J. H. PRICE, I. TITTLEY & W. D. RICHARDSON 



Ille-et-Vilaine ............ 44 



Cotes du Nord 44 



Finistere. ............ 45 



Morbihan ............ 45 



Loire Atlantique; Vendee; Charente Maritime ...... 47 



Gironde; Landes ........... 49 



Basses Pyrenees ........... 49 



Iberia 50 



Northern Spain: Guipuzcoa to Pontevedra . ...... 50 



Portugal 51 



Atlantic southern Spain .......... 52 



Discussion ............. 53 



Trends in distribution ........... 53 



Origins of populations and distribution foci ....... 54 



Reproductive patterns ........... 55 



Validity of earlier data .......... 55 



Variations between Britain and the adjacent continent ..... 57 



Periodicity of biological phenomena ........ 58 



Acknowledgements. ........... 58 



References ............. 59 



Synopsis 



Padina pavonica is one of the few British marine algae for which there is a sufficiently long history of data, 

 relatively plentiful and reliable, to permit conclusions as to distributional variation in time and space. The 

 species reaches its current northern limits along southern British and Irish shores; there is strong circum- 

 stantial evidence that past, maybe ephemeral, populations occurred considerably further north. Contraction 

 in distribution range, possibly one aspect of periodic or irregular regional response to environmental 

 changes at range periphery, seems generally indicated. There may have been similar contraction on adjacent 

 shores of Netherlands, Belgium and northern France. British foci of distribution are Devon (the earliest 

 recorded), Dorset, and the Isle of Wight. Analogous foci can be recognised along northern French shores. 

 Gametangial plants seem currently to be very rare outside the Mediterranean; even in it, gametangia 

 seldom develop on plants in the first 0-5 m of the infralittoral. On British and perhaps adjacent continental 

 shores, P. pavonica appears rarely to grow below that depth and this may accentuate gametangial scarcity. 

 Tetrasporangia are much more common than gametangia on Atlantic shores, occurring especially in July 

 to September. Basal perennation and vegetative spread remain very important. Plants seem to appear 

 slightly earlier in the year in Atlantic areas near the Mediterranean. 



Introduction 



Many past British and Irish records of benthic marine algae are unsupported by representative 

 specimens. Generally, the earlier the records, and in more recent times the more common the 

 species, the more likely is the lack of material. The delimitation beyond reasonable doubt of 

 phasic or irregular peripheral changes in distribution (expansion or contraction in range) over 

 time for any long-established species therefore requires that the plants be both conspicuous enough 

 to have been frequently noticed in the past and so distinctive that statements made in even the 

 sketchiest of published data, however early, can be confidently accepted as relating to that species. 

 Few British algae of distributional interest, the northern or southern limit or some characteristic 

 and unusual discontinuity occurring here, qualify under such stringent requirements as regards 

 data from the distant past, although more are susceptible to analysis in terms of the last 70- 

 80 years. Species such as Asparagopsis armata, Bonnemaisonia hamifera, Colpomenia peregrina, 

 and the more recent Sargassum muticum have established here during that time; recognition of 

 these events was comparatively easier since by then phycological knowledge was more extensive 

 and generally more accurate than in past eras, and the invasion by alien species is often more 

 spectacular than long-term variations in the long-established flora. Jones (1974) provides the most 

 recent broad review of distributional and floristic changes in the British marine benthic flora; his 

 examples, for reasons given above, are mostly from the 20th century and even then concern less the 



