UNITED STATES NATIONAL MUSEtTM BULLETIN 215 



In this connection, it may be noted that "weight- 

 increasing" additions are not uncommon in the plank- 

 tonic Foraminifera, a condition which would seem 

 anomalous were it not for the fact that in nearly aU 

 instances they only occxir in the later stages of develop- 

 ment, after the increase in size of the test would other- 

 wise have decreased the specific gravity and caused an 

 involuntary upward migration of planktonic species 

 adapted to greater depths. These "weight-increasing" 

 additions include the development of flanges and thick 

 walls, as in Sphaeroidinella dehiscens, a species Stubbings 

 (1939, p. 174) stated to occur most often in samples 

 from deeper water. He suggested this occurrence 

 might be due to the survival of their massive tests as 

 compared with those of more delicate species. It may 

 equally well be due to the environmental choice of the 

 species, and the development of the heavier test be 

 related to the depth at which the organism lived, not an 

 accidental character which merely allowed its preserva- 

 tion in the sediments. 



Other examples of weight increasing additions may 

 be the thickened walls of later chambers, found in 

 Pulleniatina obliqueloculata, and the marked decrease 

 in the size of the wall pores with increase in the size of 

 the test, also seen in Sphaeroidinella. The accessory 

 sheU structures or bullae, developed by the entire sub- 

 family Catapsydracinae, may be only apertural pro- 

 tection, but they also would increase the shell weight. 

 This would maintain a constant specific gravity in the 

 specimen with increase in size; interestingly, these 

 accessory features are not found in small or juvenile 

 specimens. 



Distribution of Planktonic Foraminifera 



The free suspension of pelagic animals favors their 

 wide distribution. In fact it has been stated (Chun, 

 1892, p. 120) that up to the present time no pelagic 

 forms have been discovered in either the Atlantic or 

 Pacific ocean which are not represented by parallel 

 forms in the other. 



Variations in environmental conditions are less fre- 

 quent and less abrupt in the open sea than in the shore 

 waters. Nevertheless conditions are not imiform and 

 pelagic life is accordingly not completely uniformly 

 distributed. These influencing factors are less com- 

 plex than in Uttoral areas where depth, type of bottom, 

 presence of fresh water, and high amoimts of suspended 

 sediments change rapidly with consequent influence on 

 the favma. In the open ocean the most important 

 factors are food supply, temperature, depth, light, 

 salinity, and quantity of suspended sediments. The 

 order of their importance is not certain. 



Food supply: The food supply of pelagic animals con- 

 sists of the plankton itself, the basic supply being the 

 plant portion of the plankton, or the single-celled algae 

 and diatoms. As the Foraminifera are dependent 

 upon the phytoplankton as a food source, which they 

 captiu-e by means of their radiating psuedopodia, they 

 are most abimdant where this food supply is at least 



periodically rich. In counts made in the Bay of Kiel, 

 the planktonic plant cells outntunbered the protozoans 

 by a ratio of 7 to 1 (text-fig. 1). The richest domain of 

 the plankton is the upper 100 meters of the sea water, 

 inasmuch as the plant element in it is dependent on 

 light, and the impoverishment of the plankton begins 

 below this level. 



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Figure 1. — Curves of volume of various groups of organisms in the total 

 plankton at Laboe, in the Bay of Kiel, during the year. (From 

 Hesse, Allee and Schmidt, 1937, after Lohmann). 



Aggregations of plankton also appear in certain areas, 

 especiaUy meeting-places of currents rich in plankton. 

 So-called "animal-streams" may appear in both open 

 sea and near coasts, sometimes with considerable regu- 

 larity. They form a veritable plankton soup and give 

 a smooth oily appearance to the surface of the water. 

 They may be dependent on wind and current; for 

 example, they appear twice a day in the harbor of 

 Messina (Haeckel, 1890, p. 85). Aggassiz (1892, p. 31) 

 reported these "winrows" of plankton, stating, "The 

 most extraordinary wim-ows I have met were off the 

 Tortugas, about 150 miles to the northward, where the 

 surface of the Gulf of Mexico for a whole day's steaming 

 swarmed with Globigerinae. It was a dead calm." He 

 considered them to occur along the track of the oceanic 

 ciu-rents. 



The composition of the plankton varies with time as 

 well as locality. For example the protozoans fluctuate 

 from a low ebb in winter to a high in late summer in 

 the Bay of Kiel, reflecting a similar seasonal fluctuation 

 in the numbers of diatoms and other phytoplankton. 



Temperature: The geographic distribution of the 

 animal communities of the oceanic pelagial is deter- 

 mined primarily by temperature. There is a sub- 

 division into oceanic communities typical of warm 

 water and those typical of cold water, roughly corre- 

 sponding to the tropical and subtropical areas on one 

 hand and the cooler waters on the other. These can 

 be further subdivided. Marine animals appear to 

 recognize an equatorial belt of water with a tempera- 

 ture above 25° C. as distinct from cooler tropical 

 waters lying on either side with temperatures from 20°- 

 25° C. Similarly, there are different communities in 

 the cold-water polar areas with temperatiu-es below 

 10° C. and those of the less cold waters with tempera- 

 tures between 10° and 15° C. The boundaries are not 

 sharply defined, and they may shift with the seasons, 

 but in general a distinction can be made. 



