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New Record on the Opisthobranch Fauna of Drifting Sargassum Seaweed from the Arabian Sea

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Introduction

Opisthobranchia, a specialized group of phylum Mollusca, is the third largest group in Gastropoda. There are more than 3,000 known species of nudibranchs (Holland, 2008) with their habitats ranging from intertidal areas to seabeds a mile deep. Most of the species are exclusively benthic with some exceptions like Scyllaea pelagica which has adapted to the pelagic environment provided by floating masses of Sargassum. Until 2013, 311 species of opisthobranchs have been reported from the Indian sub-continent (Bhave & Apte, 2013).

Preliminary work on opisthobranchs of the western coast of India was carried out by Gardiner (1903). Followed by Winckworth (1946a, b), Hornell (1909b), Gideon et al. (1957), Narayanan (1968, 1969, 1971a, b), Menon et al. (1970), Rao & Kumari (1973); Rao et al. (1974); Balani & Patel (1994), and Jagtap (2009). Several new records have been established after 2008 by Apte (2009, 2012); Apte et al. (2010); Apte & Salahuddin (2011); Bhave & Apte (2011, 2013).

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Our study forms the first record of Scyllaea pelagica from the Indian waters where we have provided an account of our observations on S. pelagica and its associated fauna on Sargassum seaweed.

Materials and Methods

Drifting Sargassum rafts were collected from Grande Island (Lat. 15° 21’ 01.73” N; Long. 73° 46’ 58.68” E, Garmin GPSMAP 78S), Goa in the months of February and March 2017. The rafts were collected intact and brought to the laboratory in indigenous conditions within a couple of hours. Live specimens were sorted, identified(Rudman, 2004), the length and weight of individual specimens was recorded. Additionally, wet and dry weights of the collected macroalgal rafts were recorded along with in situ temperature and salinity. Digital images were taken of live specimens in their natural habitat using Nikon AW130 (14 megapixels); specimens were stored in 99% ethyl alcohol. The identified specimen was deposited in the Bombay Natural History Society (BNHS) collections. Remaining associated macroalgal fauna was preserved in 5% formaldehyde for further analysis.

Results

Taxonomic classification

Phylum: Mollusca

Class: Gastropoda

Subclass: Heterobranchia

Infraclass: Opisthobranchia

Order: Nudibranchia

Suborder: Dendronotina

Family: Scyllaeidae

Genus: Scyllaea

Species: Scyllaea pelagic Linnaeus, 1758

Description

S. pelagica is a robustly built nudibranch with a dorso-ventrally flattened body and grows up to a size of 45mm. The anterior portion has a pair of rhinophores enclosed within two large flattened rhinophore sheaths. There are two pairs of dorsolateral lobes, with a characteristically jagged edge, and the posterior end of the body has a flattened mediodorsal crest standing vertically. The skin is smooth except for a few small, scattered conical tubercles on the sides of the body. The inner surface of the dorsolateral lobes is lined with fine dendritic gills for respiration. The ¬¬¬ground colour is dull yellowish brown or greenish brown with some brown mottling and small white markings which provide ideal camouflage with the Sargassum weed on which it lives. There often is a row of brilliant blue spots along the flanks and sometimes dorsally. The genital opening is situated on the right side near the anterior lobe: the anus lies between the two right lobes. The organism swims with its foot upwards by contracting its longitudinal muscles on the left and right side alternately (Marcus & Marcus, 1963), an important ability for a nudibranch which drifts about the oceans.

Reproductive biology

S. pelagica is hermaphroditic where two individuals come together to exchange sperm through their genital opening. Fertilization is always internal. Eggs are laid in spiral jelly coated masses on the weed or the floating substratum¬¬. The eggs hatch into planktonic trochophore-like larvae (Kaestner, 1967).

Distribution

S. pelagica occurs globally in pantropical waters clinging to floating masses of Sargassum sp. where it is well camouflaged.

Observations

Table 1. Showing mean length and weight (mean ± SD) of collected S. pelagica

Raft Date Length Weight Sample size (n)

Mean SD Mean SD

1 28/02/2017 2.5 0.7 0.9 0.3 9

2 14/03/2017 2.8 1.0 1.0 0.5 10

3 29/03/2017 2.2 1.0 0.8 0.7 16

Mean 2.5 0.9 0.9 0.5 35

Ecological Role

Nudibranchs provide important ecological functions, playing an important role in marine food chains, both as predator and prey. Drifting masses of Sargassum form a unique ecosystem(Weiss, 1968) in themselves as every associated organism is assigned a specific ecological niche. S. pelagica associated with these macroalgal rafts are known to graze on the hydroids living in the same habitat (Hickman, 1973). Thus, it may be concluded that it plays a salient role in the food chains of the floating Sargassum rafts and aids in the transfer of nutrients from the air-water interface to the depths of the pelagic column.

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