Last month, a herd of 19 manatees briefly made a splash on the national news circuit when they got stuck in (and were subsequently rescued from) a storm drain in Florida. The group swam into the drainpipe in an apparent attempt to find warmer water during a cold snap – manatees are sensitive to cold water, and temperatures below 68 degrees Fahrenheit can hurt or even kill them.
Florida manatees – marine mammals that are about 10 feet long and weigh 800 to 1,200 pounds – also face threats from other sources, including boat collisions, habitat loss, and red tides, or harmful algal blooms. Concentrations of the dinoflagellate Karenia brevis are the most common cause of red tides in the Gulf of Mexico; these microorganisms produce a suite of toxins, called brevetoxins, which are harmful to humans and other vertebrates, and can kill hundreds of manatees at a time. (The overall population of Florida manatees is estimated at 5,000 to 6,500 individuals, so hundreds of deaths during a single event is a big deal.).
Although many Florida manatees that encounter red tides succumb to the toxins the algae produce, some manage to survive (sometimes with the help of humans). A team of scientists working in Florida reported the results of a study they conducted on the adverse effects of red tide exposure on surviving manatees in a recent issue of the journal Aquatic Toxicology.
The researchers collected blood samples from 12 manatees that were rescued from a red tide, and from 11 free-ranging manatees from a different, red tide-free location. They measured several parameters that indicate immune response, as well as the concentration of brevetoxin in each manatee’s plasma.
Some immune system indicators were the same regardless of whether or not the manatees had been exposed to the red tide, but others, particularly lymphocyte proliferation (a measurement of the body’s ability to defend itself against pathogens), showed that manatees that endured a red tide displayed reduced immune system functioning. The scientists also found that the manatees with the highest concentrations of brevetoxin in their plasma tended to have the lowest lymphocyte proliferation scores. As the authors point out, the effect of red tides on manatee lymphocyte proliferation “has the potential to result in an immunosuppressed animal that could likely exhibit greater susceptibility to other stressors.”
Florida manatees encounter many hazards in their environment; according to this research, even threats that don’t prove immediately lethal may have lasting negative consequences.
Source: Walsh CJ, Butawan M, Yordy J, Ball R, Flewelling L, de Wit M, Bonde RK. (2015) Sublethal red tide toxin exposure in free-ranging manatees (Trichechus manatus) affects the immune system through reduced lymphocyte proliferation responses, inflammation, and oxidative stress. Aquatic Toxicology 161: 73-84. doi: 10.1016/j.aquatox.2015.01.019