Red Flag for Fish.

Brevetoxin Induces More--And Worse--Effects

In the Gulf of Mexico, population explosions of the marine dinoflagellate Gymnodinium breve are responsible for chronic, persistent "red tides," so named for the reddish-brown color the blooms give the affected waters. G. breve produces brevetoxins, potent neurotoxins that kill finfish and accumulate in filter-feeding animals such as shellfish. Because brevetoxins are fat-soluble and bioaccumulate in organisms, adverse effects might also be expected to occur in the eggs and developing embryos of fish and higher vertebrates. So researchers Karen L. Kimm-Brinson and John S. Ramsdell of the Center for Coastal Environmental Health and Biomolecular Research in Charleston, South Carolina, decided to study the toxins' developmental effects in aquatic species [EHP 109:377-381]. They microinjected eggs of medaka fish with one of the most common and potent brevetoxins, PbTx-1, and witnessed significant developmental abnormalities that impaired both hatching success and survival of medaka larvae.

The researchers designed their approach to reproduce the transfer of contaminant from mother fish to egg. Fertilized eggs collected from laboratory-bred females were microinjected with varying amounts of PbTx-1 reconstituted in a droplet of triolein fish oil. The egg microinjection method had already been used to characterize the adverse effects of known bioaccumulants such as ciguatoxin (a related polyether toxin also produced by a dinoflagellate) and the pesticide DDT in medaka fish. But comparable data for brevetoxin had not been reported until this study.

The results in this study were striking, say the researchers. The lowest observable effect on embryos occurred at a PbTx-1 dose concentration between 0.1 and 0.9 nanogram (ng) per egg--equal to 0.1-0.9 parts per million (ppm)--which induced excessive tail and body twitching in the early stages of development. The range of 0.1-3.0 ng/egg caused curvature of the spine, which became more pronounced with increased dose concentration. The most deformed of those embryos hatched out in an abnormal, head-first fashion and could not swim well enough to survive. Doses between 3.1 and 4.0 ng/egg produced malpositioned eyes and hernias in the brains and brain membranes of embryos. Eggs injected with concentrations of PbTx-1 above 4.0 ng failed to hatch.

Blooms of G. breve are toxic to fish at a concentration as low as 200,000 cells per liter, which produces approximately 5 ppm brevetoxin. At this 5 ppm concentration, Kimm-Brinson and Ramsdell concluded that abnormal developments similar to those in the lab experiment might occur if the animal had a bioaccumulation and egg-transfer factor of one-fifth the toxin in the water. This one-fifth bioaccumulation and egg transfer factor is well within the range found for other fat-soluble contaminants.

The results identify a new spectrum of adverse effects from red tides, which often persist from late fall to early spring, the breeding season of many Gulf fish species. Although adverse effects on larval life stages had been suspected, this new research suggests that the spawning success of fish also may be compromised. Further, sublethal exposure to brevetoxins throughout the life of many marine organisms may cause developmental toxicity to manifest in subsequent generations as with DDT, polychlorinated biphenyls, and other contaminants that accumulate in fatty tissues. Because the developmental processes of lower and higher vertebrates are similar, the potential exists for similar cumulative risks to marine mammals and even to humans.