Ontogenetic Changes in Vulnerability of the Prawn Fenneropenaeus indicus to UV-B Radiation Help Explain Ontogenetic Habitat Shifts

Abstract

The Indian Ocean white prawn (Fenneropenaeus indicus H. Milne Edwards, 1837) is distributed across the Indian Ocean, from southern Africa to northern Australia and the Red Sea, and is also present in all of South-East Asia, where F. indicus is a major commercial fishery species. F. indicus has been domesticated for aquaculture and is extensively cultured in farms throughout South-East Asia, India, the Middle East, Saudi Arabia, and eastern Africa (Ling et al. 1999). F. indicus is a non-burrowing prawn species preferring a sandy or muddy bottom, and although active at both day and night, it has been shown to display clear daily rhythms in activity, being less active at night (Natarajan 1989). F. indicus has a complex ontogenetic developmental cycle involving 12 stages distributed in three planktonic larval forms (6 nauplii stages, N-1 to N-6; 3 zoea stages, Z-1 to Z-3; and 3 mysis stages, M-1 to M-3). F. indicus develops from hatching to the post larva (PL) stage over 10 to 18 days in natural conditions (Muthu et al. 1978) and 13 to 14 days under hatchery conditions (Silas et al 1985). As many decapods, F. indicus also has a tri-phasic life cycle (Pitman and McAlpine 2003). The prawn matures and breeds mostly in offshore coastal waters, and the larvae are advected in the neuston until recruiting to their estuarine habitats (Forbes and Cyrus 1991). The PL grows in coastal estuaries, backwaters, or lagoons with a particular preference for mangrove habitats (De Freitas 1986; Mohan and Siddeek 1996; Ronnback et al. 2002), where they spend the juvenile and sub-adult stages, before migrating to open coastal habitats for breeding. Larvae are often present in the neuston, except in the period of maximum solar radiation. Seasonal studies off Kochi (Arabian Sea, India) showed that F. indicus larvae are rare in the plankton from May to September (George 1962; Rao 1964). Whereas late larval stages, PL, and juveniles inhabit shaded environments, such as mangrove creeks, the larvae are pelagic and can be subjected to high irradiance in the water column in tropical and subtropical regions. Moreover, this species is reared in shallow aquaculture ponds and floating cages (Walford and Lam 1987), which can also receive high solar radiation in tropical and subtropical regions. High light intensities have been reported to be detrimental for the survival of larval stages of penaeid prawn, which are reared under dim light conditions to increase spawning and early survival success in aquaculture (Wurts and Stickney 1984). The shaded habitat of the PL, in mangrove creeks, the low abundance of planktonic larvae during the months of peak solar radiation, and the light-sensitivity of early life stages all point at a vulnerability of F. indicus larvae to high solar radiation and particularly ultraviolet-B (UV-B) radiation as the most damaging component of solar radiation (Hader et al. 2007; Hansson and Hylander 2009). Because of its commercial importance, the physiology, growth, and behavior of F. indicus have been extensively studied (Kutty et al. 1971; Colvin 1976; Emmerson 1984). The resistance of PL to changes in environmental conditions, Communicated by Charles Simenstad