Manatee stressors

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.

Florida manatees at Crystal River National Wildlife Refuge, located on central Florida's gulf coast. 

(Image by U.S. Fish & Wildlife Service/David Hinkel via Flickr)

Ocean acidification

Last week, there was a flurry of news reports on the effects of ocean acidification on shellfish stocks in the U.S. (in response to a paper on the subject published in Nature Climate Change), and the news wasn’t good – according to one report, “U.S. shellfish producers in the Northeast and the Gulf of Mexico will be most vulnerable to an acidification of the oceans.” Ocean acidification refers to the global phenomenon of decreasing pH in marine waters as a result of human-induced carbon dioxide emissions dissolving in the ocean, and it can have a big effect on marine organisms. Acidic ocean water holds less calcium carbonate – meaning it’s less available for the organisms that need it to build their exoskeletons and shells – and it can even dissolve already formed shells.

New research recently published in the journal Aquatic Toxicology suggests that the mechanisms behind the negative effects of ocean acidification are not always straightforward – ocean acidification can weaken organisms’ immune systems and make them more susceptible to other, more sporadic threats. 

A team of Swedish researchers exposed tanks of Norway lobsters (a small lobster native to northern Europe) to saltwater with a pH lowered to the level predicted to occur by 2100. They measured the lobsters’ immune response to an injection of a common marine bacteria, and the amount of bacteria that persisted in the lobsters for 24 hours after the injection – if the lobsters’ immune systems were unaffected by the experiment and working well, they expected to see fewer bacterial cells after 24 hours. 

In order to test the lobsters’ response to other stressors that might occur in the presence of acidic ocean water, the scientists also exposed some of the lobsters to low oxygen, or hypoxic, conditions and high levels of manganese, a heavy metal that can have toxic effects at high exposures. (Scientists predict increasingly frequent intermittent periods of hypoxia as well as increasing levels of bioavailable manganese in marine environments as the world’s climate continues to change.)

Based on counts of immune cells, acidic ocean water on its own did not appear to affect the immune response of Norway lobsters; however, when the lobsters were exposed to hypoxia or manganese as well as acidic conditions, they had fewer immune cells than lobsters that didn’t experience hypoxia or high manganese levels. (Lobsters that were exposed to manganese under current pH levels also had fewer immune cells.)

Bacterial counts, however, told a slightly different story – the lobsters in the acidic condition tanks that didn’t experience any additional stressors weren’t able to reduce their bacterial loads, suggesting that their immune cells, though not reduced in number, were not functioning properly. (The lobsters in the hypoxic and high manganese conditions also had high bacterial loads, as did the lobsters exposed to manganese but not acidic ocean water; only lobsters in water with current pH levels and either no additional stressors or low oxygen were able to reduce the amount of bacteria in their bodies.)

As the acidity of the world’s oceans continues to increase, lobsters and other marine organisms may find themselves in an increasingly challenging environment in which they struggle to fight off infections, build their exoskeletons and shells, and survive.

Norway lobsters are typically about seven to eight inches long, including claws and tail. 

(Image by Hans Hillewaert via Wikimedia Commons)