46 June 2012 Water quality requirements for culture of the green sea urchin, Strongylocentrotus droebachiensis Atle Mortensen1*, Sten I. Siikavuopio1 and Philip James1 Sea urchin roe is a highly prized seafood in a number of countries and is considered one of the most valuable seafoods in the lucrative Japanese market (Hagen 1996). Demand for high quality sea urchin gonad over the last three decades has led to extensive exploitation of wild sea urchin populations all over the world, with resulting over-fishing and declining fisheries stocks in many countries (Andrew et al. 2002). This, together with the global increase in the popularity of sushi, has led to increased interest in aquaculture of a range of sea urchin species, including the coolwater species Strongylocentrotus droebachiensis (Figure 1). This species has a broad arctic-boreal distribution and is found across the whole northern hemisphere from latitudes as high as Greenland in the north and as far south as Cape Cod on the east coast of the USA, southern Scandanavia, northern UK, Oregon on the west coast of the USA, and northern Japan. Because of its wide distribution and excellent taste and market value, there has been extensive research on the biology, ecology and fisheries of the species. More recently it has also been identified as one of the sea urchin species to have the best potential for aquaculture. Consequently much of the research investigating various aspects of sea urchin aquaculture has focused on S. droebachiensis. The aquaculture of any new species presents a range of challenges. In addition to having a suitable feed, either natural or manufactured, and available holding system technology, it is essential to understand the environmental requirements of any cultured species. In this article, we summarize some of the research that has been carried out on water quality requirements of S. droebachiensis related to the potential culture of this species. Sea Urchins and Aquaculture Sea urchins are ancient and primitive organisms that lack many of the organs found in higher animals (Figure 2). They have no specialized respiratory system or circulatory system, no heart, no blood vessels and no oxygen binding molecules in their body fluids, nor do they have any specialized excretory organs. Basically, sea urchins consist of a mouth, a digestive system or gut tube, gonads, also known as the roe, and a primitive nervous system, all surrounded by a hard, calcareous shell, also known as the test. On the outside of Fig. 1. The green sea urchin Strongylocentrotus droebachiensis (Photo by Vidar Mortensen). the test are spines, podia (tube feet) and some gripping devices called pedicellaries (Figure 2). The limited number of specialized organs means that sea urchins have a limited capacity to regulate their internal bodies when faced with variations in the external environment. This means they are, to a much greater extent than higher animals, at the mercy of their surroundings. Because the composition of their body fluids, particularly the ceolomic fluid found in the cavity inside the test, mirrors the composition of surrounding seawater, sea urchins are known as conformers and are generally intolerant to changes, rather than living in stable environments where the conditions remain relatively constant. For example, sea urchins have a very limited ability to osmoregulate and consequently are never found in fresh water (Barker and Russell 2008). In fact, sea urchins can be used as indicator organisms for environmental degradation, such as water pollution, in much the same way as canaries were used to detect poisonous gases in coal mines in earlier days (Meriç et al. 2005). Because of the sensitivity of sea urchins to changes in water quality and the inability to regulate their internal en-
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