The research described in this thesis concerned the perhaps best known and studied common gastropod from the open North Sea of which only limited information was available. With the present research more insight has been obtained concerning this long-lived, off-shore snail species, which existence is threatened by human activities. To date, no other laboratory exposure studies with Buccinum undatum of different developmental stages than those reported in this thesis have been performed.Summary of resultsLaboratory maintenance and rearing (Chapter 2)The attempts to optimize conditions to study adult whelks in the laboratory have resulted in a successful method for the reproduction of B. undatum in captivity and for the rearing of the off-spring. Juvenile whelks with a well-known background became subsequently available for the exposure studies to realize the objectives of the studies.Laboratory exposure studies (Chapter 3)Imposex was induced in juveniles at concentrations > 7 ngSn/l, which showed B. undatum to be less sensitive than N. lapillus. On the other hand this indicates that TBT contamination of the North Sea can be substantial.This research has clearly shown age-related differences in sensitivity for the development of imposex. Developing juvenile whelks were affected by TBT, whilst fully matured adult females were not, when exposed for 11 months. Adolescent females showed a deviant response to TBT from juveniles which had just hatched, this points to a sensitivity where the stage of development influences the trigger for the formation of male sexual organs.Exposure of egg-masses with developing whelks ( in ovo ) to TBT did not result in an increased masculinization.Two year old juveniles had not yet developed gonadal tissue. The formation of a penis and a vas deferens in TBT-exposed juveniles can therefore not be related to or influenced by any steroid hormone produced in the gonads. This points to a different, possibly neuroendocrinological, mechanism.TBT inhibited general cytochrome P450 activity in whelks. If certain cytochrome P450 iso-enzymes are involved in steroid hormone metabolism or synthesis in whelks, TBT might influence these processes.Although TBT exposure resulted in imposex, the consequences for populations of B. undatum showed no sterilization effects on reproduction as opposed to the development in N. lapillus . However, indications were found that exposure to TBT can reduce growth which might affect reproduction indirectly when juveniles take longer to mature.No other masculine sexual organs than a penis and a vas deferens were formed in females after two years of TBT-exposure. Histologically these structures consisted of the same cell types normal for males. Juvenile whelks first developed their secondary sexual organs, followed by the differentiation of primary sexual organs.The research has contributed to the decision of the Marine Environmental Protection Committee (MEPC) of the International Maritime Organization (IMO) to ban the application of TBT by 1 stJanuary 2003 and to ban its presence by 1 stJanuary 2008 (Meeting MEPC 42, 1998).Method for the identification of masculine sexual development in B. undatum (Chapters 2, 3, 4 & 5)The laboratory and field studies demanded the development of a non-invasive method to investigate masculine sexual development or imposex. This method has been put forward in the OSPAR-SIME (Oslo and Paris commissions-Substances in the Marine Environment) workshop of the Joint Assessment Monitoring Programme (JAMP) to be used as guideline for TBT-specific biological effects monitoring in off-shore species (Davies et al. , 1997b).Both laboratory and field studies resulted in a comprehensive overview of the possible stages of male sexual development/imposex in B. undatum .Histological analysis of whelks from the laboratory and field studies showed the structure of a penis and a vas deferens in animals showing masculine development and imposex females to be similar to the cell types found in males, however, in adult imposex females the penis remained smaller than the penis of an adult male. It was found that no other sexual organ than already visible with the eye was present, which validated the method from the first paragraph.Imposex and organotin levels in the Eastern Scheldt (Chapters 4 & 5)The ban on the use of TBT based anti-fouling paints on ships 95% vs 40%). Beam trawl fishery is considered to affect populations of B. undatum adversely in the North Sea.