Live sjiecimens of M. lateral is. A. papyria, N. 

 vibex, and T. plehehis indicated bottom conditions 

 by their presence or absence and by their abun- 

 dance in rehition to otlier live molhisks. On the 

 basis of tlie occuri-ence and distribution of these 

 four species, bottom environments were classified 

 as healthy or marginal. Healthy stations were 

 those where indicator species were less than 50 

 percent of all live mollusk species present; at mar- 

 ginal stations, the indicators represented 50 per- 

 cent or more of all live species present. Unhealthy 

 stations were those where no living mollusks were 

 collected (fig. 1 and tables 2— i). The number of 

 living mollusks was generally higher at healthy 

 than at marginal stations except at marginal sta- 

 tion 9-3 where M. lateralis and TV. vihex were un- 

 usually abundant. Furthermore, on the basis of 

 station classification the entire area of the bay 



was divisible into healthy, marginal, and unhealthy 



zones. 



The four species selected as indicators for the 

 bay, and perhaps the crown conch as well, may be 

 useful for biological evaluation of the environ- 

 ment in estuarine water of the southeastem and 

 Gulf States. Table 5 represents a summary of eco- 

 logical literature and shows the extreme ranges of 

 environmental conditions that these five mollusks 

 can tolerate under natural conditions. 



HEALTHY STATIONS 



Eight stations, or about 18 percent of those hav- 

 ing live mollusks, were classified as healthy. The 

 average incidence of indicator species at these 

 stations was 27 percent, and the average niunbers 

 per station were 11 species and 225 individuals 

 (table 2). 



Table 2. — Biotic and physical characteristics of healthy benthic stations in Hillsborough Bay, Fla., August and September 1963 



Station 



Species ' Individual ' 



Indicator 

 species ' 



Deptli 



Mean sediment 

 grain size 



Sediment 

 sorting 



Sediment type Bottom vegetation 



Bottom 

 salinity 



Number Number Percent 



C 9 192 33 



C-2 10 328 40 



C-3 9 213 44 



10-15 17 512 18 



10-16 8 23 13 



10-19 12 71 17 



10-22 10 88 33 



10-23 10 353 20 



Mean 11 225 27 



Range 13-44 



Coarse silt OraciZarta sp. 



Fine sand OracilaTiasp. 



Fine sand Oracilaria sp. 



Fine sand OraciZarJa sp. 



Very fine sand None 



Fine sand None... 



Fine sand Oracilaria sp.. 



Fine sand.- QracUaria sp.. 



Fine sand. 



P.p.i. 



18.19 

 18.66 

 16.36 

 21.82 

 22.62 

 22.95 

 22.43 

 21.26 



20.52 

 18. 19-22. 95 



• Collected alive. 



Table 3. — Biotic and physical characteristics of marginal benthic stations in Hillsborough Bay, Fla., August and September 1963 



Station 



Species * 



Individual  Indicator 

 species ' 



Depth 



Mean sediment 

 grain size 



Sediment 

 sorting 



Sediment type 



Bottom 

 vegetation 



Bottom 

 salinity 



Number Number Percent M. 



C-1 4 25 50 1.0 



C-4 1 2 100 1.0 



C-6 4 13 75 4.2 



C-6 4 202 76 2.0 



C-8 1 71 100 .7 



C-8-1 1 142 100 .7 



C-8-2 1 89 100 .3 



7-1 1 4 100 2.0 



8-3 3 13 50 3.6 



8-» 1 2 100 3.9 



8-8 2 8 60 Z8 



8-9 5 57 60 1.0 



9-1... 1 1 100 .3 



9-3 6 925 60 2.0 



9-4 3 6 67 2.4 



9-9.. 1 4 100 2.8 



10-14 8 267 60 1.0 



10-20... 3 6 100 3.9 



Mean 3 102 80 1.9 



Range 60-100 .3-4.2 



2.32 

 2.95 

 2.58 

 2.88 

 2.61 

 2.95 

 6.28 

 3.04 

 4.09 

 5.23 

 3.14 

 2.51 

 2.63 

 2.38 

 2.26 

 3.37 

 2.68 

 2.88 



1.8 



1.2 



1.0 



1.0 



1.1 



1.6 



2.6 



1.1 



2.2 



2.7 



1.4 



.5 



.7 



.8 



.7 



1.8 



.8 



1.6 



Fine sand 



Fine sand 



Fine sand 



Fine sand 



Fine sand 



Fine sand 



Medium silt 



Very fine sand. 



Coarse silt 



Medium silt 



Very fine sand. 



Fine sand 



Fine sand 



Fine sand 



Fine sand 



Very fine sand. 



Fine sand 



Fine sand 



. None 



. None 



. None 



. None 



. None 



. None 



. None 



None 



None 



None 



None 



None 



None 



None 



None 



None 



Oracilaria sp. 



None 



P.p.t. 

 18.33 

 15.69 

 19.78 

 18.96 

 .74 

 1.16 

 3.69 

 17.79 

 19.78 

 20.60 

 16.56 

 18.78 

 17.25 

 18.37 

 18.51 

 17.70 

 20.61 

 22.92 



3.09 

 2. 25-5. 28 



1.4 



.5-2.7 



Very fine sand. 



15.90 

 . 74-22. 92 



' Collected alive. 



196 



U.S. FISH AND WILDLIFE SERVICE 



