Heavy metal pollution has become a great source of concern, especially through contamination of soil and groundwater via point and non-point sources (Edwards, 2002). Metals can persist in the soil for a long period of time, hence creating long-term hazards. Soil contamination with metals results in accumulation and subsequent toxicity to plants (Zayed et al. 1998; Gimmler et al. 2002), microbes and invertebrates (Owojori et al. 2009a). In metal-contaminated industrial soils, reports of loss of invertebrate species diversity are commonly reported (Spurgeon et al. 1994; Jones 1991). In earthworms, heavy metals can cause spermatozoa damage thereby reducing the earthworm sperm count (Cikutovic et al. 1993; Reinecke and Reinecke 1997), which negatively affects reproduction (Spurgeon et al. 1994). Abnormal environmental metal concentrations have been reported to adversely affect the feeding, growth, reproduction, and general physiology of molluscs (Bonally De Calventi 1965; Calabrese et al. 1977).
One way by which organisms interact with metals in the environment is through bioaccumulation. Metals are non-degradable and tend to bioaccumulate in soil biota. For example, Owojori and Siciliano (2012) reported a substantial accumulation of cadmium and lead in soil mite, Oppia nitens. In two genera of earthworms, Amynthas and Metaphire exposed to a metal-contaminated site, cadmium's bioaccumulation factor was reported to be more than ten (Wang et al. 2018). In a study where snails, Helix aspersa was exposed to a metal-polluted site and compared to a non-polluted site, a substantial accumulation of copper, zinc, lead and cadmium was reported (Gomot de Vaufleury and Pihan 2000). The bioaccumulation potential of soil biota, demonstrates that bioaccumulation is a major process through which increased levels of metals are transferred across food chains, creating ecological and public health problems. Therefore, it is important to always determine the capacity for bioaccumulation and the toxic effect of heavy metals on organisms, especially edible ones like snails, to assess potential risks to human health. Moreover, it is important to note that molluscs can accumulate higher metal ions concentrations than other invertebrate groups (Beaby and Eaves 1983).
In a study of invertebrates in the vicinity of the Avon-mouth smelting works (UK), the snail, Helix aspersa was not found in regions of extremely high metal pollution that are close to the factory (Jones 1991). One way to explain this phenomenon was that there was direct poisoning of snails by metals ingested with food. However, in laboratory experiments, where snails and slugs were exposed to very high heavy metal concentrations through food, they have proved to be tolerant (Marigomez et al. 1986). The authors did not find any effect of Zn or Cu in diets on the mortality of the slug Arionater at concentrations as high as 1000 mg/kg. Similarly, Russell et al. (1981) found no mortality in Helix aspersa that were fed on a diet containing up to 1000 mg Cd/kg food. This high tolerance in the molluscs, is possibly due to the efficient binding of metal ions by metallothioneins (Taylor et al., 1988) and deposition in insoluble intracellular granules (Howard et al. 1981). For example, Beeby and Richmond (1989) suggested that the snail shell could act as a temporary ‘sink’ for Pb. Terrestrial molluscs may also possess other physiological mechanisms to regulate metal assimilation from food sources (Berger and Dallinger 1989).
Most of these studies were conducted with temperate snail species, and very few studies have been conducted on the toxicity of metals to tropical snail species. Notably, among the snail species studied in the tropics are the giant land snail, Archachatina marginata. Their metal bioaccumulation tendencies in the field have been widely reported (Wegwu and Wigwe 2006; Ebenso and Ologhobo 2008). Some studies have also reported metal toxicity to A. marginata (Otitoloju et al. 2009). Ugokwe et al. (2020) also studied the effect of contamination on another tropical land snail species, Limicolaria aurora, where it reported induction of oxidative stress enzymes when exposed to waste leachates containing metals compared to the non-exposed snails. Archachatina papyracea is another known giant land snail species, but metal toxicity to this snail and metal accumulation is largely unknown. In Nigeria and many other countries in the South, Pb and Cd are introduced into the environment through paint flakes, burning of fossil fuels which contains these metals as anti-knocking additives, and through metal mining activities (Yakeen and Onifade 2012; Adeyi and Babalola 2017; Yahaya et al. 2021). Therefore, metal contamination of the environment is a plausible occurrence through these activities, especially in sites close to these industrial activities. Snails purchased in the local market and eaten in these regions are often gathered from the wild and farmlands (Babatunde et al. 2019); however, studies have shown that snails gathered from farmlands close to industrial activities have a high accumulation of metals (Onuoha et al. 2016). Therefore, it is important to assess the effect of some metals on the toxicity and accumulation of local snail species in the area.
The land snail Archachatina papyracea is often found in garden soils throughout Nigeria. Therefore, a study assessing the effects of Pb and Cd in the local population of snails may be critical to assessing the environmental and human impact of the metal contamination in an area. For this study, we assessed the accumulation of Pb and Cd, and the toxicity of these metals based on survival and weight change to the tropical snail species, A. papyracera. The choice of heavy metals, Pb and Cd used for this study was because Pb and Cd, though non-essential metals, are often found in high concentrations in abandoned mining and smelting sites, and most industrial activities such as in the auto-mechanic industry (Dudka et al. 1995; Nica et al. 2012, Abidemi et al. 2011). Moreover, compared with other metals, the toxicity of cadmium and lead can be very high (Owojori and Siciliano 2012; Ziyou et al. 2016; Mahmutovic et al. 2018).