod. You can see a couple of low-resolution photographs on my web site at http://people.stfx.ca/btaylor/WebPage/Barry%20Taylor.htm. Are these bodies familiar to anyone?
It looks like lipid storage structures. Copepods, adults as well as nauplii, are known to store lipids in droplets and oil sacs which are highly variable in size, color and abundance.
See this publication for informations:
http://www.int-res.com/articles/meps2006/307/m307p273.pdf
There may be several possibilities, but lipid storage is a good one. Parasites also cause some variable structures in copepods, and I suspect algal cells that haven't been digested could also.
I incline to the former hypothesis, but it should not be difficult to make a differential diagnosis. If it is not digested algae or parasites should be fairly simple extract them from the nauplia. Lipid storage structures are quite common in copepods, mainly from cold waters. I frequently observed similar inclusions in copepods from mountain lakes, but also from lowland lakes in fall-winter
Those are almost certainly lipid droplets. Their disappearance also supports that conclusion. When animals die, the lipid droplets typically start to leak out.
Thanks everyone for your assistance. The strong consensus is that we are looking at lipid droplets, and the paper provided by Dr. Azemar suggests the same thing. My curious students were relieved to hear your answers too.
I agree with Kalevi Salonen. Although the bad resolution does not give us any certainty about these red inclusions, their disappearance is a clue to suspect oil droplets.
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