188 PROVASOM [CHAP. 8 



contrary, extracts of the few stony corals tested had little or no antibacterial 

 action (Burkholder and Burkholder, 1958a). Antibacterial action was also 

 found in extracts of unidentified "hydroids" (Allen and Dawson, 1959). Since 

 most corals have endozoic zooxanthellae and harbor epizoic algae, it is not 

 known whether the living tissues of the hydrozoan or the associated flora 

 produce the antibacterial substances. 



d. Antialgal substances 



Algae produce, besides antibacterial antibiotics, auto-inhibitors and sub- 

 stances inhibiting the growth of other algae (heteroinhibitors). The phenomenon 

 is widespread among fresh-water algae ; the literature is critically discussed by 

 Proctor (1957); Hartman (1960) has tabulated the data on the interacting 

 organisms, including all the species studied by Lefevre et al. (1952). These 

 substances are found in filtrates of old cultures and it is likely that they were 

 mostly released by living mature cells or during autolysis, but excretion by 

 healthy cells is not excluded. The rate of production of these inhibitory sub- 

 stances has not been followed experimentally and their nature is unknown ; 

 those found by Lefevre et al. are, in general, heat-labile. 1 Presumably marine 

 algae behave similarly. 



e. Crustacean inhibitors 



Unicellular algae apparently release inhibitors for Crustacea : old cultures of 

 Chlorella vulgaris inhibit filtering rate and growth of Daphnia magna (Ryther, 

 1954). Tigriopus japonicus and the brine shrimp Artemia salina utilize as food 

 only a limited assortment of marine flagellates; others are unsuitable (Provasoli, 

 Shiraishi and Lance, 1959). The unsuitability of these organisms as food may 

 reflect nutritional incompleteness, but the possibility that nutritionally adequate 

 food organisms are unsuitable because they produce inhibitory substances is 

 not excluded. In the sea two relationships between phytoplankton and zoo- 

 plankton have been observed : an inverse relationship, i.e. heavy grazing by 

 zooplankton (Harvey, 1934) or avoidance of phytoplankton bloom by predators 

 (Hardy, 1936). Avoidance may indicate that some species of phytoplankton 

 produce inhibitors, or, more simply, obnoxious substances (perhaps tastes and 

 odors, or other substances repressing the feeding reaction). 



E. Poisons 



It is well known that from time to time spectacular algal blooms discolor 

 the sea; some of them lead to mass mortality of marine invertebrates and 

 fishes. Though commonly known as "red tides" because many of these blooms 



1 The blue-green algae are notorious for suppressing other algae. The fresh-water Nostoc 

 muscorum produces in bacteria-free culture a "dihydroxy-anthraquinone" which inhibits 

 growth of several algae (Cosmarium, Phormidium, Euglena), baker's yeast, and bacteria 

 (Jakob, 1961). 



