Soft sediments ( > 80 percent silt and clay) 

 and lack of vegetation account for the few 

 shellfish in deep dredged canals. 



Old Tannpa Bay is more brackish ( < 25 p.p.t.) 

 than Boca Ciega Bay, but sediments are 

 mostly sandy and submerged vegetation is ex- 

 tensive. Bottom samples there contained 97 

 species of live mollusks in 52 fan^ilies. Al- 

 though the number of species in Old Tampa 

 Bay was 62 percent less than in Boca Ciega 

 Bay, the cause is lower salinity rather than 

 dredging or pollution. 



Only 35 species of live mollusks were col- 

 lected in Hillsborough Bay, where salinity is 

 about the same as in Old Tampa Baybut where 

 dredging and pollution are responsible for 

 widespread accumulation of soft sediments 

 and elimination of bottom vegetation in water 

 more than a few feet deep. Furthernnore, no 

 live mollusks were collected at 19 of the 45 

 stations sampled. On the basis of these 19 

 stations and the incidence of four species in 

 other samples (Mulinia lateralis, Annygdalum 

 papyria, Tagelus plebeius , and Nassarius vi- 

 bex ), we divided Hillsborough Bay into three 

 life zones --healthy, marginal, and unhealthy 

 (fig. 3). These four species apparently are 

 tolerant to a high degree of pollution. They 

 made up less than 50 percent of the species 

 present at healthy stations and 50 percent or 

 more of the species present at marginal 

 stations; no living mollusks were found at 

 stations designated unhealthy. Most of the 

 healthy areas are between Hillsborough and 

 Tampa Bays where pollutants are somewhat 

 diluted and much of the bottom remains 

 sandy and vegetated. The term "healthy" is 

 partially nnisleading, however, because many 

 species found in Old Tampa Bay have not been 

 found in any part of Hillsborough Bay. 



Echinoderms and Polychaetes 



Work this year on taxonomy of echinoderms 

 raised the number of species recorded from 

 Tampa Bay to 36. Lowell Thomas, University 

 of Miami, identified the brittle stars. Elisa- 

 beth Deichmann, Harvard University Museum 

 of Comparative Zoology, identified the sea 

 cucumbers. 



Polychaete worms from stations within 

 Tampa Bay have been sorted to family or 

 lower taxonomic level. Thirty-nine families 

 and more than 140 species were recorded. 



Southern Quahog 



Growth of a clam population in lower Boca 

 Ciega Bay was recorded for the fifth year. 

 Average length of individuals in the sample 

 was 5 mm. greater than in 1967 and nearly 

 twice the mean length of clanns measured in 

 1964. A poor set on the bed is illustrated 

 again this year by the scarcity of small clams 

 (fig. 4). 



SHEll IINGTH IN MIOPOINt CLASSES 



23' 



SHELL lENCTH Cl'^t^HES]) 



(^SMM^ 



Figure 4, — Size frequency distribution (May 1968 — sample 

 size 520) and average shell length (arrow) of the 

 southern quahog (M. campechiensis ) from a population 

 in Boca Ciega Bay, Fla,, 1964-68 (1 mm. =0.04 inch). 



Table 2. — Biometric data for six large southern quahogs from Boca Ciega Bay, Fla. 



We also collected more than 100 large clams 

 in northern Boca Ciega Bay where a record- 

 size clam was reported in 1964. The specimen 

 was 168 mm. long and weighed 3 kg. (kilo- 

 grams). Our collection had two living and two 

 recently dead clams whose length exceeded that 

 of the record specimen (table 2). We deter- 

 mined the age of the clams by counting annual 

 growth lines on the shell; the distance between 

 successive annuli showed yearly increments of 

 growth. To help identify the annuli, we used a 

 diamond-toothed saw to cut shells trans- 

 versely from the umbo to the ventral edge 

 (fig. 5). The estimated age of the six largest 

 clams in the collection was 14 years or more 

 (table 2), The largest living clam had a shell 

 length of 172 mm. (fig. 6) and an estimated age 

 of 20 years. 



Lugworm 



A large marine baitworm commonly called 

 the lugworm ( Arenicola c ristata ) was intro- 

 duced this year as a prospect for aquaculture. 

 The worm is good bait for spotted sea trout, 

 sheepshead, and red and black drums. The 



