Ecology and Biodiversity of Sea Cucumbers (Holothuroidea de Blainville, 1834) in India and Its Checklist

1. Introduction

Sea cucumbers (Echinodermata, Holothuroidea), comprising approximately 1,800 species, inhabit oceans worldwide, thriving in environments ranging from shallow intertidal zones to the ocean's depths [1,2,3]. A large-scale investigation into echinoderm taxonomy began in the 19th century, utilizing samples collected from expeditions such as Valdivia, Siboga, Challenger, and Albatross, among others [4]. Koehler & Vaney made notable collections of Holothuroidea de Blainville, 1834 in the years 1905 and 1908 [4]. Several noteworthy contributions to the current understanding of Indian holothurian biodiversity have been made by [5], [6,7], [8], and [9]. These contributions include the documentation of distribution patterns, the identification of taxonomic errors, and the description of new species.

On the other hand, holothurians are extensively used in the food industry, particularly in Asian countries, where they are traditionally processed into beche-de-mer or trepang [10]. Moreover, their bodies are rich in various bioactive compounds such as phenols, collagen, amino acids, saponins, and chondroitin sulfate, making them valuable in both the cosmetic and pharmaceutical sectors [11,12]. Due to their commercial significance, sea cucumbers have faced excessive exploitation over the years, leading to declines in their populations as a result of overharvesting, illegal trade, and poaching [13]. Consequently, the Government of India has listed all holothurians under Schedule 1 of the Indian Wildlife Protection Act of 1972 to safeguard their populations [13].

This review aims to identify any previously undocumented species in Indian reports, thereby enriching our understanding of regional biodiversity. The review also addresses nomenclatural criteria, evaluates the extent of taxonomic inflation, and updates the checklist of Holothuroidea from India. It encompasses information on the ecology, biodiversity, in particular distribution, and abundance, making it a valuable resource for researchers and conservationists engaged in marine biodiversity studies.

2. Materials and Methods

Records detailing the distribution and abundance of holothurians in India were collected from a variety of sources, including websites, checklists, testimonials, articles, and other available resources. For each reported species, scientific names were verified using the WoRMS database. All corrected names were subsequently updated, resulting in the creation of a new checklist. Statistical analyses were conducted using Primer software and Excel spreadsheets. The author meticulously reviewed scientific publications from 1887 to 2024 to ensure the accuracy and currency of the checklist. The checklist reflects significant updates on nomen dubium, unaccepted names, synonyms, and other critical taxonomic terms following [3]. To interpret the diversity, dominance, and similarity, the authors organized the data by different regions, including the West Coast, East Coast, Andaman and Nicobar Islands, and Lakshadweep (Figure1).

3. Results

3.1. Diversity

The prevalence of sea cucumbers along the coastlines of the Indian Exclusive Economic Zone (EEZ) is noteworthy, attributed to the diverse habitats that support their thriving populations. An updated checklist reveals that a total of 187 species have been identified in Indian waters through previous research, encompassing 7 orders and 21 families.

The Andaman and Nicobar Islands exhibit the highest number of species, totaling 109, attributed to their remarkable biodiversity (Table 1). Figure 2 and Figure 3 display images of the cucumber photographed from the South Coast of Andaman.

The East Coast ranks as the second richest marine biodiversity region, harboring approximately 102 species (Table 2). A significant concentration of these species has been recorded in the Gulf of Mannar area.

Subsequent to this, Lakshadweep (Table 3) has been documented to host a total of 39 recorded species.

The coast with the least biodiversity was the west coast, which recorded a total of 34 species. Among these, the highest number was found along the coast of Kerala. The checklist for the West Coast is provided in Table 4. Additionally, there are 11 species from India for which the exact locations remain unknown (Table 5).

Among these recorded sea cucumbers, the order Holothuriida represents the highest proportion, comprising 26.73% of the species, followed by Dendrochirotida at 22.99%. The least reported species belong to the order Persiculida, which constitutes only 2.67% of the total (Figure 4). Notably, five species—Holothuria (Halodeima) atra, Holothuria (Lessonothuria) pardalis, Holothuria (Mertensiothuria) leucospilota, Holothuria (Semperothuria) cinerascens, and Stichopus hermanni—were observed across all selected coastal zones in this review.

The dominance of 176 species, excluding the 11 species with uncertain locations, is illustrated in Figure 5, which encompasses the West Coast, East Coast, Andaman & Nicobar Islands, and Lakshadweep. The station with lower abundance and higher diversity is represented by the lower line. In the k- k-Dominance curve analysis, the Andaman and Nicobar Islands represented a high degree of species dominance, followed closely by the East Coast of India. At the same time, the West Coast exhibited the lowest levels of species dominance.

The Shannon-Weiner diversity index (H’) provides a comprehensive measure of biodiversity by considering both the number of species present and their relative abundance, allowing for a more accurate assessment of overall diversity across research sites. The highest diversity is observed in the Andaman and Nicobar Islands (H' = 2.23) and the West Coast (H' =2.14). The East Coast shows modest variety (H' = 2.06). In contrast, Lakshadweep Island has relatively low diversity(H' = 1.39). The Andaman and Nicobar Islands and the West Coast, with their rich biodiversity and favorable environmental conditions, offer conducive habitats for a wide range of species, resulting in higher diversity indices (Table 6).

The highest species richness (S =18) was found in the Andaman and Nicobar Islands, followed by the East Coast (S = 13), West Coast (S = 12), and Lakshadweep (S = 8). The Andaman and Nicobar Islands also have the greatest Margalef diversity (d) at 3.638, followed by the West Coast at 3.07, the East Coast at 2.6, and Lakshadweep at 1.91. Pielou's evenness (J') evaluates both species richness and diversity. Pielou's evenness was highest on the West Coast (0.86) and lowest in Lakshadweep (0.67). These findings show that the Andaman and Nicobar Islands have both the highest species richness and Margalef diversity, indicating a greater variety of species in this region compared to the others studied. Additionally, the West Coast exhibits high Pielou's evenness, reflecting a more balanced distribution of species compared to the other three locations. By using the Bray-Curtis similarity measure, the degree of resemblance between data from existing documents can be quantified. The similarity indices of the four main stations are given in Figure 6. The results showed that the Andaman and Nicobar Islands and the East Coast had a higher level of resemblance (70%), while the West Coast and Lakshadweep exhibited a slightly lower similarity (50%). There is an overall 10% similarity shared by each location.

3.2. Ecology

Sea cucumbers represent one of the largest and most diverse groups of benthic invertebrates found in both intertidal and subtidal zones, contributing significantly to the overall biomass in slope and deep-sea (abyssal) benthic environments [52]. While the majority of species are sedentary or exhibit slow movement across the seafloor, some possess the ability to swim periodically, and a few are primarily bathypelagic, spending much of their lives as free-swimming organisms in the depths of the ocean [53]. The external body of the sea cucumber is elongated both lengthwise and vertically. Its pentamerous symmetry is often recognized by the presence of five meridian-bearing ambulacra [54]. Depending on the species, the tentacles may be uniform in size or vary. Their shape is a key characteristic for identification; some can be dendritic, growing quite large in the order Dendrochirotida Grube, 1840, whereas the peltate tentacles of the Aspidochirotida Grube, 1840, have a central stalk with a small, branched disc. Molpadida Haeckel, 1896, species display digitate tentacles, and pinnate tentacles are found in the Apodida Brandt, 1835 [55]. Additionally, conical or fleshy papillae are present on the body’s thick, slimy surface. Podia aid in movement, and microscopic ossicles or spicules are spread throughout the body [54,56]

Sea cucumbers represent one of the largest and most diverse groups of benthic invertebrates found in both intertidal and subtidal zones, contributing significantly to the overall biomass in slope and deep-sea (abyssal) benthic environments [52]. While the majority of species are sedentary or exhibit slow movement across the seafloor, some possess the ability to swim periodically, and a few are primarily bathypelagic, spending much of their lives as free-swimming organisms in the depths of the ocean [53]. The external body of the sea cucumber is elongated both lengthwise and vertically. Its pentamerous symmetry is often recognized by the presence of five meridian-bearing ambulacra [54]. Depending on the species, the tentacles may be uniform in size or vary. Their shape is a key characteristic for identification; some can be dendritic, growing quite large in the order Dendrochirotida Grube, 1840, whereas the peltate tentacles of the Aspidochirotida Grube, 1840, have a central stalk with a small, branched disc. Molpadida Haeckel, 1896, species display digitate tentacles, and pinnate tentacles are found in the Apodida Brandt, 1835 [55]. Additionally, conical or fleshy papillae are present on the body’s thick, slimy surface. Podia aid in movement, and microscopic ossicles or spicules are spread throughout the body [54,56]

3.2.1. Bioturbation & Nutritional Cycle

Sea cucumbers are often referred to as the "earthworms of the ocean" due to their crucial role in bioturbation [57]. Most aspidochirotids are deposit feeders, making them essential bioremediators in coastal mariculture. They help decrease organic load and redistribute surface sediments. Their excreta contain inorganic nitrogen and phosphorus, which boost the productivity of benthic organisms and support ecosystem growth in nutrient-poor environments, such as coral reefs [58]. Their feeding and excretion activities also raise seawater alkalinity. Sea cucumbers play a significant role in the ecosystem by not only redistributing organic debris but also altering it through their stomachs, which act as miniature bioreactors that rapidly absorb nutrients. This makes them some of the most influential bottom bioturbators, as they continuously stir and mix sediment [59].

There is growing interest in utilizing these animals as bioremediators within Integrated Multi-Trophic Aquaculture (IMTA) and polyculture systems, as they thrive in areas impacted by mariculture where protein-rich waste accumulates [60]. Holothuroids play a significant role in influencing primary production within reef and aquaculture ecosystems. Their waste products release essential nutrients such as ammonium and phosphate, which enhance the growth of microalgae and seagrasses, even as they consume these resources through deposit feeding [61]. Research on specific species, such as Apostichopus japonicus Selenka, 1867, and Australostichopus mollis Hutton, 1872, indicates that the benefits of nutrient recycling can sometimes be outweighed by their grazing activities [62]. The exclusion of Holothuria scabra has been shown to reduce seagrass growth, highlighting its contribution to nutrient cycling [63]. Additionally, certain species like Holothuria tubulosa Gmelin, 1791 assist in the decomposition of detritus [64]. While suspension feeders like Cucumaria frondosa are likely to recycle nutrients as well, there is still limited knowledge regarding their specific roles in these processes [61].

3.2.2. Symbiotic Relationship of Holothuroids

Holothurians have strong links to coral reefs, seagrass beds, and seaweeds because their sediment bioturbation promotes healthy growth. They enhance primary production, infauna biodiversity, and infauna biomass [65]. Moreover, over 200 species of parasitic and commensal symbionts from nine different phyla, including gastropods, arthropods, flatworms, and polychaetes, have been found in sea cucumbers [66]. Most studies have concentrated on aspidochirotid species. Symbiotic organisms benefit from these hosts through shelter, transportation, or nutrition, while the hosts generally remain unaffected. However, certain species, such as the shrimp Periclimenes imperator [68] and the Harlequin crab Lissocarcinus orbicularis [66], exhibit host specificity and even territorial behaviors. Additionally, hosts protect chemical defenses, such as saponins, and also provide spawning sites. Recent research has revealed new associations, including those with diatoms and opisthobranchs, highlighting the ecological significance of holothuroids as hosts for symbiotic relationships [68].

4. Discussion

The present review has compiled checklist information on 187 species of holothuroids from the Indian seas, encompassing 7 orders and 21 families. The distribution of species within the class Holothuroidea in Indian waters is uneven, with the order Holothuriida (26.73%) having the highest percentage of species. This implies that specific ecological factors or environmental conditions may favour the abundance and diversity of holothuroids in this order.

The Andaman and Nicobar Islands showed a significant dominance of certain species and a higher diversity of holothuroids compared to other coastal regions included in this study. [69] made a similar observation in earlier studies. Holothurian habitats display a wide range of diversity, covering environments such as coral reefs, seagrass meadows, rocky shores, sandy beaches, muddy shores, and mangrove beds. The Andaman Islands are especially notable for hosting all these diverse habitats [70]. This variety of habitats may explain the greater abundance of holothurians in this region compared to other parts of India. The east coast and the Andaman Islands share a high similarity (70%) in species distribution, whereas the west coast and Lakshadweep Island show a 50% similarity. Notably, 102 species are found on the East Coast, while 75 species are present in the Gulf of Mannar. The Gulf of Mannar has an extensive range of echinoderms, thanks to its abundance of favorable habitats such as sandy bottoms, seagrass beds, and reefs [42]. The coral reefs and other macrofauna, particularly those found in the Gulf of Mannar and the islands of Andaman and Nicobar, are key reasons for the striking resemblance between these two locations [21]. The Andaman and Nicobar Islands and the West Coast, with their rich biodiversity and favorable environmental conditions, provide suitable habitats for many species, leading to higher diversity indices. [71] found that the Andaman and Nicobar Islands have high diversity, followed by the East Coast, West Coast, and Lakshadweep Islands. However, in this study, the West Coast shows a higher level of diversity compared to its eastern counterpart. This variation in diversity can be linked to the unique geographical features and ecosystems present in each area. In 2017, [4] published a checklist of echinoderms in India, identifying a total of 179 holothuroids. However, subsequent reclassifications have emerged, including the grouping of the families Synallactidae and Stichopodidae into the distinct order Synallactida, as well as the renaming of the order Aspidochorida to Holothuriida. These reclassifications have resulted in changes in the classification of specific species. For instance, Stolus rapax has been reclassified as Pseudostolus rapax, while other species, such as Orphnurgus glaber and Oneirophanta conservata, have been reassigned to the order Synallacida.

In addition to the recent checklist, it is worth noting that [46] reported the first sighting of Bohadschia atra in Indian waters, with specimens collected from Kadmat and Agatti Islands in Lakshadweep. More recently, this same species has also been documented in the Andaman and Nicobar Islands [27]. Bohadschia atra is characterized by its brownish-black coloration accentuated with red spots and numerous brown podia on its ventral side. [51] noted the initial presence of Holothuria (Microthele) fuscogilva along the Gujarat Coast. This species displays a beige or white color with yellowish undertones on its ventral side, while its dorsal surface is marked by black or dark brown mottling. These findings highlight the expanding distribution ranges of these species.

5. Conclusions

The rising demand for holothurians in the food industry has become a significant concern, contributing to a decline in their populations. Although these sea cucumbers are classified as scheduled and protected, they continue to be subject to relentless exploitation driven by high demand, particularly in neighboring countries like India. Illegal poaching remains a persistent issue, posing a serious threat to the sustainability of their populations and the fragile marine ecosystems they inhabit. The lack of up-to-date species checklists complicates the accurate assessment of biodiversity within the Indian marine ecosystem. Insufficient information presents a considerable barrier to developing effective conservation strategies and managing natural resources responsibly. Moreover, it hampers the identification of species at risk of extinction that require urgent attention. Therefore, it is essential to allocate resources and expertise to update these checklists to enhance conservation efforts in India regularly. This study offers the most recent checklist of the class Holothuroidea within Indian waters.