226 L. HTLLIS-COLINVAUX 



dispersion, perhaps even those resulting from very slow processes like 

 the movements of ocean plates. 



A. Present distribution 



Representatives of the genus Halimeda grow wherever there are 

 warm seas, sufficient light and appropriate substrate. The band of sea 

 known as tropical, and delimited by lines of latitude 23-5° north and 

 south of the equator, therefore provides a rough model of its worldwide 

 distribution. Warm waters, however, are only approximately delimited 

 by a fixed number of degrees of latitude, since their extent is determined 

 by ocean currents, which in turn are aff"ected by the rotation and 

 orbiting of the earth and the topography of intervening land masses. 

 The tropical band of sea is asymmetric. It is broad on the western side 

 of the oceans, thin on the eastern side, a pattern which is most marked 

 for the Atlantic and Pacific Oceans. The shores of island groups such as 

 Bermuda and southern Japan, as a result, are bathed by warm waters, 

 although geographically they are north of the Tropic of Cancer. Both 

 aff'ord habitats for Halimeda. But Peru, located within the tropics, has 

 cold waters along its coast and is without Halimeda. The precise 

 latitudinal extent of tropical water shifts seasonally, generally moving 

 to the north in the middle months of the calendar year, to the south 

 at the end and beginning of the year. Halimedae at the northern and 

 southern limits of the generic range, therefoi'e, are exposed to a 

 seasonally fluctuating environment. The usual temperature of the 

 tropical water is in the neighbourhood of 25 °C. 



All the species of Halimeda are restricted to this asymmetric 

 tropical band except tuna and cuneata (Fig. 75), and their distribution 

 within this band is summarized in Table XIX. Halimeda tuna, both the 

 typical and large-segmented forms, also grows in the Mediterranean 

 where it is the only Halimeda species. Halimeda cuneata, alone among the 

 species, occurs in a band of cooler water, to about 20 °C, which corre- 

 sponds to the subtropical zone. This particular taxon appears to be 

 absent from the tropical region where the other Halimeda species occur, 

 although our knowledge of its precise distribution is confused because 

 other species, particularly discoidea, have commonly been identified as 

 cuneata (Hillis, 1959). Initially it appeared to be restricted to the 

 southern subtropics, where it is known from the tip and western shores 

 of South Africa, southern Madasgascar, south-west Australia and 

 south-east Australia (Fig. 75; Hillis, 1959), but during the International 

 Indian Ocean Expedition it was collected in the northern hemisphere 

 near Okah, at the mouth of the Gulf of Cutch on the north-western 



