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



Periodicity of biological phenomena 



Changes in the periodicity of biological phenomena such as presence of upright fronds and 

 production of viable or inviable reproductive structures cannot yet be traced in detail along the 

 whole length of the coastlines within the northern Atlantic/Mediterranean distribution ofPadina. 

 Certain areas of the Mediterranean, however, are known to yield sporangial plants over much 

 more of the year than is normally true for Britain, and (see Introduction) within a few Mediter- 

 ranean regions, populations at some depths quite commonly include high proportions of game- 

 tangial plants. To some extent, it is possible to begin tracing the changes in length and placement 

 during the average year of the period when recognisable upright fronds ofPadina can be detected, 

 in contrast to the perennating basal parts of the alga. By the entrance to the western basin of the 

 Mediterranean, upright fronds are more or less consistently present all the year round in the infra- 

 littoral, although Feldmann (1937) has indicated the probable involvement of overlapping genera- 

 tions in this. Atlantic areas north of and nearest to the Mediterranean (Portugal; Cadiz) not 

 surprisingly show more of a tendency toward the Mediterranean periodicity than do areas further 

 north. The differences are relatively slight, and the existence of a real trend cannot be confirmed 

 without extended study of the situation at selected spots from Britain to the western Mediterranean. 

 Chance differences in behaviour between the different years of observation could have led to 

 recognition of trends more apparent than well-founded. In England, Padina has been shown to be 

 present as upright fronds for longer periods, persisting well into the winter or appearing early in 

 spring, in some years than in others. Reasons for these variations in phenomena are not fully 

 understood, but more likely they concern environmental aspects to which the plants are responding 

 than spontaneous variations within the alga itself, although both may be implicated. There seems 

 to be relatively little real variation in time of appearance of upright fronds in any area between 

 Britain and northern Portugal. Variations of this kind seem on present evidence to occur south 

 of northern Portugal. 



There is overall need for careful long-term scrutiny of all shores or local microniches actually or 

 potentially capable of supporting P. pavonica, especially in southern Great Britain but also else- 

 where in the island group and throughout adjacent continental shores. Particularly desirable is a 

 more detailed search of suitable infralittoral areas in southern Britain and in the transitional 

 N. Spain/Portugal/S. Spain area. It remains to be established whether the apparent rarity of game- 

 tangial material in all but a few Mediterranean locations is due to simple lack of adequate 

 observations, to actual loss of potential, or to environmental constraints that are quickly reversible 

 in isolation from the natural living conditions. In view of the demonstration (Edwards 1973, see 

 Introduction) in a similar case in Ceramium that the potential is merely suppressed, being realised 

 in very few individuals in the field but readily expressed in culture, additional field observations and 

 culture studies on British south coast material of Padina pavonica would be of considerable bio- 

 logical interest. We would like to think that, when and if further studies on aspects of Padina in 

 this area are carried out, this paper will provide an authentic basis against which distributional or 

 biological variations can be assessed. 



i 

 Acknowledgements 



We are grateful to Mr R. Ross, Mr J. F. M. Cannon, Mr P. W. James, and Mr J. R. Laundon, British 

 Museum (Natural History), for critically reading the manuscript. Without assistance provided by many 

 colleagues, it would not have been possible to conduct so detailed a survey of the available information. 

 For their help in clarifying the situation in different areas of the British Isles, we wish to thank Dr H. 

 Blackler, Dr G. T. Boalch, Dr E. M. Burrows, Mr C. Doeg, Dr W. F. Farnham, Dr M. D. Guiry, Dr 

 J. Hayward, Dr C. E. L. Hepton, Dr D. E. G. Irvine, Mrs L. M. Irvine, Dr D. M. John, Dr M. W. Parke, 

 F.R.S., Dr G. Russell, and Mrs M. A. Wilson. The aid of the curators of collections in the various institu- 

 tions listed as abbreviations in the introduction to Section 2 of this paper is also acknowledged. Dr Paul 

 C. Silva commented cogently on nomenclatural points; Dr E. Launert was helpful in tracing obscure 

 Portuguese locations; Dr L. B. Liddle provided useful discussion and comment; Dr E. Ramon was kind 

 enough to translate relevant portions of her thesis from Hebrew into English, apart from placing at our 

 disposal information on the Israeli coasts. It is a pleasure to acknowledge our debt to all those named 

 above, as well as to Mr S. I. Honey for his onerous library and herbarium labours. Photographs have been 



