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INTRODUCTION
Cirripedes play an important role in the structure and function of intertidal ecosystems. Cirripedes are the most successful fouling animals because of their prolific settlement and wide distribution (Khandeparker & Anil, 2007; Maréchal & Hellio, 2011). Their life cycle includes six nauplius and one non-feeding cypris larval stages (Høeg & Møller, 2006). Cirripede cyprids are adapted to locate suitable substratum for attachment and metamorphosis to juvenile barnacles (Lagersson & Høeg, 2002; Aldred & Clare, 2008; Maruzzo et al., 2011). Cypris attachment and metamorphosis is also referred to as 'cypris settlement' (Clare & Matsumura, 2000). Metamorphosis is defined as the subsequent sequence of morphological, physiological and biochemical events that transform a permanently attached cyprid into a sessile filter feeding juvenile (Thiyagarajan, 2010).
Although the substratum selection, attachment and settlement mechanisms of cirripede cyprids have been extensively studied, little attention has been given to the intervening and critical series of metamorphic events from cyprid to juvenile barnacle that follow cypris settlement (Høeg & Møller, 2006; Anil et al., 2010; Thiyagarajan, 2010).
The few early observations on the morphological changes in balanomorphan metamorphosis either did not use laboratory-reared animals or failed to provide precise timing of metamorphic events (Doochin, 1951; Daniel, 1958). Walley (1969) described the detailed histological changes of Semibalanus balanoides metamorphosis by relying on field-sampled specimens without determining the precise sequence or age. Glenner & Høeg (1993, 1998) and Takenaka et al. (1993) focused on specific organs rather than the overall course of the metamorphosis of laboratory-reared cirripedes. Only a few studies provide timing of metamorphic events. Kühl (1950) divided the metamorphosis of Amphibalanus improvisus into a series of characteristic phases. Maruzzo et al. (2012) presented an accurate timeline and detailed description of morphological events of cyprid to juvenile barnacle in the model species of A. amphitrite (Darwin, 1854). Høeg et al. (2012) observed the cypris metamorphosis and described the timing of the overall series of events in four cirripede species including the suspension-feeding pedunculate (Lepas sp. and L. anserifera), sessile Thoracica (Megabalanus rosa) and parasitic Rhizocephala (Sacculina carcini). At present, only two accurate descriptions of metamorphosis in pedunculated barnacles of Thoracica are available...