i 5 2 DISCOVERY REPORTS 



LOCAL DISTRIBUTION 



The abundance of some species on the Patagonian Shelf and round the Falkland Islands allows closer 

 investigation of the factors which control their local distribution. 



Pyura legumen and Paramolgula gregaria are two species of simple ascidian commonly collected in 

 the area. Text-figs. 68 and 69 show the stations at which they were taken. P. legumen occurred close 

 to the coast of South America and of the Falkland Islands, but not on the main part of the Patagonian 

 Shelf. P. gregaria was found to be widely distributed on the Shelf, occupying not only the main part 

 but also the inshore areas where P. legumen occurred. 



P. legumen has a stalk and requires for its attachment firm solid objects. Matthews (1934) has 

 shown that such objects are common only in certain areas and are absent over most of the main part 

 of the Shelf, from which we have seen P. legumen also to be absent. 



P. gregaria is a stalkless species with a broad base and is able to live on a substratum of fine deposits ; 

 its presence over a large part of the shelf corresponds to the wide distribution of fine deposits. This 

 explanation of the different occurrence of these two species seems more satisfactory than any based on 

 differences in depth, temperature or water currents. 



Most species of the family Polyclinidae live attached to hard substrata, but on the Patagonian Shelf 

 there are species which lie on the surface of the sand only lightly attached or quite free. Aplidium 

 fuegiense and A. falklandicum may be taken as typical of these, and their wide distribution on the shelf 

 is shown in Text-fig. 70. Most of the records are from stations where the bottom material consists 

 largely of fine sand. A. fuegiense is a particularly successful species in the area, to judge from its 

 abundance, and it has a shape modified to suit its habitat. The colony is long and narrow, the expanded 

 base lying slightly embedded in the sand and the tapering distal part floating freely in the water. 

 A. falklandicum, with a similar local distribution, sometimes has a similarly shaped colony, but is at 

 least occasionally solidly attached. 



In the family Clavelinidae Sycozoa sigillinoides has a distribution on the Patagonian Shelf (Text- 

 fig. 71) similar to that of the two polyclinid species. The form of its colony is quite different, however, 

 the heads, which alone contain the zooids, being borne on stalks, but the result is the same, namely the 

 zooids are kept free of the sandy bottom. 



Three compound species of the family Styelidae occur in the Patagonian region : Polyzoa opuntia, 

 P. reticulata and Alloeocarpa incrustans (Text-fig. 72). All three species need solid firm objects for 

 attachment and tend to be restricted to coastal waters where such objects are common, or to other 

 areas with gravel and large fragments. P. opuntia in particular, which has large heavy colonies of 

 upright habit, and a small basal area, requires a firm substratum. The fact that this species and Pyura 

 legumen have similar local distributions may be related to their similar needs in this respect. Polyzoa 

 reticulata, unlike P. opuntia, forms colonies spreading horizontally over flat surfaces, and the two species, 

 which are closely related, occupy slightly different ecological niches. 



GEOGRAPHICAL SPECIATION 



Among Antarctic and Subantarctic ascidians there are several instances of geographical speciation. 

 In some cases the process has led to the complete separation of species, each species occupying a 

 different region. An example of this is the group of closely related species Pyura legumen, P. georgiana 

 and P. bouvetensis. Anatomical features show P. georgiana and P. bouvetensis to be a pair of closely 

 related species and P. legumen to be more distantly related to both. The common ancestor of all three 

 species probably first split into a Subantarctic population which became P. legumen and an Antarctic 

 population which later divided into P. georgiana and P. bouvetensis. The geographical separation of 



