THE DISTRIBUTION OF PADINA PAVONICA 7 



Naples), dioecious gametophytes at 0-0-5 m in July were exceedingly rare (8-7 % of sample) com- 

 pared with sporophytes (91-3%) and became rarer as the season progressed (September 4-6%: 

 954%; October 0%: 100%). Since the percentage of Mediterranean gametophytes increased 

 rapidly with depth, the limiting factor for their development in British waters may be precisely 

 that all detected British populations grow in the shallow depths (0-0-5 m) at which gametangial 

 development is least likely to be manifest. The November to March periodicity of Florida game- 

 tophytes of P. vickersiae and the slight bias towards greater numbers ofP.japonica gametophytes 

 in winter at Oahu may have little temperature significance for P. pavonica; however, since the 

 Florida and Hawaii sea temperature regime is generally higher than the other areas considered, all 

 gametophytes of species of Padina may be postulated to develop to a seasonal pattern that reflects 

 restriction to the same narrow band of temperatures. Ramon (1969c) concluded that, for P. 

 pavonica, there is a threshold temperature above which gametophytes appear. She correlated type 

 of gametophyte with increasing temperature, the first formed above the threshold being pre- 

 dominantly typical monoecious development. In warmer waters, there is an increasing tendency 

 for firstly male unisexual and then, at even higher temperatures, female unisexual gametophytes 

 to form. The latter, for instance, were detected in the relatively high summer temperatures (27- 

 30 C) in Israeli coastal waters. Probably the situation is not quite so straightforward as this, some 

 aspects of light levels also being involved, as they are in P.japonica (Allender, 1977). 



Tetrasporangia, by contrast, are not uncommon amongst British Padina specimens, particularly 

 those collected in the inclusive period July to September; they can be detected to some extent 

 throughout the year. Phasing of the production seems to be true for the genus wherever it appears, 

 although times of maxima may vary. Table 1 collates all available data on distribution in space and 

 time of vegetative and tetrasporangial plants of P. pavonica on the coasts of Great Britain. 

 Interesting general tendencies are revealed. The sheer numbers of records for each county almost 

 certainly reflect the length of historical period over which records for that area exist. Devon (most 

 records) is by far the earliest recorded major focus; Dorset is next, and the Isle of Wight 

 (Hampshire) is the most recent. Despite this, the three focal areas all possess large enough 

 numbers of records to be comparable; they reveal a surprisingly consistent average level of 

 tetrasporangial to total records for the year - 42 % (Wight), 41 % (Dorset), and 45 % (Devon). July 

 to September inclusive cover the vast majority of records for each focal area and therefore for the 

 whole of British Padina. The odd records outside the main period show some variation with area, 

 probably largely due to chance observation or collection, not to any difference in pattern. Note 

 that although the total numbers of records for the three principal months show a decrease from 

 the peak in August to the September figure, the percentage of tetrasporangial to total records 

 shows a steady increase from July (36%), through August (39%), to September (49%). The 

 figures for June (22%) and October (71 %) neatly supplement this progression, but are based on 

 too small a sample to be unreservedly accepted as continuing the perceived trend. The predomi- 

 nance of both vegetative and tetrasporangial records in the July-September period cannot be 

 wholly a result of collecting habits; the discrepancy in numbers of records between that period and 

 the rest of the year is too large for both tetrasporangia (65 % of 80 records = 8 1 -25 % of the total) 

 and vegetative data (157 of 188 records; 83-5%) to be accounted for solely on that basis. See 

 below for further comment on this. 



The analogous distribution of vegetative and tetrasporangial records in the north-east Atlantic 

 outside Britain appears in Table 2. Geographically, information is less consistent than for Great 

 Britain, but it is clear that analogous foci of distribution can be recognised (see Fig. 1 [map]). 

 Although in southern England the Isle of Wight and Dorset foci could be considered almost 

 continuous, they are treated separately for reasons of clarity. Northern France is similar; all 

 records from Calvados westwards to Ille-et-Vilaine could be considered as representing a single 

 focal area, but it is more convenient to recognise two - one to the east (the departements of Calvados 

 and Manche), the other formed principally by the Channel Islands and Ille-et-Vilaine. The third 

 distributional focus, further to the west, comprises Finistere and Morbihan. There are few other 

 areas further south that are identifiable with certainty as distributional foci until the Provincia de 

 Cadiz; the considerable information for the Basque coast and for southern Portugal tends to 



