The Living Foraminifera Lecture 3 The Living Foraminifera

The Living Foraminifera The micropaleontology is concerned with the dead shells of foraminifera and classification of fossil species proceeds upon the morphology of the hard parts or test. Foraminifera are single-celled animals (phylum Protozoa) and belong to the same class Rhizopoda distinguished by temporary extrusions of the protoplasm or pseudopodia. Foraminifera differ from Amoeba in that the pseudopods are fine and hair-like together to form a spreading, reticulate network, as well as in agglutinated or calcareous test.

Orbulina universa, a sand – sized single chambered test surrounded by delicate spines

The most primitive order, the Allogromida, the test is an organic, gelatinous sac-like structure, 1-10 µm thick, composed of glycoprotein and referred to as tectin. It is an internal structure surrounded by ectoplasm immediately external to the primary cell membrane which contains the dense so-like endoplasm. In advanced foraminifera such as the globigerinids, the organic layer becomes an inner lining to the calcareous test. Most structures occur in the endoplasm. The overall organization of the cell is controlled by the nucleus.

NUTRITION The food of foraminifera includes: unicellular algae, especially diatoms, other protozoans and small metazoans including crustaceans such as copepods. Solid food may be carried through the aperture and digested with the help of enzymes secreted by the lysosomes or partly digested in place.

REPRODUCTION A life cycle is termed heterophasic when it characteristically contains two different phases, or types of reproduction and maturation. Among some foraminifera there is this same alternation of an asexual followed by a sexual generation. The benthic genus Rosalina, reproduce only asexual in culture. The young gamonts with the larger proloculus are termed the megalospheric generation, while the individuals with smaller proloculi are called the microspheric generation. Thus the two morphologically distinct tests are termed dimorphism.

Dimorphic pairs are found among smaller and larger benthic foraminifera, but have not been recognized in planktonic genera. The microspheric generation with the smaller proloculus is termed B form, whereas the megalospheric phase (with larger proloculus) is called the A form. There are genera (Cibicides, Triloculina and Elphidium) in which a third generation commonly occurs, which is called A1, the second megalospheric generation.

The protoplasm of a foraminiferid comprises a single cell differentiated into an outer layer of clear ectoplasm and an inner layer of darker, coloured endoplasm. The ectoplasm surrounds the test and gives rise to numerous thread-like (filose) or branching (reticulose) pseudopodia along which the food material is drawn and the debris expelled.

Foraminiferid life history (diagrammatic)

The complete cycle for Elphidium crispum takes two years in the shallower parts of the English Channel, although it may be delayed at deeper stations. Asexual reproduction reaches a peak in April. All the protoplasm leaves the test and chambers are added at the rate of about two per day. Sexual reproduction begins early in the second spring as temperatures begin to rise. The gametes conjugate outside in open sea to produce zygotes and the B form then develops and matures during the second summer.

Life cycle of Elphidium crispum (duration 2 years)

Alternation of generations in Myxotheca arenilga after Grell (1973) Alternation of generations in Patellina corrugata after Grell (1973)