Victor Axiak

Imposex in Hexaplex trunculus (Gastropoda: Muricidae): first results from biomonitoring of tributyltin contamination in the Mediterranean

Imposex (the occurrence of penis and vas deferens in females) in Hexaplex trunculus collected from Malta (Central Mediterranean) in 1992 is reported. This phenomenon is related to the levels of tributyltin (TBT) in the organisms as well as in sediments. Imposex indices (relative penis size and vas deferens sequence) are used to biomonitor coastal contamination by TBT for the first time in the Mediterranean. In its response to TBT, H. trunculus is ranked as one of the most sensitive neogastropod species studied so far. Accumulation of TBT and its metabolites in the digestive glands, gonads and the rest of the body are described, and sex-related differences are shown. No preferential female mortalities are recorded in populations exposed to high levels of TBT. However, a reported shift in the size frequency distribution of H. trunculus in contaminated sites, towards bigger snails, may suggest reduced reproductive potential.

Imposex as a biomonitoring tool for marine pollution by tributyltin: some further observations

Imposex, i.e., the imposition of male sexual characteristics on female neograstropods, has been used worldwide as a bioindicator of pollution by the antifouling agent TBT as well as to assess the related ecological impact. The recent total ban on the use of TBT in the maritime industry has been partly based on the use of such a biomarker. This is mainly based on the assumption that no other pollutant, or environmental stress, is able to induce such a biological response. Nonetheless, several authors had challenged this idea. The present paper will present further field observations on imposex in Hexaplex trunculus from Malta (Central Mediterranean) which may help clarify this issue. Furthermore, the incidence of imposex in this species will be related to body burdens and to the environmental levels of organotins as measured analytically. First histological observations will also be presented which may help clarify the ecological significance of this response in this species. Finally a number of alternative scenarios regarding the relationship between imposex and TBT will be discussed.

Evaluation of environmental levels and biological impact of TBT in Malta (central Mediterranean)

Chemical monitoring for butyltins in bulk seawater, surface microlayer and superficial sediments determined that the two main local sources of marine contamination by tributyltin (TBT) compounds in Malta are ship-repairing dockyards and marinas. In bulk seawater, TBT values ranged from below the detection limit of 5 ng Sn l(-1) to 300 ng Sn l(-1); in sediments derived from the most polluted areas, TBT concentrations as high as 1500 ng Sn g(-1) were measured. At TBT levels found in local harbours, several sublethal biological responses are possible and were observed, including a significant reduction in MFO enzyme system activities of fish; digestive cell atrophy in the oyster Ostrea edulis; and induction of imposex in the snail Hexaplex trunculus. The latter two responses are evident at TBT concentrations below the environmental quality standard (20 ng TBT l(-1)). The ecological implications of these results are discussed and it is concluded that exposure of marine organisms to TBT in local harbours may be expected to lead to a reduction in the long-term survival of a number of such organisms. For these reasons, both the use of TBT-based antifouling paints for pleasure boats as well as drydock practices need to be controlled by appropriate regulations and their enforcement.

Sewage pollution impact on Mediterranean rocky-reef fish assemblages

The effects of sewage outfalls on subtidal fish assemblages were studied along the NW coasts of Malta (Sicily channel, Mediterranean Sea) by means of underwater visual census. The presence of two spatially distinct outfalls discharging untreated wastewaters allowed to use a balanced symmetrical after control/impact (ACI) design that consisted of two putatively impacted locations and two controls, with four sites nested in each location. Surveys were performed in 2006 at two random dates. The study highlighted significant changes at both assemblage and individual species levels. Fish assemblages structures were different between controls and sewages, where total abundance of fish were higher. The responses of individual species to sewage pollution were mostly related to an anomalous increase of two small opportunistic species i.e. Gobius bucchichii and Parablennius rouxi and to a decrease of species of the genus Symphodus, particularly S. roissali and S. ocellatus. Moreover in correspondence of the outfalls, significant changes of the fish size distribution were detected for several species. These results support the use of fish assemblages as biological indicators for marine coastal waters and demonstrated the possibility to obtain sharp signals of environmental impact from some individual fish species.

Laboratory and field investigations on the effects of organotin (tributyltin) on the oyster, Ostrea edulis

Abstract Specimens of Ostrea edults collected from coastal sites exposed to antifouling tributyltin (TBT) were found to exhibit shell thickening as indicated by a particular shell thickness index. It is suggested that this effect was qualitatively different from that reported for other bivalves and known to be specifically caused by TBT. Nonetheless, laboratory exposure experiments showed that nominal levels of 10 ng/l of TBT in seawater significantly reduce digestive cell volume in this species, and this may lead to reduced somatic growth and thus to shell abnormalities in this species.

Bioenergetic responses of the marine bivalve Venus verrucosa on long-term exposure to petroleum hydrocarbons

Abstract A number of physiological responses of the bivalve Venus verrucosa on exposure to low levels of water accommodated fractions of Kuwait crude oil (100 μg litre −1 ) for 145 days were investigated in the laboratory. Such exposure led to significant decreases in clearance or feeding rates and in the food absorption efficiencies, as well as enhanced oxygen consumption and ammonia excretion. The integration of such physiological responses to assess the scope for growth, as well as the oxygen to nitrogen ratio, indicated that such exposure led to a significant drop in the energy available for somatic growth and reproduction and enhanced protein catabolism (this being indicative of stressed conditions). These results were also confirmed by significant reductions in several body condition indices of the exposed bivalves relative to the controls. The ecological significance of such results is discussed.

Organotin pollution in Malta coastal zone

The speciation of organotins in seawater, surface microlayer and sediments in the marine environment of Malta is described. Concentrations of tributyltin (TBT) in bulk seawater inside the harbours were as high as 0.3 μg Sn L‐1 but were below detection limits (5 ng Sn L‐1) in open sea, 1.6 km offshore. In sediments, TBT levels were highest for the yacht marinas and ranged between 0.03 and 1.5 ug Sn g‐1. Dibutyltin is more common in the surface microlayer than TBT. Diphenyl and monophenyltin were found infrequently in bulk seawater and in sediments. Tetrasubstituted organotins, namely, MenBu(4_n)Sn, where n = 1, 2, and 3, were found frequently in TBT‐contaminated sediments (0.1–9μg Sng‐1), in seawater and in the microlayer where concentrations as high as 140 μg Sn L‐1 (Me3BuSn) were measured. Direct environmental methylation of TBT and that of its debutylated analogues may play a significant role in the geochemical cycling of tin under certain environmental conditions.

Analytical aspects of the gas chromatographic determination of tributyltin and metabolites in environmental samples

Tributyltin and its metabolites were determined in environmental samples by gas chromatography using a flame photometric detection method. The analytical method involved the propylation of the organotins and the use of a recovery standard to correct for inefficient recovery. A detection limit of 5 ng Sn l(-1) for seawater, and 5 ng Sn g(-1) for sediments, was obtained when the detector was equipped with an optical filter; however, this concentration limit, which is higher than the environmental quality standard for TBT, could be improved by a factor of 100 on removing the filter. The limitations imposed by the choice of recovery and internal standards were discussed and it was shown that the possible presence of Sn(II) had no effect on the total extractable inorganic tin, as determined by the analytical protocol. The identification of unexpected mixed methylbutyltins in TBT-contaminated sediments was described, and the implications of the existence of such compounds were discussed.

Behavioral responses of a marine bivalve (Venus verrucosa) to pollution by petroleum hydrocarbons

The effects of exposure to petroleum hydrocarbons (PHC) on the burrowing behavior, valve activities and siphonal and foot reactions of the clam, Venus verrucosa were investigated in a series of laboratory experiments. Such exposure did not significantly alter the burrowing activities of this species. The foot, siphons and mantle folds failed to respond both to low and high concentrations of PHC. After 130 days of exposure to 100 μg L−1 of water-accommodated fractions of oil, both the response time and the recovery period of the siphonal response to light stimuli were significantly prolonged. Increased valve activities were also recorded. The biological significance of such altered behavior and activities is discussed.