It's been a while since we last had a foram post, so why don't we have one today? Ladies and gentlemen, I present to you: the Glabratellidae.
Glabratellids are a group of forams found in littoral habitats, first appearing in the fossil record in the Eocene (Loeblich & Tappan 1964). They secrete a calcareous test with a hyaline (glass-like) microstructure. By foram standards, glabratellids can be quite small: the smallest are well under 100 µm in diameter. They have a trochospiral body shape—that is, the body chambers are arranged in such a way that they spiral like a trochus or top shell—with a flat base. At the centre of the underside is an aperture or umbilicus. The spire may be fairly low, giving them what I always think of as a 'jelly mould' shape, or it may be high so their overall appearance is conical. In the genus Schackoinella, the test bears a spine on the outside of each of the body chambers.
The most distinctive feature of glabratellids, perhaps, is their life cycle. We know the life cycles of relatively few foram species but as a rule they show a clear alternation of generations, with both well-developed haploid and diploid individuals. Haploid individuals (gamonts) produce gametes by nuclear mitosis that fuse to form zygotes that grow into mature diploid individuals (schizonts or agamonts); these latter produce haploid embryos via meiosis. The two generations may differ somewhat in appearance, and many foram species have had their gamonts and schizonts mistaken in the past for separate species. The most consistent difference between generations in all chambered forams is that the gamonts have a larger first chamber as a result of growing from larger embryos than the schizonts. In glabratellids, gamonts are also smaller and relatively higher-spired than schizonts, and the former are sinitrally coiled (to the left) while the latter are dextrally coiled (to the right).
As the gamonts themselves reach maturity, their thoughts no doubt turn to their own posterity. Whereas in some other forams the haploid generation simply releases their gametes into the water column to find their own way to fusion, sexual reproduction in glabratellids is a somewhat more intimate affair. Mature gamonts form into pairs, joined to each other via their umbilical surfaces from which they will resorb the test. Locked in their embrace, the pair become cemented to the substrate. Gametes, again, are formed by the production of plasma membranes around individual nuclei; these gametes move by means of three flagella instead of by pseudopodia. The two parents exchange gametes of which only about a tenth fuse to form zygotes; the remainder provide a food source for their developed siblings. Again, the young schizonts grow to about two or three chambers in size before being released by the dissolution of the cement holding the parent tests together.
This cosy mode of reproduction means that glabratellids may have the potential for greater population differentiation than other broadcast-spawning Foraminifera. Tsuchiya et al. (2003), in a study genetic diversity in representatives of the genus Planoglabratella collected around Japan, found evidence for cryptic speciation in P. opercularis. Some individuals of this 'species' were closer genetically to individuals of another species P. nakamurai than to other P. opercularis, and closer inspection revealed certain details of their morphology that were more nakamurai-like than opercularis-like. It may be that we have underestimated the diversity of glabratellids, and many more species of this group remain to be discovered.
REFERENCES
Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina: chiefly "thecamoebians" and Foraminiferida. The Geological Society of America, and The University of Kansas Press.
Tsuchiya, M., H. Kitazato & J. Pawlowski. 2003. Analysis of internal transcribed spacer of ribosomal DNA reveals cryptic speciation in Planoglabratella opercularis. Journal of Foraminiferal Research 33 (4): 285–293.
How many nuclei do forams have, typically?
ReplyDeleteAs many as it takes, really. Forams are coenocytic: that is, they have multiple nuclei in a more-or-less continuous mass of cytoplasm. That's how they're able to be so complex and relatively large.
DeleteYes - but what sort of numbers do we speak of? Would a hundred be a lot? Would ten thousand?
ReplyDeleteI've been looking this one up, and the answer (as with everything with foram cell biology, it seems) is... it's complicated. Simple forams (those without multiple chambers) have only a single nucleus, as do most gamonts (the sexually reproductive generation) of more complex forams (presumably said nucleus begins to multiply just before gamete formation). Schizonts (the asexually reproducing generation), in contrast, are often multinucleate. There is often nuclear dimorphism similar to that found in ciliates, with large somatic nuclei that organise the cell functions during the vegetative period but break down at the reproductive stage, vs small generative nuclei (Glabratellide are among the forams showing such dimorphism). Metarotaliella, which I think is a fairly average sort of chambered foram, usually ends up with thirteen nuclei at maturity: one somatic nucleus and twelve generative nuclei.
DeleteLarge forams seem likely to have more nuclei. In xenophyophores, the nuclei are numerous (I haven't found an estimate of just how numerous, and I suspect that the question would be somewhat meaningless if the individual simply produces new nuclei relative to growth) and what are likely to be somatic and generative nuclei are both present in numbers. We know very little about xenophyophore life cycles, but gametes have been observed developing in special side-branches of some individuals, suggesting that the gamont generation (if they have alternating generations) is also multinucleate rather than uninucleate as in other forams.