alg.ne, bacteria, and a few fuiifji. The bacteria are 

 largely ffrif>li\lir. in that they are attached to the 

 surfaces of floating ])lants, animals, and to ])articles 

 of organic detritus. Very few occur freely suspended 

 in the water (Harvey 1935). Bacteria occur at all 

 de|)ths but are especially abundant in or close to the 

 bottom. They are generally more numerous in the 

 winter than in the summer. 



The green phytoi)lankton is composed prim.irily 

 of diatoms, dinoflagellates, and small uiiarmored 

 flagellates, but several other kinds of algae are pres- 

 ent and occasionally important. Tiie dinoflagellates 

 Xocliluia and CfnUiiiiii are luminescent and in some 

 regions may give a glow at night to the entire sea. 

 Bioluminescence is not limited to these organisms, 

 however, but occurs also in various forms of bac- 

 teria, radiolarians, sponges, coelenterates, cteno- 

 phores. nemertineans, worms, crustaceans, brittle- 

 stars, moUusks, balanoglossids, tunicates, and fish 

 (Harvey 1952). 



The most important groups of protozoan zoo- 

 plankton, other than the green flagellates which are 

 usually considered with the phytoplankton, are the 

 rhizopod Foraminifera, the actinopod Radiolaria, and 

 the ciliate tintinnids. They may be enormously 

 abundant at times. 



Among the Coelenterata, many hydrozoans have 

 medusae and larval floating stages in their life cycle, 

 but only the siphonophores, the best known example 

 of which is the Portuguese man-of-war, are pelagic 

 throughout their life cycle [ Iwlopclagic }. The true 

 jellyfish of the class Scyphozoa are often conspicuous 

 and ctenophores of the related phylum are often 

 abundant. Some of these forms are so large they are 

 called iiiacroplankton. 



The various phyla of worms are represented in 

 the plankton by only a few forms, of which the 

 chaetognath Sagitta or arrow worm and the poly- 

 chaete Tomoptcris are often abundant. Many benthic 

 worms, however, produce larvae that are temporarily 

 part of the plankton {meropelagic). 



Many molluskan and echinoderm species are 

 meropelagic and it is by means of their larvae that 

 heavy, slow moving benthic forms become widely 

 dispersed. Two groups of snails are holopelagic : the 

 heteropods that inhabit tropical and subtropical 

 waters and the pteropods which occur in cold waters 

 and are important food for whalebone whales. 



Crustaceans form one of the principal groups of 

 the net plankton, and of these the holopelagic cope- 

 pods are by far the most abundant (Digby 1954). 

 Calanus finmarchictts is one of the most noteworthy 

 species and is the principal food of the commercially 

 important herring fish. Other important crustaceans 

 that enter the plankton either as larvae or adults are 

 ostracods, cumaceans, amphipods, mysidaceans, eu- 

 phausiaceans, decapod shrimps and prawns, and stom- 



FIG. 28-3 |C| Cruitace<ns: ajeuphausiid £up/>ous/o, b) osfracod 

 ConcAoecia. c| copepod Calanus, d) amphipod, Phronemia, in 

 empty mantle of the pelagic tunicate Salpa. 



atopods. Many of these forms arc also bcntliic or 

 nektonic during a part of the life cycle. 



Among the chordates are the remarkable and 

 sometimes abundant tunicates. The eggs and imma- 

 ture stages of many fish are pelagic in that they 

 absorb just enough water shortly after being spaw^ned 

 to have almost precisely the same density as the sur- 

 rounding water. The eggs of skates and rays, some 

 of the sharks, and some other fishes, such as the 

 herring, however, sink to the bottom where they 

 remain until they hatch. 



Flotation mechanisms 



The specific gravity of sea water is 1.02 to 

 while that of naked cells or protoplasm varies 

 1.02 to 1.08. The specific gravity of the entire 



1.03, 

 fron: 



FIG. 28 3 (D) Miscellaneous: a) arrow worm Sog/7^o, b) annelid 

 Tomopterls, c) nemertean Nec/onemec/es, d) pteropod mollusk 

 Limacino, e) tunicate Oikopleura, f) pteropod mollusk Clione. 



Marine biomes 357 



