Aquatic Toxicology 77 (2006) 433–438 Application of growth-related sublethal endpoints in ecotoxicological assessments using a harpacticoid copepod Ulrika Dahl a,∗ , Elena Gorokhova b , Magnus Breitholtz a a Department of Applied Environmental Science (ITM), Stockholm University, S-106 91 Stockholm, Sweden b Department of Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden Received 30 November 2005; received in revised form 23 January 2006; accepted 24 January 2006 Abstract In ecotoxicology, there is an increasing demand for sensitive sublethal endpoints. The primary aim of the present study was therefore to evaluate the relative sensitivity and usefulness of four sublethal endpoints – development time, body length, RNA content and growth rate – in the harpacticoid copepod Nitocra spinipes, using the reference molecule Simvastatin. Development time decreased significantly at low sublethal concentrations of Simvastatin (p < 0.001; F = 13.249; 0.16–1.6 ␮g L −1 ), while RNA content and body length increased significantly at 0.16 ␮g L −1 (p < 0.001; F = 6.13) and 1.6 ␮g L −1 (p < 0.01; F = 2.365), respectively. The growth rate increased significantly at 0.16–5 ␮g L −1 (p < 0.01–0.001). Hence, significant responses of growth-related traits were observed already at 0.16 ␮g L −1 , which is about 5000 times lower than the acute toxicity (96 h-LC 50 : 810 ␮g L −1 ). These results show that all assayed endpoints are very sensitive and indicate that current ecotoxicity testing used for environmental protection activities may underestimate the risk for harpacticoid copepods and most likely for other small invertebrates, when relying exclusively on acute toxicity measurements. © 2006 Elsevier B.V. All rights reserved. Keywords: Development time; RNA content; Body length; Growth rate; ERA; Crustacea 1. Introduction Environmental risk assessment (ERA) of chemicals is one of the main elements of environmental protection activities. End- points are characteristics of an ecological component that may be affected by exposure to a stressor ( Suter, 2000). Assessment endpoints are explicit expressions of the actual environmental values that are to be protected (e.g. change in abundance and functioning of aquatic populations), while stressor endpoints are the measurable responses related to an assessment endpoint (e.g. change in survival, growth rate, reproductive potential, etc.). Important characteristics of a stressor endpoint are feasibility and reliability of determination. The stressor endpoint selection is perhaps the most critical aspect of ERA, as it sets the limits for the remainder of the process. Any component from virtu- ally any level of biological organization or structural form can be used as stressor endpoint; hence, the selection of endpoints and organisms to be used for effect characterisation is a tremen- ∗ Corresponding author. Fax: +46 8 674 76 36. E-mail address: ulrika.dahl@itm.su.se (U. Dahl). dous challenge. Current ERA relies on a number of standard test organisms, such as small crustaceans. Among those, copepods are continuously used in growth models and life-cycle tests to predict the impact of chemicals on early development and repro- duction (e.g. Hutchinson et al., 1999; Breitholtz and Bengtsson, 2001; Breitholtz et al., 2003; Breitholtz and Wollenberger, 2003; Brown et al., 2003; Chandler et al., 2004). Most commonly used stressor endpoints are variables related to growth performance (Sibly and Hone, 2002).Growth is defined as a net-difference between the catabolic and anabolic metabolism (Becker et al., 2000) and is usually expressed as a change in body size or reproduction output ( Winberg, 1971). The body size correlates with many ecological as well as life-history traits and may thus influence the abundance of species as well as population structure and dynamics (Gaston et al., 2001), and it is an important stressor endpoint that is commonly used for growth assessment (e.g. Winberg, 1971). Identification of biochemical changes, such as RNA contents, which are related to growth and metabolism (Dahlhoff, 2004),can be used to determine if an organism has been exposed to a stress, including contaminants (Yang et al., 2002). The rationale is based on the fact that the RNA content of tissues or whole organisms consists primarily of 0166-445X/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2006.01.014