WETMORE, Karen L., Museum of Paleontology, University of California, Berkeley, CA 94720-4780
Observations of responses to disruptions of normal growth patterns provide information on how mechanisms that determine chamber orientation and position may affect development in foraminifera.
Numerous asymmetrical individuals of Archaias angulatus were observed in populations from natural marine environments and in populations grown in laboratory cultures. Once the periphery of the test was damaged, successive chambers tended to reflect the shape of the damaged chamber rather than returning to the characteristic symmetrical, arcuate chamber form. Aberrant forms included bifurcated chambers, tubular forms, asymmetrical coiling, and one section of a chamber that grew back over the test while most of the chamber grew outwards. These forms appear to be the result of the way, when a new chamber is formed in Archaias angulatus, the reticulopodial network extends from the apertural face of the last-formed chamber to form the shape of the new chamber (Wetmore, 1997). Growth of a new chamber, and therefore test development, in this species is clearly contingent on the shape of the last-formed chamber.
In contrast, Heterostegina depressa regenerates normal-appearing tests even after major damage. In this species, portions of the test often get broken off. Regrowth starts with the addition of chambers, sometimes unconnected, that form in any notches in the periphery of the damaged test. Successive chambers return to a more normal arcuate form as a smooth outline is restored (Rottger and Hallock, 1982). Thus, in this species the mechanisms of chamber formation maintain the same plane of growth despite disruptions, and serve to maintain a smoothly curved outer edge to the test.
Repaired chambers were also observed in some smaller benthic foraminifera, where a hole had been covered over with a new section of test wall. In some cases, an aperture was constructed in the repaired section of a chamber other than the terminal one, but there was no indication that new chambers would grow out from this aberrant aperture.
Thus, in some species of foraminifera the final chamber appears to serve as a template for growth and irregular shapes resulting from damage are perpetuated, while in other species irregular test shapes resulting from damage are restored to a form similar to undamaged individuals.