ECOLOGY AND TAXONOMY OF Holimeda 169 



followed by new segments two or so days later. Non-rliipsalian species 

 have not grown as well, at least initially, and tuna has been the most 

 difficult to maintain under these conditions. 



At the beginning of the cultm^e work, when looking for evidence of 

 success, some growth observations were especially troubling. Examples 

 include the turning white of segments or branches which then fall off, 

 and the overburdening of thalli with epiphytes, the latter being distinct 

 from the problem of maintaining Halimeda in culture with a minimum 

 number of epiphytes. Both developments, however, are part of the 

 Halimeda growth cycle. 



Segments and branches are shed, at least in some species, rather like 

 leaves being dropped from a bush or tree m the autumn, and the growth 

 of Halimeda is thereby different from that of Penicillus capitatus 

 Lamarck and Udotea flabellum (Ellis and Solander) Lamouroux (Colin- 

 vaux et al., 1965) in which there are no dehisceable units. The shedding 

 of segments or branches also relieves the Halimeda thallus of a crop of 

 epiphytes, and produces sites where vigorous new growth can occur. 



In the reef, epiphytes are, at times, very evident on Halimeda. 

 Blue-green algae may cover much of the Halimeda thallus and contri- 

 bute a coating or matting to the surrounding sand. Matheison et al. 

 (1971), in a transect study at — 17-3m to — 21-5m in the Virgin 

 Islands, reported 17 species of epiphytes, mostly red algae, on H. 

 incrassata. These workers did not seriously include blue-green algae in 

 their study, so Cyanophyta may have occurred as well. It may be that 

 massive imposition of epiphytes is a hazard to which Halimeda popula- 

 tions are adapted, and that their reproductive and persistence strate- 

 gies will be influenced by this recurrent event. It is also possible that 

 patterns of grazing on the Halimeda beds may be reflected in blooms of 

 epiphytes. That the phenomenon occurs in nature suggests that it is 

 an ecosystem event of some significance. It is also one such event that 

 we can study in our cultures. 



A number of animals, introduced inadvertently with the thalli, 

 live successfully in the culture systems described, and some of the 

 grazers, particularly the snails, may clean the plants and aquaria of 

 some growths. Introduced animals include tube worms, anemones and 

 polychaetes, with occasional snails, bivalves and brittle stars. From 

 time to time there also have been blooms of small medusae of about 

 2 mm diameter. What are presumed to be different species of opistho- 

 branch molluscs, both identified as Elysia spp., have fed, one type on 

 Tydemania, the other on Halimeda incrassata. The Tydemania lived 

 only about three months, and too little is known of its growth pattern 

 to have assessed the impact of Elysia grazing. However, the other 



