90 percent of the fauna on both transects was collected from the 

 two stations seaward of the first reef, and more than 60 percent 

 of the species at each transect occurred exclusively at these 

 stations. 



Benthic stations sampled in this study represented a wide 

 range of environmental conditions. The intertidal zone of open 

 sandy beaches is one of the most rigorous and variable of all marine 

 habitats, requiring unique adaptations among the fauna inhabiting 

 this zone. These organisms must either be able to migrate with the 

 tides or be able to withstand exposure to great extremes and rapid 

 fluctuations in temperature, salinity, and numerous other variables 

 which may occur within a given tidal cycle. Since relatively few 

 marine animals have been able to adapt to these conditions, the 

 diversity of intertidal macrofauna is characteristically lov\?, as 

 indicated in this study. 



In shallow subtidal areas, although conditions for life 

 become more favorable with regard to many factors, the turbulence 

 of the water, suspended sediments, and the instability of the 

 substrate" create conditions which are not conducive to the estab- 

 lishment of stable, diverse benthic communities. At various times 

 of the year, especially during periods of high winds and rough seas, 

 large amounts of sand may shift on and off the shore in response to 

 strong wave surge and tidal currents, causing unusually dynamic and 

 hazardous conditions for most forms of benthic life. The low 

 species diversity and sparse numbers of animals found in these 

 localities reflect these conditions. 



In deeper water seaward of the first reef, the benthic 

 habitat becomes much less affected by wave surge and turbulence. 

 The stability of the substrate and the relative uniformity of 

 physical and chemical conditions promote the development of a 

 diverse and productive benthic commimity such as found at the 

 seaward stations. 



Differences in the composition and structure of reef com- 

 munities along each transect also reflect differences in the 

 physical environment. Most species of sponges, alcyonarians, and 

 stony corals, the most conspicuous reef fauna, are highly suscep- 

 tible to siltation. The scarcity of these forms on the first reef 

 probably resulted partly from the turbulence of the water and from 

 the relatively large amounts of sediments in suspension. The low 

 profile of the first reef, its lack of structural complexity, and 

 the presence of a thin layer of sand over much of the limestone 

 surface, contributed to the low diversity and abundance of animal 

 life at this locality. 



On the broad second reef, especially toward the outer edge, 

 the greater depths and clear waters, as well as the increase in 

 structural complexity of the reef surface, were much more favorable 

 for the existence of a rich and diverse fauna. The many holes and 

 crevices within the limestone base provided a greater surface area 

 for the attachment of sessile forms as well as shelter for various 

 motile crustaceans and echinoderms. Finally, the presence of the 

 larger sponges and corals added to this complexity and increased 

 the availability of niches for smaller organisms. 



28 



