Introduction
Sacoglossans, commonly known as sap-sucking sea slugs, are primarily mesoherbivores with a narrow diet typically limited to green algae, with a few species being carnivorous and feeding on egg masses of Heterobranchs. Gills are present in shelled sacoglossans but absent in non-shelled slugs; instead, gas exchange takes place through the body surface. The family Plakobranchidae is the most species-rich within the order Sacoglossa, representing the greatest diversity in terms of species numbers (Händeler and Wägele Reference Händeler and Wägele2007; Pruvot-Fol Reference Pruvot-Fol1954; Swennen Reference Swennen2011).
In the Plakobranchidae family, the genus Elysia Risso discovered first in 1818 is the most diverse group, encompassing about 157 species. In this group of 157 species, around 104 species have been accepted with their classification names, while 40 species are currently under taxonomic review, means they may have uncertain synonyms, misspelling, basionyms, have been reassigned to different genera, or have been synonymized with other species. (WoRMS 2025) and eight species are presently listed as ambiguous due to their unclear taxonomic position (WoRMS 2025). The species belonging to the genus Elysia are herbivores in general (Jenson Reference Jensen1997) and consume algae as their primary food source (Jenson Reference Jensen2003). Many species of the genus Elysia can house living chloroplasts within their highly branched digestive diverticula, giving them a vivid green hue. This phenomenon is known as kleptoplasty, and it describes the ability of non-photosynthetic organisms to assimilate and retain algal chloroplasts in their body for several weeks to months (Bourke et al. Reference Bourke, Walker and Willan2016; Cruz and Cartaxana Reference Cruz and Cartaxana2022; Hirokane et al. Reference Hirokane, Miyamoto, Kitaura, Nakano, Hirano, Kawane and Yusa2022; Rumpho et al. Reference Rumpho, Dastoor, Manhart, Lee, Wise and Hoober2007). These animals lack a jaw and have a single row of teeth running the length of their radula. The older teeth of this row are kept in an ascus over the animal’s life (Pruvot-Fol Reference Pruvot-Fol1954). The most characteristic feature of Elysia is its unique parapodia, which resemble wings and line the back of the foot (Buatip et al. Reference Buatip, Saengkeaw and Buatip2022; Pruvot-Fol Reference Pruvot-Fol1954). The species is devoid of a shell and oral tentacles. A few Elysia species are larger, with sizes ranging from 25 to 50 mm, while the majority are smaller than 20 mm. They are essential to the food web as major consumers and important markers of ecological diversity and health (Kieckbusch et al. Reference Kieckbusch, Koch, Serafy and Anderson2004).
Elysia species have been discovered to thrive in a wide variety of habitats, such as mudflats, seagrass beds, mangroves, rocky and sandy shorelines, and coral reefs (Apte and Desai Reference Apte and Desai2017). Reports from the last 25 years, however, indicate that certain species of Elysia, namely E. leucolegnote, E. bangtawaensis, E. singaporensis, and E. bengalensis, are reported mainly in mangrove habitats (Swennen Reference Swennen2011). Due to their occurrence in mangrove ecosystems and their shape and colour resemblance to fallen mangrove leaves, they are commonly known as mangrove leaf slugs. In India, 17 species belonging to the genus Elysia has been recorded (Table 1): E. abei Baba, 1955; E. bangtawaensis Swennen, 1997; E. bengalensis Swennen, 2011; E. hirasei Baba, 1955; E. chilkensis Eliot, 1916; E. expansa O’ Donoghue, 1924; E. grandifolia Kelaart, 1858; E. marginata Pease, 1871; E. nealae Ostergaard, 1955; E. obtusa Baba, 1938; E. ornata Swainson, 1840; E. punctata Kelaart, 1858; E. pusilla Bergh, 1872; E. rufescens Pease, 1871; E. thompsoni Jensen, 1993; E. cf. leucolegnote Jensen, 1990; and E. tomentosa Jensen, 1997 (Apte and Bhave Reference Apte and Bhave2014; Apte et al. Reference Apte, Bhave and Parasharya2010; Apte et al. Reference Apte, Bhave, Pitale, Nagale, Bhave and Prabhu2014; Apte and Desai Reference Apte and Desai2017; Balani and Patel Reference Balani and Patel1994; Bhave and Apte Reference Bhave and Apte2011; Kumar et al. Reference Kumar, Sen, Raghunathan and Venkatraman2024, Reference Kumar, Venkatraman, Shrinivaasu and Raghunathan2019; MolluscaBase 2024; Sreeraj et al. Reference Sreeraj, Sivaperuman and Raghunathan2012a, Reference Sreeraj, Sivaperuman and Raghunathan2012b; Raghunathan et al. Reference Raghunathan, Dixit, Chandra, Chandra, Raghunathan, Mondal and Dash2016; Ramakrishna et al. Reference Ramakrishna, Raghunathan, Sivaperuman, Kumar, Raghuraman, Immanuel and Rajan2010; SeaLifeBase 2024; Sea Slug Forum 2024; WoRMS 2025). The reports of the species are from the west (Goa, Karnataka at Udupi District, and Maharashtra) and east (Andhra Pradesh) coasts of India, the Gulf of Kutch, the Gulf of Mannar, the Lakshadweep Islands, and the Andaman and Nicobar Islands.
List of Elysia spp. reported from the Indian region
Denotations: P = Presence; - = Absence; P* = New record; # = ambiguity in identification from India. Citations: 1Eliot Reference Eliot1916; 2Jagtap et al. Reference Jagtap, Shenai-Tirodkar, Savant, Kulkarni and Ansari2009; 3Ramakrishna et al. Reference Ramakrishna, Raghunathan, Sivaperuman, Kumar, Raghuraman, Immanuel and Rajan2010; 4Apte et al. Reference Apte, Bhave and Parasharya2010; 5Bhave and Apte Reference Bhave and Apte2011; 6Swennen Reference Swennen2011; 7Sreeraj et al. Reference Sreeraj, Sivaperuman and Raghunathan2012b; 8Apte and Bhave Reference Apte and Bhave2014; 9Apte and Desai Reference Apte and Desai2017; 10Kumar et al. Reference Kumar, Venkatraman, Shrinivaasu and Raghunathan2019; 11Kumar et al. Reference Kumar, Sen, Raghunathan and Venkatraman2024.
The present study describes the first record of E. cf. leucolegnote from the mainland mangrove ecosystem of Karnataka, India, located in the Western Arabian Sea. It describes the external morphology, distribution, and habitat of E. cf. leucolegnote.
Materials and methods
The study was carried out in May 2024 in a dense mangrove forest area at the Kali Estuary in Karnataka, situated on the west coast of India (14°50ʹ55.27″N, 74°9ʹ44.04″E and 14°50ʹ16.36″N, 74°10ʹ8.81″E) (Figure 1). The presence of muddy sediment substratum characterized the mangrove area. During low tide, a total of 15 specimens were collected from the study sites from mud pools bounded by mangrove tree species Rhizophora mucronata, R. apiculata, Sonneratia alba, and S. caseolaris. The specimens were gently picked and placed in a clean plastic container with water from the surrounding tidal pools. After collection, specimens were taken to the lab and separated into two containers: one kept them alive for monitoring, while the other was used for microscope analysis and preservation. The Olympus Tough TG-7 Digital Camera in Macro mode was used to capture macro digital images of live specimens in their native environment. The physicochemical water quality parameters of the surrounding water were measured using a Hanna portable multiparameter probe (HI9829-01042). The Olympus Tough TG-7 camera was used to take pictures of the specimens in the lab to document their morphological characteristics and taxonomical identification. The description provided by Swennen (Reference Swennen2011) was used to validate the identity of E. cf. leucolegnote. Specimens after preservation in 95% ethanol were discovered to be damaged and could not be further processed for detailed morphological examination.
Location of the E. cf. leucolegnote from Kali Estuary, Karnataka mainland of India
Results
The Kali Estuary is recognized as an ecologically valuable area that supports a diverse range of habitats for plants and animals, including mangroves, oyster beds, seaweed, salt marshes, finfish, crustaceans, reptiles, smooth-coated otters, and several bird species. According to a recent study by Hondappanavar et al. (Reference Hondappanavar, Bhat and Verma2024), 14 mangrove species have been reported in the Kali Estuary. During the low tide, all the collected (E. cf. leucolegnote) specimens were discovered in tiny mud pools on the high muddy sediment substrate, where the S. caseolaris trees were the most dominant. The physicochemical properties and nutrients in intertidal mud pool habitats regulate salinity, dissolved oxygen levels, and nutrient retention, enhancing algal growth and providing habitat stability. These interconnected components contribute to E. cf. leucolegnote persistence in mangrove environments, emphasizing the significance of environmental conditions that influence their spatial distribution. Real-time measurements of the water quality parameters were made from adjacent areas at specimen collection locations, with the value of surface water temperature of 29.5 ± 1.5°C, salinity value of 19.5 PSU, pH recorded around 7.7 ± 0.3, and dissolved oxygen concentration of 4.8 ± 0.3 mg/l. The observation of low salinity during pre-monsoon (May) was due to the continued flow of freshwater into the estuary.
Description of the new record
Systematics
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Superorder: Sacoglossa
Superfamily: Plakobranchoidea
Family: Plakobranchidae (Gray, 1840)
Genus: Elysia (Risso, 1818)
Species: Elysia cf. leucolegnote Jensen, 1990
Material examined
Fifteen specimens were collected by Ajay Kumar on 11 May 2024 at Kali Estuary (Karnataka), 14°50ʹ55.27″N, 74°9ʹ44.04″E and 14°50ʹ16.36″N, 74°10ʹ8.81″E during low tide adjacent to mangrove roots in tiny mud pools.
Diagnosis
The species was observed alive in mangrove habitats during low tide, inhabiting tiny tidal mud pools in the Kali stuary, Karwar, Karnataka. It exhibits green colouration with a white marginal triangular line on the dorsal surface, extending along the parapodia towards the rhinophores. The white colouration was observed at the tips of the rhinophores, while two black eyes were positioned beneath the rhinophores, and the parapodium featured four yellow dots.
Description
The body was dorsoventrally flattened, with a leaf-like shape pointed at the posterior end, and two rhinophores were present at the anterior end. The eyes were small, dark, and located near the base of the rhinophores, appearing as two distinct black spots. The size and length of the species were recorded to be between 4.6 and 8.5 mm, and the length of the rhinophores varied from 1.50 to 1.59 mm. The pericardium is a tubular, elongated structure situated at the back of the head, about one-fifth of the total body length, with two large vessels extending from the pericardium. The vessels and pericardium were generally visible on the dorsal side(Figure 2).
Image of the individuals of species E. cf. leucolegnote (a); living individuals of E. cf. leucolegnote from the mud pool of mangrove habitat (b).
Distribution
Pacific and Indian Oceans with occurrences along the coasts of Hong Kong (Jensen Reference Jensen1990), Biak Numfor, Papua – Indonesia (Wardiatno Reference Mardiansyah and Wardiatno2020), Northern Territory and northern Queensland, Australia (Bourke et al. Reference Bourke, Walker and Willan2016), Verde Island Passage, Philippines (Sanchez-Escalona Reference Sanchez-Escalona2019), and Pattani Campus in Southern Thailand (Buatip et al. Reference Buatip, Saengkeaw and Buatip2022) (Figure 3).
Map showing the global distribution of E. cf. leucolegnote, including the present study.
Distribution in India
Andaman Islands, India, Bay of Bengal, Northeastern Indian Ocean (Sreeraj et al. Reference Sreeraj, Sivaperuman and Raghunathan2012b). The present distribution is the new record from mainland India from Kali Estuary in Karwar, Karnataka, India (Arabian Sea), located along the Northwestern Indian Ocean.
Remarks
The E. cf. leucolegnote specimens examined in the present study exhibit characteristics consistent with those reported in previously described species possessing similar morphological features (Bourke et al. Reference Bourke, Walker and Willan2016; Jensen Reference Jensen1990; Sreeraj et al. Reference Sreeraj, Sivaperuman and Raghunathan2012b; Swennen Reference Swennen2011). These characteristics include rhinophores, which were a pair of elongated, tentacle-like structures located in the anterior region of the specimen. They were broad at the base and tapered towards the distal end with a slight groove. Their colouration was primarily green, matching the body, with a distinct whitish marginal line along the edges. Additionally, white marginal bands are present along the parapodia on both sides of the body.
Discussion
With the report of E. cf. leucolegnote, a total of 17 species from the genus Elysia were now reported from the Indian region. The reports of E. cf. leucolegnote from mainland India in Kali Estuary, Karnataka, extend the range of occurrence of the species, which was previously documented only from Hong Kong, Singapore, Thailand, and the Andaman Islands of India.
Elysia cf. leucolegnote identified in this study exhibits a green body with a white marginal line. The parapodia bear four distinct orange or yellow dots. The original description of E. leucolegnote by Jensen (Reference Jensen1990) does not specify that the specimen from Hong Kong features four yellow spots on the dorsal side of its parapodia. However, studies conducted in different locations (Thailand, Australia, Philippines, Indonesia, and India in Andaman and Nicobar Islands) have reported individuals with yellow spots in their specimen photographs (Bourke et al. Reference Bourke, Walker and Willan2016; Buatip et al. Reference Buatip, Saengkeaw and Buatip2022; Sanchez-Escalona, Reference Sanchez-Escalona2019; Sreeraj et al. Reference Sreeraj, Sivaperuman and Raghunathan2012b; Swennen Reference Swennen2011; Wardiatno Reference Mardiansyah and Wardiatno2020). The species observed during the study were found congregated in mud pools and occurred in shaded muddy regions of mangroves. The species is commonly noted for its green appearance due to kleptoplasty, but when food is scarce, it is also observed to turn yellow and decrease in size (Swennen Reference Swennen2011).
During our observation, the specimen showed a maximum length of 8.5 mm, which was comparatively smaller than other reported regions – 18–30 mm Hong Kong, China (Jensen Reference Jensen1990); 41 mm in Pak Phanang Bay, Thailand (Swennen Reference Swennen2011); 3–14 mm in the Andaman and Nicobar Islands, India (Sreeraj et al. Reference Sreeraj, Sivaperuman and Raghunathan2012b); 4–22 mm in Darwin Harbour, Australia (Bourke et al. Reference Bourke, Walker and Willan2016); 30–35 mm in the Verde Island Passage, Philippines (Sanchez-Escalona, Reference Sanchez-Escalona2019); 6–18 mm in Biak Numfor mangrove, Papua, Indonesia (Mardiansyah and Wardiatno Reference Mardiansyah and Wardiatno2020), and around 10 mm in Pattani, Thailand (Buatip et al. Reference Buatip, Saengkeaw and Buatip2022). The comparatively small size observed during the present study indicates that specimens might not be adult and were in the juvenile growth stage, although they were comparable in size to the specimen reported by Sreeraj et al. (Reference Sreeraj, Sivaperuman and Raghunathan2012b).
Elysia cf. leucolegnote has been reported to primarily consume particular algal species such as Boodleopsis pusilla, Enteromorpha, Cladophoropsis, and Cladophora (Jensen Reference Jensen2003; Swennen Reference Swennen2011). In the present study, all the specimens of E. cf. leucolegnote were found within the mangrove ecosystem. Previous studies have reported their occurrence exclusively from mangrove ecosystems and muddy soil substrates, especially beneath mangrove roots, and mangrove ecosystems with clear waters (Bourke et al. Reference Bourke, Walker and Willan2016; Jensen Reference Jensen1990; Swennen Reference Swennen2011). However, their only occurrence outside the mangrove region was reported by Swennen (Reference Swennen2011), in tidal pool areas, where they were surrounded by sulphur bacteria and white microbial colonies present in the anaerobic water.
The Elysia Risso, 1818 genus generally exhibits an overall cryptic green colour, making taxonomic identification and the exact number of species unclear due to intraspecific variation. In the genus Elysia, there are not many distinguishable traits to identify species; papillae and/or colour markings on the body surface are commonly used to classify and identify species (Takano et al. Reference Takano, Hirano, Trowbridge, Hirano and Watano2013). Martin-Hervas et al. (Reference Martín-Hervás, Carmona, Jensen, Krug, Vitale and Cervera2023) highlighted Elysia as a challenging category in terms of systematics for several reasons, such as the absence of information on the internal anatomy and the scarcity of morphological synapomorphies that hindered the hierarchical placement of species into suggested divisions (or synonymized genera). The phylogeny study by Krug et al. (Reference Krug, Vendetti and Valdés2016), combined with morphological and developmental data, suggests that characters such as penial stylets, radular morphology, parapodial size and shape, and lecithotrophic development are all surprisingly plastic, making further complications high in the identification of Elysia species.
However, Krug et al. (Reference Krug, Vendetti and Valdés2016) addressed the taxonomy and systematics of Caribbean Elysia species using an integrative method utilizing molecular and morphological data, which marked progress in the Caribbean region’s reform of Elysia. Another extensive study by Martin-Hervas et al. (Reference Martín-Hervás, Carmona, Jensen, Krug, Vitale and Cervera2023) on European Elysia taxa using molecular and morphological examinations of the taxonomy has uncovered a new, unnamed pseudocryptic species. The study included about 60% of the valid species of Elysia in the updated phylogenetic systematics complex Elysia genus. By combining morphological and developmental data to substantiate conclusions from DNA analysis, the work addresses some unresolved taxonomic and nomenclatural issues while also pinpointing those for further revision.
Our examination of Elysia species documented in the Indian region revealed inconsistencies in the reports of four species, E. grandifolia, E. marginata, E. ornata, and E. abei. Elysia abei is exclusively documented in the Andaman and Nicobar Islands of India (Ramakrishna et al. Reference Ramakrishna, Raghunathan, Sivaperuman, Kumar, Raghuraman, Immanuel and Rajan2010). However, Tripathy and Mukhopadhyay (Reference Tripathy and Mukhopadhyay2015) argued that the identification of E. abei in India is a misidentification and requires revision. Baba (Reference Baba1955) discovered an appearance of fine median denticulation along the cutting edge of certain radular teeth, and its absence in others, as the only reliable distinguishing feature between E. abei and E. amakusana, emphasizing the importance of conducting a thorough taxonomic analysis of E. abei, as external traits may lead to misidentification. Analyses by Jensen (Reference Jensen2003), Takano et al. (Reference Takano, Hirano, Trowbridge, Hirano and Watano2013), and Mehrotra et al. (Reference Mehrotra, C, Scott, Arnold, Monchanin and Chavanich2020) have also emphasized the meticulous examination of taxonomic traits in differentiating E. abei, E. amakusana, and E. japonica.
Suganya et al. (Reference Suganya, Sharma and Prabakaran2024) in their study discussed about the Elysia marginata-grandifolia complex. The taxonomic history of Elysia ornata Swainson, 1840, Elysia grandifolia Kelaart, 1858, and Elysia marginata Pease, 1871 has been revised with integrative approaches. Initially distinguished by parapodial banding and radular traits, E. grandifolia and E. marginata were synonymized with E. ornata due to morphological overlap. However, molecular analyses revealed cryptic species within the E. marginata-grandifolia complex in the Indo-Pacific, distinct from Caribbean E. ornata. This supports restricting E. ornata to the Caribbean while recognizing Indo-Pacific lineages as genetically distinct (Krug et al. Reference Krug, Vendetti, Rodriguez, Retana, Hirano and Trowbridge2013, Reference Krug, Vendetti and Valdés2016). Our observations of Elysia species that have been recorded from the Indian region have discovered that there is a great deal of void in the taxonomic expertise or research of Heterobranchia. To address misidentification and investigate potential new records or species discoveries of cryptic Elysia species, phylogenetic analysis is urgently needed. Elysia leucolegnote, E. bangtawaensis, and E. bengalensis have all been reported in Indian mangrove forests, with E. bengalensis being endemic to India and E. bangtawaensis having a limited presence range in Australia, Thailand, and India. A detailed investigation of Elysia sp. with molecular and morphological data is required to accurately identify it to the species level.
In conclusion, the documentation of newly recorded E. cf. leucolegnote denotes either a new arrival or a cryptobenthic species that was overlooked. The new report on sensitive or ecologically fragile species provides critical insights for biodiversity conservation and ecosystem management. These records contribute valuable data for conservation managers, marine ecologists, and policymakers, facilitating a deeper understanding of species distribution, habitat specificity, and ecological interactions. Given the increasing threats to coastal ecosystems from anthropogenic activities and climate change, such findings underscore the urgent need for targeted conservation strategies to protect these habitats and ensure the persistence of vulnerable species. Integrating these records into long-term monitoring programs will be essential for assessing ecosystem health and for forming evidence-based conservation policies.
Acknowledgements
We express our sincere gratitude to the Honourable Director of (CSIR)–National Institute of Oceanography (NIO), Goa, for providing the necessary facilities. We also sincerely thank the editors and reviewers for their valuable time to review the manuscript and for providing insightful comments and suggestions. This is CSIR-NIO contribution number 7443.
Author contributions
A.K.: conceptualization, methodology, visualization, formal analysis, writing – original draft, and writing – review & editing. D.R.: conceptualization, methodology, formal analysis, and writing – review & editing. C.S.M.: project administration, funding acquisition, and writing – review & editing.
Funding
This research was supported by the CSIR, India, through funding grants under the Mission on Carbon Capture, Utilisation, and Storage (CCUS) for the project ‘Improving Carbon Stock Assessment Using Advanced Carbon Measurements in Mangrove Habitats Along the West Coast of India’ (HCP0048).
Competing interests
The authors declare none.
Data availability
All data analysed in this study are included in the article. Additional information will be made available upon request.
