Gill histology of Pandalus danae under acute salinity shock By Melina G. Wettstein ABSTRACT Thalassinidean shrimp, commonly known as mud and ghost shrimp, are disrupting Washington’s profitable Pacific oyster aquaculture industry by disturbing sediment. As the sediment loosens, the oysters sink, destroying the bed. In an attempt to replace harsh chemicals as pest control, a brine solution to osmotically eliminate these shrimp has been researched. Shrimp, like many other invertebrates, are osmoconformers and cannot control their internal pressure, so extreme changes in salinity cause acute death. Unfortunately, when this high salinity water is pulled back to the ocean at high tide, subtidal invertebrates, like the coonstripe shrimp Pandalus danae may be impacted. Here we show the death rates of P. danae in varying salinities, and gill dehydration due to osmotic pressure through histology. Mortality rates were high for all salinity treatments. All the gills processed through histology showed intense dehydration. Lethal exposure time varied by salinity treatment, but the amount of dehydration did not. Our results show that runoff from the brine treatments to kill mud shrimp may impact P. danae and other osmoconformers in the subtidal. This study provides information about collateral ecological impacts on other organisms in the brine control method. This study only showed the impacts of acute salinity, and did not take into account the currents or geography of the subtidal in a real ecosystem, which must be further studied.
INTRODUCTION Washington State aquaculture is incredibly important to the economy, contributing $184 million in 2010 (Washington Sea Grant 2015). Unfortunately for the industry, oyster beds are full of pests, specifically thalassinidean shrimp like mud and ghost shrimp (Biffarius arenosus). Burrowing shrimp have an indirect negative effect on survival of oysters by bioturbation and sediment destabilization (Feldman 2000). The oysters need a solid substrate to attach to and grow on, which is loosened by the mud shrimp as they burrow. It is not uncommon for aquaculturists to pull sinking oysters from sediment in infested beds (Smith 1996). Suspended sediments 6 fieldnotesjournal.org
from shrimp digging negatively impact oyster’s filter feeding mechanisms as well (Feldman 2000). Through a special permit, oyster farmers typically use carbaryl, a pesticide that is the most effective method for removing shrimp from oyster beds (Feldman 2000). On the downside, newly settled subadult Dungeness crabs (Cancer magister) are directly exposed to the acutely toxic chemical spray, causing mass mortality. For this reason, oyster farmers have been pushed away from traditional pest control methods (Feldman 2000). In saltwater systems, organisms must either expend energy to maintain their internal salt concentrations, or they must be able to tolerate some variation as the environment changes around them (Shumway 1977). Crustaceans are typically osmoconformers, meaning their osmolarity, or internal salt concentration, is at the same salinity as the water they inhabit (Foster 2010). Changing salinity is dangerous for osmoconformers because they lack the internal mechanisms to balance internal water and salt concentrations, so water diffuses freely in or out of their cells as needed to achieve equilibrium. Any changes cause stress on the proteins, enzymes, and internal processes of the organism. Rapid changes in salinity can lead to either internal damage in a hyperosmotic system or dehydration in a hypoosmotic system. Because gills are optimized for gas exchange, they have an extremely large surface area and thus high water transport (Freir 2008). This means that in a high salinity shock, this organ is likely the point of failure, especially for crustaceans with gills that are always exposed. In the aquarium industry, reef-keepers often use rapid salinity change to shock or kill crustacean parasites or kill them before introducing new corals into their tanks, as many crustaceans are prone to salinity shock (Lougher 2016). Therefore, applying, high salinity water over oyster beds at low tide could be used for crustacean pest removal. Researchers and aquaculturists are testing 60 to 80 practical salinity unit (psu) brine solution in a new venture to rid oyster beds of thalassianian shrimp without harsh chemicals—a targeted osmotic attack (unpublished). The oysters can close up to survive, but the shrimp are left exposed to the brine. In an acute high salinity treatment, the targeted
