Micrographia Morphological characterization of the digestive tube of hawksbill sea turtle (Eretmochelys imbricata) hatchlings Radan Elvis Matias de Oliveira 1,2,3 * , Fernanda Loffler Niemeyer Attademo 2 , Ana Caroline Freitas Caetano de Sousa 4 , João Vitor de Oliveira Gurgel 4 , Marcela dos Santos Magalhães 5 , Carlos Eduardo Bezerra de Moura 1 , Ana Bernadete Lima Fragoso 2,3 , Flávio José de Lima Silva 2,3,6 and Moacir Franco de Oliveira 1 1 Postgraduate Program in Animal Science, Federal University of the Semi-Arid Region, Mossoró, Rio Grande do Norte 59625-900, Brazil; 2 Center for Environmental Studies and Monitoring – CEMAM, Areia Branca, Rio Grande do Norte 59655-000, Brazil.; 3 Cetáceos da Costa Branca Project, University of the State of Rio Grande do Norte – PCCB-UERN, Mossoró, Rio Grande do Norte 59610-210, Brazil; 4 Department of Animal Sciences, Federal University of the Semi-Arid Region, Mossoró, Rio Grande do Norte 59625-900, Brazil; 5 Department of Morphology, Federal University of Amazonas, Manaus, Amazonas 69080-900, Brazil and 6 Doctoral Program in Development and Environment (PRODEMA), Federal University of Rio Grande do Norte (UFRN), Natal, Rio Grande do Norte 59064-741, Brazil Abstract Morphological studies concerning the digestive system can further information on animal diets, thus aiding in the understanding of feeding behavior. Given the scarcity of information on sea turtle digestive system morphology, the aim of the present study was to describe the digestive tube (DT) morphology of Eretmochelys imbricata hatchlings to further understand the diet of these individuals in the wild. DT samples from 10 stillborn turtles (undefined sex) were analyzed at the macro and microscopic levels. The esophagus, stomach, small intestine (SI), and large intestine (LI) are described. Histologically, the DT is formed by four tunics, the mucosa, submucosa, muscular, and adventitia or serosa. The esophagus is lined by keratinized stratified squamous epithelium, while the remainder of the DT is lined by a simple columnar epithelium. The esophagus mucosa is marked by conical, pointed papillae. The stomach comprises three regions, the cardiac, fundic, and pyloric and is covered by neutral mucous granularcells. The intestinal mucosa presents absorptive cells with micro- villi, neutral and acidic goblet cells, and mucosa-associated lymphoid tissue. The SI is significantly longer than the LI ( p value = 0.006841). These morphological findings are strong indications of adaptations to a carnivorous diet in this hawksbill turtle age group. Key words: Cheloniidae, histochemistry, morphology, morphophysiology, scanning electron microscopy, Testudines (Received 27 February 2022; revised 8 June 2022; accepted 2 July 2022) Introduction The hawksbill turtle (Eretmochelys imbricata) displays a complex life cycle and population structure presenting migratory behavior, moving hundreds to thousands of kilometers between feeding and spawning areas (Bowen, 1995). This species is globally distributed in tropical waters and, to a lesser extent, in subtropical waters (Mortimer & Donnelly, 2008), and currently faces several threats, such as pollution, habitat loss, climate change, and human inter- actions (Saeki et al., 2000; Bugoni et al., 2001; Fujihara et al., 2003; Wilcox et al., 2018; Rizzi et al., 2019; Oliveira et al., 2021). According to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, hawksbill turtles are classified as “critically endangered”. Marine debris ingestion represents a major cause of mortality for this species, exposing animals to gastroenteric disorders and additional toxin exposure (Andrady, 2011; Rizzi et al., 2019). This, in turn, causes nutrient digestion and absorption deficits, resulting in malnutrition and decreased growth and reproductive rates, and even death (Nelms et al., 2016). Several studies indicate that hawksbill turtles are omnivorous in both the juvenile and adult stages, feeding preferentially on porifera, cnidarians, and some algae, although when feeding mainly on porifera when adults, playing an important role in coral reef maintenance worldwide (Rincon-Diaz et al., 2011; Carrión-Cortez et al., 2013; Von Brandis et al., 2014). Some hypotheses indicate that when young, these animals present a completely carnivorous diet in open waters, as described for other young turtle species, such as the green turtle (Chelonia mydas) and the loggerhead sea turtle (Caretta caretta), which feed on zooplankton, mollusks, and crustaceans (Reich et al., 2007). Morphological digestive tract studies can identify the type of food that animals consume and metabolize, consequently aiding in the understanding of feeding behavior (Ku et al., 2001; Tarakçi et al., 2005; Magalhães et al., 2010; Chen et al., 2015). Morphology studies, including those of the digestive system, have become increasingly relevant due to global warming, as this phenomenon may directly affect the diet of several species *Corresponding author: Radan Elvis Matias de Oliveira, E-mail: radan_elvis@hot- mail.com Cite this article: Oliveira REM, Attademo FLN, Sousa ACFC, Gurgel JVO, Magalhães MS, Moura CEB, Fragoso ABL, Silva FJL, Oliveira MF (2022) Morphological characteri- zation of the digestive tube of hawksbill sea turtle (Eretmochelys imbricata) hatchlings. Microsc Microanal 28, 2138–2149. doi:10.1017/S143192762201234X © The Author(s), 2022. Published by Cambridge University Press on behalf of the Microscopy Society of America Microscopy and Microanalysis (2022), 28, 2138–2149 doi:10.1017/S143192762201234X Downloaded from https://academic.oup.com/mam/article/28/6/2138/6995539 by guest on 25 January 2023