A comprehensive checklist of the earthworms (Annelida: Clitellata: Megadrili) of Sri Lanka, a component of the Western Ghats – Sri Lanka biodiversity hotspot

A comprehensive checklist of the earthworm species known from Sri Lanka, an island country in the Indian subcontinent, is presented. In total, 81 species and subspecies belonging to 20 genera and 8 families are recorded. Of these, 58 are Sri Lanka endemics, 2 near endemics and 21 exotics. The checklist includes the type locality, any significant subsequent generic placements, and the distributional pattern with province and districts for each species/subspecies within Sri Lanka.


Introduction
Sri Lanka is an island nation in the shallow Indian continental shelf, spread over an area of 65,584 km 2 (between 9 o 50'to 5 o 55'N and 81 o 53' to 79 o 31'E). This forms the second largest island in the Indian Ocean after Madagascar (Samaranayake and Moyle 1982;Illangasinghe et al. 1999;Szederjesi et al. 2019) (Fig. 1). This continental island is very rich in biodiversity with high endemism; most of the endemic flora and fauna are confined to the wet zone natural ecosystems of the country (Gunatilleke et al. 2008). The Western Ghats mountain range of the southwest portion of India, along with Sri Lanka, is represented as one of the important biodiversity hotspot in the world (Myers et al. 2000;Mittermeier et al. 2011). Present day Sri Lankan Island has been a part of the Indian plate during the Gondwanan break-up and drifted away in Miocene from the mainland India to form an island (Biswas 2008). The Sri Lanka Island had several extended periods of land connection with mainland India during the Burdigalian age (~20-16 Mya) and the recent Pleistocene epoch as part of a series of glacial periods (Bossuyt et al. 2004;Epa et al. 2012;Sudasinghe et al. 2020).
Topographically, Sri Lanka can be divided into four regions namely, i) highland, situated in the south centre, ii) surrounding intermediate region of uplands, ridges and valleys at a lower elevation, iii) former region is in turn bounded by an outer or lower zone of low lands, and iv) surrounded by a coastal fringe consisting of sandbars, lagoons and islets (Illangasinghe et al. 1999). The north region and most of the eastern parts of the country is drier (Lindström 2011). Situated north of equator, the general climate of Sri Lanka can be described as tropical, with uniformly high temperatures throughout the year without major seasonal changes; the mean annual temperature in the lowland is around 27 o C but at the central highlands it is <15 o C (Illangasinghe et al. 1999;Lindström, 2011). Sri Lanka receives the monsoon rainfall, the average annual rainfall is about 2000 mm but it is not uniform over geographical area of the island (Illangasinghe et al. 1999). The south-western part of the country receives most rainfall, especially between May and October (Lindström 2011). Based on the amount of rainfall the country has been divided into three different ecological zones viz., dry (<1900 mm rainfall), intermediate (1900-2540 mm) and wet zones (2540-5686 mm) (Perera 2001). On account of Sri Lanka's topographic and climatic heterogeneity it has rich ecosystem diversity, among these most important with relation to the earthworms are the following -coastal ecosystems, natural/semi-natural grasslands, and various kinds of natural forests (Gunatilleke et al. 2008). Forests still cover 25% of the Sri Lanka's total land area (Gunawardene et al. 2007) and most of it is situated in the dry zone (Perera 2001). Major forest types are mangroves, tropical forest groups such as lowland wet evergreen, moist evergreen, dry mixed evergreen, submontane and montane forests and thorn scrub forests (Gunatilleke et al. 2008). The tropical rain forests are found only in the southwest quarter of the country and much of these are fragmented (Gunawardene et al. 2007).
Taxonomic studies on the earthworms of Sri Lanka were started in the second quarter of the 19 th century by the description of Megascolex caeruleus by Robert Templeton in the year 1844. Eventually this represents the very first species scientifically described from the Indian subcontinent. Studies on the earthworm fauna in Sri Lanka had been very uneven since Templeton's discovery of the first earthworm. The initial active period, though brief, was towards the third quarter of the nineteenth century by Schmarda (1861). He described 4 new species from the island viz., Perichaeta brachycycla (now Megascolex brachycyclus), Perichaeta cingulata (M. cingulatus), Perichaeta leucocycla (M. leucocyclus) and Perichaeta viridis (Megascolex ? viridis), of which, generic status of Perichaeta viridis is still uncertain. Towards the end of the nineteenth century , 26 new species were described from Sri Lanka (Beddard 1886(Beddard , 1890Rosa 1892Rosa , 1894Michaelsen , 1899, followed by another active period from 1901-1925, during which 23 new species were described (Michaelsen , 1907Stephenson 1913Stephenson , 1916 (Fig. 2) and the sporadic period during 1926-1948. Michaelsen is the pioneer among the taxonomists worked on the islands earthworm fauna, who described 30 new valid species, followed by Stephenson (13 species), Gates (8), Schmarda (4), Rosa and Beddard (3 each) and Templeton (1 species). In this moment it is worth to note that no nationwide taxonomical studies on earthworm were carried out after the independence of Sri Lanka from the colonial powers. Only three published materials are available on Sri Lankan earthworm fauna since its independence, they are of Samaranayake (2013), Wikramaratne and Krishnarajah (2013) and Szederjesi et al. (2019). First two were short-term studies on the diversity and ecology and not pure taxonomical, and the classification they used were of old and published in a symposium proceeding. Even though, Samaranayake (2013) added 4 exotic species, which were not reported earlier from the country.
Despite the high earthworm diversity, a complete list of earthworms of Sri Lanka has not been published after  comprehensive work on the Oligochaeta of India and neighboring countries. Moreover, many changes in nomenclature and taxonomy of earthworms have occurred since . The aim of this study is, therefore, to prepare a checklist that updates nomenclature and distribution of Sri Lankan earthworms, and to provide a base for further investigations. Apart from their importance in identification, checklists form a source of useful information on the species distribution and endemicity, so that conservation strategies can be developed (Narayanan et al. 2016). As a result, 81 valid species and subspecies are recorded. Much is still to be learned about the earthworm diversity of the region. In this context, it is therefore hoped that this checklist shall be useful as most of Sri Lankan native earthworm species are associated with various habitats in the highlands and rain forests, which are among the most threatened ecosystems today.

Methods
This checklist is based on an exhaustive review of the entire literature on the earthworm fauna of Sri Lanka. The species entries include -1) scientific name along with author and year of publication; 2) original and pertinent literature; 3) synonyms (mainly from the original literature and in the case of exotic species, from Blakemore (2012); 4) type locality (TL); 5) type material (TM) information on the type specimens of each species is from Reynolds and Wetzel (2017), unless reference is specified; 6) distribution (D) within Sri Lanka is arranged as province (P) (ordered from north to south) followed by districts (Dt) (listed alphabetically) together with references (if a locality record is just a region, e.g. Sri Lanka, the locality is written as in the original publication); 7) distribution elsewhere (E), if applicable; and 8) remarks (R) on the taxonomic status of species, if necessary. Over the period of time, the names or spellings of collected localities have changed and this has created confusion about the distributional records of a number of species. In such cases the new name, as found in the literature, is given in brackets after the old name.
The museum abbreviations are as follows: The classification of megascolecoid earthworms at the family level is not yet stabilized (Fender and McKey-Fender 1990;Blakemore 2005Blakemore , 2013Pavlíček et al. 2012;Narayanan et al. 2016). Blakemore's (2013) recent proposal to place megascolecoid earthworm taxa among several subfamilies appears to be a step in the right direction. Generally we have followed the classification as given by Gates (1959Gates ( , 1972. However, based on Blakemore (2013), genera Argilophilus and Pontodrilus formerly of the family Acanthodrilidae have been placed in family Megascolecidae. Even though Dichogaster genus is placed under the family Benhamiidae (James and Gamiette 2016;Tiwari et al. 2020), however, previously it was in the Octochaetidae (Julka 1988). According to Gates (1960) Indo-Sri Lankan species of Notoscolex were not congeneric with those of Austro-New Zealand section of the genus. The genus Notoscolex Fletcher, 1886 belongs to the Australia-New Zealand region (Blakemore 2000). However, we have included Sri Lankan species of the genus under Notoscolex Fletcher, 1886 (sensu lato), following .  placed Nellogaster from Sri Lanka in Notoscolex, but recently Szederjesi et al. (2019) retained it in Nellogaster; hence we followed the same. Species recorded only from Sri Lanka are considered endemic, whereas those with extended distribution to neighbouring India are referred to as near endemic. Exotic species are those, which have been introduced in Sri Lanka from other countries or zoogeographical regions.

Discussion
Until recently it is thought that Sri Lanka had 63 species of earthworms, of which 48 are endemic . The present checklist reveals the occurrence of 81 species and subspecies of earthworms belonging to 21 genera and 8 families from Sri Lanka. It is rather a high species diversity in view of Sri Lanka's small geographical area. It is noteworthy that earthworm fauna of Sri Lanka possesses a close relation to the Western Ghats Mountains of India, especially of the Kerala state (Narayanan et al. 2016).
About 83% (67 species, including both native and exotic) of the species belong to Megascolecidae family and rests of the families are represented by very few species. Family Benhamiidae is represented by 4 species, Lumbricidae represented with 3 species and Moniligastridae and Ocnerodrilidae are represented by 2 each, whereas Almidae, Rhinodrilidae and Eudrilidaeare represented by single species each. Among these, Benhamiidae, Eudrilidae, Lumbricidae, Ocenrodrilidae and Rhinodrilidae are exotic to Sri Lanka. Genera with higher number of species and subspecies are Megascolex (37 species) followed by Notoscolex (14), together they form nearly 63% of all the species recorded from the country.
Province wise distribution of species shows that Central province tops the list with 58 species, followed by Western with 28, Sabaragamuwa with 11 and as of now no species has been reported from the North Western province (Fig. 3). Difference in number of species is definitely due to uneven exploration intensity. From the figure 3, it is clear that except Central province, much of the other provinces are not surveyed systematically. Several areas are yet to be explored in the country, especially the biodiversity-rich national parks, wildlife sanctuaries, which along with other forest tracts of the Sri Lanka are considered refugia of the relic biota of the former Indian plate.
As in the case of many other taxa, earthworms of Sri Lanka and Western Ghats of India show an exceptionally high level of endemism (71.6% and about 77% (Narayanan et al. 2020(Narayanan et al. , 2021 respectively), which may be due to the fact that these two areas of the Indian plate, which were the part of the ancient super continent Gondwanaland, had never been fully submerged under the sea in the various geological time periods (Illangasinghe et al. 1999;Julka and Paliwal 2005). Of the estimated 9 genera with endemic taxa in Sri Lanka, Megascolex is dominant with 36 species and subspecies followed by Notoscolex (14 species), Drawida (2), Argilophilus (2), Glyphidrilus, Pontodrilus and Perionyx with one species each. Taxonomic history of Plutellus and Argilophilus is complicated (see Tiwari et al. 2020). Gates (1972), recommended placing Oriental species of Plutellus in Argilophilus. However, the presence of the North American Argilophilus in the Indian subcontinent is dubious. Later Blakemore (2006) commented that the Indo-Oriental Argilophilus species should be placed in another genus, probably a new one. Further molecular work should be carried out to clarify the status of these species and thus require a taxonomic revision of species of Plutellus and Argilophilus. Megascolex insignis and Perionyx ceylanensis are the near endemics, since these are also known to occur in India. A number of genera (Megascolex and Notoscolex) have very ancient lineages that might have their origin in Gondwanaland. Given the high rate of endemism, earthworm fauna of Western Ghats-Sri Lanka biodiversity hotspot is of immense bio-geographical significance.
Several species are known only from the original description, and most of them were recorded more than a century ago. Since then studies on the earthworm fauna of the country have been limited and no further studies have been carried out after the independence of this island country. Hence, due to the lack of revisionary works, the taxonomic status of these species, their level of morphological variation etc. cannot be considered as confirmed. The Western Ghats and Sri Lanka have similar levels of endemism among various taxa such as trees, bryophytes, land snails, odonates, fishes, amphibians and reptiles, though the fauna of the wet evergreen forest zones have been found to be quite distinctive and forms endemic clades, where as the fauna and flora of lowland dry forests seems more similar (Bossuyt et al. 2004;Gunawardene et al. 2007;Sudasinghe et al. 2020). Earthworms are also not an exception for this; Megascolex and Notoscolex species show high level of endemism in Sri Lanka and Western Ghats south of Palghat (Palakkad) gap (Narayanan et al. 2016). Recent studies on various taxa indicate that Sri Lankan fauna is derived from evolutionary diverse ancestral stock from the Indian mainland mostly before the Pleistocene epoch and underwent in-situ radiation within the island (Bossuyt et al. 2004;Biswas 2008). Hence further comparative studies are to be conducted between the earthworms of the Western Ghats of India and Sri Lankan highlands in order to understand the evolutionary history of the earthworms. Sri Lanka has been densely settled since early times, even though much of the areas were almost covered by tropical forests, however in the last 150 years extensive deforestation has taken place as a result of European colonisation, urbanisation, logging, tea and rubber plantation expansion (Samaranayake and Moyle 1982;Lindström 2011). According to McNeely et al. (1990) the rate of forest and wildlife habitat loss in Sri Lanka is considered as one among the highest in southern Asia. Hence, many species described early in the last century might have gone extinct. Systematic collection would help to know the present-day existence of these species in Sri Lanka. Further, faunistic surveys of under explored and unexplored areas in the country would bring to light additional species.