PHYTOPLANKTON 



the water (i.e. they are heterotrophic), or on dissolved organic food. It is 

 probable that some of the bottom-living forms, which have no chlorophyll, 

 feed only in this way, but many of the planktonic ones can probably feed in 

 both ways. The planktonic dinoflagellates can thrive in waters with less 

 nutrient salts than can the diatoms so that they are generally most abundant 

 after the diatoms die out, and indeed the heterotrophic ones may be partly 

 utilizing the disintegration products of the diatoms — but more about this 

 succession of species in Chapter 9. Like the other plants previously mentioned 

 they reproduce by simple division, but the two new cells nearly always break 

 away so that they conthuie to live singly. Sometimes, however, in the genus 

 Ceratiiim, they remain attached to each other to form a chain when they are 

 said to be 'in catena'. 



Many dinoflagellates arc phosphorescent, but as they are so small and 

 often so numerous, the light they emit is like an undefined 'cloud of light' 

 rather than the brighter easily defined pinpricks of light caused by a number 

 oi planktonic animals. One that deserves spiccial mention in this connection 

 is called Noctiluca sciiitillaiis. It is an unusual dinoflagellate, so swollen that it 

 is almost spherical, relatively common, and about 1/15 inch across with the 

 typical dinoflagellate part a mere fraction of the total (Plate IX). This is a 

 producer of the well-known phosphorescence in the sea and hence its name. 

 It is entirely heterotrophic and it feeds on diatoms and the smaller animals 

 in the plankton which can sometimes be seen whole inside it. 



Other members of the phytoplankton are the threadlike filaments of 

 some of the blue-green algae (Cyanophyceae or Myxophyceae) such as 

 Oscillatoria and Aiiahciui which are found more often in inshore waters than 

 in the open sea, and indeed the members of this group are much more 

 typically freshwater than marine. Two other important marine organisms of 

 the phytoplankton are Pluicocystis a member of the Yellow-green Algae 

 (Chrysophysae), and Halosphacra one of the Xanthophyceae. Halospliaera is 

 a bright green sphere, big enough to be seen by the naked eye as it is the 

 size of a pin's head; it is an oceanic species often brought into the northern 

 North Sea in great numbers in early spring. It does not just divide into two 

 as do so many of the other species of phytoplankton, but the living tissue 

 inside divides and redivides until the whole sphere is full of minute spores 

 each with flagellae. These burst free and swim about — as members at the 

 nanoplankton — until they eventually grow into new spheres. Pliaeocystis 

 (Plate IX) is similar in many ways but instead of being a very rigid sphere it 

 forms a large gelatinous capsule of no particular shape. Numbers of these 

 stick together to form a gelatinous mass, and if this is sufficiently dense 

 makes the water unpalatable to herring which may change their course to 

 avoid patches of it. Sometimes it is abundant near the shore and becomes 



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