World Aquaculture 47 Fig. 2. The anatomy of a sea urchin. (Photo by Vidar Mortensen, drawing by Oddvar Dahl.) vironments, it is crucial to understand the water quality tolerance limits of sea urchins in aquaculture holding systems. This is particularly important for intensive culture of sea urchins, where levels of harmful substances can be elevated well above concentrations that normally occur in natural environments. The Importance of Oxygen and Water Movement The single most important water quality parameter in any aquaculture holding system is a continuous and sufficient supply of oxygen, inasmuch as this is a prerequisite for the survival, growth and well-being of all animals. In most species of sea urchins, oxygen is taken up by five pairs of external gills that are thin-walled projections of the body cavity located around the mouth. In other species, including S. droebachiensis, oxygen is taken up through the podia and transported internally through radial water canals that connect all podia (Figure 2). The movement of water inside radial water canals is facilitated by cilia. Because of the lack of respiratory pigments, oxygen is transported only as dissolved gas, giving a transport capacity 200 times lower than that of fish blood (Steen 1965). The internal organs of sea urchins are bathed in the coelomic fluid and receive oxygen from this fluid by simple diffusion, although the movement of oxygen may be enhanced by cilia on the inside of the coelomic cavity. The uptake of oxygen and amount of dissolved oxygen (DO) in the coelomic fluid of sea urchins is closely related to ambient DO concentration in the surrounding seawater. Thus, sea urchins are oxygen conformers and oxygen uptake may be restricted by insufficient oxygen supply (Giese at al. 1966, Johansen and Vadas 1967, Webster and Giese 1975, Spicer 1995). Accordingly, Siikavuopio et al. (2007b) found that gonad growth of S. droebachiensis was strongly affected by oxygen saturation in the ambient water. When oxygen saturation was reduced from 100 percent to 63 and 42 percent, gonad growth was reduced by 39 and 48 percent, respectively. Although not conclusive, these data suggest a linear relationship between growth performance and oxygen saturation. This effect has also been described in another echinoderm, the sand dollar Mellita quinquiesperforata (Lane and Lawrence, 1979). A linear correlation between growth performance and oxygen saturation is not surprising for animals that lack oxygen binding compounds in their body fluids. With most teleosts, on the other hand, growth is not affected until oxygen saturation falls below a certain threshold level, often around 70 percent (Jobling 1994), as a result of the oxygen binding characteristics of fish hemoglobin. Above the threshold level the oxygen carrying capacity of hemoglobin is fully exploited and oxygen uptake and growth are independent of oxygen saturation. The relationship between growth and oxygen saturation in sea urchins is also influenced by water movement. In the wild, sea urchins are often larger and have greater roe content in areas of high water movement (current) compared to those found in areas of lower water movement (James et al. 2007). Water movement increases the gonad growth of captive adult Evechinus chloroticus (James 2006) and somatic growth of juvenile S. droebachiensis. Increased growth is thought to be a result of improved oxygen availability. However, whether this is caused by an increase in oxygen present in seawater (greater DO concentration) or a greater flow of water across the surface of the animal (where most oxygen uptake occurs) is unclear. When exposed to very low flow rates, the oxygen consumption of S. droebachiensis increases dramatically compared to the oxygen consumption of urchins held in very high flow rates. This indicates that urchins actively extract oxygen, as much as is possible for a sea urchin, from the surrounding seawater when flows are low and the metabolic cost reduces their growth. Further research is required to fully understand the complex interaction between oxygen consumption and water movement in sea urchins but it is clear that any sea urchin aquaculture holding system should provide high dissolved oxygen levels and a means of ensuring sufficient water movement across the surface of animals held within the system to allow optimal access to dissolved oxygen available in the seawater supply.
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