113 



prey upon the larvae of their own kind as they settle if the population of 

 adults is dense enough (Ursin, 1960). The parents may die in a relatively 

 short time, and a new group of species may then re-occupy the habitat of 

 the first group. One observation substantiating this surmise is that on the 

 shallow, sandy bottoms from 11 to 26 meters, there is one very large 

 assemblage of both living and dead moUusks, occurring either living or 

 dead at most of the stations. One group of stations may contain a number 

 of living species found only at these stations, although these same species 

 may also be found dead at another group of stations. The latter stations 

 in turn contain a different set of living species, which may be found dead 

 among the living population of the first set of stations. One argument 

 against this premise, is that environmental conditions seem to be very 

 uniform throughout the area where the 11 to 26 meter assemblage is 

 found. Salinities are constant, sediment-size is relatively uniform, and 

 temperature variations are not too great at these depths. If the environ- 

 mental conditions are constant, then larval settlement should be more or 

 less random and periodical. The strong tidal currents, which sweep back 

 and forth between the northern and central Gulf constitutes one factor 

 which does not remain uniform. If larval settlement should take place 

 when the currents are sweeping north, certain species may tend to settle 

 only in the northern part of the environment. Alternatively, if larvae are 

 produced during the southern trend of the tidal currents, settlement may 

 only take place to the south although these intervals may be too short 

 to be effective. 



Of the 21 species of mollusks which were significantly found together 

 in this environment (Table IV), 12 species or 577o are suspension feeders, 

 4 or 20"/o are predators, 3 or 14^0 are deposit feeders, and 2 or 97o are 

 believed to be scavengers. As could be predicted in this environment, the 

 abundant animals (qualitatively) are suspension feeders, well-adapted to 

 the hard sand substratum which contains relatively little interstitial 

 organic matter. A preliminary analysis of probable feeding types for all 

 of the living mollusks taken at the computor-paired stations (fig. 17) 

 gave the following organization: 40 species or 45°/o are suspension feeders, 

 26 species or 30°/q are predators, 17 or 19°/o are deposit feeders, 4 or 

 approximately 4''/o are algae feeders, and 2 or about 2^0 are possibly 

 parasitic or commensal. These percentages are not too meaningful, since 

 the samples were not quantitative. Again the suspension feeders predom- 

 inate. The fact that intensive upwelling takes place nearby and occasionally 

 high inshore surface productivity (fig. 10) occurs in this area where these 

 typical stations were taken (fig. 17), may also account for the high per- 



8 Vidensk. Medd. fra Dansk naturh. Foren. Bd. 126. 



