238 BULLETIN OF THE BUKEAU OF FISHERIES 



A complete study of the relation of various metals to fouling has been made 

 recently by Parker (1924), who submerged panels of zinc, iron, aluminum, tin, lead, 

 and copper. He found more or less fouling on aU of them except the copper, and 

 only a small amount on zinc. He explained this difference on the basis of ionization 

 of these metals in salt water and the solubility of the resulting compounds. Thus, 

 he states: 



The poisonous effects of these metals on marine animal life will depend upon the intrinsic 

 toxicity of their ions, relatively high for all heavy metals, and the solubility of their hydroxides 

 and basic carbonates in sea water. These solubilities in the case of 'Fe, Sn, and Al are in amounts 

 inappreciable; in other words, these metals in sea water are not surrounded by a layer of poisonous 

 ions, and hence animals may grow upon them. In the case of Zn and Cu, on the other hand 

 the corresponding compounds are appreciably soluble in sea water, and the poisons thus liberated 

 prevent the growth of animals upon these metals. 



His experiments with metal couples, however, have shown results that indicate 

 a means of preventing fouling, even if an unpractical method. He found that by 

 coupling copper with metals higher in the electromotive series this metal can be 

 rendered chemically inactive in sea water, and under such circumstances animals 

 will grow freely upon it. Similar results were obtained %vith other couples, so that 

 Parker concludes that "marine animals will grow upon any heavy metal, provided 

 that metal does not hberate ions or soluble compoimds. " Conversely, it would 

 accordingly be apparent that any electrolytic action causing ionization would serve 

 to prevent fouling. 



LIGHT AND COLORS 



During the course of the examination of the second ship observed in dry dock it 

 was observed, as previously noted in this report, that fouling was most severe in the 

 region of the rim and beneath the bilge keels of the ship. This increase in amount 

 of fouling on lightly or moderately fouled ships in all areas that might be considered 

 as "shaded" has been one of the most outstanding points noted during the whole 

 investigation. More than 50 per cent of all examinations showed such results very 

 strikingly. Other explanations have been offered to explaia this intensification of 

 growth in restricted areas, as, for example, the protection aft'orded in such locations. 

 The ^vTiter, however, has held that the main factor was the influence of light. 

 This contention no doubt was iafluenced greatly by previous knowledge of various 

 biological studies on related phenomena. 



The reaction to hght of animals and plant organisms has long been a favorite 

 study of biologists, because of the fact that most organisms react to this stimulus, 

 as well as because of the ease with which the stimulating agent can be controlled. 

 Lord Avesbury (Su- John Lubbock, 1904) was one of the first to demonstrate the 

 fact that animals of many sorts react to Ught of different colors, finding, for example, 

 that bees "prefer" blue flowers and that the tiny water fleas, Cladocera, gather in 

 the region of the red if given a choice of aU the colors of the spectrum. 



More recently Mast (1911) and others have shown that reaction to light is a 

 property common to almost all living things, both plant and animal. He showed, 

 among other experiments, that the larvae of one of the hydroids (Eudendrium) 

 common on ships' bottoms react negatively to Ught, while the spores of certain 

 plant forms (algae), also common on ships' bottoms, are positive in their reaction to 



