Taxonomic Significance of Pseudopodial Development in Living Planktonic Foraminiferida: ABSTRACT

Abstract

Axopodial development in the Foraminiferida is of prime importance taxonomically. Classification revisions End_Page 642------------------------------ proposed recently by Bovee and Jahn designate axopodia to separate sarcodines at a subclass level. Yet these are only one of several types of pseudopodia seen in planktonic Foraminifera. Living Globigerinidae collected off the southern California coast were observed in agnotobiotic cultures for periods up to three months. Healthy globigerinids exhibit a gradual change in the morphology and function of their pseudopodia which is related to the development and secondary calcification of the whole organism. The pseudopodia differ considerably from those of benthonic Foraminifera, except in the bidirectional protoplasmic movement. Newly formed pseudopodia are short, radiate, anastomosing, and motile, emanating from the sarcode through the test pores. Development of an internal axial core greatly lengthens individual pseudopodia, thereby increasing exposed protoplasmic surface, and providing support for flotation of the organism. The initial axial core is extremely flexible (probably entirely proteinaceous), resulting in tenuous pseudopodia which anastomose distally on contact with one another. Such pseudopodia are readily regenerated following loss by an actively metabolizing young foraminifer. During development, the pseudopodia increase slightly in width and become brittle as calcification hardens the inner core. At this stage, pseudopodia may be broken by handling, rather than tangled as in young specimens. Anastomosing occurs only in a few specimens by means of slender, temporary connections near the pseudopodial tips. There the protoplasm and granular contents commonly flow undisturbed beyond the tapering end of the solidified core to form an actively motile, retractable probe. The pseudopodial granular contents range from numerous large, angular particles in new pseudopodia, to relatively few regular and flattened shapes after calcified cores have developed. Calcification increases pseudopodial core diameter while decreasing pore-opening diameter. This restricts the flow of protoplasm along the pseudopodia, and limits the size and shape of transported particles. Finally, as the axial cores become thicker and heavier, globigerinid pseudopodia no longer anastomose. The cores, originating in protoplasm, are trapped by their expansion in the gradually closing pores to form the spines referred to in the literature. The pseudopodia of planktonic Foraminifera thus originate as specialized rhizopodia, which develop into axopodia as growth of the organism proceeds. With such a pseudopodial continuum occurring within one species, classification based on the pseudopodial differences outlined by Bovee and Jahn would cut across specific life cycles. Accordingly, the proposed taxonomy is considered inadequate with regard to the Foraminiferida. End_of_Article - Last_Page 643------------