Carapace asymmetry: A possible biomarker for metal accumulation in adult olive Ridleys marine turtles?

Highlights

• Developmental instability index can determine the asymmetry with just a carapace photo.

• More asymmetric adult turtles presented higher concentrations of some of the analyzed metals.

• Ridley's carapace asymmetry could be use as biomarker for some metals accumulation.

Abstract

The Olive Ridley marine turtle (Lepidochelys olivacea) is characterized by individual morphological variability in the number and shape of scutes. The influence of pollutants on developmental instability and one of its consequences, the asymmetry of individuals, has been demonstrated in several species, especially invertebrates and some birds. However, the use of this asymmetry as a biomarker of contamination in adult individuals has never been explored. We developed an index to quantify developmental instability (DIx) based on the number and relative size of costal carapace scutes. The link between DIx and inorganic elements concentrations was explored in various tissues of stranded turtles from the Southern Mexican Pacific. The relationships between adult contamination and DIx could directly or indirectly reflect (i) the disruption of metal elimination in the adult stage dependent on embryonic perturbation and thus determining DIx, (ii) the difference in metal absorption dependent on DIx status, or (iii) DIx linked to other unknown factors.

Introduction

The impact of anthropogenic pollutants on marine ecosystems is of global concern (Depledge et al., 2013; Duarte, 2014; Ericson et al., 2013). Among the many different kinds of contaminants, some of the most studied are inorganic elements due to their toxicity. In marine environments, these elements occur naturally at low concentrations; however, anthropogenic activities can increase their concentrations to high levels that may pose a threat to marine ecosystems with deleterious results (Hansen et al., 2016). Regarding non-essential elements, lead (Pb) and cadmium (Cd) are among the most studied and associated with diverse pathologies in different marine species (Boughammoura et al., 2013; Nunes et al., 2014; Rana, 2014; Sonne et al., 2009).

In addition, other non-essential elements such as titanium (Ti), thallium (Tl), lithium (Li), and strontium (Sr) are a growing concern due to their tendency to induce physiological effects after long-term exposure (Lehmann and Lee, 2013; McPherson et al., 2014; Rodriguez-Mercado and Altamirano-Lozano, 2013; Shi et al., 2013). For instance, titanium nanoparticle (Ti-NP) have increasingly been widely used for different purposes such as plastic, pharmaceuticals, cosmetics and food colorants industries among others (Bakare and Udoakang, 2016). These new applications of Ti-NP, however, call into question their biological effects (Chen et al., 2014). Studies have revealed that a high (20 to 50 mg/mL) chronic exposure to Ti-NP can cause inflammation, fibrosis, lung tumors and are possibly carcinogenic to humans (Bakare and Udoakang, 2016; Chen et al., 2014). The same authors mention that evidence indicates that Ti-NP can impair cellular DNA repair and induce genotoxicity. Most of these non-essential elements have been identified having an interaction and effects on Ca regulation (Khandwala and Van Uum, 2006; Lehmann and Lee, 2013), and therefore, deleterious effects on the skeletal tissue metabolism of vertebrates (Boughammoura et al., 2013; Emiroglu et al., 2010; Lall and Lewis-McCrea, 2007; Pigatto et al., 2016; Rana, 2014).

Meanwhile, inorganic essential elements, such as aluminium (Al), arsenic (As), cobalt (Co), chrome (Cr), copper (Cu) and selenium (Se), play an essential role in certain metabolic processes. However, when their concentrations accumulate beyond physiological requirements, they become toxic in many species such as fish (Bears et al., 2006), rats (Mahieu et al., 2005), primates (Clauss and Paglia, 2012), and reptiles (Marco et al., 2004; Perrault et al., 2013). For instance, Se, being essential for cell functions, is recommended for daily consumption in humans, but at higher concentrations, it may cause cardiovascular disease, arrest the cell cycle, and break the DNA structure (Sun et al., 2014). Arsenic, on the other hand, is highly toxic in many species. In humans and some experimental animals, in high and acute doses, this element tends to accumulate in liver > kidney > muscle and brain (ATSDR, 2007). Accumulation of As depends on many factors, but marine species (e.g., jellyfish, cephalopods, some fish) are known to have a high concentration of this element (Bears et al., 2006; Fujihara et al., 2003; Williams et al., 2006). In many species, the digestive tract is an effective barrier to avoid Al absorption, and in a normal organism, the small amount absorbed is soon excreted in urine (Drüeke, 2002). However, when exposure is very high or when there is kidney dysfunction, Al begins to accumulate in various tissues such as bone, brain, liver, and kidney (Drüeke, 2002; Mahor and Ali, 2015; Scheuhammer, 1987).

Developmental stability (DS) is the ability of an organism to withstand perturbations during its development (Băncilă et al., 2012; Beasley et al., 2013; Bishop et al., 1998; Mast and Carr, 1989). DS has been used as an indicator index of individual fitness and is considered an environmental stress biomarker in different species related, among others, to certain pollutants such as organochloride and metals (Băncilă et al., 2012; Beasley et al., 2013; Ho et al., 2009; Jawad et al., 2016). A common approach for assessing DS is through fluctuating asymmetry (FA) analysis (Beasley et al., 2013; Dongen, 2006; Parsons, 1989; Sanchez-Chardi et al., 2013). For instance, Eeva et al. (2000) reported increased levels of FA in the tarsus of Parus major (passerine birds) populations exposed to metals compared to other sites with less metal exposure. Bishop et al. (1998) also found higher rates of developmental abnormalities in eggs and hatchlings of Common Snapping turtles (Chelydra serpentina) exposed to a higher concentration of organochlorine and total mercury. In this sense, geometric morphometric tools have also proved to be more sensitive and able to detect a larger signal of FA compared to studies that use only linear/meristic measures (Beasley et al., 2013).

In sea turtles, a high frequency of carapace bilateral asymmetry has been described in Olive Ridley (Lepidochelys olivacea) in the Indian Ocean (Deraniyagala, 1939), Western Africa (Carr, 1957), British Guiana (now Guyana) (Pritchard, 1966), Surinam (Hill, 1971) and on the Pacific coast of Mexico (Frazier, 1983). Developmental instability generally reflects a non-normal development caused by perturbations of the developmental process, while developmental stability reflects the capacity to avoid or reduce such perturbations by developmental means (Moller, 2006). The asymmetric condition of L. olivacea carapace can be considered fixed at the hatching stage, because costal carapace scutes do not show any significant allometric variance during the different life stages of marine turtles (Mast and Carr, 1989; Wyneken, 2001). In Olive Ridleys, egg manipulation for protection purposes and high incubation temperatures during embryonic development may promote asymmetry in turtles (Ayres-Fernández and Cordero-Rivera, 2004; Davis and Grosse, 2008; Lazic et al., 2013; Mast and Carr, 1989; Navarro Sánchez, 2015). Regarding developmental instability and pollutants, two studies on chelonian have been published (Bishop et al., 1991; Bishop et al., 1998). These works focused on eggs, developing embryos, hatchlings, and the FA of snapping turtles (Chelydra serpentina serpentina), with high rates of developmental abnormalities related to higher polychlorinated aromatic hydrocarbon concentrations in eggs.

High metal concentrations in Olive Ridley marine turtle (Lepidochelys olivacea) populations from the Southern Pacific coast of Mexico have been previously documented (Cortés-Gómez et al., 2014). In many species, the ability of an individual to escape pollutant threats differs based on behavioral or physiological adaptations. For example, the individual may have some physiological capabilities to be more or less affected by pollutants based on its capacity to concentrate them more or less or to detoxify better (Walker et al., 2005). To assess the effect that pollutants can pose to individuals, is imperative to anticipate and try to settle different biomarkers, either of exposure, effect or sensibility (Aguirre and Lutz, 2004; Labrada-Martagon et al., 2011). Additionally, there is a demand for species that can serve as bioindicators of pollutants and for nondestructive sampling techniques (Beasley et al., 2013; Burger et al., 2007). As previously mentioned, the influence of pollutants on developmental instability and one of its consequences, the asymmetry of individuals, has been demonstrated in several species, but the reverse link has never been explored: can carapace asymmetry at the adult stage be used as a biomarker of contamination in individuals?

To explore this premise, the aims of this paper were: (1) to establish the concentrations of 16 inorganic elements in liver, kidney, muscle, brain, bone, blood, and egg components (albumin, yolk and eggshell) from stranded dead Olive Ridley turtles; (2) to develop a method to use carapace morphologies obtained from photographs of L. olivacea to quantify the morphological plasticity that could be a marker of developmental instability; and (3) to examine if there is a relationship between inorganic elements and asymmetry of the carapace of this species, and if this information can be used as a biomarker for these pollutants in this species.