Endemic Species of Elasmobranchs (Sharks and Rays) in South America: A Review

2.1 Diversity, endemism, and species conservation status

Endemic species are found exclusively in a specific geographic region and are not found naturally in other parts of the world. The methodology used, in this chapter, to evaluate the diversity and endemism of elasmobranchs in South America was conducted through in-depth research carried out on the International Union for Conservation of Nature (IUCN)’s Red List of Threatened Species platform, which allowed the use of series of filters (e.g., habitat occupied by the species, taxonomic description, and area of occurrence, among others). Additional research on manuscripts was also carried out. From the data obtained, it was possible to reach a broad overview of the endemic species that occur in coastal and island environments, in addition to an assessment of their population status. A global total of 1194 elasmobranchs evaluated by the IUCN’s Red List of Threatened Species was verified; among these, 256 correspond to sharks and rays that occur around South America, of which 38 (∼15%) are endemic species. Among the marine elasmobranch’s endemic to South America, 68.4% correspond to rays, and 31.6% were represented by sharks. The species were distributed into 16 families, with a greater number of families being found for batoids than for sharks, with the Rajidae family representing the largest number of skates (Table 1).

The Western Atlantic Ocean has the highest number of records of endemic species, with the majority attributed to Brazil, which hosts 22 species, and only one representative exclusive each for Argentina and the Falkland Islands. In the portion of the South American continent bordered by the Pacific Ocean, Chile is the country with the highest number of endemic records, cataloging nine species, while Colombia, Ecuador, and the Galapagos Islands together have six representatives. In the assessment of the conservation status of species in South America conducted by the IUCN, nearly half of the endemic marine elasmobranch species (40%) are classified within some category of extinction threat. The majority of the species have been assessed as Least Concern (31.5%), followed by the Vulnerable and Data Deficient categories, representing ∼27 and ∼16% of the species, respectively. Four species (10.5%) are classified as Endangered, while the Menni’s Skate (Dipturus mennii) is the only species categorized as Critically Endangered, with its geographic distribution in Brazil.

The type of habitat in which the species occur was also verified, with the same species being present in two types of habitats: Marine Neritic (MN), characterized as a region of the marine environment that is located on the continental shelf, and Marine Deep Benthic (MDB), an environment associated with the ocean floor which may extend beyond the continental shelf. Among the two types of habitats mentioned, most species (n = 16) are restricted to the Marine Deep Benthic habitat. Of the total endemic species evaluated, were verified the occurrence of six species with representatives in island environments (e.g., Lutz’s Stingray Hypanus berthalutzae, which occurs in the Fernando de Noronha archipelago and in the Rocas Atoll). In general terms, endemism can be caused by physical, climatic, or biological barriers that limit the distribution of a species or separate it from the original group. The more specific the environment, the greater the degree of endemism, that is, the greater the number of endemic species. In this sense, it is of great importance to identify and characterize the areas of endemism in each environment, since these can be included in the selection of priority areas for nature preservation.

2.2 Anthropogenic threats

A series of human activities are currently threatening marine ecosystems and, consequently, resident or migratory species in all oceans. For elasmobranchs, all this pressure, direct or indirect, can strongly affect the populations of rays and sharks. In this sense, aspects related to fishing pressure, pollution and environmental degradation will be detailed and discussed here.

2.3 Fisheries

Shark and ray populations have declined significantly in recent years as a result of extensive human exploitation throughout the world’s oceans, putting some species at risk of extinction. These species are captured in some fisheries as target species and/or as bycatch, both by artisanal fishing and by large-scale industrial fishing [9]. Due to the great diversity of habitats of these animals, they end up interacting and are susceptible to a wide variety of fishing gear (e.g., bottom trawling, surface trawling, bottom longline, pelagic longline, and trolling; Figure 1).

In all South American countries, sharks and rays are consumed both locally and for export. For example, Brazil, where sharks and rays are sold as “cação” without identifying the species, is one of the countries that consumes the most sharks. It is also one of the countries that exports the most fins to the Asian market [10].

Although fishing for some species is prohibited, they end up being fished as bycatch and, in many cases, returned to the sea. Bottom trawling is responsible for capturing 19 species of endemic rays, followed by gillnets responsible for catching nine endemic species, while longlines are responsible for catching five species (Table 2). Some species are subject to fishing pressure from different fishing gear, such as Hypanus marianni and Pseudobatos horklii [11]. Endemic rays such as those of the genus Sympterygia that are listed as critically endangered by the IUCN and that live close to the substrate, are species captured by artisanal and industrial fishermen who practice bottom trawling throughout their distribution and also interact with other fishing gear [11, 12]. Uncontrolled fishing of some endemic species with low distribution can lead to the decline of these populations and extinction. An example is the ray Pseudobatos horklii, which has suffered great fishing pressure throughout its distribution and is classified as critically endangered by the IUCN [11].

2.4 Pollution

Pollution can be defined as the introduction of any substance or energy into the environment at levels that cause deleterious effects on human health and other living organisms and interfere with the functioning of part or all of the ecosystem [13, 14]. Thus, the accumulation of pollutants in natural environments, especially in aquatic ecosystems, has become one of humanity’s greatest concerns in recent decades, since water is an important means of chemical transport, constantly influenced by human activities that occur in terrestrial and river systems [15], in addition to being an excellent solvent.

Pollutants can also have different properties, such as stock pollutants, which include plastics and other non-biodegradable materials, synthetic chemicals, and/or heavy metals, which tend to accumulate in the environment and in living organisms over time and along the food chain [16]. Among the stock pollutants, plastic is a global concern. Most plastic waste reaches aquatic environments through irregular disposal, and estimates indicate that the discharge of plastic waste into rivers, lakes, and oceans reaches 23 million tons per year [17]. Plastic can be carried by rivers, ocean currents, or even pushed by the wind with ease due to its weight and structure, and can accumulate in true floating islands, such as the Pacific Plastic Vortex, which covers approximately 1.5 million km² [18]. This is extremely worrying since plastic is a persistent material in the environment and has natural removal rates on the scale of decades to centuries [17]. There are currently several records in the scientific literature on the negative interaction of elasmobranchs with plastic waste in the South Atlantic, especially entanglement, such as that recorded in tiger sharks, Galeocerdo cuvier (Péron and Lesueur, 1822) [19], blue sharks, Prionace glauca (Linnaeus, 1758) [20], sharpnose sharks, Rhizoprionodon lalandii (Valenciennes, 1839) [21] and manta rays, Mobula birostris (Walbaum, 1792) and M. cf. birostris [22]. Many of these elasmobranch entanglements are caused by ghost fishing gear (74% of animals), followed by polypropylene straps (11% of animals) and other entanglement materials such as circular plastic debris, polyethylene bags and rubber tires (1% of total entangled animals) (Figure 2) [23].

Plastic also ends up being mistakenly consumed by elasmobranchs, especially microplastics. In addition to the potential for bioaccumulation and biomagnification, these plastic wastes also interact with chemical pollutants, which adhere to the plastic particles that facilitate the dispersion of these pollutants [24]. The presence of chemical pollutants has also been constantly observed in aquatic environments, which commonly present a mixture of metals, persistent organic pollutants (POPs), pharmaceuticals, cosmetics, personal hygiene products, surfactants and pesticides [24, 25]. These chemical pollutants also have the capacity for bioaccumulation and biomagnification in elasmobranchs, and many can function as endocrine disruptors or be proven to be carcinogenic and mutagenic. Malformations, for example, have been observed in the Spotback Skate, Atlantoraja castelnaui (Miranda Ribeiro, 1907) [26] and in blacktip sharks, Carcharhinus falciformis (Bibron, 1839) [25] and associated with the presence of chemical pollutants in regions of the South Atlantic. These negative effects resulting from pollution are worrying in the short and long term, as they can interfere with the ability of these animals to compete for food, space and reproductive partners, reducing the persistence capacity of elasmobranch populations, since sharks and rays are strategist K species, which commonly take a long time to reach reproductive age, compared to teleosts and other taxa, and effectively contribute to the maintenance of the size of their populations.

2.5 Environmental degradation

The introduction of pollutants into aquatic environments influences environmental conditions such as dissolved oxygen concentration, pH and turbidity, often reducing the quality of these environments. The discharge of domestic sewage, for example, can lead to an increase in the concentration of nutrients that cause excessive growth of algae, which in turn reduce the passage of light and oxygenation of aquatic environments and often produce toxic compounds such as hydrogen sulfide (H2S) and ammonia (NH3), creating dead zones [15]. In freshwater environments, there are numerous reports of eutrophic environments due to excess nutrients from waste from human activities, and in recent years, the number of reports of macroalgae blooms in marine environments has been increasing, with species of the genus Ulva and the genus Sargassum being the algae commonly involved in the green tides and golden tides, respectively [15]. Another effect of the deposition of nutrients from human activities in aquatic ecosystems is nutrient-induced bioerosion, which, in addition to causing degradation of the coral reef structure, also intensifies the effects of ocean acidification [15] and can sometimes cause the death of coral reefs, areas intensively used by sharks and rays for resting, reproduction and feeding.

Changes in the physical structure of the environment, caused by urbanization and the construction of dams, canals, and ports, and by exploitative activities such as predatory fishing with explosives or trawl nets, as well as gas and oil exploration, modify natural landscapes, often reducing the quantity and quality of available aquatic habitats. Habitat characteristics play a fundamental role in the structuring of aquatic communities, influencing the distribution of species and the presence of mesopredators and top predators such as sharks and rays [27]. Species with highly specialized life histories and limited spatial or environmental distribution are more vulnerable to habitat changes. Many shark and ray species use shallow coastal areas as nurseries to protect their neonates and juveniles from large predators and adverse environmental conditions. Thus, the reduction in the quantity and quality of habitats often compromises feeding, reproduction, and shelter areas, which compromises the recruitment of some species and further contributes to the reduction of biodiversity.

Competition for space and food with invasive alien species also compromises the availability and quality of habitats. Biological invasions have complex direct and indirect impacts, often observed in the long term, when invaders are well-established and have large spatial ranges. Invasive alien species affect the richness and abundance of native species, change the behavior of these animals, alter the phylogenetic diversity among communities, and modify trophic networks, increasing the risk of extinction of native species [28]. Overall, there is little direct evidence that invasive alien species threaten elasmobranchs; however, there are several evidence of how invasive alien species interfere with the trophic dynamics of communities, compromising the food supply and the integrity of the natural ecosystems that elasmobranchs depend on [28].

3.1 Alternatives for conservation

As previously mentioned, elasmobranch populations are being directly affected by several anthropogenic threats, as well as marine ecosystems, making the group highly vulnerable to various human activities. In this sense, the development of research aimed at investigating the biology, ecology, movement pattern, and habitat use of sharks and rays is being heavily employed through the use of several non-lethal techniques, in the hope of understanding the population dynamics of sharks and rays and, consequently, contributing to the conservation of the group. Among the various non-lethal methods employed in fisheries science, we highlight the study of blood cells and their relationship with pollutant concentrations in the environment, underwater observation (visual census), investigation of movement patterns and habitat use, through the extensive use of electronic tags (telemetry), and even the observation of animal diversity and behavior through Baited Remote Underwater Video (BRUV) and Drones.

3.2 Ecotoxicology

The vulnerability of elasmobranchs has given rise to a new field of study regarding the concentrations and effects of chemical substances of anthropogenic origin on these species. The bioavailability of these substances, that is, their availability for absorption by organisms, is a risk, especially for aquatic organisms, since these animals live submerged and in permanent contact with a mixture of chemical substances [24]. These bioavailable substances can be absorbed through gill respiration and the ingestion of food and water, in the case of marine fish, or through the tegument, the tissue that covers the body of these animals, bioaccumulating in these organisms over time, or biomagnifying through the food chain, especially in top predators, such as sharks and aquatic mammals [29].

The bioaccumulation and biomagnification of substances in elasmobranchs occurs in different tissues, especially in adipose tissues and organs such as the liver, the main organ involved in the detoxification process [30]. Sharks, in particular, have large, fat-rich livers and are very susceptible to the accumulation of lipophilic substances, such as metals and persistent organic pollutants (POPs), and tend to have higher metal concentrations compared to teleosts of similar trophic levels [24]. In South America, high concentrations of POPs were observed in the liver and muscle of blue sharks, Prionace glauca, at concentrations higher than those permitted for human consumption by the European Commission (maximum of 200 ng g−1 ww), as well as high concentrations of Mn, Cd, Fe, and Cu [31]. Maternal transfer of polycyclic aromatic hydrocarbons (PAHs) has also been observed in elasmobranchs in the South Atlantic, more precisely in the endemic Brazilian guitarfish Pseudobatos horkelii, which presented an average rate of PAHs discharged to offspring of 13%, with low molecular weight PAHs presenting the highest rates of maternal transfer [32].

Responses of organisms to contact with these exogenous substances have been used as biomarkers of the health of these animals and the environment in which they live. In South America, the activities of some of these biomarkers have begun to be investigated in sharks and rays, such as studies on the activity of cholinesterases (ChE), enzymes that play an essential role in neuronal and motor functions, in brain and muscle tissues of blue sharks. The brain of blue sharks contains atypical ChE, exhibiting mixed properties of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), differently from bony fish, which implies different detoxification mechanisms [31]. The activity of the enzyme ethoxyresorufin-O-deethylase (EROD), an essential biotransformation enzyme for the detoxification of a wide range of xenobiotics, was also investigated in the catshark, Schroederichthys chilensis, captured in the South Pacific Ocean, in conjunction with the determination of fluorescent aromatic compounds, to indicate the occurrence of exposure to PAHs. EROD activity and fluorescent aromatic compounds showed significant differences between study areas, with catsharks captured in PAH-contaminated areas presenting EROD activity values 4.2 and 2.4 times higher than the uncontaminated reference study area [33]. The induction of EROD can produce deleterious side effects, such as DNA-PAH adducts (binding between DNA molecules and PAHs), which are crucial factors in the development of cancer in fish and are also linked to deficiencies in both reproduction and the immune system.

Genotoxic responses have also been observed through blood samples collected by non-lethal methods in tiger sharks, lemon sharks, and nurse sharks, sampled in a conservation area in the South Atlantic. Significant variations among species, which are also part of different trophic levels, were observed and associated with the presence of pollutants in the water column, suggesting that contact with pollutants such as metals and surfactants, even in small quantities, can cause genotoxic effects in sharks (Figure 3) [34]. These biomarkers do not provide information about impacts at more complex organizational levels, such as populations, communities and/or ecosystems, but they enable the detection of harmful effects early on at basal levels, which occur on a scale of minutes to weeks, and it is essential to have a correlation between their frequencies and concentrations with the intensity of exposure to the stressor [35].

Relationships between metal and metalloid concentrations and the frequency of genomic damage have also been investigated in tiger sharks, nurse sharks, and lemon sharks, with Al, As, and Zn showing a significant effect on the frequencies of genomic damage for all three species. Interspecific variations were also observed in this study, with Zn influencing the frequencies of binucleate cells and Al the frequencies of total damage and micronuclei in tiger sharks and lemon sharks, and As influencing the frequencies of binucleate cells and notched nucleus in nurse sharks, while showing a strong and positive correlation with most of the metals analyzed [36]. The high frequency of genomic damage over a long period of time is worrying because it can lead to disturbances in development and reproduction that can compromise the fitness of the organisms.

In general, there are still few studies focusing on pollutant levels and their effects on elasmobranchs, with most studies focusing on economically important sharks and rays, highlighting the lack of studies on species with low economic value and critical conservation status. There are also few studies using non-invasive methodologies, with muscle and liver being the most analyzed organs in current ecotoxicological studies. Therefore, the knowledge about the impacts of pollution on the health of these animals is urgent, since pollution of aquatic environments is a growing problem, especially in developing countries, constituting an additional stressor for many elasmobranch species that are already under threat of extinction due to fishing exploitation and habitat loss [37]. Thus, understanding the negative effects associated with pollution can greatly contribute to the implementation of public policies aimed at the conservation of elasmobranchs and their essential habitats.

3.3 Visual census

Visual census is a method used to observe individuals in an aquatic environment directly, through free diving or scuba diving [38]. Visual censuses serve to collect relevant information on spatial ecology, abundance, density, distribution, and population structure, distribution by sex and size, in addition to providing information on behavior in the wild [38, 39]. Visual census has been widely used, especially for endangered species or in marine protected areas, as they do not involve capture [38, 39]. Visual census can be performed in transects, intensive search, or stationary points depending on the objective and species studied. In transects, divers follow a parallel line, noting all the individuals observed. Transects can be used in different environments such as shipwrecks and reef environments [38].

During the visual census, it is possible to evaluate the different behaviors performed by sharks and rays (e.g., resting, swimming, feeding, and reproduction) and correlate them with the environment in which they are being monitored (e.g., type of substrate, depth, temperature, and tide, among others). The Atoll Biological Reserve (Brazil) is an ecological sanctuary for several migratory and resident species. Since it is a marine protected area, where only research activities are carried out, a series of studies involving non-lethal methods have already been carried out to investigate the populations of sharks and rays (e.g., Lemon Shark, Negaprion brevirostris, G. cirratum, and H. berthalutzae). The Rocas Atoll is an island ecosystem in the Western Atlantic Ocean, which is strongly linked to the sea regimes. During periods when the tide is rising or falling, there is intense movement of sharks and rays, either toward the interior or exterior of the Rocas Atoll.

H. berthalutzae is a species of stingray endemic to South America (Brazil), which has been extensively studied using non-lethal methods in the Rocas Atoll Biological Reserve and the Fernando de Noronha Archipelago [38]. To expand the knowledge available in the literature on Lutz’s stingray regarding population structure, behavior, and seasonality, a monitoring period of approximately one decade was carried out at Rocas Atoll. The visual census was performed only inside the atoll, where it was verified that most of the time, the specimens remained at rest, camouflaged in the sandy substrate. Individuals moving on rocky substrates were observed less frequently (Figure 4). A predominance of females over males was also observed (sex ratio of 1 female: 0.14 males), in addition to adult individuals predominating in the underwater observations. The results reported for the Rocas Atoll were different from the data obtained in the Fernando de Noronha Archipelago. In this location, there is a greater predominance of young individuals in shallow areas, in addition to a greater balance in the proportion between males and females.

In general terms, the visual census has proven itself over the years to be an extremely efficient, low-cost, and highly recommended technique for investigating shark and ray populations on oceanic islands, clear water, and shallow areas, favoring the implementation of the methodology and providing data acquisition on wild animals, thus contributing to the conservation of populations.

3.4 BRUVS and drones

Recent technological advancements have significantly enhanced the development of non-lethal and non-invasive sampling methods for assessing fish assemblages. This is particularly beneficial in hard-to-reach environments, areas that present risks to samplers, and ecological sanctuaries with high preservation values. Furthermore, research that minimizes impacts on organisms and their habitats is particularly encouraged for endangered species and areas identified as hotspots, where the removal of animals from their environment could have significant effects on their populations, especially in the case of endemic species. In this context, the use of underwater video cameras for inventorying and monitoring fauna and their habitats has been on the rise, as they provide permanent records that do not impact the natural environment [40].

Baited Remote Underwater Video Surveys (BRUVs) have yielded highly satisfactory results on fish and elasmobranch research assemblages in various marine ecosystems [41]. This equipment can be constructed using readily available materials, featuring a relatively simple design that consists of a compact video camera housed in a waterproof case, attached to a base made in various shapes, and directed to a box with baits.

The number of cameras associated with the structure will depend on the objectives of the investigations: Single-BRUVs are equipped with a single camera and are less effective at estimating organism sizes compared to Stereo-BRUVs, which are constructed with two cameras. The latter not only provides more accurate length estimates but also expands the field of view of the monitored area [42]. The cameras capture underwater images of the species surrounding the equipment, primarily attracted by the bait contained in the boxes, although some animals may also approach the structure out of curiosity about the new presence.

Similar to underwater visual censuses, BRUVs enable the assessment of biodiversity, abundance, and spatial distribution of species, as well as the inference of important data regarding the population structure of a region [43], all without the presence of samplers in the water. Consequently, recent studies have focused on species behavior in their natural habitats using the autonomous recordings obtained from BRUVs, thereby generating much more reliable data on the natural behaviors of these species [44].

The BRUVs have been successfully utilized, particularly in Marine Protected Areas (MPAs), especially in fully protected zones, with the aim of monitoring biodiversity and assessing the effectiveness of these locations [45]. On the other hand, in areas with a high degree of environmental impact, BRUVs are employed to measure the extent of habitat degradation reflected in the species community at each site. In anthropogenically impacted areas of Pernambuco, Brazil, Lutz’s stingray (Hypanus berthalutzae), an endemic species vulnerable to extinction, was documented in images produced by BRUVs in the region [46]. The distribution of Lutz’s stingray extends to the oceanic islands of the Atol das Rocas and the Fernando de Noronha archipelago, which are important marine protected areas (MPAs) for the conservation of this species (Figure 5).

Furthermore, the use of BRUVs allows for a greater number of samples to be collected in a shorter time frame and facilitates research in deeper environments due to the autonomy of the equipment, eliminating the need for divers to conduct the work. Globally, the use of BRUVs in scientific studies is on the rise due to their operational advantages and low cost.

However, it is important to note that, like all research methods capable of estimating species abundance and diversity, their use should be coupled with different methodologies to mitigate the sampling biases inherent to each method, as well as to allow for comparisons regarding the precision of the results obtained [47].

Within the same framework of faunal inventory and monitoring of underwater regions using autonomous equipment, remotely operated vehicles (ROVs) represent a more technically advanced data collection tool. In addition to capturing images with an attached video camera, ROVs are equipped with sensors and tools to perform specific tasks, such as collecting samples. The ability of ROVs to operate at depths inaccessible to divers, combined with their capacity to enter confined and narrow spaces, makes this equipment extremely valuable across various sectors, including scientific research.

In marine fauna investigations, ROVs differ from BRUVs in that they do not rely on bait as an attractant. Instead, they are capable of performing movements and actions controlled by an operator remotely, which is particularly beneficial for investigating species in deep regions where endemic species may be identified that would likely go unseen by other methods [48]. However, due to their greater autonomy, efficiency in data generation, and application across various research fronts, the cost of acquiring and operating ROVs remains a barrier to the development of academic research.

Drones, or unmanned aerial vehicles (UAVs), have gained popularity due to their ability to efficiently capture aerial imagery and the wide range of price points available for acquisition. Consequently, these technologies offer a broad array of functionalities that can be applied in various activities and environments.

Drones are model aircraft equipped with an autopilot system and can be either remotely controlled by an operator or fly autonomously, following predefined routes or control programs. Although the images generated by drones provide a perspective from above the water, these devices have become an important tool in research for the identification and monitoring of marine megafauna species. Additionally, they capture behaviors in natural habitats, area usage patterns, and estimates of population structure within ecosystems.

Large pelagic elasmobranchs, such as mobulids and whale sharks, are easily detected by these devices when they migrate to the ocean’s surface [49]. Drones can access areas that would be extremely challenging to reach, particularly in regions with difficult navigation or usage restrictions, and they also have lower logistical costs compared to some traditional research methods. In addition, drones are an essential tool for species conservation due to their ability to monitor the recovery of degraded areas and to document illegal activities in marine protected areas. This capability aids in identifying issues and facilitating the prosecution of those responsible. Technological innovations have not only reduced the size of these devices but also lowered acquisition costs, allowing for an expansion in the number of studies conducted with drones worldwide [49].

3.5 Electronic tagging

The movement patterns of aquatic organisms provide a suitable understanding of ecological and evolutionary processes that occur across different spatial and temporal scales. Sharks and rays should optimize their movements to balance the energetic and life history requirements with the costs of movement, such as reproductive and trophic migrations [50]. The choice of specific habitats within an ecosystem can be crucial to the survival and reproductive success of a species. In the early 1950s, the use of conventional (plastic) tags made it possible to investigate the movement of elasmobranchs. However, due to the limitations of the data obtained by tag-recapture work, biotelemetry techniques were developed in the hope of expanding monitoring both on a geographic scale and in greater detail of habitat use.

Acoustic telemetry is a frequently used method to assess the presence and movement information from aquatic animals, including skates and rays. This tool involves the use of transmitters implanted in the animals to be tracked and a hydrophone (usually called to as an acoustic receiver) that will have the capacity to detect the signal emitted by the transmitters. The data from acoustic telemetry are usually collected in two different ways, be it for active tracking or passive tracking, and each technique has its singular advantages and limitations [51].

Active tracking normally uses specific transmitters that emit continuous acoustic pulses and the signal is monitored by a receiver and hydrophone carried onboard the tracking vessel. Tracking invariably takes place over short durations of a few hours to a few days. The main objective of this technique is to provide detailed, fine-scale (m) tracks of animals by following the signal from the transmitter. The severe nature of this fieldwork means that it can normally only be conducted for a brief time, being therefore, essential that a rapid method of transmitter attachment is used to minimize stress and, consequently, not promote unusual behaviors by the study animal during the tracking [52]. Passive telemetry is based on the use of an array of acoustic receivers (omni-directional hydrophone) installed in underwater stations capable of detecting the presence of an individual, with a coded transmitter, for a long period of time. However, it is not possible to know the location of the individual, which carries the acoustic transmitter, if it is not close to the array of receivers.

Transmitters used in acoustic telemetry (active or passive) can be attached to sharks and rays externally or internally. External tagging can be done in several ways. For sharks, it is extremely usual to attach transmitters to the dorsal fin of the specimen, while for rays, this attachment can be done to the pectoral fin, tail, or even the blowhole (Figure 6a). Internal tagging involves a surgical procedure in which a small incision (∼4 cm) is made in the animal’s abdominal cavity, the transmitter is inserted, and the incision is sutured (Figure 6c and d). The choice of tagging technique (internal or external) should be based on the ecology of the species. In this sense, it is very important to evaluate the biology and behavior of the animal to choose the best form of tagging, ensure the retention of the transmitter, and consequently, the success of the monitoring.

An additional tool, which is widely used in research around the world, is satellite telemetry (Figure 6b). Individual movement patterns are recorded remotely while the tag is fixed to a specimen, and the data are transmitted via satellite. The satellite transmitters can record data of estimated geographic position, depth, and ambient temperature [53]. The main advantage of this technique is the possibility of monitoring an individual, for a long time, on a large spatial scale, without the need for the installation of receivers. The tagging and monitoring techniques reported here have revolutionized studies of behavior and habitat use for elasmobranchs, enabling the acquisition of valuable data using a non-lethal method.

4.1 Challenges and perspectives

The endemic species of elasmobranchs (sharks and rays) in South America face numerous challenges and opportunities for conservation. Here’s an overview of the key challenges and perspectives. Many species are targeted for their meat, fins, and cartilage, leading to significant population declines, and unsustainable fishing practices exacerbate this problem. Combined with this, coastal development, pollution, and habitat degradation (such as the deforestation of mangroves and the destruction of coral reefs) threaten critical breeding and nursery habitats for these species. Rising sea temperatures and ocean acidification can impact the health of marine ecosystems by affecting food availability and habitat suitability for elasmobranchs, which, without enforcement of fishing regulations and illegal fishing activities, contribute to the decline of endemic species. Another key factor is that many endemic species lack comprehensive research data, hampering effective conservation strategies, and this knowledge gap can make it difficult to assess population health and trends. Finally, as coastal communities expand, interactions between humans and elasmobranchs can lead to conflicts, further complicating conservation efforts.

From perspectives, we can initially mention the conservation initiatives that culminate in the growing awareness and defense of marine conservation aimed at protecting elasmobranch habitats and populations, in addition to the creation of marine protected areas (MPAs) as crucial for the preservation of biodiversity. Promoting sustainable fishing practices can also help restore populations of overexploited species, and certification programs and community-led management initiatives are gaining momentum. Investment in research can help fill knowledge gaps and the development of citizen science projects, and partnerships between governments, NGOs, and academic institutions can improve monitoring efforts. In this sense, local community involvement in conservation efforts promotes management and can lead to more effective management of marine resources, as well as strengthening legal structures and international agreements can help protect elasmobranchs, combined with collaborative efforts between countries. South American species are essential for transboundary species management.

Addressing the challenges faced by endemic elasmobranchs in South America requires a multifaceted approach that includes sustainable practices, strong legal frameworks, community engagement, and dedicated research efforts. By prioritizing these areas, it is possible to create a more favorable environment for the survival of these unique and vital species.