Why Do Nummulites Have Large Tests?

[matthyaouw]

I have recently learned that single celled foraminiferas such as nummulites are capable of growing tests upto 160mm across. Can anyone suggest what advantage such a large test would provide for a cingle celled organism? One thing that really puzzles me is why the organism devotes the energy to creating so many new body chambers, when one is barely bigger than the last. Having so many body chambers of such similar size is not common in all foraminiferans, so why in nummulites?
I have tried to google this, however all I am able to find is basic descriptions of the species, and things about nummulitic limestones and palaeoclimatological evidence gathered from foraminiferans.

Below is a picture of a nummulite:
http://www.ut.ee/BGGM/eluareng/foramin3.jpg

(Fact of the moment: The great pyramids of giza are constructed almost entirely from fossil nummulites)

 

[matthyaouw]

No one care to speculate?
I had a thought a day or two ago- It could be a deterrent to predators. Such a large shell would deter the average filter feeder, and with the organism probably taking up less than 1% of the shell, it would be barely worth eating the whole shell for the tiny amount of nutrients avaliable, and the organism itself would be quite difficult to extract from the shell and eat.

 

[Moonbear]

It would ONLY be speculation. Your thought about it being a deterrent to filter feeders sounds plausible. Another related possibility is it keeps it from floating away (foraminiferans usually get carried along with the current, which is why they get gobbled up so easily by the filter feeders), and if it blends in with the pebbles on the sea floor, even better.

 

[DocToxyn]

My thoughts on the predator avoidance theory brings up a few issues. Is this large size attained after a "lifetime" of building or does the foram immediately invest in such a large structure? If it only reaches this size at later stages in its life (perhaps even post-reproductive), it seems less likely that it is done in order to deter predators. Such a tactic would be better employed early in life so that the organism has a better chance of reaching reproductive age and passing on its genes. Or perhaps it employs other tactics early on?

As far as published theory I found this abstract proposing that the compartmentalization may play a role in cytosolic metabolism.

also this citation: Ungaro, S., (1994) Nummulite morphological evolution
In In: Matteucci, et al. (eds.) Studies on Ecology and Paleoecology
of Benthic Communities, Boll. Soc. Paleont., Ital., Spec. Vol. 2,
Mucchi, Modena
Volume:
Pages: 343-349

neither are in available in my closest library...darn medical books take up all the space .

 

[Moonbear]

DocToxyn, can you check that link? It doesn't take me to an abstract, but to a publisher's homepage. It seems to indicate my university is registered (very oddly, using your link, I get a page that tells me what my university username and password for the site is...usually I either link into things directly without a site telling me how the university is registered, or I have to call the reference desk for a password if the publisher wants to be more restrictive of access). Or, just post the citation without abstract and I can locate the abstract myself.

 

[DocToxyn]

Sorry, I got to that page from a google search and somehow got past the publisher hompage to the abstract. The link in my reply did take me to the hompage as you said, so I don't think it had to do with some subscription my university has...anyway here's the citation and abstract

Journal: Lethaia
Publisher: Taylor & Francis AS, part of the Taylor & Francis Group
Title: Multicellular-like compartmentalization of cytoplast in fossil
larger foraminifera
Author: Carles Ferràndez-Cañadell
Issue: Volume 35, Number 2 / June 01, 2002
Pages: 121 - 130

Foraminifera are usually between 0.1 and 1 mm in size, thus falling within the range of the largest eukaryotic cells. However, some fossil and extant foraminiferal species reach diameters of more than 100 mm. One hypothesis of how these gigantic sizes could have been attained by these unicellular organisms is the temporary compartmentalization of cytoplasm into smaller volumes of effective metabolism, as reported for several recent species. Evidence of this phenomenon is shown in fossil genera of larger foraminifera belonging to five families of Cretaceous to Oligocene age. Alternative interpretations are discussed.

 

[matthyaouw]

I've not had much time to look into this yet with some uni deadlines that were somewhat pressing, so hopefull I'll do some more research over the weekend.
One thing I have learned however is that I was incorrect about the size of the organism. Forams actualy occupy every body chamber they have at once, rather than just the newest and largest one.