1 Introduction

The surge in plastic production worldwide is an alarming environmental issue. In 2020, global plastic production was estimated to have reached 367 million metric tons (MMT) (Plastics Europe 2021). Borrelle et al. (2020) projected that in 2016 alone, approximately 23 MMT of plastic waste was generated globally; this figure is expected to rise to an annual input of 90 MMT into aquatic ecosystems by 2030. Plastic debris does not remain confined to coastal areas but rather disperses across the oceans (Chenillat et al. 2021). The Secretariat of the Convention on Biological Diversity (SCBD 2012) reported that over 80% of the detrimental environmental impacts on marine life are attributed to plastic debris. Furthermore, 914 species have been documented to be adversely affected by marine debris through entanglement or ingestion (Kühn and van Franeker 2020). Ingestion of debris by marine organisms can obstruct the gastrointestinal tract (GI tract), causing malnutrition due to dietary dilution and even death (Kühn et al. 2015; Nelms et al. 2016). In the context of microplastics or nanoplastics, there is the added risk of trophic transfer within marine food webs (Chae et al. 2018; Nelms et al. 2018).

Sea turtles are particularly vulnerable to the detrimental effects of entanglement and ingestion of marine debris (Kühn et al. 2015). Out of the seven sea turtle species, six are classified as ranging from “vulnerable” to “critically endangered” in the International Union for Conservation of Nature Red List; the seventh species, the flatback sea turtle (Natator depressus), is categorized as “data deficient” (IUCN 2022). For the entire duration of life, loggerhead turtles (Caretta caretta), hawksbills (Eretmochelys imbricata), olive ridleys (Lepidochelys olivacea), kemp’s ridleys (Lepidochelys kempii) are omnivore, foraging on gastropod, crustaceans, bivalve, algae, etc., in common (Jones and Seminoff 2013). On the other hand, adult green turtles (Chelonia mydas) are herbivores, feeding on seagrass, algae, etc. (Jones and Seminoff 2013). Flatbacks forage on soft-bodied invertebrates, and leatherbacks (Dermochelys coriacea) feed on primarily jellyfish (Jones and Seminoff 2013). The migratory pattern and oceanic–neritic developmental pattern could be diverse between species (Bolten 2002). It is noteworthy that the frequency of occurrence (FO%) of plastic debris ingestion, meaning the percentage of plastic debris detected turtles among analyzed all species of sea turtles, in sea turtles is high compared to other marine species, such as marine mammals and seabirds (Kühn and van Franeker 2020).

Lynch (2018) published a comprehensive literature review and global meta-analysis to ascertain the extent of plastic ingestion in sea turtles. The study suggested standardized unit as g/kg among various units of plastic ingestion data (e.g., pieces/turtle, milliliters/turtle, g/curved carapace length). The analysis yielded two principal insights: (i) the Central and Northwest (NW) Pacific, in conjunction with the Southwest Atlantic Ocean, are hotspots for plastic ingestion, and (ii) green and hawksbill sea turtles are particularly susceptible to plastic ingestion. However, there was no exploration of the properties of the ingested plastics.

In the pursuit of a more granular understanding, several recent studies have investigated the characteristics and origins of debris ingested by sea turtles. For instance, Roman et al. (2020) undertook a literature review to pinpoint the types of debris that have the most fatal impact on marine megafauna, including cetaceans, pinnipeds, sea turtles, and seabirds. Moreover, Moon et al. (2022) investigated the diverse attributes (e.g., shape, color, polymer type, and original purpose) of plastics ingested by sea turtles in Korean waters. Despite these efforts, there remains a conspicuous dearth of studies that offer an exhaustive account of the characteristics of ingested debris.

In this study, we endeavor to bridge this gap by providing an updated review on the quantity of plastic ingestion by sea turtles and FO%, incorporating findings from articles published since Lynch’s review in 2018. We performed a comprehensive literature review focusing on the characteristics of plastic debris ingested by sea turtles, extracting and analyzing data on the characteristics of plastics, such as shape, original usage, color, and polymer type. Additionally, we explored geographical variations in the quantities and characteristics of ingested plastic debris in sea turtles and juxtaposed these characteristics with beach debris data reported by the Ocean Conservancy (2018).

2 Methods

2.1 Search Strategy

For the analysis, we obtained data from a database provided by a recent review article (Lynch 2018), supplemented by a literature search on sea turtles and plastic ingestion. We employed Google Scholar as the search engine, with the following search terms: “marine”, “ocean”, “sea”, “turtle”, “ingest”, “plastic”, “debris”, and “litter”; the search was conducted until February 28, 2022. Articles that were deemed not relevant to the topic were excluded from the study.

We omitted articles that investigated microplastics smaller than 1 mm, which we refer to as small microplastics. While the information on small microplastics is valuable, its inclusion could introduce outliers in terms of numbers, considering that the majority of articles in our database dealt with plastic debris larger than 1 mm. In cases where a study presented data on both small and large plastics (larger than 1 mm), only data pertaining to plastics larger than 1 mm were incorporated. It is worth mentioning that a review on microplastic ingestion by sea turtles would be beneficial in the future, as research on this topic is gradually increasing. Furthermore, reports, review articles, and newsletters were excluded to avoid data redundancy. Ultimately, we collected 110 journal articles.

2.2 Data Collection and Analysis

We primarily obtained data on the quantity and %FO of plastic ingestion from supplementary information provided by Lynch (2018), supplemented with data from 26 studies published subsequently on sea turtles. We used the concentration unit of a gram (dry weight) of plastic per kilogram sea turtle body weight (g/kg wet weight) to analyze the quantity of ingested plastic, in line with the approach proposed by Lynch (2018). For sea turtle carcasses, plastic debris was sampled from the entire GI tract, only the stomach, intestine, or fecal. For alive turtles, non-destructive samples, such as fecal and lavage were collected to identify plastic debris. When a study provided both lavage and fecal data for the same turtles, fecal data were used to calculate the mean values of plastic ingestion, including g/kg, %FO, and other characteristics. Since the length and passage time of the intestine are considerably longer than the stomach, we considered that fecal samples comprehensively encompass plastic ingestion than lavage. In total, we included data from 58 articles for the analysis of the quantity of ingested plastics, and from 100 articles for %FO. In this study, we presented the overall levels and frequencies of plastic ingestion in sea turtles, along with comparisons among different continents or sea turtle species.

To characterize the ingested plastics, we extracted data on their shape, origin, color, and polymer type from articles that included such details. Specifically, 24, 20, 11, and 9 articles provided numerical data on shape, origin, color, and polymer type, respectively.

We re-categorized the shape data into six groups: pellet, film, fiber, foam, fragment, or others. Some articles classified ingested plastics by shape (e.g., fragment, film, and fiber), while others used origin as a classification (e.g., rope, net, and plastic bag) or hard to categorize (e.g., ballon). To allow for a comprehensive analysis of shape, we integrated origin data into shape categories. For example, items like hard plastics, caps, straws, and cups were classified as fragments, while film packaging, plastic bags, bottle labels, tapes, leaflets, food wrappers, and soft plastics were categorized as films. The fiber category included items such as entangled wires, twine, synthetic fibers, nets, ropes, and fishing lines. Expanded polystyrene (EPS; commonly known as Styrofoam) and foamed packaging were classified as foam, and both pellets and nurdles were categorized as pellets. Comparisons of the shapes of ingested plastics were also made among continents and sea turtle species.

Despite the limited data available, we also analyzed the original usage of the ingested plastics. We reorganized information on original usage into the following categories: consumer (single-use, reusable), fishery, industry, or others; others included debris where the originating product could be identified, but its specific origin was unknown (e.g., EPS). Table S1 of Supplementary Material provides detailed information on specific origins included in each category.

We also compared the characteristics of marine plastic debris ingested by sea turtles with coastal beach debris, based on the beach monitoring results of the International Coastal Cleanup (ICC) reported by Ocean Conservancy (2018). In the ICC report, items such as cigarette butts, plastic beverage bottles, plastic bottle caps, straws, and stirrers were categorized as fragments, while food wrappers were categorized as films. Other items such as take-out containers, beverage cans, and glass beverage bottles were categorized as other debris, as their specific shapes and plastic compositions could not be determined.

For color and polymer type analyses, only articles providing numerical data on colors or polymer types of ingested plastics were included. Articles that grouped colors together (e.g., black/brown, blue/green) and did not provide numerical data were excluded. We standardized the color categories as follows: white, transparent, green, yellow, black, blue, red, multicolored, and others (including orange, gray, brown, etc.). In the polymer type analysis, EPS was considered as polystyrene (PS), and polymers found in only one study were classified as others.

3 Results and Discussion

3.1 Geographical Comparison

Sea turtles are migratory animals (Wallace et al. 2010), so it is challenging to determine the exact range of movement for every individual. Thus, only approximate species-specific ranges can be estimated. Different species have distinct habitats and ranges of movement, which implies variations in the extent of plastic pollution exposure. Therefore, a geographical comparison of plastic ingestion needs to be interpreted considering this fact. Due to the lack of information regarding sea turtle movements in the collected literature, this study solely compared the regions where sea turtles were found.

In our analysis, we categorized the study regions of sea turtles into five continents and oceans for geographical comparison. A large proportion of the studies were concentrated in the Americas (53 studies, accounting for 47%) and Europe (22 studies, accounting for 19%). These were followed by Asia (14 studies, 12%), oceans (12 studies, 11%), Oceania (11 studies, 10%), and Africa (1 study, 1%). Despite the extensive geographical areas of Asia and Africa, the number of studies conducted in these regions is limited. There is a pressing need for additional research in these regions to address this disparity.

3.2 Species Analysis

We identified the species of sea turtles that were the subjects of the articles. Green sea turtles were the most studied, with 77 studies, followed by loggerhead (57 studies), leatherback (17 studies), olive ridley (12 studies), Kemp’s ridley (10 studies), hawksbill (10 studies), and flatback (4 studies) sea turtles. Of particular importance, green and loggerhead sea turtles have been the subjects of the majority of studies. This is likely due to their broad geographical distribution, making them ideal indicators for monitoring plastic pollution. These species are among the top six marine species with the highest frequency of plastic entanglement or ingestion (SCBD 2012).

3.3 Quantity of Debris Ingestion

We expanded upon the previous review by Lynch (2018), assessing the quantity of plastic ingestion for green, loggerhead, olive ridley, and leatherback sea turtles. The data on Kemp’s ridley, hawksbill, and flatback sea turtles’ plastic ingestion (g/kg) have not been reported since Lynch’s (2018) review. Our analysis comprised a comprehensive dataset from 58 studies, encompassing 3,280 turtles. Detailed results are listed in Table S2 of Supplementary Material. The data revealed that sea turtles, on average, ingested 0.39 ± 0.75 g/kg of plastic, with individual ingestion ranging from 0 to 3.67 g/kg.

The top three regions with the greatest plastic ingestion by species were the NW Pacific (1.2 g/kg), Central Pacific (1.2 g/kg) and NW Atlantic (0.69 g/kg) for green sea turtles; North Central Pacific (0.9 g/kg), West Indian Ocean (0.68 g/kg) and Northeast (NE) Atlantic (0.32 g/kg) for loggerhead sea turtles; NW Atlantic (2.3 g/kg), Mediterranean (0.16 g/kg), and NE Atlantic (0.003 g/kg) for leatherback sea turtles; NW Atlantic (0.004 g/kg) alone for Kemp’s ridley sea turtles (no plastic was detected in Kemp’s ridley sea turtles in the NE Atlantic); North Central Pacific (0.22 g/kg), Southwest Atlantic (0.03 g/kg), and NW Pacific (0.0004 g/kg) for olive ridley sea turtles; and North Central Pacific (3.1 g/kg) and SW Atlantic (0.38 g/kg) for hawksbill sea turtles. For flatback sea turtles, none of the studies reported specific quantities of plastic ingestion. Since different species show distinct preferences for prey and habitats even within the same region, the amount of plastic ingestion can vary. Schuyler et al. (2014a) found differences in FO% based on the diet habit of sea turtles, while Schuyler et al. (2016) revealed varying probabilities of plastic ingestion depending on whether turtles inhabited neritic or oceanic regions. Species-specific variations in plastic ingestion quantities and FO% among sea turtles in the same region have been found not only in this study but also in prior research (Lynch 2018). Also, the wide range of investigation period in collected literature could potentially result in variations in plastic pollution trends within the same region.

Sea turtles are long-lived animals. Identifying the relationship between plastic ingestion quantities and sea turtle age is an exceedingly intriguing topic. Their age can be approximately estimated using curved carapace length (CCL) or straight carapace length (SCL). However, most of the studies do not provide CCL and ingestion quantities of raw data for each individual. Though we did not conduct correlation analysis in this review, certain studies have presented a negative correlation between the amount of plastic ingested by sea turtles and their CCL (Clukey et al. 2017; Moon et al. 2022; Nunes et al. 2021). This indicates that as the size of sea turtles increases (with higher ages), plastic ingestion quantities decrease. Conversely, there are studies showing the opposite trend (Domènech et al. 2019; Wedemeyer-Strombel et al. 2015). If more individual-specific information amasses, allowing data aggregation, it would be possible to ascertain the link between plastic ingestion quantities and sea turtles’ size.

3.4 Frequency of Debris Ingestion

A total of 100 articles provided clear records of the %FO of plastic ingestion (Table S3 of Supplementary Material). Of the 8888 sea turtles studied, 54 ± 36% had ingested plastic. This was similar to the 52% estimated by Schuyler et al. (2016) in their global risk analysis that considered factors such as debris mapping, and sea turtle distribution, life history stage, and species.

The %FO of plastic ingestion was 88 ± 18% among five flatback, 64 ± 34% among 5232 green, 61 ± 45% among 98 olive ridley, 50 ± 41% among 81 leatherback, 48 ± 33% among 2852 loggerhead, 35 ± 36% among 24 hawksbill, and 20 ± 26% among 235 Kemp’s ridley sea turtles.

By region, 75% of sea turtles in the NW Indian Ocean (n = 92), 74% in the North Central Pacific (n = 154), and 74% in the NW Pacific (n = 66) ingested plastic (Table S3 of Supplementary Material). The NW Indian Ocean, which displayed the highest %FO, is located adjacent to Africa and Southwest Asia. The plastic entering the Indian Ocean is estimated to account for 15–20% of the global emissions per year (Pattiaratchi et al. 2022). Furthermore, around 7% (6 MMT) of the total marine fishery production takes place in the Western Indian Ocean (FAO 2020). Even though the plastic debris emission and generation potential in the NW Indian Ocean are not higher than that in the Pacific Ocean, sea turtles in the NW Indian Ocean showed a similar FO%. The sample size of the regions may be not a reason for this result. (NW Indian (n = 92), N Central Pacific (n = 154), and NW Pacific Ocean (n = 316)). To thoroughly assess plastic pollution in sea turtles, it is imperative to record both %FO and g/kg, which represent ingestion quantities. The prominence of %FO in NW Indian might not reflect the complete picture, as g/kg data for NW Indian turtles is missing. Additional research on plastic ingestion recording quantities is needed in this region.

The high %FO of sea turtles in the North Central Pacific is in agreement with the findings of Lebreton et al. (2018), who reported a substantial quantity of plastic debris in the same region. This extensive plastic pollution in the North Central Pacific is likely due to active fishery activities and the significant release of plastic waste from Asian regions into the ocean. Approximately, 18% (15 MMT) of the world’s marine fishery production originates from the North Central Pacific region (FAO 2020), and East Asian countries contribute significantly to plastic emissions into the ocean (Meijer et al. 2021), which are subsequently transported by ocean currents into the North Central Pacific.

The high %FO of sea turtles in the NW Pacific may also be attributed to substantial plastic input from East Asia. About a quarter of global plastic debris is released from East Asia (Borrelle et al. 2020), and there are substantial fishery activities in this region, with a fishing production of 20 MMT (FAO 2020), which could be a major source of debris.

3.5 Shape of Ingested Debris

Plastic debris undergoes weathering in the environment, leading to its fragmentation into smaller particles, and causing the prints and markings on the plastic to fade. Some studies identify the origin of plastic debris; however, many studies categorize fragmented plastics as “hard plastic” without specifying their origin. Therefore, we converted origin information into shape information where applicable. Twenty-four articles reported the shape of the ingested plastics. A standardized protocol for classifying the shape of plastics is lacking, so the shape of plastics ingested by sea turtles has been documented in various ways. For instance, Domènech et al. (2019), Matiddi et al. (2017), Pham et al. (2017), and Yaghmour et al. (2018) followed the Marine Strategy Framework Directive guidelines (MSFD 2017) for classification. In our study, we reclassified or converted the origin data from each article into shape classifications following guidelines proposed by the INDICIT Consortium (2018), including categories such as pellet, film, fiber, foam, fragment, and others. The shapes of the ingested plastics are summarized and compared by region in Table 1 and Fig. 1. The shape composition is represented as a count-based percentage.

Table 1 Data on the shape composition (%) of plastic debris ingested by sea turtles extracted from the literature review
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Fig. 1
figure 1

Shape composition (%) of plastic debris ingested by sea turtles in different global regions. Shapes were classified as fragment, film, fiber, foam, and pellet. The data represent proportions by plastic particle count, except for Fukuoka et al. (2016) and Nunes et al. (2021), who used mass-based proportions. Descriptions of each reference can be found below the figure [Brazil (a): Rizzi et al. (2019), Brazil (b): Nunes et al. (2021), Brazil (c): Petry et al. (2021), Brazil (d): Poli et al. (2015), USA: Texas: Choi et al. (2021), USA: Florida: Eastman et al. (2020), Japan: Fukuoka et al. (2016), Oman: Sinaei et al. (2021), South Korea: Moon et al. (2022), UAE (a): Yaghmour et al. (2018), UAE (b): Yaghmour et al. (2021), Croatia: Lazar and Gracan (2011), Greece: Digka et al. (2020), Italy (a): Camedda et al. (2022), Italy (b): Matiddi et al. (2017), Italy (c): Di Renzo et al. (2021), Spain (a): Domènech et al. (2019), Spain (b): Solomando et al. (2022), Spain (c): Tomás et al. (2002), New Zealand: Godoy and Stockin (2018), Azores: Rodríguez et al. (2022), Hawaii and American Samoa: Clukey et al. (2017), North Atlantic Subtropical Gyre: Pham et al. (2017), Réunion island: Hoarau et al. (2014)]

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We observed some differences in the shapes of plastic debris ingested by loggerhead and green sea turtles. Loggerhead turtles tended to ingest significantly more foam-shaped plastics compared to green turtles (Wilcoxon rank sum test, p < 0.05), whereas green turtles ingested more film-shaped plastics, but not significantly (Wilcoxon rank sum test, p > 0.05) (Fig. 2). The proportion of fiber and fragment-shaped plastic were similar between those species. Both species ingested negligible numbers of resin pellets and other shapes. The feeding characteristics of each species may contribute to these differences, with loggerhead sea turtles being omnivorous throughout their lives, feeding on fish and crustaceans among other food items, whereas green sea turtles undergo a dietary shift from omnivorous to herbivorous, primarily consuming seagrass and algae. Consequently, loggerhead sea turtles may ingest foam debris, such as EPS buoys, while foraging for invertebrates (Fukuoka et al. 2016).

Fig. 2
figure 2

Comparison of the shape compositions (%) of plastic debris consumed by loggerhead (Cc, n = 1005) and green (Cm, n = 735) sea turtles. The numbers above bars represent mean values and asterisks on bars indicate significant differences between species. Pellets and other shapes are excluded from this figure due to the extremely small proportion

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To identify whether the characteristics of ingested plastic are reflective of the plastic debris found in the marine environment, we compared ingested debris with beach debris data from the ICC report (Ocean Conservancy 2018). We extracted and converted the number of the top ten items from each national beach into shape data, as described in the Methods. In most countries, fragments constituted more than half of the beach debris (Fig. 3). For example, in the United Arab Emirates (UAE), fragments accounted for 99% of beach debris. Generally, sea turtles most often ingested fragments (42%). Sea turtles from oceans (Hawaii and American Samoa, Azores, and Réunion Island) ingested a high proportion of fragments. However, sea turtles from Asia ingested a relatively higher frequency of other shapes, especially films and fibers, compared to America, Europe, and the oceans. The overall compositions of film, fiber, and foam plastics were relatively higher in the digestive tracts of sea turtles compared to coastal beaches (Figs. 1 and 3), suggesting that beach debris may not fully represent the debris ingested by sea turtles.

Fig. 3
figure 3

Shape composition (%) of plastic debris on beaches in various countries in 2017 Ocean Conservancy (2018). Shapes were classified as fragment, film, and foam. All data represent proportions by plastic particle count

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This discrepancy between debris from beaches and sea turtles may be attributed to several factors. First, the selective feeding behavior of sea turtles may have caused them to preferentially ingest plastics resembling their natural prey. Second, plastics may have been ingested accidentally while foraging. Third, beach debris may differ from floating or sunken debris at sea, which is the primary habitat of sea turtles. Duncan et al. (2019) reported a higher abundance of sheet, thread, and dark-colored plastics ingested by sea turtles compared to those found on beaches. Similarly, Schuyler et al. (2014b) found that flexible and translucent debris was more abundant in green, hawksbill, and flatback sea turtles than on beaches. Additionally, the characteristics of plastic debris in the water column may differ from those on beaches due to variations in floating behavior or the tendency to drift ashore, influenced by the density or morphology of individual debris. For instance, EPS particles are likely to drift ashore with waves due to their low density. Thus, understanding the extent of sea turtles’ exposure to plastic debris requires information on underwater plastic pollution, as sea turtles primarily encounter such debris in the water column. However, existing studies on marine plastic debris in the water column are limited, underscoring the need for further research.

Regional plastic pollution can be closely linked to the characteristics of plastics observed in marine organisms inhabiting those areas. The proportions of plastic shapes found in sea turtles were not distinctly consistent among countries sharing the same sea area, such as Europe (Italy, Spain, Croatia, and Greece), the Americas (Brazil and the United States), and NE Asia (Korea and Japan). However, sea turtles from Asia ingested more film than Europe, and less fragment than Oceans in proportion (Kruskal–Wallis test by ranks, p < 0.05). Further studies providing numeric data on plastic shapes of ingested plastic and water columns are needed to better understand the impact of local plastic pollution on sea turtles.

3.6 Original Usage of Ingested Debris

Synthesizing origin information from articles proved challenging due to the use of a variety of categories (n = 20), with only some overlapping among articles. Thus, we reclassified plastics based on their original use into six categories: consumer (single-use), consumer (reusable), fishery, industry, other, and unknown. We analyzed 20 articles for original usage. Debris of unknown origin accounted for 65 ± 43% of the total debris. Most articles focused on the shape or texture of ingested plastics rather than the origin. Nevertheless, six studies ascertained the origin of more than half of the plastic debris. Among these, single-use plastics represented 22% and fishery debris 48%. For instance, on the Brazilian coast, 64% of the plastics found in dead sea turtles were single use (Rizzi et al. 2019). Off the UAE coast, 70% of plastics ingested by sea turtles were fishery-related plastics (Sinaei et al. 2021). These findings indicate that single-use and fishery debris affect sea turtles.

Roman et al. (2020) conducted a recent review attempting to categorize the origin, material, and shape of ingested debris affecting marine megafauna, including cetaceans, pinnipeds, sea turtles, and seabirds. The debris was categorized into various types, such as plastic (hard, film-like, fragments), thread, balloon/latex, rubber, foamed synthetic, paper, metal, glass, rope/net, and fishing debris. The study revealed that film-like plastics and hard fragments were the primary items linked to sea turtle fatalities. Although the review included origin categories like balloon/latex, rope/net, and fishing debris, it remained challenging to determine specific items posing a threat to sea turtles. Moreover, the quantity of plastic ingested by sea turtles was not considered.

It was difficult to integrate data on ingested plastics from the literature, primarily because most studies provided information on shape without accompanying photographs. For example, determining whether to classify “woven plastic bag” or “mesh bag” in the same or different categories was unclear. Knowledge of the origin of plastic debris is critical for policymaking to mitigate the negative effects of plastic pollution on marine ecosystems. We recommend that future research includes detailed information on ingested debris to help identify the sources.

3.7 Color of Ingested Debris

Eleven articles contained quantitative data on the colors of ingested debris. However, integrating color data was complicated due to the diverse color categories employed across the studies. For example, Bessa et al. (2019) used eight color categories, whereas Moon et al. (2022) used 13. To facilitate a comparative color analysis, we consolidated the color data into the following categories: white, transparent, green, yellow, black, blue, red, multicolored, and others (orange, gray, brown, etc.). Standardizing color categories is vital for effective monitoring and comparisons of marine plastic debris in both biotic and abiotic environments.

White (34 ± 14%) and transparent (31 ± 24%) were the predominant colors found in sea turtles, followed by black (10 ± 7.8%), green (7.5 ± 7.2%), blue (6.5 ± 6.1%), yellow (2.4 ± 3.6%), red (1.3 ± 1.4%), multicolored (2.3 ± 3.2%), and others (5.1 ± 5.6%) (Table 2). Several studies, which we excluded from the analysis due to the absence of numerical data, also reported a prevalence of lighter colors like transparent or white. There are two possible explanations for the prevalence of light-colored plastics. First, sea turtles might selectively ingest light-colored plastics due to their resemblance to prey such as jellyfish (Schuyler et al. 2012). Second, the abundance of light-colored plastics in the environment can lead to a higher likelihood of ingestion. Santos et al. (2016) noted that beach plastic and ingested plastic by sea turtles comprised predominantly light-colored plastics. Further research is needed to explore plastic debris from the mid-water column, where sea turtles frequently encounter plastic debris. Additionally, further studies are required to understand sea turtles’ behavioral responses to different colors of plastics.

Table 2 Data on the color composition (%) of plastic debris ingested by sea turtles extracted from the literature review
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The color categories in the surveyed articles were diverse, which indicates a need for simplification and standardization. Most studies presented color information of plastic debris graphically or as the %FO of colors. A few articles offered numerical data for each color category, while others provided information only for major colors. As noted above, some studies used combined color categories such as black/brown. For improved data standardization and comparability, we advise that researchers provide numerical values for all relevant variables.

3.8 Polymer Types of Ingested Debris

We compiled data on the types of polymers from nine journal articles, which yielded a total of 18 distinct polymer types found in sea turtles. Among these, poly(dimer acid co-alkyl-polyamine), polyisobutylene, polyisoprene, polyacrylate copolymer, polyvinyl fluoride, polyvinyl, and acrylonitrile butadiene styrene were each reported in only a single article, which were categorized as others. The majority of the studies reported polymer types as count-based percentages, with the exception of Jung et al. (2018), who employed mass-based percentages.

In sea turtles, polyethylene (PE; 65 ± 24%) emerged as the most prevalent polymer, trailed by polypropylene (PP; 20 ± 17%), PE/PP copolymer (3 ± 6%), polyamide (PA; 2 ± 5%), poly(ethylene-vinyl acetate) (1 ± 4%), nylon (1 ± 2%), polyurethane (PU; 1 ± 3%), PS (2 ± 4%), polyester/polyethylene terephthalate (PES/PET; 0.3 ± 0.6%), polyvinyl chloride (PVC; 0.2 ± 0.6%), and others (1 ± 1%) (Table 3).

Table 3 Data on the polymer type composition (%) of plastic debris ingested by sea turtles extracted from the literature review
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PE and PP, which were identified as the most prominent polymer types in sea turtles, collectively account for over half of the total polymer demand, as reported by Plastics Europe (2021). PE is versatile and has various applications in plastic bags, food packaging, agricultural films, and pipes, whereas PP is frequently used for food packaging and hinged caps (Plastics Europe 2021). Notably, PE and PP were also the primary polymer types found in nets and ropes ingested by sea turtles, highlighting the impact of marine-sourced plastics on sea turtles (Moon et al. 2022). These findings underscore that the widespread use of these polymers substantially contributes to the accumulation of plastics in marine ecosystems. Plastic could pose significant threats to marine life by reducing an individual’s capacity to forage food, digest nutrients, perceive starvation, evade predators, and take part in reproduction, and migration (SCBD 2012). It also functions as a route that allows the transfer of chemicals to marine life (Ugwu et al. 2021). Sea turtles have ingested a variety of polymers that maybe included numerous additives, and the resulting effects cannot be predicted. It emphasizes further research on the physical and chemical effects of plastic on sea turtles.

4 Conclusion

Marine plastic debris exerts detrimental effects on marine wildlife, with sea turtles being particularly vulnerable. Due to their global distribution, high %FO, substantial ingestion of plastic, and the prolonged retention of plastics in their digestive tracts, sea turtles serve as effective bioindicators for plastic pollution (Savoca et al. 2022). While prior review articles concerning sea turtles primarily analyzed %FO and quantities of ingested plastics, there is a pressing need to investigate additional plastic attributes such as shape, color, and polymer type, to gain more granular insights into the nature of consumed plastics. In this review, we refreshed the data from previous reviews concerning the %FO and g/kg of ingested plastics in sea turtles and also broadened the knowledge base by analyzing the shape, original usage, color, and polymer type of the plastic debris that sea turtles consume. Moreover, we performed a comparative analysis of the characteristics of ingested plastics across different regions and sea turtle species and juxtaposed these data with those of plastics found on beaches.

Consolidating the categories of shape, original usage, color, and polymer type of plastic items from various articles was challenging due to a lack of methodological consistency, making it difficult to synthesize data on quantities or characteristics at a local or global scale. We excluded a number of studies from our collected database owing to ambiguous descriptions, blending of multiple categories, or the absence of numerical data. It is imperative to standardize the reporting of plastic characteristics to minimize data loss. We advocate for future research to delineate the characteristics of plastics with greater precision, thereby facilitating the consolidation of data. Data on ingested plastics and their sources are invaluable for guiding policy development aimed at regulating the production, usage, and waste management of plastics to effectively reduce the ingestion of plastics by marine organisms (Roman et al. 2020).