Clare Island Survey — Marine Algae. 15 33 



Head it is found on Hat, sloping, or vertical, semi-exposed rocks, which face 

 north or north-east. At Portlea it affects similar situations, but shows a 

 decided preference for vertical rocks (these being at the same time the more 

 shaded). The plants are attached by a strong basal disk, and are usually found 

 on mussels, but they occur also on limpets and bare rock. The vertical area 

 covered by the alga is rather extensive, reaching at times from the bottom of 

 the F. spiralis zone down to about low-water mark, but more often forming 

 a band below the upper Porphyra. It is frequently interrupted by patches of 

 other vegetation. At Old Head, Fucus vesiculosus, var. cvesieidosus is the chief 

 intruder ; but portions of the Porphyra and Callithamnion arbuscula associa- 

 tions above, and the Nemalion vegetation below, are also interspersed amongst 

 it. A modified form of the association occurs on boulder-beaches, the plant 

 hanging in profusion from the lower part of boulders surmounted by 

 Himanthalia (Plate II). 



The form of the alga when growing as an association on exposed coasts is 

 very distinct. Short and tufted, not more than 3 to 4 inches long, it consists 

 of narrow or rounded fronds, with a number of small lateral proliferations. 

 Though narrow, it is very distinct from var. sarniense, which is thinner and 

 paler in colour, and resembles Gvaeilaria multipartita in form. With increased 

 shelter the plant becomes larger, till at length the pool-form, with fronds a 

 foot or more in length, is reached. The islanders distinguish the larger and 

 small forms as Dillisk and Crannogh respectively, and always associate the 

 latter with the presence of "small shells" — i.e., mussels (see Economic Section). 



Borgesen remarks that the Ehodymenia association prefers places where 

 fresh water oozes from the rocks. This is not the case on Clare Island. As a 

 pool or streamlet plant, Rhodymenia enjoys an admixture of fresh water ; but 

 when growing as above described, it was always found in a purely marine 

 habitat. 



Laurencia-Gigartina Association. 



Near the low-water line on exposed coasts there often exists a dense 

 carpet composed of the two red algae, — Laureneia pinnatijida and Gigartina 

 mamttlosa. Gigartina occupies the lower part and goes down into the sub- 

 littoral region, whilst Laureneia extends more or less in an upward direction. 

 The band formed by the latter is often conspicuous, so much so that it was 

 at first regarded as a distinct association. Further examination at spring- 

 tides showed that it was frequently connected with a zone of Gigartina, and 

 that in the more exposed places the latter species was dominant. It 

 gradually became evident that their relative abundance was largely a matter 

 of exposure and slope, and that these two species of similar habit could be 



K,I.A. PROC, VOL. XXXI, E 15 



