454 BULLETIN OF THE BUREAU OF FISHERIES. 



Kjellman's limitation of the littoral region has not proved altogether satisfactory, 

 since many marine algae range far above the highest tide mark, especially along coasts 

 wet by the spray from heavy surf, and other species are able to live in water that is 

 brackish or, indeed, actually fresh. Rosenvinge (1898, p. 189) pointed out that the 

 upper boundary of the littoral region should be considered as that level at which 

 marine algal vegetation begins, and Borgesen (1905, p. 709) agrees with this view. 

 The littoral region can then best be defined as the zone extending from low-water mark 

 to the highest point where marine algae cease to grow. As a matter of fact, however, 

 in sheltered waters the upper limit will generally coincide closely with high-tide mark. 



The line between the littoral and sublittoral regions is not always easily deter- 

 mined, for conditions vary in different localities. It is not safe to limit arbitrarily the 

 upper boundary of the sublittoral to the lowest water or neap tide mark, for many 

 species characteristic of the sublittoral will grow a little above such a line. Rosen- 

 vinge and Borgesen agree in placing the boundary between the littoral and sublittoral 

 somewhat above the lowest tide mark. It is probably very near to the average low- 

 water level. 



The lower limit of the sublittoral region varies greatly in its depth from the sur- 

 face and can not be defined with exactness. It merely marks the gradual diminution 

 of vegetation until a bottom is reached that is devoid of plant life. There is no sharp 

 line showing the lower boundary of the sublittoral, such as defines its upper limit at 

 low-water mark. Consequently there is no line marking the upper limit of an elittoral 

 region or depth from which plant life is absent. Indeed, to speak of an elittoral region 

 is to use a negative expression, and the term is not important in descriptive studies on 

 the distribution of algae. 



The line of greatest significance in determining regions of marine vegetation is 

 that between the littoral and sublittoral, near the level of average low water. Above 

 and below this boundary the life conditions differ more than at any other point between 

 the upper and lower limits of marine algal life. Exposure to the air, to rain, and to 

 the heat and drying influence of untempered sunlight introduce very important fac- 

 tors in the littoral region which are not present in the sublittoral and make this line 

 of separation a most significant one. For these reasons the littoral and sublittoral 

 regions are natural divisions, and further subdivisions are of far less import and, indeed, 

 can hardly be made under ordinary conditions, although some authors have attempted 

 to define a supralittoral region above the littoral. 



Certain of the Cyanophyceae and Chlorophyceae and a few of the Phaeophyceae and 

 Rhodophyceae are most commonly found only in the upper region of the sublittoral 

 either just below the lowest tide mark or in shallow water. For these a separate zone 

 might be distinguished; but there are so many species of the Phaeophyceae and Rho- 

 dophyceae which are present in both shallow and deep water that the limits of such 

 a zone, at least in the Woods Hole region, is not easily determined, since there is a 

 very complex overlapping of species. For these reasons we have not attempted to 

 separate and designate regions of the sublittoral further than to qualify the term with 

 the words " upper" or " lower" in certain instances where species are very clearly 

 restricted in their habits. 



When the algae of the littoral and sublittoral regions are studied closely, certain 

 groups of species will be found in more or less close companionship, with definite rela- 



