IDENTIFICATION OF MARINE GROWTHS 15 



Species of sea lettuce ( Ulva) may also be met with on ships' hulls, 

 although, so far as present experience goes, they always occur in 

 association with some of the other green forms. One of the common- 

 est kinds grows in broad sheets of about the thickness of tissue paper 

 and is readily recognised (Fig. 3), but some kinds of Ulva produce 

 relatively narrow ribbons that are not always easily distinguished 

 from some types of Enteromorpha. 



Of the brown seaweeds responsible for fouling one of the very 

 commonest is Ectocarpus, of which there are a number of different 

 kinds. Its tresses are usually dark brown, and when spread out 

 are seen to be richly branched (Figs. 6 and 41). Some kinds are 

 less than an inch long and, if there is a dense growth oi Enteromorpha, 

 for instance, may be completely hidden beneath it. Other kinds of 

 Ectocarpus may grow to a length of several inches. Under the 

 microscope the contents of the cells appear brown or yellowish- 

 brown, although if the vessel has been for some hours in a fresh-water 

 port the brown colour will have leached out and the contents may 

 look green. Some or many of the shorter branches will be found to 

 end in oval or elliptical structures, consisting usually of several rows 

 of minute cells, much smaller than those composing the otlier parts 

 of the seaweed (Fig. 28). These distinctive structures are the 

 reproductive organs of Ectocarpus. 



Other brown seaweeds that may occur on ships' hulls are coarser 

 and less richly branched, but these are rare and have not usually 

 reached their full growth, and it would require an expert to name 

 them. The most frequent is Scytosiphon, with hollow strands 

 resembling those of Enteromorpha. The bladder-wTack {Fiicus) and 

 the oarweeds (Laminaria) (Fig. 5) are likely to settle only on a 

 heavily fouled vessel which has made a long sojourn in port. They 

 would almost certainly become detached when the ship began to 

 move through the water. 



Of the red seaweeds likely to occur on ships' hulls the two most 

 easilj^ recognised are Ceramium (Fig. 7) and Polysiphonia (Fig. 8). 

 Both appear as richly branched tresses, usually 1-3 in. long ; those 

 of Ceramium are mostly bright red, those of Polysiphonia often dark 

 purple, sometimes appearing almost black. When the tips of the 

 threads of Ceramium are examined under a hand-lens (Fig. 42) they 

 usually appear forked, the two arms of the fork often being curved 

 inwards like a pair of tongs. The threads themselves show numer- 

 ous cross bands which are of a darker colour than the intervening 

 portions. These two features are quite distinctive of Ceramium. 

 Under a microscope ( x 100) the banded appearance is seen to be due 

 to the fact that the large cells forming the threads of this seaweed 

 are covered at regular intervals by sheets of smaller cells. 



The threads of Polysiphonia are usually coarser. The upper 

 («*.e., younger) parts, when viewed with a hand-lens, show an 

 arrangement of cells in horizontal tiers which is very characteristic 

 and is even more obvious at a magnification of x 50 (Fig. 43). The 



