Tuesday, June 7, 2011

Polar lobe in Nassarius fossatus

Nassarius fossatus is a marine snail. Snails, or gastropods, belong to the Spiralia - a large group of animals with spiral cleavage. Nassarius fossatus has unequal cleavage, which means that one of the cells at the two-cell and four-cell stage is larger than the others. The first four cells in a spiralian embryo are denoted as A, B, C and D. The D cell is the largest in unequal spiral cleavage. There are several mechanisms by which unequal cleavage can be accomplished. Nassarius does this via the so-called polar lobe, which is shown in these pictures. A polar lobe is an anucleated protuberance which forms at the vegetal pole during first, second, and sometimes subsequent cell divisions. It then fuses with one of the cells, making it larger than the others.

The top picture shows polar lobe formation during the first cell division. One can see two polar bodies. Polar bodies are the tiny sister cells of the oocyte which are produced during meiosis, contain discarded DNA and mark the animal pole of the embryo (up in the first three pictures). The opposite pole of the embryo is the vegetal pole. The two cells at the animal pole are the first two blastomeres. What looks like a third cell at the vegetal pole is the polar lobe, which at this stage is nearly completely cinched off from either blastomere. Subsequently the polar lobe fuses with one of the blastomeres (second picture from top), so that by the end of the first cell division one of the blastomeres (called CD) is noticeably larger than the AB cell (third picture from top). Polar lobe also forms at the second cell division (not shown). At the four-cell stage blastomere D is the largest, blastomere C is the second largest, while A and B cells are about the same size (bottom picture). The first three pictures are lateral views, while the bottom picture is a polar view. It is the first time I have heard of and observed unequal spiral cleavage, and I think it is remarkable. I also liked these eggs because the egg capsules they are laid in are very beautiful when viewed under the dissecting microscope (see picture by Janelle Urioste).


  1. I have stumbled upon your blog and find it most interesting! I have learned a lot as well since I'm new to most embryology terms and development types...I have been however, looking closely at a Nassarius (Reticunassa) species from Japan...Nassarius fraterculus. Have you heard of any publications dealing with this species in the US...have you spot it in Oregon?
    I have noticied that embryos developing with long light to dark cycles can grow very fast, faster than those in unlit incubators (19 degrees both treatments)...do you know of any case where this has been reported...examples on the effects of temperature are abundant but light effect on development rates is somewhat scarce...
    and...finally...i have seen some embryos from different capsules undergo development into pediveligers to finally escape from the capsule as crawling juveniles...but i have seem embryos (from different female capsules) retaining the veliger stage...however haven't been able to catch or see any freeswiming larvae in my trials so far...Im sure the capsules are all from the same species...have you heard of alternating developmental modes? direct-planktonic?
    Sorry for the long post...
    I have posted a video that shows the capsules and juveniles of this species...for the first time i think...(http://youtu.be/4PkmA82nujw)

    your feedback will be greatly appreciated

    all the best,


  2. This blog has been really helpful, since im kind of new in the Embryology field. However i have been following some capsules of Nassarius fraterculus and have noticied a few interesting things.

    Have you spotted this species in Oregon? Do you know any publications on the life history of this species?

    (http://youtu.be/4PkmA82nujw) this is a link to a video on some observations i have made so far...

    Have you heard or seen alternation of developmental modes? the embryos in these capsules develop directly but i have seen capsules from different females of the same species to retain the veliger stage...

    Your feedback would be greatly appreciated!