STUDIES nsr foraminifera 



small or microscopic and all pelagic microscopic animals 

 are plankton (Hesse, Allee, and Schmidt, 1937, p. 233). 



Special Characteristics of Planktonic Organisms 



"Living matter is heavier than sea water; its specific 

 gravity ranges from 1.02 to 1.06, averaging about 1.04. 

 Special adaptations are consequently required to pre- 

 vent animals from sinking. This distinguishes pelagic 

 creatures from animals of the benthal and gives them 

 certain features in common; these appear in various 

 groups by convergent evolution". (Hesse, Allee, and 

 Schmidt, 1937, p. 223). 



The state of suspension may be brought about either 

 by a reduction in specific gravity or by added resistance 

 offered to the water by the animal. 



Reduction in specific gravity: This may be ac- 

 complished by economy in use of skeletal material. 

 According to Rhumbler (1911), Orhulina universa from 

 surface waters has a thin shell with walls from 1.28^ to 

 18m, whereas specimens from the bottom have walls up 

 to 24;u in thickness. The planktonic Glohigerinas of the 

 surface waters are distinguished by thin-walled shells 

 from the smaller cold-water species, such as Globigerina 

 pachyderma, which may live at greater depths. The 

 amount of calcium carbonate in the shells is also re- 

 duced in various genera and species by an increase in 

 size of pores, by enlargement of the aperture, or by the 

 development of supplementary apertures. 



Specific gravity of planktonic organisms may also 

 be reduced by taking up relatively large amounts of 

 sea water, as is done by jellyfish. The absolute surplus 

 of weight remains the same, but the relative difference 

 is reduced by an increase in the volume of the organism. 

 Invertebrate marine animals may take up water from 

 their surroundings without injury since then- body 

 fluids are isotonic with sea water. Storage of fighter 

 materials is an even more effective method of weight 

 reduction used by some planktonic organisms. This 

 would include internal storage of water of less salinity, 

 of fat globules or even air bubbles. These various modi- 

 fications of the protoplasm represent a possible field for 

 research in the Foraminifera, for as yet no data are 

 available as to possible differences in the composition 

 of the protoplasm in planktonic and benthonic Fora- 

 minifera. 



Added resistance to sinking: Increased friction 

 with the water and resistance offered by the surface is 

 obtained by increase in size in the horizontal plane of 

 a sinking body. This method is most effective for small 

 animals, such as Protozoans, which have a high value 

 of surface-weight proportion. This may be accom- 

 pUshed by a flattening of the body itself, as in the de- 

 velopment of a radial test, with elongate or clavate 

 chambers, or by the development of lateral projections, 

 such as the spines so characteristic of the Orbulinidae. 



Emifiani (1954, p. 153) stated: 



The capacity of a certain foraminifer to live in a water of 

 certain density depends obviously, upon its specific weight; this, 

 in turn, depends upon (a) the specific weight of the protoplasm 



and its inclusions, (b) the specific weight of the test and (c) the 

 ratio of the mass of the protoplasm and inclusions to the mass 

 of the test. If the first two factors are assumed to be roughly 

 constant for all species, the important factor appears to be the 

 third one; i. e., the ratio of the mass of protoplasm and inclusions 

 to the mass of the test. For a given locality, species in which 

 this ratio is the largest will prefer shallower habitats, while 

 species with a smaller ratio will occupy deeper habitats. . . . 

 If the specific weight surpasses a certain limit, which depends 

 upon the density of the water, the foraminifer may not be able 

 to live within a reasonable distance from the surface and may 

 find itself in a zone too deep for efficient nutrition . . . muta- 

 tions of pelagic species toward a decrease of the ratio mass of 

 protoplasm to mass of test are more probably deadly, as are 

 mutations of benthonic species in the opposite direction. 



A foraminiferal species will change its depth habitat during its 

 lifetime if growth processes modify the ratio above mentioned. 



Samples of various species were checked by size groups, 

 and only Orhulina universa showed an appreciable differ- 

 ence between the size groups. This is (p. 154) "ex- 

 plained by the fact that in this species, while the mass of 

 protoplasm increased proportionally to the cube of the 

 diameter of the test, the mass of the test increases 

 proportionally to only the square of the diameter, 

 the thickness of the wall remaining approximately 

 constant. Therefore the animal grows progressively 

 lighter and progressively migrates toward the surface". 

 Some theoretical assumptions could be made on this 

 basis. It could be stated that there is a mechanical 

 sifting, that is, a movement of the animals to the 

 depths at which they can maintain themselves. It has 

 been demonstrated that there is such a vertical sorting 

 according to size in the radiolarians, with smaller 

 species in the warmer surface waters, larger ones below. 

 In part the reverse is true of the Foraminifera, due to 

 the difference in development of the organism, for the 

 larger the specimen the greater the mass ratio of proto- 

 plasm to test. 



It will be noted that the species restricted to the sur- 

 face waters are those in which the chambers increase 

 rapidly in size as added, and which have consistently 

 thin shells, large primary apertures and, in the case of 

 Globigerinoides, numerous secondary openings as well. 

 This shows a distinct correlation between the characters 

 of these species and the adaptation necessary to main- 

 tain them in the surface waters they prefer. It would 

 suggest that fossil species with similar appearance, 

 probably inhabited similar levels in the ocean. The 

 converse is true of Orbulirta which apparently can live 

 equally well in the higher layers which its increased size, 

 and therefore decreased specific gravity, causes it to 

 occupy in its later growth. EmUiani stated that the 

 shell wall of the specimens he examined remained con- 

 stant in thickness throughout development. However, 

 in many samples one can find specimens of Orhulina. 

 with many concentric layers developed, suggesting that 

 some specimens of this species had increased the spe- 

 cific gravity by an addition of shell material and thus 

 regained the lower environmental zone. Rhumblers' 

 comments cited above on the relative thickness of wall 

 of surface specimens of Orhulina universa and those 

 from the bottom tend to bear out this supposition. 



