It isn't an oocyte of a zygote! I am not sure what this is. It may be anything from an egg mass to a large (single-celled) protozoan. I do like the UFO in outer space feel of this image.
An individual from the plankton is a plankter.
There are many ways to catagorise plankton:
1) size: pico(0.2-2 um), nano(2-20 um), micro(20-200 um), meso(0.2mm-20mm), macro(>20mm)
2) nutrition: phyto(plants), zoo(animals) - however species that do both are not uncommon, plus bacteria and viruses are also very numerous members of the plankton!
3) life cycle: holo(whole life cycle as plankton), mero(part of life cycle as plankton)
4) systematic groups: major taxonomic groups
5) protozan and metazoan
So I work on metazoan holoplanktonic mesozooplankton, mainly crustacea. See how quickly we get lost in jargon. I prefer to say bugs!
Hope that all that useless info hid the fact I couldn't ID the blob!
Alex