In the field, both covering materials and shelters are commonly available to sea urchins. The behavioural decision between covering and sheltering thus obviously exists. However, no information is available on the preference between the two natural behaviours. Two conditions were designed with covering and sheltering materials to comparatively describe the diel trade-off between covering and sheltering behaviours in male and female Strongylocentrotus intermedius. In the present study S. intermedius had an obvious diel trade-off between covering and sheltering behaviours, preferring covering over sheltering when both behaviours were possible. Sex did not significantly affect the trade-off between the two behaviours, no matter whether covering materials were inside or outside the shelters. The rhythm of covering behaviour was not well correlated with intensity of light in the trade-off with sheltering behaviour. The present study provides new insights into behavioural ecology of sea urchins.Sea urchins are a group of ecologically important marine organisms, both as grazers and prey (Pearse, 2006). Covering and sheltering are two common behaviours of sea urchins in both the field and the laboratory. It has been a paradigm that sea urchins do not have a behavioural choice between covering and sheltering behaviours because they are not mutually exclusive. However, behavioural decision-making is very common in animals according to their physiological conditions and reproductive demands, selecting a single behaviour from a number of possible responses (Mowrey & Portman, 2012). For example, our previous studies indicate that sheltering behaviour of Strongylocentrotus intermedius showed significant plasticity with and without food availability, indicating a clear trade-off between sheltering and foraging behaviours (Zhao et al., 2013b).The sea urchin Paracentrotus lividus has an obvious habitat preference for Posidonia oceanica seagrass beds over rocky habitats (Pinna et al., 2012). Pinna et al. (2012) clearly emphasized the importance of sheltering behaviour in sea urchins, which has also been well documented to function as avoidance of predators (Nelson & Vance, 1979) and insolation . It must be noted, however, that covering behaviour also commonly exists in habitats of sea urchins that provide the potential for sheltering behaviour (Crook, 2003). This raises an interesting question of whether the habitat preference of sea urchins is due to the potential for sheltering behaviour (Pinna et al., 2012) or for covering behaviour (Dumont et al., 2007). However, the behavioural choice between sheltering and covering has never been investigated in sea urchins either in the field or in the laboratory. Biological sex has been well documented to affect behavioural decision-making in both invertebrates (for example, Barrios et al., 2008) and vertebrates (for example, Kaciuba-Uscilko & Grucza, 2001). Consequently, we were also strongly motivated to test whether sex differences exist in the potential choice between the two behaviour...
