Effects of Cadmium on Locomotor Activity Rhythms of the Amphipod Gammarus aequicauda P. S. Morillo-Velarde • J. Lloret • A. Marı ´n • F. J. Sa ´nchez-Va ´zquez Received: 5 October 2009 / Accepted: 14 June 2010 / Published online: 6 July 2010 Ó Springer Science+Business Media, LLC 2010 Abstract Behavioural responses are linked to complex biochemical and physiologic changes and may act as sen- sitive indicators of the sublethal effects of pollutants. This article investigates changes in the locomotor activity rhythms of the amphipod Gammarus aequicauda exposed to cadmium (Cd) as a model to study the effect of pollu- tants on an ecologically important species. Under a 12:12 h light-to-dark cycle, G. aequicauda showed a strict noctur- nal rhythm, with 90.2 ± 0.4% of their total daily activity occurring during the night. Under constant darkness, cir- cadian rhythms persisted for 10 days, with a mean peri- odicity of 24.32 h, thus confirming endogenous control. Exposure to sublethal concentrations of Cd (0.16, 0.20, 0.24, and 0.28 mg l -1 ) did not change the nocturnal activity patterns of G. aequicauda, although their swim- ming activity during the night was significantly decreased by exposure to concentrations of 0.24 and 0.28 mg Cd l -1 . In conclusion, locomotor activity bioassays using the amphipod G. aequicauda appeared to be a sensitive indi- cator of Cd contamination, and sensitivity and tolerance to Cd in short-term bioassays may depend on the time of the day tests are carried out. These results provide further support for the idea that behavioural end points in amphi- pods are useful indicators of pollutant exposure and that future studies should take circadian rhythms into consideration. Basic biologic functions (e.g., feeding and locomotor activity) show circadian rhythmicity that is usually syn- chronised to cyclic environmental events, such as the day/ night cycle. Thus, animals exhibit distinct diurnal, or nocturnal, daily patterns of behaviour. To examine the internal origin of these daily activity rhythms, animals are usually kept under constant conditions and isolated from any external time cue (i.e., constant dark), in which cir- cadian rhythms run freely with a periodicity close to, but slightly different from, 24 h (Aschoff 1981). Although circadian rhythms have been widely investigated in ter- restrial vertebrates and some aquatic animals, such as fish (Lopez-Olmeda and Sa ´nchez-Va ´zquez 2010), little is known about marine invertebrate chronobiology. Amphipods are considered key estuarine species in many Mediterranean coastal food chains. Habitats in which amphipods occur may be polluted, and because amphipods can be easily manipulated, they constitute an optimal model species for toxicity assessment in laboratory bioas- says (Oakden et al. 1984; Thomas 1993; Costa et al. 1998; Ce ´sar et al. 2000; Prato and Biandolino 2009). Locomo- tion, which is required by gammarids to find food, avoid predation, and mate, like other biologic functions may show a typical rhythmicity for the species considered. Any pollutant that interferes with locomotor activity and/or its rhythmicity may decrease the fitness of the affected organisms (Mills et al. 2006) and have potentially negative consequences for their natural population. Thus, locomotor activity may be seen as a behavioural marker of great importance. However, it is controversial as to whether such a rhythm is generated endogenously by an internal timing system or biologic clock or if it is driven exogenously by environmental cycles. To assess the impact of pollutants in aquatic ecosystems, various protocols have been developed using small P. S. Morillo-Velarde (&) Á J. Lloret Á A. Marı ´n Department of Ecology and Hydrology, University of Murcia, 30100 Murcia, Spain e-mail: piedad.smv@um.es F. J. Sa ´nchez-Va ´zquez Department of Physiology, University of Murcia, 30100 Murcia, Spain 123 Arch Environ Contam Toxicol (2011) 60:444–451 DOI 10.1007/s00244-010-9563-9