Late last year a Japanese researcher continued a line of environmental research into the potential adverse environmental effects of manufactured nanoparticles on a species of small fish, their eggs, and hatch-lings. The study exposed Medaka fish and their offspring to latex nanospheres in various sizes and at various water salinity levels. The fluorescent particles ranged in size from 39.4 nanometers to 42,000 nanometers in diameter and exposure times varied. 

The study presented some interesting findings:

· Adult Medaka were dissected after exposure to 39.4 nm particles at 10 mg/l for 7 days and certain organs were then examined under a microscope. The liver, intestines, gonads, gills, kidney, ovary, testis and brains of the fish all exhibited nanoparticle uptake. No mortality was experience in this test.

· Medaka eggs were exposed to 39.4 nm particles under the same conditions. Nanoparticle uptake was observed in the eggs’ oil droplets, envelopes, and yolks following that order of accumulation. Larvae spawned from the eggs showed nanoparticle accumulation in the yolk and gallbladder, but not the liver. No mortality was observed in this test either.

· Additional Medaka eggs were also exposed for 3 days to 39.4 to 42,000 nm particles. Of all the various sizes of particles used, 474 nm particles were adsorbed and accumulated most by the eggs. Larger and smaller particles showed less adsorption and accumulation.

· Increasing levels of water salinity increased nanoparticle adsorption by the Medaka eggs.

· Increasing levels of water salinity also increased mortality. There was no mortality at lower salinity levels, but all of the fish died at high salinity levels.

The researcher concluded nanoparticles entered the Medaka through the membrane of the gills and/or intestine, then entered the bloodstream, and were transported and deposited into various organs. The olfactory neuron migration route as previously theorized in other fish experiments was ruled out. Moreover, the researcher found “[s]alinity may affect the bioavailability of nanoparticles to penetrate membranes.”  He theorized, the potential toxicity of nanoparticles is complex and “depends on multiple physicochemical and environmental factors” such as particle size, chemical composition, surface structure, solubility shape and aggregation. He concluded with the familiar refrain that more studies are needed in this area, and advocated a precautionary approach: “until more is known about the environmental effects of nanomaterials, the release of manufactured nanomaterial into the environment must be avoided as far as possible.”

S. Kashiwada, “Distribution of Nanoparticles in the See-through Medaka (Oryzias latipes),” Environmental Health Perspectives Vol. 114, No. 11, Nov. 2006.