118 



events during the Tertiary of Middle America furnished the setting for the 

 recruitment of this multitude of Caribbean species into Pacific coastal 

 community complexes. 



Buchanan (1958) describes a community in similar depths off Accra, 

 Ghana which he terms the "silty-sand community", occurring in from 

 36 to 45 meters (20-25 fathoms). This community is characterized by a 

 giant Foraminifera, which so far has not appeared in shallow water in the 

 eastern Pacific. The only important molluscan species were a Cardium 

 and a Phos, while the crustaceans consisted of several species of decapod 

 shrimp. Most of the macro-invertebrate species also ranged into the off- 

 shore "coarse-sand community" to depths of about 90 meters (50 fathoms). 

 As can be seen, the kinds of animals in Buchanan's community bear 

 little resemblance to those found in equivalent depths in the Gulf of Mexico 

 or Gulf of California although water temperatures are slightly lower off 

 Ghana. Longhurst (1958) in his discussion of benthic communities off 

 Sierra Leone, West Africa gives no list of invertebrates restricted to inter- 

 mediate shelf depths, but rather finds strict dependence upon sediment 

 size and estuarine influence. It is evident from his data, that relatively 

 few invertebrate species live in depths of 27 to 65 meters compared to the 

 number of species living in similar depths in the Gulf of California. In a 

 discussion of the quantitative aspects of his study, Longhurst (1957) 

 did find that there was a decrease in number of species and individuals 

 with increasing depths. He also stated that there was little difference 

 between his region and similar depths in temperate, boreal and Arctic 

 waters in terms of the biomass, and individuals and species per square 

 meter. He reported that 800 species of animals were taken by grab and 

 dredge off Sierra Leone, which differed only slightly from Jones (1951) 

 list of 757 species from the Isle of Man in the British Isles, although com- 

 parisons of this sort are not too meaningful as the level of systematic 

 knowledge of the two areas are quite different. 



Almost equal numbers of suspension, deposit and carnivorous feeders 

 were produced from a preliminary analysis of the feeding types of the 

 mollusks and a few other invertebrates found in the northern sand-bottom, 

 intermediate shelf assemblage. Of the 68 species selected for his survey, 

 17 are probably deposit feeders, 16 are suspension feeders, 15 are predators, 

 2 scavengers, 3 are algae feeders, and the rest are unknown as to feeding 

 habits. Although a sand bottom is usually characterized by an excess of 

 suspension feeders, these sands are at greater depths and less affected by 

 turbulence than most sandy areas, possibly allowing more organic matter 

 to accumulate on the bottom. The alternating strong currents which sweep 



