OPISTHOBRANCH

ABSTRACT


INTRODUCTION
Opisthobranchs are shell-less molluscan gastropods found in both marine and freshwater ecosystems, and approximately 6000 species were reported throughout the world (Wagele et al. 2008). The distribution range of these fascinating animals stretches from the Indo-Pacific tropical region and the east coast of Africa to the Hawaiian Islands. Nudibranchs belong to Phylum Mollusca, Class Gastropoda, and Subclass Opisthobranchia. Gastropod group receives more importance due to the scarcity of studies in India. Opisthobranchs are one of the least studied groups of mollusks with highly diverse body forms, cryptic coloration, diets and habitats. The works done on Opisthobranchs in India and Kerala are little and patchy, and the earlier works, dating back to the 1880s by Alder & Hancock (1864), Kelaart (1858Kelaart ( , 1859Kelaart ( , 1883, and Gardiner (1903). From the beginning, opisthobranch diversity studies in India were restricted to the East and Southeast Coasts. The early records of these gastropods from the West Coast of India are limited to the works of Eliot (1905 and1909), Gideon et al. (1957), Menon et al. (1961) and Narayanan (1968). Recently, the opisthobranchs received good attention from various parts of the country by the promising studies done by renowned researchers.
The opisthobranch diversity of Indian waters has a long history, from the very pioneer study of Alder and Hancock (1864) to the very recent and comprehensive updates from the various eminent researchers in the field of marine biodiversity, including Aptae (2009), Bhave & Apte, 2013;Venkataraman et al. (2015), Raghunathan et al. (2016); Apte & Desai (2017); Vadher et al. (2020).
Even though the rocky reefs are important ecosystems of the Kerala coast, biodiversity studies are rare in this regard. The early studies on opisthobranch diversity of the Kerala coast were restricted to a few attempts (Narayanan, 1968b. After that, the opisthobranch diversity studies received more attention through remarkable studies done by various researchers Biju Kumar (2012), Ravinesh & Biju Kumar (2013), Chinnadurai et al. (2014), Ravinesh et al. (2014), Sheeja & Padmakumar (2014 a,b), Venkataraman et al. (2015), Apte & Desai, (2017), Baiju et al. (2016), Anu et al. (2017) andSneha Chandran et al. (2017). The present manuscript may get more attention due to the scarcity of opisthobranch diversity studies from the coast of Kerala.
The present study was carried out in various rocky reef structures of Kerala, the southwest coast of India. The study was able to record 13 species of opisthobranchs belonging to three orders, eight families and nine genera. Out of 13 species recorded in the study, five species were identified as new to the Kerala coast.

Study Area
Patchy rocky reefs are present along with the intertidal areas and occasionally at sub-tidal depths of the coast of Kerala. These rocky structures offer substratum for various biotic assemblages of the marine environment. The study mainly focused on coastal and sub-tidal rocky reefs of the coast. The rocky reefs are recognized as one of the most biodiversity-rich ecosystems among the others. These ecosystems are home to various life forms ranging from algae to various groups of invertebrates and fishes. Due to the shallow nature of the coast, extensive macroalgal communities are prominent include Bryopsis plumose, Caulerpa peltata, Caulerpa racemosa, Caulerpa taxifolia, Chaetomorpha antennina, Valoniopsis pachynema, Ulva prolifera, Ulva lactuca, Dictyota bartayresiana, Lobophora variegate, Sargassum wightii, Corallina officinalis and Kappaphycus alvarezii. The algal diversity of the rocky reefs will attract and supports various groups of invertebrates and fishes.
Vizhinjam is located at 08°22′36′′ N and 76°59′32′′ E. The reef substratum is typically granite boulder rocks that are covered with mussel beds (Perna perna). The mussel beds provide secondary hard substratum for many marine animals.
Thirumullavaram is situated along the Kollam coast (08°3'49.2'' N and 76°33'05.1'' E in the Kollam district of Kerala state. The shore substratum is partially formed by the combination of laterite rocks and sand and subjected to heavy wave action. Velliyamkallu (Thikkodi, Kozhikode) is considered a subtidal rocky reef, a massive rock that has witnessed many battles in pre-independent India and was once a vantage point for the Marakkar to attack the invading Portuguese. The rocky island is situated between 11°66'32.97" N and 75°54'16.81" E, 10 to 14 km away from the Thikkodi coast. Scientifically it can be defined as "Marine Nearshore Supra tidal Rocky Reef. This zone includes areas above Mean Higher High Water (MHHW) that are affected by wave splash and overwash but does not include areas affected only by wind-driven spray. This zone is subjected to periodic high wave energy, exposure to air and often to variable salinity. The name 'Velliyamkallu' means the white rock because it looks white due to the bird's droppings.

Methodology
Sampling was done from the rocky reef areas of the coast. Live specimens were collected by snorkeling and with the aid of SCUBA, handpicked specimens from the intertidal areas, inshore rocky reef, reef platforms, mussel beds, and subtidal reef areas of Vizhinjam, Thirumullavaram, and Velliyamkallu (Thikkodi, Kozhikode) of the Kerala coast between the depth range of 2 to 10 m. Sample collection was carried out from November 2018 to March 2019. These rocky reefs are partially formed by rocks and sand substratum and subjected to heavy wave action.

RESULTS AND DISCUSSION
Present study was able to record 13 species belonging to the 9 Genera, 8 Families, and 3 Orders. Among these, five of them are noticed as a new record from the Kerala coast (Table 1). Of the 3 Orders reported in the study, Nudibranchia with maximum species representation of 10 (77%), followed by Pleurobranchida with 2 (15%) species, and Sacoglossa was the least in this context of 1 (8%) species ( Figure 1). While considering the family-wise species distribution (Figures 2 and 3, and Table 3), Dendrodorididae was noticed with the highest species, the contribution of 3 (23%), followed by Facelinidae, Chromodorididae, and Discodorididae with 2 (15%) species in each and Pleurobranchidae, Bornellidae, Phyllidiidae and Plakobranchidae with 1 (8%) species on each.  While categorizing the species according to the number of specimens observed in each observation, these thirteen species were grouped into 3 categories as common, rare and uncommon. Out of 13 species observed, uncommon 38% (5 species), rare 38% (5 species), and regular 24% (3 species) ( Figure 4).
Comprehensive studies on the biodiversity of marine slugs were restricted to a few eminent researchers in the country. Among these, a few of them need to be discussed. Opisthobranchs are found in both marine and freshwater ecosystems and its approximately 6000 species were reported throughout the world (Wagele et al. 2008). From the beginning, Opisthobranch diversity studies in India were restricted to the East and Southeast coast. Records on these gastropods from the West Coast of India are limited to the works of Eliot (1905 and1909), Gideon et al. (1957), Menon et al. (1961), and Narayanan (1968). Venkataraman et al. (2015) provided the identification of Opisthobranch species, based on the collective experience of the authors who have been working for several years on marine biodiversity. Apte & Desai (2017) detailed study on about 361 species of sea slug species of Indian waters with their detailed descriptions. Vadher et al. (2020)   validated the total sea slug fauna of the Gujarat coast with a report of 95 species belonging to 62 genera and 29 families. Studies on opisthobranch diversity the Kerala coast were limited to a few studies. Narayanan (1968 b) reported the three opisthobranchs to belong to Pleurobranchidae (Pleurobranchus (Susania) ceylonicus), Doridae (Platydoris tabulate) and Hexabranchidae (Hexabranchus flammulatus) collected from Thankassery Coast (Kerala State) on the South-west Coast. Valdes et al. (1999), in that study reported one of the species, Chromodoris mandapamensis was collected from Muttom, Kerala. Ravinesh & Bijukumar (2013) mentioned six Opisthobranch species in the study of intertidal biodiversity associated with the natural rocky shore and sea wall of the Kerala coast. Chinnadurai et al. (2014) recorded the long-tailed sea slug Stylocheilus longicauda for the first time from the Southwest coast of India. This was the first record of the long-tailed pelagic sea slug Stylocheilus longicauda (Gastropoda: Opisthobranchia) from the Arabian Sea off Narakkal, Vypeen Island, Kochi, Southwest coast of India. Ravinesh et al. (2014) recorded as first time the sea slug species Polybranchia orientalis (Sacoglossa: Caliphyllidae) from the Southwest

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It is a small slug. The species with dark orange color body and light orange foot with a gelatinous texture. The species are mostly found in rocky reef habitats of protected to exposed sites, including rock pools and under rocks on lagoon reefs and pinnacles.

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The slug with translucent cream-white in color with a network of orange markings on all the body and scattered sub-epidermal opaque white granules. The color pattern extends to the rhinophore sheaths. The lobe-like oral tentacles, rhinophoral papillae, and dorsolateral processes are translucent white and have a subapical orange ring. Posterior to the rhinophores, there are six pairs of dorsolateral processes. The rhinophore sheath has tall and stalked with branches distally into elongate papillae. The slug can be identified by the bright red tips of the cerata. The slug species are mostly able to see under an overhanging rock in less than 1 m of water. Moridilla brockii is characterized by a translucent orange foot and clusters of orange cerata. There is a central row of large rolled cerata, which are in some specimens, translucent at the base, then white, and then orange. The rhinophores are orange, and translucent at the base. The oral tentacles are translucent orange, then translucent white in the last third, then orange again. In some specimens, there is a broad white band at the dorsal midline between the cerata clusters, going from the orange tip of the tail to the rhinophores, then to the upper half of the oral tentacles. When disturbed M. brockii unrolls the large rolled cerata and points them at the source of the disturbance.

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Characterised by the bright orange median line on the head which forks anteriorly, with a branch running up the anterior edge of each oral tentacle. Another orange line runs along the posterior edge of each oral tentacle and then runs along each side of the body below the cerata. The oral tentacles, rhinophores, tentacular foot corners, and cerata are all tipped with yellow, and there is a broad orange band on the rhinophores. Originally described in India, it has been reported two or three times. The only other record is from a photo that seems to be of this species from Hong Kong.

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The gills of this Chromodoris are lined with red or reddish-orange. The rhinophores are uniformly red or reddish-orange with a white stalk. The body is pale violet with rounded areas of creamy yellow pustules and deeper violet and red markings. These marks follow the contours of bilaterally symmetrical yellow pustules. The gills, usually 8 simple pinnate structures, are tricolored: the upper half is red or orange (both rachis and pinnae), and the lower half has a pale violet internal rachis and anus area with white pinnae.

Rare
Goniobranchus cavae (Eliot, 1904) (Figure 3.6 They all have large purple or purple-brown spots or marks, ringed with white, often reticulate brownish background, and a purple border. The mantle base is lined with a blue tint, and the rhinophore (are the horn-like structures that project from the front of the sea slug. "Rhino" means nose and "phore" means carrier. They are functioning as the organ for the sense of smell), gills are also lined with blue color. Gujarat, Tamil Nadu, Andhra Pradesh, Andaman Islands, Nicobar Islands.

Uncommon
Tropical to warm temperate Indo-West Pacific.

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The species is characterized by having an irregular color pattern of dark blotches over a reddish ground body color. Body dorso-ventrally flattened, soft and slimy, with a broad and smooth mantle, and thin and wavy mantle edge. Head extremely small, bearing a pore-like mouth. Rhinophores are bulbous, lamellated, with thick stalks. Rhinophora pockets with simple, thin, slightly elevated rims. Branchial plumes extended over the posterior part of the dorsum, their circle interrupted by the anal papilla. Each gill is finely subdivided. The Red Sea to Australia, Hawaii, Japan, South Pacific.

Rare
Lakshadweep, Andhra Pradesh. 180 The largest sea slug species recorded in the study. The species was observed while grazing over the Gayliella fimbriata algal mat. It is also seen in shallow pools and undersides of the rocks. They prefer muddy reefs with turbid water. These slugs produce a powerful toxin when they are disturbed and can make a severe burning sensation on the skin. The surface is extremely warty and dark brown with white patches. The lower part of the body has white spots, which is diagnostic for the species. Rhinophores are stalked. The stalk is dark brown, and the rhinophores are light brown in color. Gills are light brown, leafy, and highly branched.
Throughout the Indo-West Pacific. Tropical and warm temperate Indo-West Pacific.

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The species are mostly seen on seaweeds on rocky reef substratum. Broad, soft body with lots of bumps and pimples and distinctive electric blue spots. The tubercles are large and pinktipped. The blue spots are more easily noticeable than the black spots, which are more numerous. The mantle is straw-coloured with thick club-like rhinophores, and large feathery gills. The animal is generally beige with a purplish or pinkish tinge.

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The second largest sea slug species recorded in the study is usually seen in shallow pools and under rocks. It prefers rocky substrate. Brown mottling is distinct on the foot. The species can autotomize large parts of the mantle or sometimes the entire mantle skirt if disturbed. Gills are highly frilled. The species was re-designated as S. fragalis by Dayrat (2010) in his comprehensive review of basal discodorids.
Tropical Indo-West Pacific.
Andaman and Nicobar Islands, Gulf of Mannar.

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This species can be characterized externally by its relatively softly mantle, covered with softly pointed tubercles, mottled with light brown colors, and a few small dark brown patches. The underside of the mantle is translucent white with scattered large brown spots. The specimen was recorded from the mussel beds of the Vizhinjam coast while snorkeling. As per the ecology, the species may be observed in intertidal to the shallow subtidal, rocky reef, sandy sediment, and under rocks.

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Body long with a pair of very large 'wings' (called parapodia). The overall body color of the slug may be shades of green, yellow, or even white. The species shows remarkable color variation. It is believed that the color depends on how much and what seaweed is in the animal's digestive system. But all have a black margin on the edge of the parapodia, with an orange or yellow margin next to the black. There is a pair of long thick tentacles with tips in the same color banding as the body edge. The body has either black or white spots. The parapodia are often held in ruffles so that the animal resembles seaweed. Found feeding on Bryopsis plumosa (Jensen, 1981) and Caulerpa racemosa.   Valdes et al. (1999), reported three species of opisthobranchs along with the redescription of Chromodoris trimarginata (Winckworth, 1946) in that study, one of the species, Chromodoris mandapamensis was collected from Muttom, Kerala. Ravinesh & Bijukumar (2013), mentioned six opisthobranch species in the study of intertidal biodiversity associated with the natural rocky shore and sea wall of the Kerala coast. Ravinesh et al. (2014), recorded the sea slug Polybranchia orientalis (Sacoglossa: Caliphyllidae) from the coasts of Vizhinjam, Kovalam, and Thirumullavaram for the first time. Sheeja and Padmakumar (2014 a, b) recorded two species belonging to Pleurobranchomorpha (Berthellina citrina (Rüppell & Leuckart, 1828) (Mollusca, Gastropoda)) and Plancobrancha (Plakobranchus ocellatus Hasselt, 1824 (Sacoglossa, Opisthobranchia)). Chinnadurai et al. (2014) recorded the long-tailed sea slug Stylocheilus longicauda for the first time from the Southwest coast of India. This was the first record of long-tailed pelagic sea slug Stylocheilus longicauda (Gastropoda: Opisthobranchia) from the Arabian Sea off Narakkal, Vypeen Island, Kochi, Southwest coast of India. Baiju et al. (2016) reported six species of opisthobranchs from the coast of Vizhinjam while studying the biodiversity associated with mussel fishery. Anu et al. (2017) recorded four opisthobranch species as an associated fauna of mussel beds (Perna perna) of Vizhinjam, South coast of Kerala, India, including two new records to Kerala coast. Sneha Chandran et al. (2017) reported 15 species of opisthobranchs to belong to 4 families through the countries' first citizen science initiative, which includes two new records to India and five new records to the West coast of India. Present study recorded thirteen species of opisthobranchs and taxonomically identified under 3 Orders, 8 Families, and 9 Genera. The study specifically recognized the presence of eight new records on the Kerala coast.
The present study results prove that rocky reef ecosystems are under-exploited in the sense of Opisthobranch diversity. In-depth studies are required to assess the diversity of one of the least studied animals from the coast. Even in the short span of the study period, we were able to record thirteen species of sea slugs belonging to 8 Families and 9 Genera.

CONCLUSION
Compared with other areas of marine biodiversity d o c u m e n t a t i o n , t h e k n o w l e d g e a b o u t t h e opisthobranchiate faunal diversity of the Indian subcontinent is too little to interpret. The research gap will become more prominent in the Kerala scenario. A little research was carried out on this group of animals. The recent and sudden changes in the coastal ecology are due to the pressure from both natural and anthropogenic sources. The biodiversity of coastal ecosystems rocky reefs is subjected to silent loss of biodiversity. The concern is that we may lose many species without being aware of their existence in our coastal rocky reef ecosystems. The present attempt was able to record 13 species of opisthobranchs belonging to 3 orders, 8 families and 9 genera. This manuscript deals with the strictly rocky reef ecosystem-based biodiversity study. Present study results prove that rocky reef ecosystems are under-exploited in the sense of opisthobranch diversity.