4.3.4.2 laboratory Toxicity Studies 



Extensive part per billion acute and chronic laboratory toxicity studies 

 of TOT with oysters have been conducted in France and England (See Tables 

 4.10 and 4.11). laboratory bioassays determined that concentrations of 9 

 ug/1 TOT was lethal to half of the larvae of the Pacific Oyster exposed in 

 48 hours and that exposures to 2.4 ug/1 would cause a significant 

 reduction in the growth of the spat which had settled on a substrate. 

 These same concentrations were also found to cause a significant reduction 

 in the rates of growth of mussel spat Mytilus edulis . Clam spat Venerupis 

 decussata and the spat of the European flat oyster Ostrea edulis . Studies 

 reported by Alzieu (1986) from France were able to delineate embryogenesis 

 and larval developmental effects over a range of TOT concentration 

 exposures, see Table 4.12. 



Other serious reproductive effects have been found in other molluscs which 

 accounts for the demise of local populations in England of gastropods 

 Nucella lapillus . (the dogwhelk) and Nassarius obsoletus (the mud snail) . 

 The dogwheUcs are predators, feeding mainly on barnacles, mussels and 

 limpets. Populations of the common dogwhelk, Nucella lapillus . around the 

 South-West peninsula of England were studied by scientists of the UK 

 Marine Biological Association for occurrence of "imposex" (imposed sex) 

 which is the superimposition of male characteristics notably a penis and a 

 vas deferens, on female individuals of the species. These researchers 

 found a high occurrence of imposex in populations close to centers of 

 boating and shipping activities. The occurrence of imposex correlated 

 significantly with the concentration of body burdens of tin in dogwheUcs 

 (up to 2 ug/g dry tissue) . Laboratory studies have confirmed that 

 exposure to TOT will induce imposex. The ecological significance is that 

 the field observations of Nucella lapillus populations around South-West 

 England indicate that those populations which exhibit marked imposex show 

 signs of decline in numbers, populations contain fewer females than 

 expected and that juveniles and deposited egg capsules are scarce or 

 absent, indicating a lowering of the reproductive capacity of those local 

 populations. Also females examined had oviducts clogged with decomposing 

 eggs that they could not release. The newly formed male reproductive 

 tissues blocked these oviducts. Subsequently laboratory studies suggest 

 that environmental concentrations of about 20 ng/1 of TOT as Sn (56 ng/1 

 TOT) seems to initiate imposex (Bryan et al., 1986; Bryan et al., 1987; 

 Gibbs and Bryan, 1986; Gibbs and Bryan, 1987; Gibbs et al., 1987). See 

 Figure 4.3, from Gibbs et al., 1987 for description of stages, and Figure 

 4.4 which is a photograph of a female dogwhelk which has developed three 

 penises (photograph courtesy of P.E. Gibbs, U.K. Marine Biological 

 Association) . 



IV-20 



